the basaltic shergottite northwest africa 856 petrology and chemistry we report on the discovery of a new shergottite from south morocco this single stone weighing 320 g is referenced as northwest africa nwa 856 with djel ibone as a synonymous name it is a fresh fine grained basaltic rock consisting mainly of two pyroxenes total similar to68 vol 45 pigeonite en 61 16 wo 9 22 fs 26 68 23 augite en 46 26 wo 34 29 fs 21 43 and plagioclase converted to maskelynite similar to23 vol ab 43 57 or 1 5 an 54 36 accessory minerals include merrillite cl apatite pyrrhotite ilmenite ulvospinel silica stishovite and glass amorphous k feldspar and baddeleyite amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths in addition melt pockets similar to2 vol were recognized with relics of maskelynite pyroxene and both dense silica glass and stishovite occurring as both grains and submicrometer needles the compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of similar to50 vol maskelynite the meteorite is highly fractured at all scales

oxygen isotope measurements of individual unmelted antarctic micrometeorites abstract we present oxygen isotope measurements of 28 unmelted antarctic micrometeorites measuring 150 250 m long axis collected in the south pole water well the micrometeorites were all unmelted and classified as either fine grained scoriaceous coarse grained or composite a mix of two other classes spot analyses were made of each micrometeorite type using an ion microprobe the oxygen isotope values were measured relative to standard mean ocean water smow and range from 18o 3 to 60 and 17o 1 to 32 falling along the terrestrial fractionation line tfl within 2 errors several analytical spots comprising multiple phases were made on each particle variability in the oxygen isotope ratios was observed among micrometeorite types between micrometeorites of the same type and between analytical spots on a single micrometeorite indicating that micrometeorites are isotopically heterogeneous in general the lowest isotope values are associated with the coarse grained micrometeorites whereas most of the fine grained and scoriaceous micrometeorites have an average 18o 22 suggesting that the matrix in micrometeorites is isotopically heavier than the anhydrous silicate phases the oxygen isotope values for the coarse grained micrometeorites composed mainly of anhydrous phases do not lie along the carbonaceous chondrite anhydrous mineral ccam line as observed for olivines pyroxenes and some kinds of chondrules in carbonaceous chondrites suggesting that coarse grained mms are not related to chondrules as previously thought our measurements span the same range as values found for melted micrometeorites in other studies although four of the micrometeorites have oxygen isotope values lying along the tfl close to the region where the bulk ci carbonaceous chondrites are found 21 particles have very enriched 17o and 18o values that have not been reported in previous analyses of chondrite matrix material suggesting that they could be a new type of solar system object the parent bodies of the micrometeorites with higher 18o values may be thermal metamorphosed carbonaceous asteroids that have not been found as meteorites either because they are friable asteroids that produce small particles rather than rocks upon collision with other bodies or because the rocks they produce are too friable to survive atmospheric entry

ordinary chondrite related giant 800 μm cosmic spherules from the transantarctic mountains antarctica abstract in order to identify the parent bodies of cosmic spherules melted micrometeorites with porphyritic olivine po and cryptocrystalline cc textures we measured the oxygen isotopic composition of 15 giant 800 m cosmic spherules recovered in the transantarctic mountains antarctica with ir laser fluorination mass spectrometry and we conducted a characterization of their petrographic and magnetic properties samples include 6 8 and 1 spherules of po cc and barred olivine bo textural types respectively eleven spherules 70 of the total 4 6 po and 6 8 cc and the bo spherule are related to ordinary chondrites based on oxygen isotopic compositions olivines in ordinary chondrite related spherules have compositions fa 8 5 11 8 they are ni poor to ni rich 0 04 1 12 wt and tend to be richer in cao than other spherules 0 10 0 17 wt ordinary chondrite related spherules also have high magnetite contents 2 12 wt one po and one cc spherules are related to previously identified 17 o enriched cosmic spherules for which the parent body is unknown one cc spherule has an oxygen isotopic signature relating it to cm cr carbonaceous chondrites the majority of po cc cosmic spherules derive from ordinary chondrites this result exemplifies how the texture of cosmic spherules is not only controlled by atmospheric entry heating conditions but also depends on the parent body whether be it through orbital parameters entry angle and velocity or chemistry mineralogy or grain size of the precursor

composition of back arc basin volcanics valu fa ridge lau basin evidence for a slab derived component in their mantle source abstract samples dredged from 2 localities near the crest of the valu fa ridge an active back arc basin spreading centre in the lau basin consist of highly vesicular lava fragments of andesitic composition the samples are characterized by rare euhedral an 85 plagioclase phenocrysts in a hypocrystalline groundmass of an 60 plagioclase laths brown glass and rare subhedral clinopyroxene samples from within and to a lesser extent between the dredge hauls show remarkable isotopic and chemical homogeneity with 87 sr 86 sr 0 70330 2 143 nd 144 nd 0 51303 2 206 pb 204 pb 18 65 2 207 pb 204 pb 15 55 1 208 pb 204 pb 38 34 4 sr 165 ppm rb 7 ppm cs 0 17 ppm k 3300 to 4200 ppm ba 96 ppm and ree lree depleted with 12 18 chondritic abundances on sr nd pb pb and sr pb plots the volcanics lie just within or on the edge of the morb fields overlapping with island arc volcanics from the marianas and tonga compared with morb and ocean island basalts the samples show alkali element enrichment relative to ree and higher cs relative to rb the isotopic and geochemical characteristics of the valu fa ridge volcanics clearly indicate a minor but significant slab derived component in the back arc basin mantle source

mineralogically mature sandstones in accretionary prisms abstract plate tectonic interpretations of sandstone composition are made in the hope that detrital framework modes of sandstone suites provide information about the tectonic setting of basins of deposition and associated provenances dickinson et al 1983 the underlying assumption that provenance tectonics are directly associated with tectonics of depositional and post depositional sedimentary settings is not however universally true as sandstones from two contemporary accretionary prism complexes nias island indonesia and barbados west indies illustrate mineralogically immature sandstones rich in feldspar and volcanic lithic fragments are expected in such tectonic settings but both nias island and barbadian sandstones are mineralogically very mature dominated by quartz in b th cases this discrepancy between inferred and actual tectonic setting has been explained by postulating that sands from a provenance petrogenetically unrelated to the arc trench system e g continental or recycled orogen sands have been introduced into the accretionary prism by sedimentary or tectonic processes sandstones which are transported sedimentologically or tectonically across the boundaries of sedimentary tectonic provinces and which therefore occur in tectonic settings petrogenetically unrelated to the tectonics of the sediment source area are important components of at least some active accretionary complexes the possibility of trans boundary sedimentary and or tectonic transport must be allowed for in making plate tectonic interpretations based on sandstone petrog aphy especially in reconstructions of ancient orogenic belts

provenance mixing in an intraoceanic subduction zone tonga trench louisville ridge collision zone southwest pacific abstract dredging on the lower slope of the tonga forearc at the intersection between the louisville ridge hotspot chain and the trench yielded both late tertiary volcaniclastic sediments and late cretaceous slightly tuffaceous pelagic sediments petrography and phase chemistry of volcanic debris of tertiary age samples indicates derivation from a low k tholeiitic arc volcanic glass has a low k2o content and shows an fe enrichment trend plagioclase grains are high in an and low in or components pyroxene grains calcic clinopyroxene orthopyroxene and pigeonite have low tio2 contents and show an fe enrichment trend alkali feldspar and biotite are absent the composition of the tertiary samples is similar to other tonga forearc sediments and is consistent with their derivation from the adjacent magmatic arc the character and composition of volcanogenic debris in cretaceous age samples indicates derivation from an intraplate alkali igneous source plagioclase compositions show an increasing or component with increasing ab alkali feldspar is a rare additional component of these samples calcic clinopyroxene has high tio2 contents and is titanaugite amphibole is also rich in tio2 and is kaersutite minor volcanic glass is rich in alkalis samples were dredged from the seaward slope of a small terrace on the lower trench slope a planar reflector located 2 6 km below the terrace is marked by an abrupt jump in seismic velocity and is interpreted as the top of an essentially undamaged late cretaceous guyot of the louisville chain that was subducted at the tonga trench about 0 5 ma this guyot is the likely source of the late cretaceous intraplate igneous detritus collected at the dredge site some dredge samples yielded a mixed volcanic arc intraplate provenance and or late tertiary late cretaceous ages this probably represents the mixing of the cretaceous seamount and tertiary magmatic arc sources during post collision slumping of the wedge of lower slope material as it re establishes its critical taper

accretion rate of cosmic spherules measured at the south pole micrometeorites are terrestrially collected extraterrestrial particles smaller than about 1 mm which account for most of the mass being accreted to the earth1 2 compared with meteorites micrometeorites more completely represent the earth crossing meteoroid complex3 4 and should include fragments of asteroids comets mars and our moon as well as pre solar and interstellar grains3 6 previous measurements of the flux of micrometeoroids that survive to the earth s surface have large uncertainties owing to the destruction of particles by weathering7 8 9 inefficiencies in magnetic collection or separation techniques7 8 9 low particle counts10 11 poor age constraint7 8 9 12 13 or highly variable concentrating processes12 13 here we describe an attempt to circumvent these problems through the collection of thousands of well preserved and dated micrometeorites from the bottom of the south pole water well which supplies drinking water for the scott amundsen station using this collection we have determined precise estimates of the flux and mass distribution for 50 700 m cosmic spherules melted micrometeorites allowing for the expected abundance of unmelted micrometeorites14 in the samples our results indicate that about 90 of the incoming mass of submillimetre particles evaporates during atmospheric entry our data indicate the loss of glass rich and small stony spherules from deep sea deposits7 8 and they provide constraints for models describing the survival probability of micrometeoroids15 16

the leonard award address presented 1996 july 25 berlin germany the elemental composition of stony cosmic spherules five hundred stony cosmic spherules collected from deep sea sediments polar ice and the strato sphere have been analyzed for major and some minor element composition typical spherules are products of atmospheric melting of millimeter sized and smaller meteoroids the samples are small and modified by atmospheric entry but they are an important source of information on the composition of asteroids the spherules in this study were all analyzed in an identical manner and they provide a sampling of the solar system s asteroids that is both different and less biased than provided by studies of conventional meteorites volatile elements such as na and s are depleted due to atmospheric heating while siderophiles are depleted by less understood causes the refractory nonsiderophile elements appear not to have been significantly dis turbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with the earth typical spherules have cm like composition that is distinctively different than ordinary chondrites and most other meteorite types we assume that c type asteroids are the primary origin of spherules with this composition type s asteroids should also be an important source of the spherules and the analysis data provide constraints on their composition a minor fraction of the spher ules are melt products of precursor particles that did not have chondritic elemental compositions the most common of these are particles that are dominated by olivine the observed compositions of spherules are in consistent with the possibility that an appreciable fraction of the spherules are simply chondrules remelted during atmospheric entry

micrometeorite flux on earth during the last 50 000 years the flux of micrometeorites is estimated by using cosmic spherule counts from a seafloor area of 2 50 m 2 from the indian ocean the spherules are recovered from sediment samples in close spaced locations from the indian ocean after sieving 293 kg of sediment the terrestrial age of the spherules has a range of 0 50 000 years the spherules have a size range of 57 750 m average size 265 92 m the diameter of the spherules increases from scoriaceous barred cryptocrystalline glassy types the time averaged flux of the spherules is 160 tonnes yr a sizeable mass 60 resides in the 300 m fraction the slope of distribution is similar to that of deepsea spherules dss but significantly different from other collections which have lower average diameters it is observed that a significant population of cosmic dust resides in the higher sizes which can be recovered by sampling large areas in time and space the proportions of textural types observed are similar to that of unbiased collections this collection comprises particles that are larger than other unbiased collections and the proportion of textural types does not vary with size further this collection samples the flux over a longer time span indicating that the textural types of cosmic dust that have been raining on the earth during the last 50 ky have been constant regardless of size major element chemistry of a majority of the spherules shows elemental ratios that are close to a cm or ci chondritic parent body a single spherule 0 2 of the population suggests an achondritic parent body large unbiased collections spanning large areas temporally and spatially expand the inventory of the earth crossing meteoroid complex and provide valuable inputs for models on cosmic dust accretion

modal composition and detrital clinopyroxene geochemistry of lithic sandstones from the new england fold belt east australia a paleozoic forearc terrane the new england fold belt of southeast australia is divided by the peel fault system into two time equivalent zones showing contrasting depositional structural and metamorphic histories these are inferred to represent a western magmatic arc frontal arc succession and an eastern subduction complex assemblage the composition of the early to middle paleozoic clastic rocks from both east and west of the peel fault system indicates derivation from an undissected volcanic arc detrital quartz is absent or present only in minor amounts and always as monocrystalline grains lithic fragments are almost exclusively volcanic and normally dominant over feldspar which is invariably plagioclase now largely albitized to the west of the fault the distribution of lathwork microlitic felsitic and vitric rock fragments as well as variation in the amount of detrital quartz indicates changes in provenance through time and two cycles of basaltic to silicic volcanic activity have been identified the composition of detrital calcic clinopyroxene grains particularly their low ti and al contents and their ti mn na ratios indicates that they crystallized mainly from subalkaline probably calc alkaline volcanic arc magmas the dominantly volcanogenic provenance of sandstone from east of the fault and the low ti content of most detrital clinopyroxene grains indicate that the volcanic chain that lay to the west of the peel fault was the most likely source for the major portion of the clastic debris in the eastern rocks however the presence of ti rich detrital clinopyroxenes identical in composition to those in the metabasalts of the subduction complex indicates that uplifted segments of the complex provided an additional sediment source

martian meteorite dhofar 019 a new shergottite dhofar 019 is a new martian meteorite found in the desert of oman in texture mineralogy and major and trace element chemistry this meteorite is classified as a basaltic shergottite olivine megacrysts are set within a groundmass composed of finer grained olivine pyroxene pigeonite and augite and maskelynite minor phases are chromite ulvospinel ilmenite silica k rich feldspar merrillite chlorapatite and pyrrhotite secondary phases of terrestrial origin include calcite gypsum celestite fe hydroxides and smectite dhofar 019 is most similar to the elephant moraine eeta 79001 lithology a and dar al gani dag 476 489 shergottites the main features that distinguish dhofar 019 from other shergottites are lack of orthopyroxene lower ni contents of olivine the heaviest oxygen isotopic bulk composition and larger compositional ranges for olivine maskelynite and spinel as well as a wide range for pyroxenes the large compositional ranges of the minerals are indicative of relatively rapid crystallization modeling of olivine chemical zonations yield minimum cooling rates of 0 5 0 8 c h spinel chemistry suggests that crystallization took place under one of the most reduced conditions for martian meteorites at an f o2 3 log units below the quartz fayalite magnetite qfm buffer the olivine megacrysts are heterogeneously distributed in the rock crystal size distribution analysis suggests that they constitute a population formed under steady state conditions of nucleation and growth although a few grains may be cumulates the parent melt is thought to have been derived from partial melting of a light rare earth element and platinum group element depleted mantle source shergottites eeta79001 lithology a dag 476 489 and dhofar 019 although of different ages comprise a particular type of martian rocks such rocks could have formed from chemically similar source s and parent melt s with their bulk compositions affected by olivine accumulation

chondrules in antarctic micrometeorites previous studies of unmelted micrometeorites 50 m recovered from antarctic ice have concluded that chondrules which are a major component of chondritic meteorites are extremely rare among micrometeorites we report the discovery of eight micrometeorites containing chondritic igneous objects which strongly suggests that at least a portion of coarse grained crystalline micrometeorites represent chondrule fragments six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine grained matrix these particles suggest that at least some coarse grained micrometeorites cgmms may be derived from the same parent bodies as fine grained micrometeorites the new evidence indicates that contrary to previous suggestions the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites

sedimentary record of explosive silicic volcanism in a cretaceous deep marine conglomerate succession northern antarctic peninsula lower cretaceous conglomeratic strata exposed on southern sobral peninsula were deposited on a deep marine apron in the back arc larsen basin close to its faulted boundary with the antarctic peninsula magmatic arc the succession is dominated by amalgamated beds of clast supported conglomerate which together with minor intercalated sandstones consist of varied but largely basaltic to andesitic volcanic material and clasts derived from the palaeozoic triassic meta sedimentary basement of the arc most of the volcanic clasts are thought to have been derived from lithified volcanic successions or older synvolcanic deposits rather than from sites of coeval eruption these mixed provenance strata enclose a number of intervals consisting mainly of inverse normally graded conglomerate and graded stratified pebbly sandstone in which the sand fraction is dominated by crystals and vitric grains considered to have been redeposited in the immediate aftermath of explosive silicic arc volcanism like syneruption deposits on non marine volcaniclastic aprons these intervals are more sand prone than the enclosing strata and appear to show evidence of unusually rapid aggradation plagioclase from one such interval has yielded 40ar 39ar ages concordant at 121 ma similar to those obtained from the non marine cerro negro formation deposited within the magmatic arc it is suggested that the two successions can be viewed as counterparts both recording a history of mainly basaltic to andesitic volcanism punctuated by relatively infrequent explosive silicic eruptions whereas the cerro negro formation consists mainly of syneruption deposits most of the volcaniclastic material delivered to the eruption distal deep marine apron appears to have been derived by normal degradation processes only rare silicic eruptions were capable of supplying pyroclastic material rapidly enough and in sufficient quantities to produce compositionally distinct syneruption intervals

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

compositional suites of terrigenous deep sea sands of the present continental margins the compositional variability shown by sands of present continental margins was determined through conventional microscopic techniques this allowed the establishment of a descriptive classification of deep sea terrigenous sands based on a restricted number of commonly used parameters the fundamental sand types include 1 qf suite sands in which quartzose and or feldspathic grains predominate carbonate l bearing sands on average richer in lithics occur within this suite 2 ql suite in which quartzose and or lithic grains predominate little compositional diversification is apparent within this suite 3 fl suite in which lithic and or feldspathic grains predominate two compositional members can be identified based on the k feldspar plagioclase ratio 4 lv suite consists of sands in which volcanic grains largely dominate the lithic types vitric and serpentine sands occur within this suite the suites are related to distinct source area types defined as follows 1 cratonic block 2 sedimentary orogenic belt 3 volcanic orogenic belt 4 volcanic arc some sands cannot be easily assigned to a single compositional suite because petrologic parameters overlap with an adjacent suite such critical compositions are commonly arkosic using roundness of the framework components it is possible to define a category identified as matured sands which are greatly enriched in resistates

numbers types and compositions of an unbiased collection of cosmic spherules abstract abstract micrometeorites collected from the bottom of the south pole water well spww may represent a complete well preserved sample of the cosmic dust that accreted on earth from 1100 1500 a d we classified 1588 cosmic spherules in the size range 50 800 pm the collection has 41 barred olivine spherules 17 glass spheres 12 cryptocrystalline spherules 11 porphyritic olivine spherules 12 relic grain bearing spherules 3 scoriaceous spherules 2 i type spherules 1 ca al ti rich cat spherules and 1 g type spherules we also found bubbly glass spherules spherules with glass caps and ones with sulfide coatings particles that are absent from other collections a classification sequence of the stony spherules scoriaceous relic grain bearing porphyritic barred olivine cryptocrystalline glass and cat is consistent with progressive heating and evaporation of fe from chondritic materials the modem day accretion rate and size distribution measured at the spww can account for the stony spherules present in deep sea collection through preferential dissolution of glass and small stony spherules however weathering alone cannot account for the high accretion rate of i type spherules determined for two deep sea collections the spww collection provides data to constrain models of atmospheric entry heating and to assess the effects of terrestrial weathering

heavy mineral analysis and clinopyroxene geochemistry applied to provenance analysis of lithic sandstones from the azuero sona complex nw panama abstract the volcaniclastic sandstones of the middle eocene to lower miocene tonosi formation were deposited within the forearc realm of an island arc system which today is part of the southern central american landbridge both heavy mineral analyses and geochemical analyses of clinopyroxenes clearly reveal differences in the composition between sediments of the lower and upper tonosi formation indicating changes within the tectonic setting as well as island arc evolution during time of deposition based on characteristic heavy mineral suites three lithogroups can be determined and assigned to the lower and upper tonosi formation respectively provenance analyses indicate that the sandstones of the tonosi formation were generally derived from an island arc setting the abundance of a mafic heavy mineral suite relative to heavy minerals derived from metamorphic and sialic intrusive rocks indicates a convergent plate boundary setting for provenance of the lower tonosi formation this result is supported by the chemistry of clinopyroxenes within the heavy mineral suite which indicates evolved basaltic magma types sourced the detrital grains within the hb ol id cpxbrown opx cpxgreen diagram most samples of the upper tonosi formation plot in the field for divergent plate boundaries the geochemical analyses also show that clinopyroxenes of the upper tonosi formation additionally exhibit affinities to within plate alkaline magma types as an integration of our results we present a provisional two stage model for the tectono sedimentary evolution of the western panamanian forearc area during the deposition of the tonosi formation

fe mg fe mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies abstract we examine fe mg mn relations of meteorites as represented on plots of molar fe mg ratio vs molar fe mn ratio and use them to consider whether it is meaningful to classify achondrites as primitive or evolved first we demonstrate that fe mg fe mn plots can be used to identify some of the major processes that have affected meteorite parent bodies during both the nebular condensation and accretion and the planetary igneous stages of their evolution examination of compositions of chondrites on this diagram reveals distinct patterns that result from three nebular processes 1 volatility controlled mn mg fractionation 2 metal silicate fractionation under highly reducing conditions and 3 subsequent oxidation leading to variations in feo mgo ratio ordinary chondrites ocs show a characteristic pattern that records all three of these processes and suggests that the latter two are coupled examination of the compositions of achondrites on this diagram shows that all of those that have been classified as primitive lodranites acapulcoites winonaites iab silicate inclusions brachinites and ureilites are easily distinguished from those that have been classified as evolved sncs heds and lunar basalts the mn mg ratios of the former are chondritic while those of the latter are superchondritic hence they plot in two graphically distinct regions of the fe mg fe mn diagram a simple melting calculation reveals that this distinction results principally because the former are residues and the latter are melts or cumulate products of melts rather than because the latter are more evolved additionally however some of the groups classified as primitive achondrites show fe mg fe mn patterns similar to that of ocs constant but distinct mno mgo ratios over large ranges of feo mgo ratio we argue that these trends reflect primary heterogeneity resulting from the same nebular processes as those that affected ocs and therefore preserve evidence that more of the materials that accreted to form planetesimals experienced these processes than is apparent from known chondrites this evidence is preserved principally because these meteorites are residues in contrast groups commonly classified as evolved achondrites show fe mg fe mn patterns that reflect only igneous processing and any evidence for primary heterogeneity of their parent bodies has been obliterated again this is principally because these meteorites represent melts rather than beause they are more evolved usage of the terms primitive and evolved to describe achondrites should be clearly defined

petrology and geochemistry of antarctic micrometeorites abstract the petrology and geochemistry of twentythree chondritic dust particles with masses of 1 47 g sizes 100 400 m were recovered from blue ice near cap prudhomme antarctica and studied by inaa asem empa and optical microscopy sample selection criteria were irregular shape and for a subsample black color with the aim of studying as many unmelted micrometeorites mms as possible of thirteen unmelted mms six were phyllosilicate dominated mms and seven were coarsegrained crystalline mms consisting mainly of olivine and pyroxene the remaining ten particles were largely melted and consisted of a foamy melt with variable amounts of relic phases scoriaceous mms thus of the black particles selected an astonishing portion 40 by number consisted of largely unmelted mms although unmelted most phyllosilicate mms have been thermally metamorphosed to a degree that most of the phyllosilicates were destroyed but not melted the original preterrestrial mineralogy is occasionally preserved and consists of serpentine like phyllosilicates with variable amounts of cronstedtite tochilinite like oxides olivine and pyroxene the crystalline mms consist of olivine low ca pyroxene tochilinite like oxides and occasional ni poor metal relics in scoriaceous mms consist of the same phases mineral compositions and the coexistence of phyllosilicates with anhydrous phases are typical of cm and cr type carbonaceous chondrites however the olivine pyroxene ratio 1 and the lack of carbonates sulfates and of very fe poor refractory element rich olivines and pyroxenes sets the mms apart from cm and cr chondrites the bulk chemistry of the phyllosilicate mms is similar to that of cm chondrites however several elements are either depleted ca ni s less commonly na mg and mn or enriched k fe as br rb sb and au in mms as compared to cm chondrites similar depletions and enrichments are also found in the scoriaceous mms we suggest that the depletions are probably due to terrestrial leaching of sulfates and carbonates from unmelted mms the overabundance of some elements may also be due to processes acting during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere most mms are coated by magnetite of platy or octahedral habit which is rich in mg al si mn and ni we interpret the magnetites to be products of recondensation processes in the high 90 km atmosphere which are therefore probably the first refractory aerominerals identified

a unique high mn fe microgabbro in the parnallee ll3 ordinary chondrite nebular mixture or planetary differentiate from a previously unrecognized planetary body the study of planetary materials in chondritic meteorites constrains the compositional diversity of materials in different nebular environments and provides information on the degree of differentiation of early planetary bodies we studied a unique microgabbro fragment from the parnallee ll3 unequilibrated ordinary chondrite the fragment which was originally identified by its ophitic to sub ophitic texture exhibits features characteristic of lunar and terrestrial tholeiitic basalts extreme compositional zoning in clinopyroxene wo 10 en 65 fs 25 to wo 15 en 2fs 83 a multiply saturated major element composition similar to mid ocean ridge basalt with 3 1 wt na 2o and 0 15 wt k 2o and uniformly enriched rare earth elements c 10 c1 a high bulkmno feo ratio 0 064 distinguishes the microgabbro from other basaltic rocks and suggests the precursor material formed or reached equilibrium in a reducing environment however the absence of fe metal and the extreme enrichment of feo up to 40 wt in late crystallizing pyroxferroite requires the last crystallization event to have occurred in a relatively oxidizing environment we suggest the microgabbro formed by partial melting in a planetary body after removal of metallic fe examination of possible planetary source materials such as alkali rich eucritic material a volatile depleted c1 carbonaceous chondrite or h group chondrite shows that multiple stage fractionation is required to produce a melt with the feo mgo ratio of the microgabbro from these materials the increasing number of planetary igneous fragments observed in unequilibrated ordinary chondrites uoc suggests the picture of uoc as primitive assemblages of unprocessed material is overly simplistic

ordinary chondritic micrometeorites from the indian ocean extraterrestrial particulate materials on the earth can originate in the form of collisional debris from the asteroid belt cometary material or as meteoroid ablation spherules signatures that link them to their parent bodies become obliterated if the frictional heating is severe during atmospheric entry we investigated 481 micrometeorites isolated from 300 kg of deepsea sediment out of which 15 spherules appear to have retained signatures of their provenance based on their textures bulk chemical compositions and relict grain compositions seven out of these 15 spherules contain chromite grains whose compositions help in distinguishing sub groups within the ordinary chondrite sources there are seven other spherules which comprise either entirely of dusty olivines or contain dusty olivines as relict grains two of these spherules appear to be chondrules from an unequilibrated ordinary chondrite in addition a pop porphyritic olivine pyroxene chondrule like spherule is also recovered the bulk chemical composition of all the spherules in combination with trace elements the chromite composition and presence of dusty olivines suggest an ordinary chondritic source these micrometeorites have undergone minimal frictional heating during their passage through the atmosphere and have retained these features these micrometeorites therefore also imply there is a significant contribution from ordinary chondritic sources to the micrometeorite flux on the earth

crumbs from the crust of vesta achondritic cosmic spherules from the south pole water well ten glass cosmic spherules cs from the south pole water well collection were analyzed by electron microprobe nine of them have fe mn and fe mg ratios in the range typical of chondrites one of them sp37 3 along with up to six other previously analyzed cs have nonchondritic fe mn and fe mg ratios that agree well with values typical of either basaltic howardite eucrite and diogenite hed meteorites or martian basalts but not of lunar samples sp37 3 also contains an anorthite relic grain anorthite has not previously been reported in cosmic spherules but is well known in hed meteorites the much greater frequency of heds among hand sized meteorites suggests but does not prove that hed precursors are more likely for the nonchondritic spherules we estimate that hed like micrometeorites constitute 0 5 0 4 of the total population of micrometeorites in the south pole water well a fraction that translates to a flux of 1 6 0 3 x 10 8 g hed micrometeorites m 2 y the ratio of hed like objects to carbonaceous objects is about 100 times less in micrometeorites than among hand size specimens we infer that the comparative mechanical weakness of carbonaceous precursor materials tends to encourage spherule formation

petrology and chemistry of the picritic shergottite north west africa 1068 nwa 1068 we report on the petrology and chemistry of north west africa 1068 nwa 1068 a shergottite recently recovered in morocco this meteorite has a total known mass of about 577 g and comprises 23 fragments the largest fragment is a greenish brown rock devoid of fusion crust it displays a porphyritic texture consisting of a fine grained groundmass and olivine grains excluding the impact melt pockets and the minor carbonate veins produced by terrestrial weathering modal analyses indicate the following mineral proportions 52 vol pyroxenes 22 maskelynite 21 olivine 2 phosphates merrillite and chlorapatite 2 opaque oxides mainly ilmenite and chromite and sulfides and 1 k rich mesostasis olivines with various habits occur as clusters often associated with chromite or single crystals ranging in size from 50 mum to 2 millimeters megacrysts these crystals originate probably from disrupted cumulates with strong affinities with peridotitic shergottites

late precambrian volcaniclastic deposits of the avalonian eastport basin newfoundland appalachians petrofacies detrital clinopyroxene geochemistry and palaeotectonic implications abstract the eastport basin contains over 5 km of well preserved marine volcaniclastic deposits that comprise parts of the late precambrian avalonian connecting point and love cove groups of the eastern newfoundland appalachians basin infilling was dominated by deep sea resedimentation of pyroclastic and epiclastic sediments four petrographically and texturally distinct petrofacies have been designated viz tuffs tuffaceous sandstones and conglomerates feldspathic sandstones and conglomerates polymict sandstones and conglomerates a broad compositional spectrum of volcanogenic detritus a copious supply of pyroclastic basaltic andesitic dacitic and rhyolitic ejecta and the ubiquity of trace plutonic and metamorphic rock fragments are diagnostic provenance attributes of the basin fill and these indicate derivation from an evolved magmatic arc developed on a transitional or possibly continental crust detrital calcic clinopyroxenes and calcic amphiboles from the upper part of the connecting point group are low in ti and al display calc alkaline and calc alkaline to tholeftic affinity of the parental magma and plot in the field of orogenic basalts discriminant function analysis demonstrates that basalts and andesites were the main source of the detrital clinopyroxene the clinopyroxene major element chemistry is consistent with the evolved volcanic arc provenance suggested by petrographic data both datasets independently support other sedimentological and stratigraphic evidence that the eastport basin records sedimentation adjacent to an active avalonian cadomian volcanic arc the remnants of which are preserved in part within the presently adjacent predominantly volcanic love cove group

carbonaceous micrometeorites from antarctica over 100 000 large interplanetary dust particles in the 50 500 micrometers size range have been recovered in clean conditions from approximately 600 tons of antarctic melt ice water as both unmelted and partially melted dehydrated micrometeorites and cosmic spherules flux measurements in both the greenland and antarctica ice sheets indicate that the micrometeorites deliver to the earth s surface approximately 2000x more extraterrestrial material than brought by meteorites mineralogical and chemical studies of antarctic micrometeorites indicate that they are only related to the relatively rare cm and cr carbonaceous chondrite groups being mostly chondritic carbonaceous objects composed of highly unequilibrated assemblages of anhydrous and hydrous minerals however there are also marked differences between these two families of solar system objects including higher c o ratios and a very marked depletion of chondrules in micrometeorite matter hence they are chondrites without chondrules thus the parent meteoroids of micrometeorites represent a dominant and new population of solar system objects probably formed in the outer solar system and delivered to the inner solar system by the most appropriate vehicles comets one of the major purposes of this paper is to discuss applications of micrometeorite studies that have been previously presented to exobiologists but deal with the synthesis of prebiotic molecules on the early earth and more recently with the early history of the solar system

the volatile element enrichment of chondritic interplanetary dust particles abstract interplanetary dust particles idps collected in the stratosphere provide new information on the chemical and isotopic composition of material in the vicinity of the earth and thus possibly of asteroids and comets the available data on major minor and trace element abundances mainly obtained with pixe and syxfa suggest the presence of at least two groups of chondritic idps which differ significantly in their volatile element content the ubiquitous enrichment of volatile lithophile siderophile and chalcophile elements is most plausibly explained by atmospheric contamination and thus the question arises of the factual relationship of all properties of interplanetary dust particles to early solar system processes

oxygen isotopes in cosmic spherules and the composition of the near earth interplanetary dust complex abstract a long standing controversy in the micrometeorite community regards the relative contribution of primitive asteroids or comets and of evolved asteroids to the interplanetary dust cloud we compiled and studied a large set of oxygen isotopic data from the literature on cosmic spherules from different collections covering different influx periods within the last 1 myr cosmic spherules micrometeorites melted during atmospheric entry are the most abundant micrometeorites in worldwide collections according to several models they are representative of the composition and origin of micrometeorites 50 m in size spherule statistics 136 spherules 50 2280 m in size indicate that at least 20 of the micrometeoroid complex is fed by asteroids observed in the inner asteroid belt the ordinary chondrite and secondarily the hed parent asteroids likely belonging to the s type and v type spectral classes respectively another 60 or more is related to primitive objects of the solar system with carbonaceous chondrite compositions either primitive asteroids belonging to the c d or p type spectral classes in the outer asteroid belt or comets contribution from terrestrial planets has not been identified yet oxygen isotopes also document that the composition of the micrometeoroid complex is different from that of macroscopic meteoroids since the latter is dominated by materials from evolved and differentiated asteroids rather than primitive asteroids or comets cosmic spherule statistics show that the contribution of ordinary chondrite material to the composition of the micrometeoroid complex increases with micrometeorite size thereby documenting a continuum between meteorites and micrometeorites the transition in terms of relative abundance of the two cosmic spherule populations occurs around 500 m in size

evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the mesozoic chugach accretionary complex south central alaska the gulf of alaska is one of the largest accretionary complexes on earth in this study we examined the earliest phase of accretion in the mesozoic mchugh complex and valdez groups exposed in se alaska the oldest preserved fragment the mesomelange assemblage is jurassic ca 160 140 ma and consists of an 3 km thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the wrangellia and the talkeetna oceanic arc a tectonic erosion event affected the forearc just prior to ca 120 ma and was likely caused by seamount collision ridge subduction or both this was followed at 105 ma by mass wasting of sandstone and conglomerates preserved as the graywacke conglomerate assemblage ca 105 83 ma heavy minerals indicate continued flux from arc sources but with significant changes suggesting a larger more diverse catchment area erosion of deeper crustal sources provided high mg diopside and garnets to the trench faster sediment flux was caused by rock uplift triggered by final accretion of the wrangellia peninsula terrane to north america the start of large scale accretion in alaska roughly coincided with the initiation of shimanto complex accretion in japan and can be understood as primarily linked to sediment supply driven by plate margin tectonics rather than climatically induced erosion onshore

oxygen isotopic and chemical compositions of cosmic spherules collected from the antarctic ice sheet implications for their precursor materials abstract bulk chemical compositions and oxygen isotopic compositions were analyzed for 48 stony cosmic spherules melted micrometeorites collected from the antarctic ice sheet using electron and ion microprobes no clear correlation was found between their isotopic compositions and textures the oxygen isotopic compositions showed an extremely wide range from 28 to 93 in 18o and from 21 to 13 in 17o in 18o 17o space most samples 38 out of 48 plot close to the terrestrial fractionation line but 7 samples plot along the carbonaceous chondrite anhydrous mineral ccam line three samples plot well above the terrestrial fractionation line one of these has a 17o of 13 the largest value ever found in solar system materials one possible precursor for this spherule could be 16o poor planetary material that is still unknown as a meteorite the majority of the remaining spherules are thought to be related to carbonaceous chondrites

extra terrestrial influx rates of cosmogenic isotopes and platinum group elements realizable geochemical effects we present estimates of realizable influx of selected cosmogenic isotopes 3he 10be 26al and 14c and platinum group elements ir os and re on the earth via influx of meteoroids we define realizable as the particulate fraction of mass influx which is included in standard geochemical analyses of terrestrial sediments this is the surviving mass fraction of size 0 5 1 cm which gets mixed with terrestrial samples and is analyzed in normal terrestrial assays of sediments larger surviving meteoroid fragments of the order of cm to m or larger obviously belong to the non realizable flux category since i their distribution in terrestrial samples would be very patchy resulting in a highly variable density matrix and ii they would also generally except in wide diameter cores be excluded from sediment cores we estimate the realizable influx of meteoritic particles based on a recent model describing production of smaller size fragments arising during the atmospheric entry of meteoroids implicit in this work is the assumption that the secondary fragments are not subject to much heating and therefore most of the initial 3he and noble gases would be preserved in the secondary fragments since after break up they are not subjected to much heating under this assumption production of small size fragments in the ablation process constitutes a safe landing mechanism for parts of the meteoroid in this paper we show that the meteoritic ablation fragmentation process produces a significant flux of 3he platinum group metals and cosmogenic 26al in fact measurements of these isotopes in ocean sediments should allow a reasonable estimate of temporal variations in the flux of meteoroids of 50 cm to 5 m radii which produce most of the secondary fragments in the size range 1 cm we would like to state here that as pointed out in our previous work stratospheric collections would be biased towards collection of the primary incident extra terrestrial particles whereas terrestrial accumulations representing large area time accumulations as in the case of ocean sediments would efficiently sample the fragmented particles

compositional affinities of volcanic fragments in sedimentary rocks using electron microprobe analysis abstract the identification of clasts of volcanic origin in terrigenous detrital rocks is generally based on the observation of textural criteria in the absence of quantitative analytical data on the chemical composition of such fragments it is often quite difficult to identify their specific origin however it is now possible to use a defocused microprobe beam to obtain high precision analyses of microcrystalline elements the application of this technique to the groundmass of various lavas shows that the distribution of certain elements varies as a function of whole rock composition three ternary diagrams using the most discriminant parameters k 2 o na 2 o cao fe 2 o 3 na 2 o k 2 o mgo and sio 2 cao 10 mgo 10 are proposed for the identification of different groundmass compositions ultrabasic basic intermediate and acid the study of reworked clastic material from the upper ordovician sediments of sardinia shows that fragments of volcanic groundmass can preserve their initial geochemical characteristics the application of this method to microcrystalline fragments contained in proterozoic sandstones of the armorican massif shows that it can be an additional approach for the identification of siliciclastic source areas

subduction of a late cretaceous seamount of the louisville ridge at the tonga trench a model of normal and accelerated tectonic erosion the hotspot generated louisville ridge is a 4000 km chain of seamounts typically 2 2 5 km high and 10 40 km in diameter and an underlying crustal swell 1 5 km high and 100 km wide trending nnw across the southwestern pacific the northwest end of the ridge collides with the north trending tonga trench 26 s which just north of that point is exceptionally deep 10 8 km and lacks both a turbidite wedge and a bordering accretionary complex the collision zone is moving rapidly southward multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2 3 km beneath the lower western trench slope which is interpreted as the flat summit of a subducted guyot motuku of the louisville chain projected eastward the summit plain passes 1 2 km above the trench floor dredging of the nearby inner trench wall recovered uppermost cretaceous maestrichtian oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted louisville seamount the principal effects of hotspot ridge collision with a sediment starved trench are 1 the impacting seamounts are subducted rather than accreted and 2 although some seamount rocks are temporarily accreted the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km m y accelerated tectonic erosion is related to 1 fracturing shearing and general weakening of arc substrate rocks as they are lifted by the swell penetrated by impacting seamounts and left to collapse as the ridge moves away 2 a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell 3 a possible elevation of the subduction decollement to account for the removal of as much as 30 000 km of material from a 400 km sector of the trench in 1 million years and 4 a reduction in supply of arc derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge normal tectonic erosion in the tonga trench is apparently minor and we conclude that the bulk of the 37 000 km of material which fills subducting grabens each million years is arc derived volcanic and pelagic sediment

melted micrometeorites from antarctic ice with evidence for the separation of immiscible fe ni s liquids during entry heating we report the discovery of four large 50 m cosmic spherules css and a single scoriaceous micrometeorite smm that contain evidence for the separation of immiscible fe ni s liquids during atmospheric entry heating the particles contain segregated fe rich regions dominated by either ni s bearing fe oxides or iron sulphides and have textural relations that suggest these separated from the silicate portions of the particles as metallic liquids the oxides which may be hydrous are thought to result from alteration of metal and sulphide the compositions of the silicate portions of the css are equivalent to spherules without fe rich regions implying that metallic liquids are exsolved during the heating of most spherules but completely separate the single smm has a very different composition from other scoriaceous particles and the occurrence of an exsolved metallic liquid probably indicates extreme reduction during entry heating the pyrolysis of carbonaceous materials is the most likely explanation for reduction and suggests that the precursor material of this particle was unusually c rich this smm might be therefore an appropriate candidate for a large melted anhydrous or smectite interplanetary dust particle idp the exsolution of immiscible fe ni s liquids during entry heating will result in systematic changes in the compositions of the remaining silicate melt

provenance of volcaniclastic beach sand in a magmatic arc setting an example from lipari island aeolian archipelago tyrrhenian sea lipari an active volcanic island in the aeolian magmatic arc is an excellent area to determine the effects of multiple source lithology climate weathering transport and depositional environment on epiclastic sand composition volcaniclastic sand samples from 12 modern beaches were petrographically characterized using the gazzi dickinson method and the proportions of source rocks in combination with topography in associated coastal drainage basins were quantified using gis several types of bedrock in the 12 drainage basins that are the likely prominent sources for sand at each sampled beach were recognized and divided into two categories of provenance lithotypes lavas and pyroclastic rocks ranging in composition from basaltic andesitic to andesitic to rhyolitic volcanic lithic fragments from lipari beach sand consist of colourless and black glassy volcanic fragments with lathwork felsitic vitric and microlitic textures moreover high amounts of detrital less durable minerals such as pyroxene olivine and fe oxides illustrate how the analysed sands preserve the source rock s provenance signals applying the concept of sand generation index we see that these lithotypes have different propensities to create detritus in terms of both grain size and composition clastic contribution from pyroclastic rock outcrops such as pumice is not found in the size ranges studied suggesting that these pumiceous source rocks probably only produce gravel or very fine sand and silt this finding has implications for the stratigraphic record because pumice clasts ranging from medium to fine grain size could be underrepresented in older volcaniclastic deposits and overrepresented in other size fractions

volcanic arc reconstruction using discriminant function analysis of detrital clinopyroxene and amphibole from the new england fold belt eastern australia discriminant function analysis dfa of ca rich clinopyroxene and amphibole from modern arc volcanics allows identification of the parent magma type basalt andesite or dacite rhyolite with a view to understanding the provenance of arc derived sandstones the dfa approach has been applied to a suite of well studied cambrian to devonian volcaniclastic sandstones of the new england fold belt of eastern australia which contain significant arc derived detrital clinopyroxene and occasionally amphibole previous provenance studies of these rocks have been based on petrography identifying two cycles of volcanism but the results of this study show this to be oversimplified instead dfa analysis shows that some units comprise both intermediate and acidic volcanic components with significant along strike or vertical variations in terms of arc derived detritus the dfa approach avoids the pitfalls of a component mixing that limits the usefulness of whole rock chemical analysis and b the qualitative as

interpretation of neovolcanic versus palaeovolcanic sand grains an example from miocene deep marine sandstone of the topanga group southern california despite abundant data on volcaniclastic sand stone the compositional spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented one of the most intricate tasks in optical analysis of sand stone containing volcanic particles is to distinguish grains derived by erosion of ancient volcanic rocks i e palaeovolcanic noncoeval grains from grains generated by active volcanism subaqueous and or subaerial during sedimentation neovolcanic coeval grains deep marine volcaniclastic sandstones of the middle topanga group of southern california are interstratified with 3000 m thick volcanic deposits both subaqueous and subaerial lava and pyroclastic rocks ranging from basalt andesite to dacite these rocks overlie quartzofeldspathic sandstones petrofacies 1 of the lower topanga group derived from deep erosion of a mesozoic magmatic arc changes in sandstone composition in the middle topanga group provide an example of the influence of coeval volcanism on deep marine sedimentation volcaniclastic strata were deposited in deep marine portions of a turbidite complex volcaniclastic apron built onto a succession of intrabasinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks distinctive petrofacies 2 5 directly overlying basalt and basaltic andesite lava flows petrofacies 2 is a pure volcanolithic sandstone including vitric microlitic and lathwork volcanic grains and neovolcanic crystals plagioclase pyroxene and olivine the abundance of quenched glass palagonite fragments suggests a subaqueous neovolcanic provenance whereas sandstones including andesite and minor basalt grains suggest subaerial neovolcanic provenance this petrofacies probably was deposited during syneruptive periods testifying to provenance from both intrabasinal and extrabasinal volcanic events deposited during intereruptive periods impure volcanolithic petrofacies 3 includes both neovolcanic 85 and older detritus derived from plutonic metamorphic and palaeovolcanic rocks during post eruptive periods the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus 48 14 and increase of plutonic metamorphic and palaeovolcanic detritus the upper topanga group calabasas formation conformably overlying the middle unit has dominantly plutoniclastic sandstone petrofacies 6 neovolcanic detritus is drastically reduced 4 whereas palaeovolcanic detritus is similar to percentages of the lower topanga group petrofacies 1 the middle topanga group sandstones are vertically organized into four

acoustic evidence of a baltic ice lake drainage debrite in the northern baltic sea abstract the sudden drainage of the baltic ice lake bil at the end of the younger dryas chron was one of the major events within the pleistocene holocene boundary in scandinavia thus far the effects it might have had on sedimentary environments have been inadequately recorded outside of central and southern sweden in the present study high quality acoustic sounding data was used to examine and describe possible traces of the event from the north eastern baltic proper and gulf of finland sediments five acoustic facies were identified two of them bearing frequent indications of sediment deformation slumping sliding and homogenization the lower deformation zone is in the glacial clay deposited in the ice proximal environment the middle zone in the ice distal glacial clay and the upper zone in late glacial clay we suggest that the middle deformation unit may have basin wide significance because it represents a debrite system originating from sediment instability due to the sudden water level drop associated with the bil drainage at the transition of the younger dryas preboreal chrons

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

exceptions to the relationship between plate tectonics and sandstone composition abstract routine use of qfl qplsmlvm and lmlslv plate tectonic provenance diagrams has led to the recognition of error populations for these sandstones the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams the anomalous sandstones can be separated into at least four categories i sandstones deposited during the transition between tectonic regimes may be derived in part from relict source rocks ii the relative enrichment of sandstones in detrital quartz by weathering and or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data iii sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields iv the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams and is an unresolved problem the first two categories represent permanent problems in the use of sandstone composition as an indicator of plate tectonic setting whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate tectonic setting

trace element geochemistry and sedimentary provinces a study from the tasman geosyncline australia abstract the abundances of la th u and hf have been determined in flysch sequences of the tasman geosyncline australia the arc derived graywackes differ from those of rifted continental margins and marginal basins in containing low abundances of la th u and hf la 9 2 1 7 ppm th 1 4 0 6 ppm u 0 52 0 2 ppm hf 2 1 0 6 ppm reflecting similar low abundances in the andesitic source rocks graywackes of rifted continental margins and marginal basins in contrast are mature and characterised by high abundances of la th u and hf la 39 9 9 ppm th 16 0 6 ppm u 3 40 0 5 ppm hf 7 9 ppm mirroring increased abundances in the granitic and sedimentary source rocks sedimentary rocks deposited in inter arc basins show wide dispersion but have characteristics intermediate between these two types the th u ratio for magmatic arc sediments is 2 7 for most other sedimentary rocks it remains uniform at 4 5 0 3 for sediments having andesitic and dacitic rocks as provenance the la th ratios are 6 7 and 4 5 respectively this ratio remains constant at 2 6 for most quartzose sedimentary rocks

meteoritic event recorded in antarctic ice during systematic sampling of volcanic ash tephra layers at a well known antarctic meteorite collection site the allan hills main ice field a band of unusually dark and rounded many spheroidal particles was discovered this debris layer bit 58 extends parallel to the stratigraphy of the ice established from the tephra bands apparently marking a single depositional event the shapes internal texture major element composition and levels of cosmogenic nuclides of particles from within bit 58 all strongly suggest that this material represents ablation debris from the passage of a large h group ordinary chondrite preliminary cosmogenic isotope dating suggests an age of 2 8 ma implying that the east antarctic ice sheet has been stable since that time the relationship of the bit 58 layer to known impact events is not clear

isotopic fractionation of iron potassium and oxygen in stony cosmic spherules implications for heating histories and sources abstract atmospheric heating alters the compositions and textures of micrometeorites to understand the changes and to test a proposed relationship between a micrometeorite s petrographic texture and its degree of heating we made elemental and multiple isotope analyses of stony cosmic spherules scs collected from the south pole water well specifically we analyzed the elemental compositions of 94 scs and the isotopic ratios of fe k and o on 43 12 and 8 of these scs respectively our results show that scs classified as strongly heated generally have lower concentrations of volatile and moderately volatile elements of the 43 spherules analyzed for fe isotopes only 5 have 57fe 5 in contrast enrichment of 41k is pervasive 41k 0 in all 12 spherules analyzed and large up to 183 the determination of k isotope abundances in scs may therefore be useful in deciphering thermal histories three of the eight scs analyzed for o isotopes are mass fractionated with 18o 30 we attribute two of these three 18o enrichments to evaporative losses of oxygen in the atmosphere and the third to the presence in the parent material of an exotic phase perhaps a sulfate or a carbonate the k isotope and o isotope data are broadly consistent with the proposed textural classification because most spherules were not heated enough to fractionate al mg or si we compared the measured mg al and si al ratios directly to those of conventional meteorites and their matrices 30 of the scs have compositions outside the range defined by the bulk and the matrix compositions of known meteorite groups but consistent with those of pyroxene and olivine rich materials and may be samples of chondrules the other 70 have mg al and si al ratios similar to those of ci cm co and cv chondrites natural variability of the mg al and si al ratios precludes the assignment of an individual scs to a particular class of c chondrite

late cretaceous pelagic sediments volcanic ash and biotas from near the louisville hotspot pacific plate paleolatitude 42 s abstract dredging on the deep inner slope of the tonga trench immediately north of the intersection between the louisville ridge hotspot chain and the trench recovered some late cretaceous maestrichtian slightly tuffaceous pelagic sediments they are inferred to have been scraped off a recently subducted late cretaceous guyot of the louisville chain in the vicinity of the louisville hotspot present location 50 26 s 139 09 w late cretaceous location 42 s longitude unknown late cretaceous rich diatom radiolarian silicoflagellate foraminiferal and coccolith biotas accumulated on the flanks of the guyot and are described in this paper rich sponge faunas are not described inoceramus prisms are present volcanic ash is of within plate alkalic character isotope ratios in bulk carbonate 18o 2 63 to 0 85 13c 2 98 to 3 83 are normal for pacific maestrichtian sediments the local ccd may have been shallower than the regional ccd because of high organic productivity in some samples late cretaceous materials have been mixed with neogene materials mixing may have taken place on the flanks of the guyot during transit across the western pacific or on the trench slope during or after subduction and offscraping about 0 5 ma

measurement of53mn in deep sea iron and stony spherules abstract cosmic ray produced 53 mn t 1 2 3 7 10 6 years was measured in individual and groups of deep sea iron and stony spherules by highly sensitive neutron activation analysis the activities found were less than 20 dpm 53 mn kg fe 10 5 10 6 dpm 53 mn sample in iron spherules except one iron spherule whose activity was 241 73 dpm 53 mn kg fe these low activities may indicate evaporative loss of 53 mn due to heating in the earth s atmosphere on the other hand all stony spherules contained 200 260 dpm 53 mn kg fe which is similar to chondritic values these spherules may be ablation debris from large objects

provenance and petrology of deep sea drilling project sands and sandstones from the japan and mariana forearc and backarc regions abstract one hundred and twenty point counts of oligocene to recent sands and sandstones from dsdp sites in the japan and mariana intraoceanic forearc and backarc basins demonstrate that there is a clear compositional difference between the continentally influenced japan forearc and backarc sediments and the totally oceanic mariana forearc and backarc sediments japan forearc sediments average 10 qfl q 0 82 p f 2 framework mica 74 lmlvlst lv and 19 lmlvlst lst in contrast the mariana forearc and backarc sediments average 0 qfl q 1 00 p f 0 framework mica 98 lmlvlst lv and 1 lmlvlst lst sediment compositions in the japan region are variable the honshu forearc sediments average 5 qfl q 0 94 p f 1 framework mica 82 lmlvlst lv and 15 lmlvlst lst the yamato basin sediments dsdp site 299 average 13 qfl q 0 70 p f 3 framework mica 78 lmlvlst lv and 14 lmlvlst lst the japan basin sediments dsdp site 301 average 24 qfl q 0 54 p f 9 framework mica 58 lmlvlst lv and 21 lmlvlst lst p f and framework mica are higher in the yamato basin sediments than in the forearc sediments due to an increase in modal potassium content of volcanic rocks from east to west on the island of honshu site 301 possesses a higher qfl q and lmlvlst lst and lower lmlvlst lv than site 299 because it receives sediment from the asian mainland as well as the island of honshu dsdp site 293 sediments in the mariana region average 0 97 p f 1 framework mica 13 lmlvlst lm and 83 lmlvlst lv due to their proximity to the island of luzon the remaining mariana forearc and backarc sediments show a uniform composition

porosities of lunar meteorites strength porosity and petrologic screening during the meteorite delivery process porosity has been directly measured for eight lunar meteorite breccias and calculated for two more on the basis of literature density measurements lunar meteorite regolith breccias display systematically low porosity in comparison to otherwise analogous apollo regolith breccias among seven meteoritic regolith breccias porosity ranges from 1 to 11 and averages 7 5 1 3 2 whereas for 44 analogous apollo samples porosities mostly calculated from literature density data the average is 25 1 7 the origin of this disparity is enigmatic but the trend probably reflects mainly a bias in favor of strong compact breccias among fragments that manage to survive the violent process of launch to lunar escape velocity 2 38 km s in addition compaction during launch may play an important role the population of lunar meteorites is clearly not a random unmodified sample of lithic materials near the surface of the parent body