uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

trace element abundances in hibonites from the murchison carbonaceous chondrite constraints on high temperature processes in the solar nebula abundances of trace elements including 14 rare earth elements ree and 8 other refractory trace elements rte have been measured in 33 hibonite and 3 perovskite bearing grains from the murchison carbonaceous chondrite the normalized abundance patterns show a good correlation with three morphological and chemical groups of hibonites sixteen platy hibonite crystals placs typically have patterns which are depleted in the relatively volatile trace elements v nb ba eu and yb group iii three placs also show large depletions in ce one plac is enriched in the ultrarefractory ree while two other placs have fractionated patterns enriched in the light ree with positive tm anomalies the placs commonly have a roll off in the abundances of the hree which is consistent with solid liquid partitioning three blue aggregates bags have patterns with low overall abundances and depletions in the ultrarefractory ree except tm group ii spinel hibonite grains shibs show a variety of patterns mainly of group ii but 4 shibs have relatively smooth patterns with anomalies in the volatile ree ce eu and yb the perovskites have patterns similar to group ii but with excesses in eu and yb the hree roll off in the placs and the igneous morphologies of the shibs indicate that these grains formed by the crystallization of melts distillation may be important for the bags and two of the placs which have large isotopic mass fractionation effects placs have the simplest trace element patterns and the largest ti isotopic anomalies and probably were the first hibonites to form in the early solar system the shibs have trace element signatures that require multiple processing episodes they are somewhat complementary to the placs in that they are enriched in the volatile elements which are depleted in the placs on average the shibs contain isotopically less anomalous ti than the placs however shibs contain 26mg excesses that are consistent with the in situ decay of 26al whereas placs generally have only small or no excesses of 26mg despite high 27al24mg while an overall relationship exists between the morphology chemistry and ti and mg isotopic systematics of meteoritic hibonites they do not as yet fit into a simple scenario for the formation of the solar system

roller bearing tectonic evolution of the juan fernandez microplate a combined gloria sidescan and hydrosweep multi narrow beam sonar survey shows striking evidence for fan shaped spreading systems bounding pseudofaults and compression ridges associated with 90 of clockwise rotation of the juan fernandez microplate in the past 4 myr a quantitative analysis of this evolution suggests that the microplate has rotated as a rigid roller bearing for most of its history driven at its edges by shear between the surrounding major plates the remarkable similarity between the juan fernandez and easter microplates despite their different tectonic settings suggests that this is a general model for microplate tectonics

magnetite melt hfse partitioning abstract results from doped hydrous experiments on natural mafic to intermediate composition lavas at 2 5 kbar pressure were combined with existing 1 atm data to evaluate the effects of composition and temperature on the partitioning behavior of the high field strength elements hfse zr nb ta and hf between magnetite and natural silicate melts magnetite composition was found to be the strongest controlling factor on partitioning behavior the partition coefficients d for zr nb hf and ta correlate with d ti ti and al content of the magnetite temperature and pressure the partition coefficients for the hfse are similar to one another for any given magnetite melt pair but range from 2 in titanomagnetite in addition the relationship between ti and the hfse changes as a function of pressure and temperature with the hfse becoming more incompatible relative to ti at lower temperatures and or higher pressures this change in the relationship between d ti and d hfse with temperature and pressure means that the expressions presented in nielsen et al 1994 nielsen r l forsythe l m gallaghan w e fisk m r 1994 major and trace element magnetite melt partitioning chem geol 117 167 191 are not valid for hydrous aluminous systems expressions were derived to describe the relationship between d hfse and temperature pressure fe 2 mg exchange ti al ratio of the magnetite and d ti these expressions reproduce the input data within 35 50 1 over a range extending from highly incompatible to compatible

trace element abundances and magnesium calcium and titanium isotopic compositions of grossite containing inclusions from the carbonaceous chondrite acfer 182 the carbonaceous chondrite acfer 182 contains ca al rich inclusions cais that differ from most cais in other meteorites many of them contain grossite caal4o7 whose modal abundance exceeds 30 vol in most of these cais grossite rich inclusions similar inclusions have been found only in the ch chondrite alh85085 and the cr chondrite acfer 059 el djouf 001 ion microprobe analyses of trace elements were made on nineteen cais from acfer 182 and two cais from acfer 059 el djouf 001 of al mg isotopes on eighteen acfer 182 and the two acfer 059 el djouf 001 inclusions and of calcium isotopes on ten acfer 182 inclusions on six of them also of titanium isotopes and one acfer 059 el djouf 001 inclusion trace element and isotopic signatures of grossite containing inclusions from acfer 182 resemble those of some inclusions from alh85085 volatility fractionated trace element abundance patterns ranging from ultrarefractory depleted similar to group ii pattern to volatile enhanced are predominant 17 cais including both inclusions from acfer 059 el djouf 001 two inclusions have ultrarefractory patterns one has a group iii pattern and one inclusion has a group iii related pattern similar to patterns found in hal type inclusions only one cai from acfer 182 but both inclusions from acfer 059 el djouf 001 have nonlinear excesses of 26mg corresponding to the initial 26al27al ratio of 5 10 5 none of the twenty inclusions has a significant intrinsic isotopic mass fractionation of mg titanium is normal in all analyzed cais and only small 48ca excesses 4 3 and 3 8 are present in two inclusions isotopes of both ca and ti show no significant intrinsic mass fractionations grossite containing inclusions from acfer 182 occur in different petrographic contexts in many cases petrographic characteristics are correlated with trace element abundances almost all inclusions studied in this work have a condensation history but no evidence for extensive evaporation processes is present many of the cais must have formed by direct gas solid condensation while some inclusions with clearly identifiable igneous textures formed from a refractory melt grossite whose formation was the subject of controversy in the past probably condensed from a reservoir with a higher than chondritic ca al ratio and depleted in the highly refractory trace elements or crystallized from a ca al rich melt the two ultrarefractory inclusions from acfer 182 have extremely high refractory trace element abundances sometimes exceeding 105 ci the lack of isotopic fractionation indicates a formation at very high temperatures by condensation rather than evaporation the lack of 26mg excesses in almost all inclusions from acfer 182 is best explained by heterogeneous distribution of 26al in the primitive solar nebula grossite rich inclusions having volatility fractionated ultrarefractory depleted trace element abundance patterns and lacking 26mg excesses could be the complement of platy hibonites placs from murchison with group iii or ultrarefractory patterns and likewise no 26mg excesses

evolution of the juan fernandez microplate during the last three million years during the spring of 1983 the r v thomas washington surveyed an area located north of the antarctic nazca pacific triple junction at 35 s magnetic and seabeam bathymetric data collected during the survey confirmed the existence of the juan fernandez microplate this paper presents an analysis of the magnetic anomaly data the western boundary of the microplate is a fast spreading center which has existed since 2 ma and where the accretion rate has been about 14 5 cm yr for the last 0 7 ma the eastern boundary of the microplate is characterized by a slow spreading center which separates the juan fernandez and nazca plates the accretion rate has been about 7 0 cm yr between 0 7 and 0 4 ma and about 1 6 cm yr for the last 0 4 ma the two spreading centers are connected in the north and south by transform faults between the juan fernandez and rapanui microplates the east pacific rise is well defined between 30 and 32 s in this region the axis displays a record accretion of about 17 2 cm yr south of the juan fernandez microplate one magnetic profile oceano 7008 indicates that the opening rate is about 12 0 cm yr on the antarctic pacific ridge the birth of the microplate is dated at about 2 ma when the western boundary started to accrete the evolution of the microplate corresponds to a transfer of accretion from the eastern boundary to the western axis this is revealed by the net decrease of the opening rate from 7 0 cm yr to 1 6 cm yr observed at the eastern ridge where a small jump occurred at 0 4 ma

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

tectonic history of the pacific farallon phoenix triple junction from late jurassic to early cretaceous an abandoned mesozoic spreading system in the central pacific basin in the central pacific basin an abandoned ne trending spreading ridge is evident on the satellite gravity map and is confirmed in bathymetric and magnetic data our identification of magnetic anomaly lineations flanking the abandoned ridge suggests that between chrons m21 147 ma and m14 135 ma it was the plate boundary between the pacific plate and a microplate which we here name the trinidad microplate the configuration of the pacific farallon phoenix triple junction until chron m22 149 ma was fault fault ridge ffr after chron m22 the pacific farallon phoenix triple junction changed its configuration the reorganization of the triple junction led to the birth of the trinidad microplate in its vicinity around chron m15 136 ma the spreading ridge between the pacific plate and the trinidad microplate was abandoned and the magellan microplate was born and remained active till chron m9 128 5 ma after welding of the magellan microplate to the pacific plate the triple junction had a ridge ridge ridge rrr configuration until the major reorganization around chron m0 120 ma

a plagioclase olivine spinel magnetite inclusion from maralinga ck evidence for sequential condensation and solid gas exchange abstract we report a detailed petrography mineral chemistry and trace element study of matroc a large calcium aluminum rich inclusion cai 5 2 5 mm of irregular triangular shape the inclusion has a zonal structure the core consists of a porous plagioclase olivine ca rich pyroxene intergrowth with subordinate apatite its texture is meta gabbro like similar to other plagioclase olivine inclusions pois the mantle has variable thickness 0 1 1 5 mm and consists of a compact symplectitic intergrowth of spinel hercynite and plagioclase with abundant dispersed magnetite subordinate ca rich pyroxene and traces of sulfides the thin 5 50 m discontinuous crust of matroc consists mainly of plagioclase with some olivine and magnetite the mg fe phases of matroc are fe rich olivine has fa33 2 and high nio content similar to that in the host rock ca rich pyroxene has much lower tio2 and cr2o3 contents than that of the host chondrite and plagioclase is an55 an74 magnetites have variable compositions are poorer in al2o3 and cr2o3 and richer in nio than those in the host spinels have also variable compositions rich in feo nio and zno despite their different mineralogy both core and mantle have bulk trace element abundances similar to those in average group ii cais however the mantle is richer in nb and u and poorer in eu be b sr and li than the core all minerals have high trace element contents minerals in the core show signs of incomplete equilibration of trace elements within and between them mantle minerals are far from equilibrium with each other and the bulk system spinel and anorthite carry the trace element signature of their precursor melilite or hibonite and magnetite contains large amounts of a heterogeneously distributed remnant extremely rich in trace elements obscurite possibly of a former perovskite inclusion matroc has a complex history the poi core probably formed by reaction of an unknown precursor s of condensation origin with a vapor to form olivine plagioclase clinopyroxene apatite and an unknown phase s that vanished leaving abundant void space the spinel rich mantle is also a secondary mineral assemblage that formed by breakdown of and solid vapor reactions with a precursor or precursors possibly melilite or hibonite the abundant magnetite formed by reaction of perovskite with an oxidizing vapor and by precipitation from such a vapor all phases of the inclusion experienced the metasomatic addition of fe ni and moderately volatile elements such as v be li cr and mn similar to all other constituents of the maralinga ck chondrite phases in matroc and in the host rock are close to equilibrium in the distribution of fe mg ni and mn but far from equilibrium in the distribution of m 3 and m 4 ions the minor and trace element abundances in the magnetite of the host rock and of matroc preclude an origin by oxidation of a metal precursor

hibonite hercynite inclusion hh 1 from the lance co3 meteorite the history of an ultrarefractory cai we studied the petrography major and trace element chemistry and magnesium calcium titanium and iron isotopic compositions of a hibonite rich refractory inclusion from the lance co3 meteorite the inclusion 290 230 m in size consists of a core of hibonite crystals containing small perovskite grains surrounded by a hercynite mantle and a thin diopside rim the hercynite contains many small perovskite ilmenite and sulfide grains trace element abundances show an ultrarefractory signature in all measured phases with an increasing depletion of the rees heavier than tb with increasing mass in hibonite perovskite and ilmenite but not hercynite calcium and titanium are isotopically anomalous with large deficits in 48ca and 50ti deficits are 60 in 50ti and 11 5 in 49ti in hibonite hercynite perovskite and ilmenite in addition the titanium isotopes show a mass dependent isotopic fractionation of 8 5 amu in favor of the heavy isotopes calcium isotope deficits in hibonite are 6 9 in 42ca 5 0 in 43ca and 32 5 in 48ca relative to 40ca 44ca the deficits are smaller in the rim diopside 48ca 21 and a neighboring pyroxene grain 48ca 8 magnesium shows excesses in 26mg but the lack of an isochron relationship between 26mg24mg and 27al24mg indicates redistribution of radiogenic 26mg magnesium in hibonite is isotopically light fmg 5 3 amu the iron isotopes in hercynite are normal the data show that inclusion hh 1 experienced a complex metasomatic history including fe metasomatism sulfurization and formation of nepheline and sodalite the isotopic data provide not only a record of nucleosynthetic processes and nebular inhomogeneity but also a tracer of the relative movement of elements which may have occurred during alteration

pito and orongo fracture zones the northern and southern boundaries of the easter microplate southeast pacific abstract the easter rapanui microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on earth is occurring today recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets during the january 1987 rapanui expedition on the n o jean charcot we conducted a sea beam magnetics gravity survey of a portion of the microplate boundaries the method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships the northern terminus of the east rift or eastern boundary of the microplate is expressed as a series of parallel nw se trending valleys including what appears to be with 5890 m depth the deepest active rift axis mapped in the pacific today pito rift the northern end of the pito rift merges with an e w to 083 narrow band of linear faults interpreted to be a transform fault between the nazca and easter rapanui plates the northern triple junction between the easter rapanui nazca and pacific plates is a rff type with the two transform faults colinear along an approximately e w direction the southwestern boundary of the easter rapanui microplate is marked by a series of en echelon offsets outlined by depressions which merge into an approximately e w zone where shear must be predominant the southern triple junction is a rrf junction with an overlapping ridge system the structural data acquired during the survey provide strong constraints for kinematic models of the microplate the structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate

compressional deformation north of the easter microplate a manned submersible and seafloor gravity investigation we have investigated an e w compressional ridge in 0 9 ma lithosphere of the nazca plate just north of the easter microplate on the east pacific rise using a combination of visual observation and seabed gravity measurements from four manned submersible dives three of the dives made a 26 km long traverse across the ridge during which 14 gravity measurements were made the observations confirm the presence of active thrust faulting on the 1 1 km high south face of the ridge however the gravity precludes the existence of a simple low angle thrust and is only compatible with significant crustal shortening and thickening if the block to the south of the thrust has a lower density than the overriding block visual and sonar observations of pervasive fine scale tectonism in this area lend some support to this idea alternatively there could be oblique thrusting on a high angle fault with relatively little crustal shortening in addition a small degree of crustal shortening may take place in the form of buckling of an elastic or plastic lithosphere though this cannot explain the strong asymmetry of the ridge which we believe must imply faulting

southeast pacific tectonic evolution from early oligocene to present plate tectonic reconstructions of the nazca antarctic and pacific plates are presented from late oligocene to present these reconstructions document major plate boundary reorganizations in the southeast pacific at dirons 6c 24 ma 6 o 20 ma and 5a 12 ma and a smaller reorganization at chron 3 o 5 ma during the chron 6 o reorganization it appears that a ridge propagated into crust north of the northernmost pacific antarctic ridge between the chiloe fracture zone fz of the chile ridge and agassiz fz of the pacific nazca ridge which resulted in a northward jump of the pacific antarctic nazca pac ant naz mid ocean triple junction during the chron 5a reorganization the chile ridge propagated northward from the valdivia fz system to the challenger fz through lithosphere formed roughly 5 myr earlier at the pacific nazca ridge during this reorganization a short lived microplate the friday microplate existed at the pac ant naz triple junction the pac ant naz triple junction jumped northward 500 km as a result of this reorganization from a location along the valdivia fz to a location along the challenger fz the chron 5a reorganization also included a change in spreading direction of the chile and pacific antarctic ridges the reorganization at chron 3 o initiated the formation of the juan fernandez and easter microplates along the east pacific rise the manner of plate boundary reorganization at chron 6 o and chron 5a and possibly today at the juan fernandez microplate included a sequence of rift propagation transfer of lithosphere from one plate to another microplate formation and microplate abandonment and resulted in northward migration of the pac ant naz triple junction the associated microplate differs from previously studied microplates in that there is no failed ridge

oceanic microplate formation records the onset of india eurasia collision abstract mapping of seafloor tectonic fabric in the indian ocean using high resolution satellite derived vertical gravity gradient data reveals an extinct pacific style oceanic microplate mammerickx microplate west of the ninetyeast ridge it is one of the first pacific style microplates to be mapped outside the pacific basin suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern pacific ridges the microplate formed at the indian antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south whose conjugate is located north of the kerguelen plateau independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric also seen in multibeam data magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o 47 3 ma plate reorganizations can trigger ridge propagation and microplate development and we propose that mammerickx microplate formation is linked with the india eurasia collision initial soft collision the collision altered the stress regime at the indian antarctic ridge leading to a change in segmentation and ridge propagation from an establishing transform fast indian antarctic spreading that preceded microplate formation and kerguelen plume activity may have facilitated ridge propagation via the production of thin and weak lithosphere however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event prior to the collision the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and w shaped lineations that record back and forth ridge propagation microplate formation provides a precise means of dating the onset of the india eurasia collision and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories

edge driven microplate kinematics it is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate s motion relative to the bounding major plates are frequently located close to its margins with those plates close to the tips of propagating rifts we propose a class of edge driven block models to illustrate how slip across the microplate margins block rotation and propagation of rifting may be related to the relative motion of the plates on either side an important feature of these edge driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates according to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time pseudofault geometries and matching rotations of the easter microplate show that for most of its 5 m y history block rotation could be driven by the drag of the nazca and pacific plates on the microplate s edges rather than by a shear flow of mantle underneath

presolar isotopic and chemical signatures in hibonite bearing refractory inclusions from the murchison carbonaceous chondrite magnesium calcium and titanium isotopic compositions as well as trace element abundances have been determined by ion microprobe mass spectrometry for 37 hibonite bearing inclusions from the murchison carbonaceous chondrite these and previously analyzed hibonite bearing inclusions can be divided into three main groups based on morphological and mineralogical criteria platy crystal fragments placs generally have depletions in the relatively volatile refractory trace elements eu yb mb v ba little or no evidence of 26al i e little or no radiogenic 26mg 26mg and widely variable ca and ti isotopic compositions one plac 13 13 has excess 48ca of 104 and 50ti of 273 the largest effects yet measured in these elements four blue aggregates bags are distinctive in having low overall trace element abundances with large relative depletions in the ultrarefractory elements gd er lu zr hf y sc and mass fractionated mg isotopic compositions enriched in the heavy isotopes large 50ti deficits have been measured in three of the four bags the only bag analyzed for its ca isotopic composition also shows a deficit in 48ca spinel hibonite inclusions shibs show a wide variety of morphologies ranging from bladed hibonite crystals within spinel to spherules of hibonite and spinel to large uniform hibonite crystals within spinel shibs consisting of bladed hibonite in spinel or hibonite spinel spherules typically have ultrarefractory depleted trace element patterns although uncommon ultrarefractory enriched patterns were also found 26mg at a level consistent with 5 10 5 27al and generally small 10 ca and ti isotopic anomalies one of the shib spherules 7 170 has highly unusual systematics with a large 50ti deficit only a small 48ca deficit and no 26mg the large hibonite crystals within spinel have variable trace element patterns commonly with anomalies in the relatively volatile elements ce eu and yb 26mg is present in some of these inclusions but none of this type of shib has large ca or ti isotopic anomalies while at least four distinct components are required to model the ti isotopic compositions three components are sufficient to describe the variations in ca isotopic compositions measured thus far the systematics of plac 13 13 with large excesses of 48ca and 50ti match the predictions of the multiple zone mixing model for nucleosynthesis in supernova ejecta to a maximum neutron excess of 0 159 compositions depleted in 48ca and 50ti are unlikely to represent discrete nucleosynthetic components but represent material with a shortfall of neutron rich material relative to the solar system abundances additional components must be present but are not easily resolved within the morphological groups of hibonite bearing inclusions a few have exceptional chemical and isotopic characteristics such as shib spherule 7 170 however independent of the morphological groups there are correlations in hibonite isotopic systematics and chemistry that hold for all inclusions studied 1 no inclusion has both a large 50ti anomaly and 26mg and 2 inclusions depleted in the ultrarefractory elements have 26mg with the exception of those which have a large 50ti anomaly these features clearly establish a relationship between chemical and isotopic characteristics of the inclusions which must predate their formation several components were combined probably as mixtures of dust grains or crystals and were melted to form refractory inclusions the nucleosynthetic anomalies must be inherited from stellar environments it is possible that the depletions in ultrarefractory elements are also a memory of stellar condensation and are not necessarily related to a condensation event in the solar nebula as has generally been assumed

origin of plagioclase olivine inclusions in carbonaceous chondrites plagioclase olivine inclusions pois are an abundant group of chondrule like objects with igneous textures found in carbonaceous chondrites pois consist of plagioclase olivine pyroxene and spinel and cover a wide range of compositions between type c ca al rich inclusions cais and ferromagnesian chondrules pois are distinguished from cais by the absence of melilite lack of refractory siderophile rich opaque assemblages more sodic plagioclase and abundance of olivine and aluminousenstatite rare accessory minerals including armalcolite zirconolite rutile and sapphirine are found in several pois the petrographie and chemical properties of pois indicate that they are not condensates or evaporative residues but formed by melting or partial melting of pre existing solids seven of fourteen pois contain isotopically fractionated mg and despite their textures these pois are not isotopically homogeneous spinel is the major carrier of fractionated mg in six pois the magnitude of mg isotopic fractionation fmg of spinel ranges from 8 to 11 amu among the pois but f mg is always either positive or negative within an individual pol within a single inclusion f mg of coexisting spinel is not constant but varies by up to 7 amu isotopic fractionation of mg in silicates is less common and always positive f mg 0 pyroxenes in two pois and plagioclase in a third contain isotopically heavy mg radiogenic 26 mg is rare in pois as only two inclusions show evidence of excess 26 mg with 26 mg 27 al 4 10 6 the preservation of isotopic heterogeneity in objects whose igneous textures suggest crystallization from a homogeneous melt implies that melting was incomplete allowing survival of relict phases a comparison of the essential characteristics of pois and cais suggests that the major processes leading to formation of pois including condensation dust gas fractionation aggregation of chemically and isotopically disparate materials and partial melting are common to most cais and chondrules we present a scenario for the formation of these objects and conclude that the homogeneity of the final assemblage cai poi or chondrule is primarily a reflection of the thermal history rather than the nature of precursor materials

block rotation and deformation by strike slip faults 2 the properties of a type of macroscopic discontinuous deformation study of the general features of deformation by systems of subparallel strike slip faults shows that the governing constraints are kinematic the fault blocks must remain in contact with each other and the deformed area must fit with its surroundings as a result fault blocks that move laterally without significant internal deformation also rotate about vertical axes relative to boundaries of the fault domain by an amount that is quantitatively related to fault slip spacing and orientation areas where left and right lateral faults move simultaneously are usually divided into domains that contain faults of one kind only deformation of such multidomain areas is nonhomogeneous but the differences between domains can compensate for each other so that the regional deformation is quite simple strike slip faulting combined with block rotation is an efficient mechanism of deformation because modest offsets and rotations change the linear dimensions of faulted areas by several tens of percents which may often greatly exceed the results of other types of coeval deformation paleomagnetic data can provide an independent measure of block rotations and should be used to supplement structural data block rotation changes the initial angles between right and left lateral faults and also their orientation relative to the stress field the faults probably tend to rotate away from the axis of maximum compression rotated faults continue to move because they are weak surfaces since the deformation depends on geometric factors such as fault orientation and domain geometry it is not simply related to the stress field these properties including rotation of material units characterize discontinuous deformation in general

the kinematic evolution of the macquarie plate a case study for the fragmentation of oceanic lithosphere abstract the tectonic evolution of the southeast indian ridge seir and in particular of its easternmost edge has not been constrained by high resolution shipboard data and therefore the kinematic details of its behavior are uncertain using new shipboard magnetic data obtained by r vib araon and m v l astrolabe along the easternmost seir and available archived magnetic data we estimated the finite rotation parameters of the macquarie antarctic and australian antarctic motions for eight anomalies 1o 2 2ay 2ao 3y 3o 3ay and 3ao these new finite rotations indicate that the macquarie plate since its creation 6 24 million years ago behaved as an independent and rigid plate confirming previous estimates the change in the australian antarctic spreading direction from n s to nw se appears to coincide with the formation of the macquarie plate at 6 24 ma analysis of the estimated plate motions indicates that the initiation and growth stages of the macquarie plate resemble the kinematic evolution of other microplates and continental breakup whereby a rapid acceleration in angular velocity took place after its initial formation followed by a slow decay suggesting that a decrease in the resistive strength force might have played a significant role in the kinematic evolution of the microplate the motions of the macquarie plate during its growth stages may have been further enhanced by the increased subducting rates along the hjort trench while the macquarie plate has exhibited constant growth by seafloor spreading

the development of two oceanic microplates easter and juan fernandez microplates east pacific rise in this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid ocean ridge microplates on the east pacific rise we quantify their development in terms of the principal dimensions of the microplates and their boundaries and the temporal changes in these dimensions the results are discussed in the framework of the roller bearing microplate model of schouten and others which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates we test the specific predictions of this model for the geometry of the pseudofaults traces of the propagating tips of the microplates spreading boundaries and the histories of rotation of the microplates both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling no slip or deformation between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries juan fernandez microplate was also rotating at the predicted rate just before 2 ma when it was driven by the nazca pacific couple rather than the nazca antarctic couple that currently applies however earlier rotations were slower than predicted implying some degree of decoupling or deformation at earlier times the microplates appear to have grown from moderate sized c 100 km probably left stepping offsets of the east pacific rise since about 5 ma the nature of these offsets is currently uncertain but they may have been propagating rifts which changed from shearing by bookshelf faulting to rigid block microplate rotation on reaching a critical size once rigid block rotation began the microplates grew by seafloor spreading accompanied by propagation of their bounding spreading centres the effects of the neighbouring easter mantle plume and pacific nazca antarctic triple junction can be discerned in the bathymetry and ridge propagation rates but do not appear to have had a major influence on the microplates development

migrating transform zone and lithospheric transfer at the galapagos 95 5 w propagator at the 95 5 w propagator system along the cocos nazca plate boundary the spreading axis is offset left laterally about 35 km 1 m y this offset migrates westward 52 km m y relative to points along the nearly east west trending ridge axis transferring lithosphere from the cocos to the nazca plate as it does so the required 58 km m y of right lateral motion across this migrating transform is apparently not accommodated by a throughgoing north south transform fault but rather by an approximately 15 to 20 km wide zone of shear deformation dominated by eastward curving of east west structures to a northwest southeast trend this pattern of curvature the coinciding zone of high seismicity and the abundance of talus in the area suggest that shear is distributed over this entire zone the deformation is inferred to be taken up by a bookshelf mechanism whereby faults oriented at high angles to the shear couple have left lateral slip antithetic to the right lateral shear couple rotating individual blocks clockwise these are interpreted as reactivated abyssal hill bounding normal faults probably dipping 45 60 northeast southwest shortening due to the rotation appears to be accommodated by reverse slip on the faults and may account for the uplift of de steiguer ridge in the zone of transferred lithosphere these faults have apparently had a very complex history initially normal faults formed as abyssal hills were created at the spreading axis and subsequently they had left lateral strike slip motion in the transform zone and reverse motion uplifting de steiguer ridge strain analysis based on observed structural curvature suggests that shear is distributed nonuniformly and lithospheric transfer requires 0 4 m y

the chile ridge a tectonic framework a new chile ridge tectonic framework is developed based on satellite altimetry data shipboard geophysical data and primarily 38 500 km of magnetic data gathered on a joint u s chilean aeromagnetic survey eighteen active transforms with fossil fracture zones fzs including two complex systems the chile fz and valdivia fz systems have been mapped between the northern end of the antarctic nazca plate boundary chile ridge at 35 s and the chile margin triple junction at 47 s chile ridge spreading rates from 23 ma to present have been determined and show slowdowns in spreading rates that correspond to times of ridge trench collisions the valdivia fz system previously mapped as two fzs with an uncharted seismically active region between them is now recognized to be a multiple offset fz system composed of six fzs separated by short ridge segments 22 to 27 km long at chron 5a 12 ma the chile ridge propagated from the valdivia fz system northward into the nazca plate through crust formed 5 myr earlier at the pacific nazca ridge evidence for this propagation event includes the friday and crusoe troughs located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults this propagation event led to the formation of the friday microplate which resulted in the transferal of crust from the nazca plate to the antarctic plate and in a 500 km northward stepwise migration of the pacific antarctic nazca triple junction rift propagation microplate formation microplate extinction and stepwise triple junction migration are found to occur during large scale plate motion changes and plate boundary changes in the southeast pacific

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

the occurrence of grossite caal4o7 in chondrites the mineral grossite caal4o7 ca dialuminate also termed ca2 is a rare phase in terrestrial and extraterrestrial rocks in meteorites grossite was found in ca al rich inclusions cais from allende cv3 vigarano cv3 leoville cv3 efremovka cv3 colony co3 murchison cm2 alh 85085 ch acfer 182 ch acfer 059 e1 djouf 001 cr2 and the anomalous c chondrite adelaide in ca al rich inclusions from most of these meteorites grossite is an accessory phase however in many cais from alh 85085 acfer 182 and acfer 059 e1 djouf 001 it is a major phase in this paper we summarize the results of literature reports on grossite bearing inclusions and focus in detail on the occurrence mineralogy and chemistry of caal4o7 containing ca al rich inclusions in acfer 182 and acfer 059 e1 djouf 001 in these carbonaceous chondrites abundant cais with high modal abundances of grossite 30 vol often 60 vol were discovered to classify ca al rich inclusions with such high modal abundances of caal4o7 we suggest to introduce a new type of ca al rich inclusion grossite rich inclusions in acfer 059 e1 djouf 001 and acfer 182 grossite containing inclusions occur with different petrographic characteristics the most common type consists of a core of grossite rimmed by melilite ca pyroxene spinel hibonite in an other type anhedral polycrystalline grossite complexes of irregular shape are enclosed in melilite in some spherical ca2 inclusions abundant euhedral to subhedral grossite and or hibonite crystals are embedded in melilite in two inclusions a core of hibonite is mantled by grossite additionally some grossite bearing inclusions with unique texture and or mineralogy were found one of these inclusions is rich in zr y rich perovskite and contains phases rich in the ultrarefractory elements sc and or zr almost all of the grossite containing inclusions are rich in al2o3 50 wt even if grossite is only a minor phase based on textural appearance and or on ree contents in some inclusions core minerals with group ii patterns a condensation history for most inclusions is suggested a single inclusion rich in perovskite and other phases containing high concentrations of ultrarefractory elements appears to have formed as an early condensate from the solar nebula

erosion driven drawdown of atmospheric carbon dioxide the organic pathway rapidly eroding coastal mountain belts where steep rivers and submarine channels connect upland sources to nearby marine sinks are hotspots of organic carbon transfer from life biomass soil and exhumed bedrock into geological storage using observations from the southern alps of new zealand and taiwan we have mapped this organic pathway to geological carbon sequestration and can evaluate the magnitude and efficiency of transfers between sources and sinks we demonstrate that poc is harvested by landsliding but importantly also by common and widespread surface runoff on steep hillslopes although terrestrially sourced poc is found in many sedimentary environments associated with mountain belts and frontier basins it appears to be most abundantly trapped and preserved in marine turbidites the loss of all forms of poc in onward transport through short steep routing systems to this repository is limited this is in marked contrast to larger routing systems in which only the most resilient forms of poc survive into long term deposition

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

overlapping rift zones at the 5 5 s offset of the east pacific rise a seabeam and magnetometer survey of the pacific nazca plate boundary around 5 5 s mapped a nontransform offset whose geology and kinematics seem typical of a whole class of structures formed where fast spreading rises are laterally offset for distances less than the width of a subaxial magma chamber though the spreading center is abruptly displaced by 15 km right laterally there is no trace of strike slip faulting instead the ends of two 100 km long axial volcanic ridges veer 15 toward each other and overlap for more than 20 km in this overlapping region between 5 22 s and 5 32 s accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one sided spreading a continuous but dog legged magma chamber is inferred to underlie both rift zones and an intervening 9 km wide volcano studded basin a strong magnetic signature from the crust which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank in the past 0 5 106 years the southern axial ridge has propagated north at 40 mm yr 55 of the spreading half rate

ridge hotspot interaction the pacific antarctic ridge and the foundation seamounts the study of different magmatic provinces between the resolution fracture zone 33 s 131 w and the pacific antarctic ridge par axis 37 s 111 w suggests that similar processes of interaction between hotspot and spreading axial magmatism occurred 20 25 ma and 0 5 ma ago there is evidence of this process from the changes in composition observed in the lavas erupted near 400 300 km between the present day par axis 37 s 111 w and the eastern tip of the foundation seamount fs hotspot near 36 20 s 114 w where the last alkali enriched volcanics k ti 0 30 zr y 6 and ce yb n 4 have erupted this transitional province between the par and the fs consists of volcanic cones built on several volcanic ridges 20 ma also built on volcanic ridges are identified west 1200 km from the par axis of a failed rift propagator frp forming the eastern boundary of the ancient selkirk microplate some of these seamounts made of alkali basalts were built during the initiation of the fs hotspot 20 23 ma ago the interaction and the influence thermal of mantle plume magmatism with the ancient spreading ridge of the farallon pacific plates was responsible for the eruptions of the t morbs and andesitic lavas this situation is comparable to that presently observed on the par axis where silicic lavas are also erupted in association with t morbs

late neogene motion of the pacific plate oceanic crust between 38 s and 65 s along the pacific antarctic ridge records a complete history of neogene pacific antarctic motion we have modeled magnetic stages corresponding to portions of isochrons 1 2 2a 3 3a 4 and 5 from 17 high quality marine magnetic traverses of the ridge to determine the neogene finite rotation poles for this plate pair we chose to model velocities over time intervals of about 0 5 m y because this greatly increased the number of estimates of pacific antarctic plate velocity our analysis shows that a change in pacific antarctic relative motion occurred between chron 2a 3 40 ma and the beginning of chron 3 time 3 86 ma we used 203 estimates of pacific antarctic spreading velocity to calculate a 0 3 4 ma and 3 86 10 3 ma euler pole combined with tracks of seamounts that record the passage of the pacific plate over mantle plumes our data show that this change in relative motion corresponds to a change in the absolute motion of the pacific plate with respect to the hotspot frame of reference using these euler poles to calculate pacific antarctic africa north america circuit euler poles show that this recent change in pacific plate absolute motion corresponds to a change from strike slip to transpressive motion along the coastal california pacific north america plate boundary interestingly this age of increased tectonism agrees in time with deformation in new zealand and northern japan

tectonic evolution of ridge axis discontinuities by the meeting linking or self decapitation of neighboring ridge segments recent deep tow sea beam and seamarc ii studies indicate that ridge axis discontinuities along the east pacific rise evolve in several different ways where magmatic pulses along the spreading center happen to meet head on a low point or saddle point occurs along the axial depth profile of the spreading center where the magmatic pulses misalign overlapping spreading centers develop and two distinct evolutionary paths are possible one spreading center tip may cut through to and link with the adjacent en echelon ridge chopping off the opposing ridge tip as proposed by macdonald and fox 1983 in addition either spreading center may cut inside or outside of itself repeatedly decapitating its own ridge tip crack propagation studies show that the crack propagation force g drops significantly when the ratio of crack overlap to crack offset l w exceeds 3 applying this relation to spreading centers we suggest that propagation of the individual spreading center tips may stall when l w is greater than 3 if linkage has not yet occurred then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip decapitating the ridge tip in a process we call self decapitation subsequent magmatic pulses may be deflected or derailed from the existing path of the ridge because freezing of the axial magma chamber near ridge axis discontinuities creates a core of coherent unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber in addition the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge for propagating rifts which represent first order changes in plate boundary geometry l w remains 1 5 so that g is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced

neotethyan intraoceanic microplate rotation and variations in spreading axis orientation palaeomagnetic evidence from the hatay ophiolite southern turkey abstract insights into tectonic processes operating in ancient ocean basins are provided by analyses of fragments of oceanic lithosphere preserved as ophiolites during collisional orogenesis here we present a palaeomagnetic analysis of the upper cretaceous hatay kizil dag ophiolite of turkey that provides evidence for intraoceanic microplate rotation and variations in ridge axis orientation in a neotethyan ocean basin magnetizations at 46 sites are shown to be pre deformational in origin and rotated from the relevant reference direction a net tectonic rotation approach to the analysis of the data provides information on permissible net rotation poles and angles and allows uncertainties in input vectors to be considered results demonstrate that all levels of the ophiolite have been rotated anticlockwise by angles in excess of 90 around steeply inclined axes the hatay ophiolite formed in the same supra subduction zone spreading system as the troodos ophiolite cyprus which is known to have rotated 90 anticlockwise in an intraoceanic setting in the late cretaceous to early eocene by considering our results in the context of the known timing of the troodos rotation we infer that 50 60 of rotation of the hatay ophiolite took place as part of an areally extensive troodos microplate this phase of rotation was triggered by initial impingement of the arabian continental margin with the neotethyan subduction trench consistent with models for modern day oceanic microplate rotation in complex convergent plate boundaries the hatay ophiolite then became detached from the actively rotating microplate and was emplaced onto the arabian margin in the maastrichtian undergoing a further 30 40 of anticlockwise rotation during thrusting back stripping of rotations allows correction of the hatay sheeted dykes to their initial orientations the restored dyke trend of 020 differs from that inferred previously for the troodos sheeted dyke complex demonstrating a primary variation in orientation of neotethyan spreading axes such variability is commonly observed in modern spreading systems in marginal basins these may act as analogues for the supra subduction zone spreading inferred for many ophiolites

modelling the plate driving mechanism this paper demonstrates how sub lithospheric loading gives rise to plate interior stresses and plate boundary forces the stressing of the lithosphere resulting from plume heads in the upper mantle hot spots from low density asthenospheric upwelling beneath ocean ridges and from dense subducting slabs has been modelled by finite element analysis it is first demonstrated how a sub lithospheric load exemplified by a sub continental plume head produces local deviatoric stress in the strong upper lithosphere within a plate interior region it is shown that a 500 km wide region of anomalous density averaging 8 kg m 3 between 100 km and 400 km depths gives rise to a deviatoric tension of 75 mpa in a 20 km thick strong layer forming the upper lithosphere above the deep buoyant load the large stresses in the strong layer are caused by the shear drag and excess pressure exerted by the buoyant load when such a stress system is cut across by a zone or plane of weakness then plate boundary forces and distant plate interior stresses are produced a weak zone beneath the crest is incorporated in a model of a normal ocean ridge and this yields a ridge push force of 2 5 10 12 nm 1 referenced to old ocean floor and deviatoric compression of about 40 mpa in old ocean floor a model of a ridge underlain by an anomalously low density upper mantle plume head yields a much larger ridge push force and large deviatoric compressions of nearly 100 mpa extend into the bordering continents subduction pull slab pull and trench suction of about 4 5 10 12 nm 1 is modelled for a 300 km deep slab separated from the overriding plate by a weak fault and collision pull is shown to result from downbulging lower lithosphere beneath collision mountain ranges

plate motions and crustal deformation over the past 30 years the plate tectonic paradigm has revolutionized the earth sciences through its powerful yet elegant quantitative treatment of large scale crustal deformation and through its conceptually integrative role within the earth sciences this review summarizes several important developments in plate tectonic research for the period 1991 to early 1994 with emphasis placed on publications that have either significantly improved our understanding of plate motions and their relation to crustal deformation or have raised significant new questions revisions to the geomagnetic reversal time scale cenozoic plate motions microplate kinematics oblique convergence and deformation of forearcs and kinematic information derived from detailed bathymetric and side scan sonar studies are featured

large rotation of the easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor one of the goals of the pito93 cruise along the perimeter of the easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study using a new orienting device the geocompass 14 oriented blocks were obtained during 6 dives conducted using the ifremer dsrv nautile paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and af demagnetizations and reveals a mean characteristic remanent magnetization chrm direction within each block with both normal and reverse polarities after elimination of data from 5 blocks situated outside the microplate and one block which bears an anomalous direction we have computed a mean chrm direction at dm 49 0 im 39 0 k 10 2 95 19 8 n 7 although the mean inclination is in agreement with the inclination of the recent dipole magnetic field d 0 i 43 the average declination is significantly different from this direction which we interpret as due to a bulk clockwise rotation of 48 5 11 after the bivariate average of the easter microplate over the past 2 3 ma this value is in excellent agreement with previous estimates based on remote geophysical measurements and models

apatite liquid partition coefficients for the rare earth elements and strontium abstract sixteen sets of apatite liquid partition coefficients dap liq for the rare earth elements ree la sm dy lu and six values for sr were experimentally determined in natural systems ranging from basanite to granite the apatite melt glass assemblages were obtained from starting glasses artificially enriched in ree sr and fluorapatite components these were run under dry and hydrous conditions of 7 5 20 kbar and 950 1120 c in a solid media piston cylinder apparatus an sem equipped electron microprobe was used for subsequent measurement of ree and sr concentrations in coexisting apatites and quenched glasses the resulting partition coefficient patterns resemble previously determined apatite phenocryst groundmass concentration ratios in the following respects 1 the rare earth patterns are uniformly concave downward i e the middle ree are more compatible in apatite than the light and heavy ree 2 dreeap liq is much higher for silicic melts than for basic ones and 3 strontium and therefore eu2 is less concentrated by apatite than are the trivalent ree the effects of both temperature and melt composition on dreeap liq are systematic and pronounced at 950 c for example a change in melt sio2 content from 50 to 68 wt causes the average ree partition coefficient to increase from 7 to 30 a 130 c increase in temperature on the other hand results in a two fold decrease in dreeap liq partitioning of sr is insenstitive to changes in melt composition and temperature and neither the sr nor the ree partition coefficients appear to be affected by variations in pressure or h2o content of the melt the experimentally determined partition coefficients can be used not only in trace element modelling but also to distinguish apatite phenocrysts from xenocrysts in rocks reported apatite megacryst host basalt ree concentration ratios 12 for example are considerably higher than the equilibrium partition coefficients which suggest that in this particular case the apatite is actually xenocrystic a reversal experiment incorporated in our study yielded diffusion profiles of ree in apatite from which we extracted a ree ca interdiffusion coefficient of 2 4 10 14 cm2 s at 1120 c extrapolated downward to crustal temperatures this low value suggests that complete ree equilibrium between felsic partial melts and residual apatite is rarely established

plate tectonic reconstructions of the juan fernandez microplate transformation from internal shear to rigid rotation side scan sonar swath bathymetry and magnetic anomaly date define a detailed three phase history of the juan fernandez microplate the 6 m y history is presented in a series of discrete time steps to document the growth and reorganization of propagating spreading centers and structural feature and microplate kinematic evolution prior to the microplate the east pacific rise at the pacific antarctic nazca triple junction was offset by a long transform fault zone likely the fastest slipping transform on earth at anomaly 3a time the microplate originated from an intratransform setting between anomaly 3a 5 95 ma and anomaly 3 5 24 ma time its early development resembled a large propagating rift system and microplate core structures suggest the entire offset zone may have experienced deformation fast propagation of the east ridge dominated microplate growth until 2 6 1 9 ma when seafloor spreading became the dominant growth process the microplate rotation rate increased threefold from 9 to 29 m y 1 average from phase 1 4 2 2 6 ma to phase 2 2 6 1 1 ma of the microplate s history then reduced fourfold 29 to 7 m y 1 average phase 3 1 1 ma to present phases 2 and 3 of the microplate s rotational history support the edge driven model for microplate kinematics of schouten and others to a good approximation the pacific nazca shear couple drove microplate rotation during phase 2 but development of the southeastern boundary enabled a transfer to the nazca antarctic plate pair phase 3 west ridge propagation and reorganization of the south western boundary may have decoupled the pacific plate from the microplate thus facilitating the shear couple transfer the recent continued deceleration in microplate rotation rate and westward migration of the pacific antarctic ridge axis relative to the microplate may indicate that the process of microplate death has begun we speculate that the juan fernandez microplate will accrete to the antarctic plate perhaps within the next million years like the extinct friday microplate has done thereby accomplishing another northward migration of the pacific antarctic nazca triple junction our reconstructions illustrate that the easter and juan fernandez microplates are more similar than previously thought in terms of their origin growth rift propagation ridge segmentation and overall tectonic evolution

hibonite bearing microspherules a new type of refractory inclusions with large isotopic anomalies reported are petrographic descriptions major and trace element chemistry and mg ca and ti isotopic compositions of a new class of refractory inclusions that consist of spherules composed of hibonite and a silicate glass the distinctive features of these inclusions are excesses in 48ca and 50ti in both glass and hibonite and 26mg depletions relative to terrestrial isotopic compositions three spherules have been examined and analyzed one from the lance co3 meteorite and two from the murchison cm2 meteorite lance 3413 1 31 la3413 1 31 and murchison 7 228 mur7 228 have euhedral to subhedral hibonite crystals enclosed within glass murchison 7 753 mur7 753 has a rounded hibonite core with several small inclusions of perovskite a small fragment of glass is attached to the hibonite and an fe silicate rim is imperfectly preserved around the grain la3413 1 31 has a group ii ree pattern mur7 228 a refractory pattern with depletions in the relatively volatile elements sr ba nb v and eu and mur7 753 a pattern characterized by the prior removal of an ultrarefractory component and overall fractionation of all rees the partitioning of the lrees between hibonite and glass in mur7 228 is consistent with equilibrium hibonite liquid partition coefficients previously determined la3413 1 31 shows much less partitioning while mur7 753 shows no evidence for partitioning and preserves an unequilibrated refractory component highly enriched in gd all spherules have initial magnesium depleted in 26mg by around 3 relative to terrestrial mg but only mur7 228 shows evidence for in situ decay of 26al with an initial 26al27al of 1 7 0 7 10 5 both hibonite and glass in all three spherules show excesses of 48ca and 50ti ranging up to 40 and 20 respectively relative to terrestrial ca and ti spherules such as these are rare and the only other occurrence is in the unique chondrite alh85085 the hibonite in the spherules shows similarities to isotopically anomalous hibonite crystal fragments placs but it is unlikely that the spherules formed by remelting of placs the precursors include isotopically anomalous ca ti carriers but also isotopically normal refractory components that probably formed as condensates the spherules formed by melting of these precursors under disequilibrium conditions and rapid cooling after hibonite crystallization these inclusions must have formed early prior to the dilution of isotopic anomalies by mixing processes and in an area characterized by excesses of 48ca and 50ti depletions of 26mg and lack of 26al

new insights into the formation of fayalitic olivine from allende dark inclusions abstract although considerable progress has been made in unraveling the origin s of fayalitic olivines in dark inclusions dis many questions remain still unresolved and or controversial we combine a chemical and petrographic study of the allende dark inclusion 4884 2b amnh new york and atem studies of a fragment of the dark inclusion allende af nhm vienna and discuss an alternative way in which fayalitic olivines could have formed allende dark inclusion 4884 2b contains a few aggregates with variable proportions of transparent and feathery olivine two such objects aggregates a and b are the focus of this study as they preserve glasses that can help in deciphering the nature of the processes involved during olivine growth and subsequent olivine transformation the petrographic and chemical characteristics of aggregates a and b indicate that the forsteritic stack olivines may be pseudomorphs of clear olivine crystals the atem studies in all af suggest that fayalitic olivines may be the result of secondary processes e g metasomatic exchange reactions operating in the solar nebula transformation may have occurred through the mediation of a dry gas phase involving nonvolatile major elements such as mg and fe e g dohmen et al 1998 this mechanism could reveal olivine growth patterns e g stacked platelets due to a rapid growth regime and may have contributed to the development of their fibrous aspect while preserving the shape i e volume of the crystals this highly selective process did completely or partially transform ferromagnesian minerals but affected the fine grained mesostasis only slightly

statistical analysis of the el nino southern oscillation and sea floor seismicity in the eastern tropical pacific we present statistical evidence for a temporal link between variations in the el nino southern oscillation enso and the occurrence of earthquakes on the east pacific rise epr we adopt a zero inflated poisson regression model to represent the relationship between the number of earthquakes in the easter microplate on the epr and enso expressed using the southern oscillation index soi for east pacific sea level pressure anomalies from february 1973 to february 2009 we also examine the relationship between the numbers of earthquakes and sea levels as retrieved by topex poseidon from october 1992 to july 2002 we observe a significant 95 confidence level positive influence of soi on seismicity positive soi values trigger more earthquakes over the following 2 to 6 months than negative soi values there is a significant negative influence of absolute sea levels on seismicity at 6 months lag we propose that increased seismicity is associated with enso driven sea surface gradients rising from east to west in the equatorial pacific leading to a reduction in ocean bottom pressure over the epr by a few kilopascal this relationship is opposite to reservoir triggered seismicity and suggests that epr fault activity may be triggered by plate flexure associated with the reduced pressure

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

a history of the easter microplate 5 25 ma to present gloria and other geophysical data are used to interpret the morphology and tectonic pattern of the easter microplate we model magnetic anomaly data and calculate spreading rates for use in euler pole calculations we calculate three plate closure and best fit poles for nazca pacific nazca easter and pacific easter plate motions and compare the results with earthquake focal mechanisms and seismicity we use the three plate closure poles to rotate digital gloria and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate s east rift first started to propagate northward at around 5 25 ma the microplate has evolved in two stages the first is from 5 25 ma to around 2 5 ma and involved continuous northward propagation of the east rift while the west rift became segmented along new transform faults as it adjusted to keep the overall nazca pacific velocities constant the transpressive northern boundary migrated with the east rift tip while the transtensional southern boundary remained more or less still at some time after 2 5 ma the east rift ceased northward propagation and the southwest rift began to open up along the transtensional pacific easter transform a convergent northern boundary formed involving southward thrusting of the nazca plate over the microplate no further nazca plate lithosphere was transferred from north of the microplate to the microplate interior and the pito deep region opened by pervasively rifting apart preexisting nazca plate lithosphere

plagioclase rich inclusions in carbonaceous chondrite meteorites liquid condensates plagioclase rich inclusions in carbonaceous chondrites are of two types 1 mm cm sized ca al rich inclusions cais with 30 60 vol anorthite and up to 35 vol each of ti al pyroxene tpx melilite and spinel and 2 smaller 3 mm ca chondrules similar to those found in ordinary chondrites containing more sodic plagioclase pyroxene olivine and spinel the cais which were formerly named type i and thought to be intermediate between type a and type b cais are shown to be an independent type renamed type c with distinctive chemical and mineralogical compositions type c cais have group i or group ii trace element patterns low initial 26al27al ratios of 5 10 6 typical ak 35 55 melilite tpx with 2 12 tio2 and opaque grains osirrumopt alloy nuggets and fremdlinge smaller than 10 m the textures sequence of crystallization and concentric zonation indicate that they crystallized from melt droplets the texture of the anorthite ranges from coarse laths to a fine grained groundmass reflecting increasing retardation of its nucleation the few cais with the coarse laths cooled slowly 2 5 h from below the liquidus but the majority cooled faster from completely molten material as the elemental abundances in type c s are dominated by volatility the melts were not formed by planetary magmatic processes evaporative residues and remelted solid condensates are also ruled out because the compositions of naturally occurring experimentally produced and thermodynamically calculated residues and solid condensates differ from those of type c cais type c cais have compositions similar to those calculated for liquid condensates under high solar gas pressure 1 atm or in dust rich environments and are probably liquid condensates the few type c cais that crystallized from incompletely molten materials may have been reheated liquid condensates or mixed liquidsolid condensates

experimentally determined mineral melt partition coefficients for sc y and ree for olivine orthopyroxene pigeonite magnetite and ilmenite our current lack of understanding of the partitioning behavior of sc y and the ree rare earth elements can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set to address this problem we conducted a series of experiments using several different natural composition lavas doped with the elements of interest as starting compositions microprobe analyses of orthopyroxene pigeonite olivine magnetite ilmenite and co existing glasses in the experimental charges were used to calculate expressions that describe ree partitioning as a function of a variety of system parameters using expressions that represent mineral melt reactions versus element ratio distribution coefficients it is possible to calculate terms that express low ca pyroxene melt partitioning behavior and are independent of both pyroxene and melt composition compositional variations suggest that sc substitution in olivine involves either a paired substitution with al or more commonly with vacancies the partitioning of sc is dependent both on melt composition and temperature our experimentally determined olivine melt ree ds partition coefficients are similar to but slightly higher than those reported by mckay 1986 and support their conclusions that olivines are strongly lree depleted y and ree mineral melt partition coefficients for magnetite range from 0 003 for la to 0 02 for lu ilmenite partition coefficients range from 0 007 for la to 0 029 for lu these experimental values are two orders of magnitude lower than many of the published values determined by phenocryst matrix separation techniques

earthquake focal mechanism and oceanic thrust in easter microplate analogy with oman ophiolite previous work has suggested compression and thrust faulting along the northern boundary of the easter microplate due to its motion relative to the nazca plate inversion of the p and sh seismic waves related to a recent ms 7 1 earthquake located along the northern boundary of the easter microplate indicates a thin sliver of crust thrusting at a shallow angle towards the south giving support to this interpretation the oman ophiolite where similar thrusts have been mapped has been recently interpreted as being derived from similar microplate environment boudier et al 1997 this interpretation 1 provides possible tests to document the existence of oceanic thrusts near microplates and 2 suggests a new solution to the problem of how many ophiolites could have obtained oceanic detachment faults which will favour obduction

overlapping spreading centers implications from crack growth simulation by the displacement discontinuity method overlapping spreading centers osc s are a fundamental aspect of accretionary processes at intermediate and fast spreading centers and typically occur at deep points along the axial depth profile they have a characteristic geometry consisting of two en echelon overlapping curving ridges separated by an elongated depression the length to width ratio of this overlap basin is typically 3 1 we have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity a boundary element displacement discontinuity method was used to solve this problem our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region for small osc s there is a very good agreement between calculations and observations but for large ones the overlap basin tends to be longer than our predicted results indicate this suggests that the assumptions made in the model i e perfectly elastic isotropic and homogeneous medium are perhaps valid for the brittle lid above the magma chamber that underlies osc s with small offsets 2 km but oversimplified for osc s with large offsets our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach the deflection will be the greatest for small misalignments of the fracture systems thus even minor misalignments of the spreading centers may result in the development of osc s where the misalignment is less than the width of the cracking front the fracture systems may meet head on creating a saddle point along the axial depth profile our results support the hypothesis suggested by macdonald et al 1984 in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned

patterns of stress at midocean ridges and their offsets due to seafloor subsidence the effect of the seafloor subsidence on the horizontal stress field is investigated by combining the finite element method with a formulation that allows us to compute the two dimensional 2d horizontal stresses arising from isostatically compensated vertical loads the topographic load created by the elevation of midocean ridges relative to old ocean floor is shown to be a significant source of ridge parallel tensile stresses these may predominate over the ridge perpendicular stresses and explain observations at midocean ridge offsets such as 1 oblique normal faulting at ridge transform intersections trending up to 60 relative to the ridge axis and 2 nontransform offsets consisting of structures oriented at 45 relative to the ridge trend at midocean ridge overlaps rotation of the ridge parallel tensile stresses favours rift propagation at more than 45 relative to the ridge trend it is suggested that propagating rift tips that bend abruptly lead to partially unlocked offsets and as a result large overlaps may eventually start to rotate and evolve into a microplate

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

distribution of axial lava domes along a superfast overlapping spreading center 27 32 s on the east pacific rise 1 deep towed dsl 120 bathymetric data are used to investigate the pattern of lava dome formation along a superfast spreading portion of the southern east pacific rise epr including the overlapping limbs of a giant 120 120 km propagator near 29 s along the 670 km of the axis surveyed 1172 small domes were identified using a closed contour algorithm their abundance defined by spatial density is well correlated with the along axis relief of the ridge crest where the western and eastern limbs plunge toward the overlap zone densities are high 3 6 km 2 however where the axial depth profile is shallow and flat densities are comparably low 0 4 km 2 volcanic domes within the low abundance areas are characterized by lower ratios of height to basal radius 0 15 versus 0 22 smaller maximum heights 18 versus 40 m and a larger relative percentage of small versus large mounds the zone of high dome abundance encompasses the overlapping limbs of the rift and extends more than 100 km to the north and south beyond the overlap zone domes form dominantly during low effusion rate point source eruptions which suggests that discontinuous melt lenses underlie the ridge axis proximal to the overlapper conversely fissure fed sheet flows dominate along the more distal segments implying the presence of a more continuous axial magma lens throughout the survey area dome abundance increases systematically near second order segment boundaries within the high abundance zone some third order offsets also correlate with increased dome production but local peaks in abundance are not tied exclusively to higher order ridge offsets where dome abundance is low domes are clustered tightly near second order offsets and there is no increase in dome abundance near third order segment boundaries

meteorite 3 d synchrotron microtomography methods and applications methods of synchrotron x ray computed microtomography xrcmt are described which allow nondestructive high spatial and contrast resolution imaging of the density structures of meteorites and their components in three dimensions images of bulk chondrites to one cubic centimeter in size reveal compound chondrules chondrule matrix volumetric ratios metal and sulfide distribution petrofabrics and 3 d chondrule and calcium aluminum inclusion cai sizes and shapes images of separated chondrules and cais reveal void spaces mineral intergrowth textures and the true locations of crystal rims and cores at resolutions to 8 cubic micron volume element images of achondrites reveal mineral fabrics and crystal zoning lunar glass spherules can be searched for phenocrysts bearing deeply sourced melt inclusions a companion dvd and url contain images for classroom and research use numerical techniques for quantification of x ray computed microtomography xrcmt data and its potential applications are discussed three dimensional x ray images of meteorites provide a way to discover components of interest and to precisely slice samples to expose these components with minimal damage and loss of material three dimensional studies of petrographic features size shape texture and modal abundance of chondrites and their components as well as other meteorites have definite advantages over standard 2 d studies using randomly sliced thin sections

the relationship between ck and cv chondrites ck chondrites are highly oxidized meteorites containing abundant magnetite and trace amounts of fe ni metal although the group is predominately composed of equilibrated meteorites types 4 6 in recent years a significant number of new samples have been classified as being either ck3 or ck3 anomalous these unequilibrated cks often display a close affinity with members of the cv oxidized subgroup cks and cvs oxidized subgroup may therefore form a continuum and by implication could be derived from a single common parent body to investigate the relationship between these two groups a detailed study of the oxygen isotope composition opaque mineralogy and major and trace element geochemistry of a suite of cv and ck chondrites has been undertaken the results of oxygen isotope analysis confirm the close affinity between cv and ck chondrites while excluding the possibility of a linkage between the co and ck groups magnetites in both cv and ck chondrites show significant compositional similarities but high ti contents are a diagnostic feature of the latter group the results of major and trace element analysis demonstrate that both cv and ck chondrites show overlapping variation supporting evidence for a single common source for both groups comes from their similar cosmic ray exposure age distributions recent reflectance spectral analysis is consistent with both the cvs and cks being derived from eos family asteroids which are believed to have formed by the catastrophic disruption of a single large asteroid thus a range of evidence appears to be consistent with cv and ck chondrites representing samples from a single thermally stratified parent body in view of the close similarity between cv and ck chondrites some modification of the present classification scheme may be warranted possibly involving integration of the two groups one means of achieving this would be to reassigned ck chondrites to a subgroup of the oxidized cvs it is recognized that a full evaluation of this proposal may require further study of the still poorly understood ck3 chondrites

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

compositional and petrographic similarities of cv and ck chondrites a single group with variations in textures and volatile concentrations attributable to impact heating crushing and oxidation abstract greenwood et al 2010 gathered data on o isotopic and elemental compositions and reevaluated literature data for cv and ck chondrites they concluded that these two chondrite groups originated on the same parent asteroid with ck chondrites being metamorphosed cv chondrites which are otherwise missing types 4 through 6 to test this interpretation we have gathered new instrumental neutron activation analysis inaa data for cv and ck chondrites and reexamined their petrographic features the new inaa data like the older data show scatter attributable to weathering effects but we conclude that the refractory lithophile abundances are the same in cv and ck in agreement with the greenwood et al interpretation several volatile elements are significantly lower in ck than in cv chondrites among the elements we determine the greatest difference between cv and ck is found for br for which the cv ck ratio is 4 as and sb are about 20 lower in ck than cv and smaller differences are observed for zn ga and se it seems likely that volatiles were lost during impact heating events that also provided the heat responsible for metamorphic recrystallization within statistical uncertainty chondrules in cv and ck chondrites are the same size and have similar textural distributions a significant petrographic difference between ck and cv chondrites cited by kallemeyn et al 1991 was the much higher percentage of igneous rims around cv chondrules however we now recognize that many chondrules in ck3 8 nwa 1559 have igneous rims and in ck4 chondrites igneous rims are recognizable by their associated sulfide rich rings there are no quantifiable cv ck differences in igneous rim abundances we used ca and al maps to show that ck chondrites have cai abundances similar to those of cv chondrites it thus appears that there are no resolvable pre metamorphic petrographic differences between cv and ck chondrites we recommend that the ck designation be abandoned and that the cv group be acknowledged as spanning the range of petrologic types 3 6 we suggest that ck3 chondrites be designated cv3oxk most ck4 chondrites are highly fragmental collisional crushing appears to be much more common than in cv chondrites it seems likely that ck chondrites formed from reduced cv3 materials after the latter was impacted buried aqueously altered and annealed

geologic structure of the uppermost oceanic crust created at fast to intermediate rate spreading centers abstract geological investigations of major fault scarps tectonic windows and dsdp odp drill holes provide direct views of the uppermost oceanic crust generated at fast to intermediate rate spreading centers these areas reveal a consistent upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing inward dip toward the spreading center at which they formed and dikes in the lavas and underlying sheeted dike complex showing a similar degree of outward dip widespread fracturing faulting and hydrothermal metamorphism accompanied magmatic construction these geological relationships can be interpreted in terms of dramatic asymmetrical subaxial subsidence of upper crustal rock units that diminishes across the very narrow few kilometers wide zone of lava accumulation and dike intrusion at the ridge axis this type of crustal structure is in accord with some existing models of spreading but augments these idealized views with more realistic geological

the juan fernandez microplate north of the pacific nazca antarctic plate junction at 35 s abstract results of the r v thomas washington pascua 3 expedition provide evidence for the existence of the juan fernandez microplate just north of the junction between the east pacific rise epr and the chile fracture zone prior to pascua 3 the microplate in the region had been hypothesized from the pattern of seismicity the eastern and western boundaries of the juan fernandez microplate are well defined and represent north south trending spreading centers characterized by very slow and very fast rates of accretion respectively in agreement with the rates the eastern boundary is represented by a rift valley and the western boundary by an epr type axial ridge the northern boundary of the juan fernandez microplate is a 100 trending wide fracture zone complex which may have resulted from northward transform fault migration the fracture zone fails to meet the zone of accretion at the pacific nazca juan fernandez triple junction in this area the zone of accretion displays a double ridge with a large overlap the southern boundary of the juan fernandez microplate is still poorly constrained the plate geometry derived from seabeam differs from that derived by anderson fontana et al 1986 14 from a plate motion inversion scheme using primarily earthquake first motion solutions together with limited bathymetric and magnetic data

metamorphosed calcium aluminum rich inclusions in ck carbonaceous chondrites ck chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6 although cks are described as calcium aluminum rich inclusion cai poor objects the abundance of cais in the 18 ck3 6 we analyzed ranges from zero to approximately 16 4 during thermal metamorphism some of the fine grained cais recrystallized as irregular assemblages of plagioclase ca rich pyroxene olivine ca poor pyroxene magnetite coarse grained cais display zoned spinel fassaite destabilization and secondary grossular and spinel secondary anorthite grossular ca rich pyroxene and spinel derive from the destabilization of melilite which is lacking in all cais investigated the al mg isotopic systematics measured in fine and coarsegrained cais from tanezrouft tnz 057 was affected by mg redistribution the partial equilibration of al mg isotopic signatures obtained in the core of a coarse grained cai cg1 cai in tnz 057 may indicate a lower peak temperature for mg diffusion of approximately 540 580 c while grossular present in the core of this cai indicates a higher temperature of around 800 c for the metamorphic event on the parent body of tnz 057 excluding metamorphic features the similarity in nature and abundance of cais in ck and cv chondrites confirms that cvs and cks form a continuous metamorphic series from type 3 to 6

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

the non igneous genesis of angrites support from trace element distribution between phases in d orbigny d orbigny is an exceptional angrite chemically it resembles other angrites such as asuka 881371 sahara 99555 lewis cliff lew 87051 and lew 86010 but its structure and texture are peculiar it has a compact and porous lithology abundant glasses augite bearing druses and chemical and mineralogical properties that are highly unusual for igneous rocks our previous studies led us to a new view on angrites they can possibly be considered as cais that grew to larger sizes than the ones we know from carbonaceous chondrites thus angrites may bear a record of rare and special conditions in some part of the early solar nebula here we report trace element contents of d orbigny phases trace element data were obtained from both the porous and the compact part of this meteorite we have confronted our results with the popular igneous genetic model according to this model if all phases of d orbigny crystallized from the same system as an igneous origin implies a record of this genesis should be expressed in the distribution of trace elements among early and late phases our results show that the trace element distribution of the two contemporaneous phases olivine and plagioclase which form the backbone of the rock seem to require liquids of different composition abundances of highly incompatible elements in all olivines including the megacrysts indicate disequilibrium with the bulk rock and suggest liquids very rich in these elements 10 000 x ci which is much richer than any fractional crystallization could possibly produce in addition trace element contents of late phases are incompatible with formation from the bulk systems residual melt these results add additional severe constraints to the many conflicts that existed previously between an igneous model for the origin of angrites and the mineralogical and chemical observations this new trace element content data reported here corroborate our previous results based on the shape structure mineralogy chemical and isotopic data of the whole meteorite as well as on a petrographic and chemical composition study of all types of glasses and give strength to a new genetic model that postulates that d orbigny and possibly all angrites could have formed in the solar nebula under changing redox conditions more akin to chondritic constituents e g cais than to planetary differentiated rock

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

comparisons of gravity anomalies at pseudofaults fracture zones and nontransform discontinuities from fast to slow spreading areas published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading we investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments we find that altimetry based gravity anomalies on the mathematician bauer easter juan fernandez and northern chile ridge pseudofaults located in 75 150 mm yr full rate seafloor spreading environments are similar in amplitude and form to atlantic fracture zones with 20 30 mm yr spreading rates a 5 15 mgal positive mantle bouguer anomaly is observed on the pseudofault bounding the eastern juan fernandez microplate comparable to those at some similar age offset nontransform discontinuities in slow spreading environments our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust the latter of which remains frozen at pseudofaults we predict that any pseudofaults with age offsets more than 1 m y and not coincident with hotspot volcanism will be associated with thin and possibly unusually dense crust even in superfast seafloor spreading environments

trace element studies of silicate rich inclusions in the guin ungr and kodaikanal iie iron meteorites a devitrified glass inclusion from the guin ungr iron consists of cryptocrystalline feldspars pyroxenes and silica and is rich in sio2 al2o3 and na2o it contains a rutile grain and is in contact with a large cl apatite the latter is very rich in rare earth elements rees 80 x ci which display a flat abundance pattern except for eu and yb which are underabundant the devitrified glass is very poor in rees light rare earth elements lrees and deficits in eu and yban ultrarefractory pattern the genetic models suggested so far cannot explain what is observed and consequently we offer a new model for silicate inclusion formation in iie and related irons nebular processes and a relationship with e meteorites guin or ca al rich inclusions cais kodaikanal are indicated a sequence of condensation cas tin or refractory pyroxene rich liquids and vapor solid elemental exchange can be identified that took place beginning under reducing and ending at oxidizing conditions phosphate rutile formation alkali and fe 2 metasomatism metasomatic loss of rees from glasses

pliocene change in pacific plate motion the absolute motion of the pacific plate has been nearly constant during the past 15 myr but epp1 recognized that some change took place between 15 myr and the present and tentatively chose 1 myr for the time of the change cox and engebretson2 found a change in pacific absolute motion based on hotspot tracks and a change in pacific antarctic relative motion the pliocene deformation in california and the juan de fuca rotations led them to postulate a 5 myr age for the change the manner and timing of the change are explored more thoroughly here and we put the time for the change in motion of the pacific plate at between 5 and 3 2 myr basing the younger limit on the correlation of computed hotspot tracks with pacific and nazca hotspot traces the change may explain several observations on the neighbouring continental and oceanic plate boundaries including direct adjustments at the pacific spreading ridges a likely mechanism for the change is a large clockwise torque caused by acceleration of the sinking slab beneath the kurile trench where a fossil spreading ridge is inferred to have been subducted

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

refractory inclusions in the prototypical cm chondrite mighei abstract refractory inclusions in the mighei cm2 meteorite are very abundant 35 being found in three thin sections of the meteorite the most common are pyroxene rimmed spinel rich inclusions that occur in a range of structural and textural types some inclusions are simple aggregates of crystals or rimmed crystals ranging from loose clusters of spinel grains to distended chains of pyroxene rimmed spinel knots others are more compact and may have experienced recrystallization or melting these include nodular spinel inclusions with polygonal granular textured spinel interiors and pyroxene rims ribbons of spinel in some cases with hibonite and or perovskite that are again rimmed by pyroxene and refractory spherules four inclusions are complex aggregates of several texturally distinct objects each collectively mighei inclusions are similar to those in murchison but appear to be somewhat more altered volatility fractionated and unfractionated trace element patterns similar to those found in refractory inclusions in other carbonaceous chondrites are represented in mighei inclusions and in addition some mighei inclusions have volatility fractionated pattern types not previously found in refractory inclusions in three of the complex aggregates fundamentally different trace element patterns are present within different portions of each inclusion meaning that these objects accreted from genetically different components that never equilibrated with one another subsequent to accretion bulk inclusion magnesium isotopic compositions show minor mass fractionation effects fmg values range from 2 3 to 7 5 but the average fmg value is indistinguishable from normal 1 8 1 2 there are no correlations between magnesium isotopic composition and textural type or trace element pattern type the diverse inclusion types in mighei represent a variety of origins and histories textures suggest that the nodular and banded inclusions may have experienced solid state recrystallization whereas the porous and chain like varieties and complex aggregates have not magnesium isotopic compositions and trace element abundances suggest that the mighei inclusions are unlikely to be evaporation residues and are more likely to be the products of condensation or recondensation processes

microplate and shear zone models for oceanic spreading center reorganizations the reorganization of oceanic spreading centers is often accomplished through the process of rift propagation we first examine general rift propagation geometries for the simple two plate case in which the propagating rift instantaneously reaches full spreading rate while the other rift dies instantaneously such that the propagating and dying rifts never overlap a velocity space representation provides a framework for describing the geometry of rift propagation and its various tectonic elements and shows how both the failed rift and isochrons between the two rifts are reoriented as a consequence of purely rigid plate tectonics we then discuss the more complex geometries that result when both growing and dying rifts spread simultaneously during propagation such that a technically independent overlap region like the easter or juan fernandez plates along the east pacific rise is produced we examine the effect of the propagation rate and rise time of spreading on the evolution of overlap regions between dual oceanic spreading centers the tectonics and evolution of such regions can be modeled by assuming that they act either as rigid microplates or as zones of distributed shear rigid plate models are suggested by the reasonable success of euler vectors derived from inversions of plate motion data spreading rates transform azimuths and slip vectors both in describing available data and in predicting subsequently acquired data on the other hand recent structural data showing fabric oblique to the surrounding spreading centers might suggest shearing rather than rigid behavior in the interior of the easter microplate we use schematic evolutionary simulations for an easter style geometry to examine whether plate motion data and orientations of preexisting and present structures can be used to discriminate between rigid and shear models in a microplate model rigid plate tectonics requires that relative motion occur only along the boundaries of the overlap region while structures within the overlap zone rotate rigidly in contrast in a shear model relative motions at the boundaries differ from those predicted by rigid plate tectonics and the angular relationships of structures within the overlap region are altered by internal deformation thus inversion of relative motion data yields different results for rigid and shear behavior similarly under favorable circumstances structural trends could be diagnostic of the different cases we find that much of the easter structural data suggestive of shearing are also consistent with rigid plate tectonics we conclude that microplate models offer useful testable hypotheses for describing overlap systems and that shearing need be invoked only to explain features not describable by rigid plate tectonics furthermore the approaches used here to study current overlap regions should be useful in the analysis of similar regions formed during past oceanic plate boundary reorganizations

formation of orange hibonite as inferred from some allende inclusions we studied three fluffy type a refractory inclusions from allende that contain orange hibonite the melilite in the present samples is very al rich averaging ak6 ak14 and ak12 in the three samples studied hibonite in two inclusions unlike that in murchison has low rare earth element abundances of 10 ci in the other inclusion the hibonite melilite and perovskite have group ii like patterns the hibonite and melilite in all three inclusions studied have excess 26mg consistent with 26al 27al i 5 10 5 much of the hibonite and some of the spinel in these inclusions is corroded these phases are found enclosed in melilite but based on bulk compositions and phase equilibria hibonite should not be an early crystallizing phase in these inclusions we conclude that the hibonite and probably some of the spinel is relic reversely zoned melilite rounded spinel and isotopically heavy mg in the inclusions probably reflect reheating events that involved melting and evaporation alteration of the gehlenitic melilite gave rise to some rare phases including corundum and nearly pure cats pyroxene studies have shown that blue hibonite contains ti3 while orange hibonite does not ihinger and stolper 1986 beckett et al 1988 orange hibonite formed either under oxidizing conditions such as at oxygen fugacities at least seven orders of magnitude greater than that of a solar gas at 1700 k or under conditions reducing enough e g solar that it contained ti3 which was later oxidized in situ although v and ce oxides are volatile at the temperature and range of oxygen fugacities at which orange hibonite is known to be stable we find that a the hibonite is v rich 1 wt v2o3 and b there are no negative ce anomalies in allende hibonite this indicates that the hibonite did not form by condensation under oxidizing conditions in addition there are slight excesses of ti si cations relative to mg fe cations up to 0 1 of 0 8 cations per 19 oxygen anions probably reflecting the original presence of ti3 the results of this study strongly support the suggestion ihinger and stolper 1986 that allende hibonite originally formed under reducing conditions and was later oxidized oxygen fugacities within 2 3 orders of magnitude of that of a solar gas are implied otherwise strong ce and v depletions would be observed

apatite as a probe of halogen and water fugacities in the terrestrial planets abstract apatite preserves a record of the halogen and water fugacities that existed during the waning stages of crystallization of planetary magmas when they became saturated in phosphates we develop a thermodynamic formalism based on apatite merrillite equilibria that makes it possible to compare the relative values of halogen and water fugacities in martian lunar and terrestrial basalts accounting for possible differences in pressure temperature and oxygen fugacities among the planets we show that each of these planetary bodies has distinctive ratios among volatile fugacities at apatite saturation and that these fugacities are in some cases related in a consistent way to volatile fugacities in the mantle magma sources our analysis shows that the martian mantle parental to basaltic snc meteorites was dry and poor in both fluorine and chlorine compared to the terrestrial mantle the limited data available from mars show no secular variation in mantle halogen and water fugacities from 4 ga to 180 ma the water and halogens found in present day martian surface rocks have thus resided in the planet s surficial systems since at least 4 ga and may have been degassed from the planet s interior during a primordial crust forming event in comparison to the earth and mars the moon and possibly the eucrite parent body too appear to be strongly depleted not only in h 2 o but also in cl 2 relative to h 2 o chlorine depletion is strongest in mare basalts perhaps reflecting an eruptive process characteristic of large scale lunar magmatism