crystal morphologies in pillow basalts implications for mid ocean ridge processes abstract we present the results of a detailed petrological study of a sparsely phyric basalt mapco ch98 dr11 dredged along the mid atlantic ridge 30 41 n the sample contains microphenocrysts of olivine that display four different rapid growth morphologies comparison of these morphologies with those obtained in dynamic crystallization experiments allows us to constrain the thermal history of the sample the dendritic morphology swallowtail chain and lattice olivine is directly related to the final quenching during magma seawater interaction in contrast the three other morphologies namely the complex polyhedral crystal the closed hopper and the complex swallowtail morphology result from several cycles of cooling heating corresponding to a maximum degree of undercooling of 20 25 c during crystal growth these thermal variations occurred before eruption and are interpreted to be the result of turbulent convection in a small magmatic body beneath the ridge the results suggest that the mid atlantic ridge is underlain by a mush zone that releases batches of liquid during tectonic segregation aphyric basalts are emitted during eruptions controlled by the tectonic activity whereas phyric basalts correspond to small fractions of magma from the mush zone mobilized by reinjections of primitive magmas

dissolution kinetics of plagioclase in the melt of the system diopside albite anorthite and origin of dusty plagioclase in andesites the textures and kinetics of reaction between plagioclase and melts have been investigated experimentally and origin of dusty plagioclase in andesites has been discussed in the experiments plagioclase of different compositions an96 an61 an54 an23 and an22 surrounded by glasses of six different compositions in the system diopside albite anorthite was heated at temperatures ranging from 1 200 to 1 410 c for 30 min to 88 h textures were closely related to temperature and chemical compositions a crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt glass and remained its original euhedral shape below the liquidus whatever the temperature the crystal melt interface became rough and often more complicated sieve like texture composed of plagioclase melt mixture in the scale of a few m was developed from the surface of the crystal inward formation of mantled plagioclase if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase from these results the following two types of dissolution have been recognized 1 a crystal simply dissolves in the melt which is undersaturated with respect to the phase simple dissolution and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt partial dissolution the amount of a crystal dissolved and reacted increased proportional to the square root of time this suggests that these processes are controlled by diffusion probably in the crystal mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so called resorbed plagioclase in igneous rocks chemical compositions and textures of plagioclase phenocrysts in island arc andesites of magma mixing origin have been examined cores of clear and dusty plagioclase were clacic about an90 and sodic about an50 respectively this result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing

fine scale variability of isopycnal salinity in the california current system this paper examines the finescale structure and seasonal fluctuations of the isopycnal salinity of the california current system from 2007 to 2013 using temperature and salinity profiles obtained from a series of underwater glider surveys the seasonal mean distributions of the spectral power of the isopycnal salinity gradient averaged over submesoscale 12 30 km and mesoscale 30 60 km ranges along three survey lines off monterey bay point conception and dana point were obtained from 298 transects the mesoscale and submesoscale variance increased as coastal upwelling caused the isopycnal salinity gradient to steepen areas of elevated variance were clearly observed around the salinity front during the summer then spread offshore through the fall and winter the high finescale variances were observed typically above 25 8 kg m 3 and decreased with depth to a minimum at around 26 3 kg m 3 the mean spectral slope of the isopycnal salinity gradient with respect to wavenumber was 0 19 0 27 over the horizontal scale of 12 60 km and 31 to 35 of the spectra had significantly positive slopes in contrast the spectral slope over 12 30 km was mostly flat with mean values of 0 025 0 32 an increase in submesoscale variability accompanying the steepening of the spectral slope was often observed in inshore areas e g off monterey bay in winter where a sharp front developed between the california current and the california under current and the lower layers of the southern california bight where vigorous interaction between a synoptic current and bottom topography is to be expected

on sampling the ocean using underwater gliders surface to 1000 m and back took 5 6 h and covered 5 3 km resulting in a horizontal speed of 0 26 m s 1 seasoar undulated between the surface and 400 m completing a cycle in 11 min while covering 2 6 km for a speed of 3 9 m s 1 adjacent profiles of temperature and salinity are compared between the two platforms to prove that each is accurate spray and seasoar data are compared through sections isopycnal spatial series and wave number spectra the relative slowness of the glider results in the projection of high frequency oceanic variability such as internal waves onto spatial structure the projection is caused by doppler smearing because of finite speed and aliasing due to discrete sampling the projected variability is apparent in properties measured on depth surfaces or in isopycnal depth no projected variability is seen in observations of properties on constant density surfaces because internal waves are intrinsically filtered wave number spectra suggest that projected variability affects properties at constant depth at wavelengths shorter than 30 km these results imply that isobaric quantities like geostrophic shear are valid at wavelengths longer than 30 km while isopycnal quantities like spice may be analyzed for scales as small as a glider measures

variations in lava composition during the march 1981 eruption of mount etna and the implications of a compositional comparison with earlier historic eruptions the march 1981 eruption of mount etna occurred on the northern slopes of the volcano in an area previously inactive for more than 400 a a radial fissure system extending 7 5 km and producing 20 2 106 m3 of basic hawaiite lava opened at 2600 m a s l on 17th march and migrated downslope to 1125 m a s l where activity ceased on 23rd march gravity evidence points to the draining of a radial storage dyke located between sea level and 1 km a s l whole rock chemistry and petrography suggests that the magma in the dyke was a hybrid supporting crystal and compositional gradients developed as a result of progressively greater mixing of residual 1979 basic hawaiite magma with fresh basic hawaiite magma during permissive filling of the dyke around september 1980 these mixing gradients caused the eruption to display a significant contrast in the concentrations of al2o3 sr cao mgo and fe2o3 total between early high altitude and later low altitude products the fresh magma which entered the high level system around september 1980 ascended rapidly from a deep storage region at 16 24 km depth

the time scales of magma mixing and mingling involving primitive melts and melt mush interaction at mid ocean ridges we have studied the chemical zoning of plagioclase phenocrysts from the slow spreading mid atlantic ridge and the intermediate spreading rate costa rica rift to obtain the time scales of magmatic processes beneath these ridges the anorthite content mg and sr in plagioclase phenocrysts from the mid atlantic ridge can be interpreted as recording initial crystallisation from a primitive magma 11 wt mgo in an open system this was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma the initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt evidence that the crystals accumulated in a mush zone comes from both 1 plagioclase rims that were in equilibrium with a sr poor melt requiring extreme differentiation and 2 different crystals found in the same thin section having different histories diffusion modelling shows that crystal residence times in the mush were 11 wt partial equilibration in some crystals can be modelled as starting 1 year prior to eruption but for others longer times are required for complete equilibration this variety of times is most readily explained if the mixing occurred in a mush zone none of the plagioclase phenocrysts from the costa rica rift that we studied have mg contents in equilibrium with their host basalt even at their rims requiring mixing into a much more evolved magma within days of eruption in combination these observations suggest that at both intermediate and slow spreading ridges 1 the chemical environment to which crystals are exposed changes on annual to decadal time scales 2 plagioclase crystals record the existence of melts unlike those erupted and 3 disaggregation of crystal mush zones appears to precede eruption providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts

observations of plagioclase zoning using interference imaging abstract interference imaging techniques laser interference microscopy in transmitted light and nomarski differential interference contrast in reflected light have been used to study different types of zoning in magmatic plagioclase crystals samples studied come from a wide range of rock types and geological environments analysis of zoning profiles for individual unit zones shows that a saw tooth pattern in which an initially high an decreases as growth proceeds is the most common type of zoning configuration we recognize 2 major types of zoning type 1 small zones usually less than 10 microns thick with compositional amplitudes up to 10 an and type 2 zones up to50 microns thick with large compositional amplitudes of up to 35 an detailed analysis of zoning patterns shows that oscillatory zoning is not actually oscillatory in the sense of a sine wave it is discontinuous unit zones are characteristically bounded by discontinuities this common pattern is consistent with alternating periods of growth and periods of dissolution reaction and lack of growth or possibly very slow growth our observations firmly establish that dissolution and magmatic reaction involving both growth and dissolution are common during growth of the magmatic plagioclase which we have studied type 1 zonation appears consistent with perturbations of local conditions in a diffusion controlled boundary layer around the growing crystal type 2 zoning is probably due to large scale conditions faceted planar growth surfaces appear to be the rule in magmatic crystallization although in some instances rounded or irregular convolute zonation can occur the repetitive nature of many of the features we have observed is consistent with and suggestive of some form of self organization

improving the geoid combining altimetry and mean dynamic topography in the california coastal ocean satellite gravity mapping missions altimeters and other platforms have allowed the earth s geoid to be mapped over the ocean to a horizontal resolution of approximately 100 km with an uncertainty of less than 10 cm at finer resolution this uncertainty increases to greater than 10 cm achieving greater accuracy requires accurate estimates of the dynamic ocean topography dot in this study two dot estimates for the california current system with uncertainties less than 10 cm are used to solve for a geoid correction field the derived field increases the consistency between the dots and along track altimetric observations suggesting it is a useful correction to the gravitational field the correction is large compared to the dynamic ocean topography with a magnitude of 15 cm and significant structure especially near the coast the results are evidence that modern high resolution dynamic ocean topography products can be used to improve estimates of the geoid

pre eruption history of phyric basalts from dsdp legs 45 and 46 evidence from morphology and zoning patterns in plagioclase phyric basalts recovered from dsdp legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains megacrysts or aggregates glomerocrysts and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition almost all the plagioclase crystals are complexly zoned in most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts core mantle and rim the cores exhibit two major types of morphology tabular with a euhedral to slightly rounded outline or a skeletal inner core wrapped by a slightly rounded homogeneous outer core the mantle region is characterized by a zoning pattern composed of one to several spikes plateaus superimposed on a gently zoned base line with one large plateau always at the outside of the mantle and by in most cases a rounded internal morphology the inner rim is typically oscillatory zoned the width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow the presence of a skeletal inner core and the concentration of glass inclusions in low an zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount the resorption features at the outer margins of low an zones indicate superheating of the liquid with respect to the crystal

poleward flows in the southern california current system glider observations and numerical simulation 1 three years of continuous spray glider observations in the southern california current system ccs are combined with a numerical simulation to describe the mean and variability of poleward flows in the southern ccs gliders provide upper ocean observations with good across shore and temporal resolution along two across shore survey lines while the numerical simulation provides a dynamically consistent estimate of the ocean state persistent poleward flows are observed in three areas within 100 km of the coast at point conception within the southern california bight scb and offshore of the scb and the santa rosa ridge srr poleward transport by the flows within the scb and offshore of the srr exceeds the poleward transport off point conception suggesting that the poleward flows are not continuous over the 225 km between observation lines the numerical simulation shows offshore transport between the survey lines that is consistent with some of the poleward flow turning offshore before reaching point conception the poleward current offshore of the srr is unique in that it is strongest at depths greater than 350 m and it is observed to migrate westward away from the coast this westward propagation is tied to westward propagating density anomalies originating in the scb during the spring summer upwelling season when wind stress curl is most strongly positive the across shore wave number frequency and phase speed of the westward propagation and the lack of across shore transport of salinity along isopycnals are consistent with first mode baroclinic rossby dynamics

characterizing the semidiurnal internal tide off tasmania using glider data the spatial structure of the semidiurnal internal tide in the vicinity of tasmania is characterized using temperature and salinity data from seaglider and slocum glider deployments wavelet analysis of isopycnal displacements measured by the gliders was used to isolate the semidiurnal internal tide with a solid signal observed both to the east and to the south of tasmania the signal south of tasmania was attributed to local forcing while that to the east of tasmania was found to have propagated from the south east to the north west a result which supports previous studies indicating the presence of an internal tidal beam originating over the macquarie ridge south of new zealand displacement amplitudes were observed to be amplified in the vicinity of the continental slope with the incoming tidal beam shown to be both reflected and scattered on the continental slope and shelf and energy transferred to higher modes

multi stage modification of paleoarchean crust beneath the anabar tectonic province siberian craton abstract according to present views the crustal terranes of the anabar province of the siberian craton were initially independent blocks separated from the convecting mantle at 3 1 daldyn terrane 2 9 magan terrane and 2 5 ga markha terrane rosen 2003 2004 rosen et al 1994 2005 2009 previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the markha terrane concluded that the evolution of the crust of the markha terrane is very similar to that of the daldyn terrane to test this conclusion we present results of u pb and hf isotope studies on zircons in crustal xenoliths from the zapolyarnaya kimberlite pipe upper muna kimberlite field located within the daldyn terrane and the botuobinskaya pipe nakyn kimberlite field in the center of the markha terrane the data on xenoliths from the botuobinskaya kimberlite pipe record tectonothermal events at 2 94 2 8 2 7 and 2 ga the event at 2 ga caused pb loss in zircons from a mafic granulite u pb dating of zircons from the zapolyarnaya pipe gives an age of 2 7 ga all zircons from the studied crustal xenoliths have archean hf model ages ranging from 3 65 to 3 11 ga this relatively narrow range suggests that reworking of the ancient crust beneath the nakyn and upper muna kimberlite fields was minor compared with the daldyn and alakit markha fields shatsky et al 2016 this study when combined with dating of detrital zircons implies that tectonic thermal events at 2 9 2 85 2 75 2 7 and 2 0 1 95 ga occurred everywhere on the anabar tectonic province and could reflect the upwelling of superplumes at 2 9 2 7 and 2 ga the presence of the same tectonic thermal events in the daldyn and markha terranes rosen et al 2006a b supports the conclusion that the identification of the markha terrane as a separate unit is not valid

cannibalization of an amphibole rich andesitic progenitor induced by caldera collapse during the matahina eruption evidence from amphibole compositions the diverse range of calcic amphibole compositions in eruptive products from the ca 330 ka matahina eruption ca 160 km3 rhyolitic magma of the okataina volcanic complex taupo volcanic zone including crystal rich basalt dacite pumice from post collapse deposits reveals several pre and syn eruption magmatic processes 1 amphibole phenocrysts in the basaltic andesite and andesite crystallized at the highest pressures and temperatures p to 0 6 0 06 gpa and t to 950 c equivalent to mid crustal depths 13 22 km inter and intra crystalline compositions range from ti magnesiohastingsite ti tschermakite tschermakite magnesiohornblende and some display gradual decreases in t from core to rim both consistent with magma differentiation by cooling at depth 2 the largest amphibole crystals from the basaltic andesite to andesite display several core to rim increases in t to 70 c indicating that new hotter magma periodically fluxed the crystal mush 3 the dominant population of rhyolite amphibole is small and bladed magnesiohornblende and crystallized at low p t conditions p 0 3 gpa t 765 c equivalent to the eruptive p t conditions dacitic and low silica rhyolitic amphibole tschermakite magnesiohornblende form two distinct populations which nucleated at two different t high 820 c and low 750 c these compositional variations governed primarily by differences in t conditions during crystal growth record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive temperature therefore the diversity in amphibole compositions can be reconciled as an exchange of crystals liquid between the basaltic andesite to dacite from the mid crust and rhyolite from the upper crust which quenched against one another modifying the dacite to low silica rhyolite compositions as the eruption progressed

trapped diurnal internal tides propagating semidiurnal internal tides and mixing estimates in the california current system from sustained glider observations 2006 2012 abstract from 2006 2012 along 3 repeated cross shore transects california cooperative oceanic fisheries investigations lines 66 7 80 and 90 in the california current system 33 609 shear and 39 737 strain profiles from 66 glider missions are used to estimate mixing via finescale parameterizations from a dataset containing over 52 000 profiles elevated diffusivity estimates and energetic diurnal d1 and semidiurnal d2 internal tides are found a within 100 km of the coast on lines 66 7 and 80 and b over the santa rosa cortes ridge srcr in the southern california bight scb on line 90 while finding elevated mixing near topography and associated with internal tides is not novel the combination of resolution and extent in this ongoing data collection is unmatched in the coastal ocean to our knowledge both d1 and d2 internal tides are energy sources for mixing at these latitudes the d1 internal tide is subinertial on line 90 d1 and d2 tides are equally energetic over the srcr the main site of elevated mixing within the scb numerous sources of internal tides at the rough topography in the scb produce standing and or partially standing waves on lines 66 7 and 80 the dominant energy source below about 100 m for mixing is the d1 internal tide which has an energy density of the d2 internal tide on line 80 estimated diffusivity estimated dissipation and d1 energy density peak in summer the d1 energy density shows an increasing trend from 2006 to 2012 its amplitude and phase are mostly consistent with topographically trapped d1 internal tides traveling with the topography on their right the observed offshore decay of the diffusivity estimates is consistent with the exponential decay of a trapped wave with a mode 1 rossby radius of 20 30 km despite the variable mesoscale it is remarkable that coherent internal tidal phase is found

glider observations of the north equatorial current in the western tropical pacific the north equatorial current nec of the pacific ocean advects subtropical subpolar and tropical water masses repeat underwater glider observations of the nec from june 2009 to january 2014 along 134 3 e provide absolute zonal geostrophic velocity transport and water mass structure at length scales of 10 1000 km the nec is strongest near the surface and persistent eastward undercurrents are identified deeper than potential density surface 26 kg m 3 at 9 6 n and 13 1 n mean transport from the surface to 27 3 kg m 3 and 8 5 n 16 5 n is 37 6 sv 106 m3 s 1 with a standard deviation of 15 6 sv the transport variability is greatest deeper than 26 kg m 3 due to undercurrent variability wavelet analysis at scales of 10 80 km reveals extrema of fine scale salinity variance along isopycnals spice variance high spice variance is found in the north pacific tropical water nptw and the north pacific intermediate water npiw with a spice variance minimum between water masses at 25 5 kg m 3 a horizontal cox number ch relates salinity variance at fine scales 10 80 km to that at greater length scales 120 200 km as a function of density ch is nearly vertically uniform indicating that the stirring of mean salinity gradients enhances fine scale salinity variance nptw with an estimated horizontal eddy diffusivity of order 104 m s 2 is a useful tracer for the region and may be used to relate the fine scale salinity variance to an eddy diffusivity

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

seasonal evolution of upper ocean horizontal structure and the remnant mixed layer 1 we discuss the seasonal evolution of upper ocean thermohaline structure at small horizontal scales the upper 350 m of a 1000 km long section in the subtropical north pacific was observed in winter spring and summer with 3 14 km horizontal resolution four vertical regions had distinct density and salinity structure the mixed layer remnant mixed layer high stratification layer and permanent thermocline the remnant mixed layer consists of water from the winter mixed layer left over after restratification the remnant mixed layer was most similar to the mixed layer in winter and spring and most similar to the high stratification layer below in summer the high stratification layer had elevated stratification that varied seasonally the permanent thermocline varied little seasonally and was horizontally and vertically uniform in comparison in all seasons density ratios showed that mixed layer s differences tended to compensate in density with the strongest tendency toward compensation in winter density ratios were temperature dominated in the remnant mixed layer consistent with salt fingering salinity anomalies were largest at the surface and decayed with depth in all seasons spectra of isopycnal depth and s anomalies along isopycnals are compared between the three seasons and four vertical layers isopycnal depth variance at 30 46 km wavelengths decreased from winter to spring to summer by a factor of 2 10 in stratified regions by treating salinity anomalies as a tracer the effective isopycnal diffusivity in the remnant mixed layer was estimated to be 1 4 m2 s 1 over 30 46 km wavelengths

petrology and evolution of transitional alkaline sub alkaline lavas from patmos dodecanesos greece evidence for fractional crystallization magma mixing and assimilation petrographic mineral chemical and whole rock major oxide data are presented for the lavas of the main volcanic series of patmos dodecanesos greece these lavas were erupted about 7 m y ago and range in composition from ne trachybasalts through hy trachybasalts and trachyandesites to q trachytes to some extent the ne trachybasalts are intermediate in composition to the alkaline lavas found on oceanic islands and the calc alkaline lavas of destructive plate margins major oxide variation is largely explicable in terms of fractional crystallization involving removal of the observed phenocryst and microphenocryst phases viz olivine plagioclase clinopyroxene and ti magnetite in the mafic lavas plagioclase clinopyroxene mica and ti magnetite in the evolved lavas apatite which occurs as an inclusion in other phenocrysts or as microphenocrysts must also have been removed however mass balance calculations indicate that the chemistry of the hy trachybasalts is inconsistent with an origin via fractional crystallization alone and the complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas show that they are hybrids formed by the mixing of 80 77 ne trachybasalt with 20 23 trachyandesite it is estimated that the mixing event preceded eruption by a period of 12 h 2 weeks suggesting that mixing triggered eruption combined fractionation and mixing cannot explain the relatively low mgo contents of the hy trachybasalts and it is concluded that assimilation also occurred assimilation and especially addition of volatiles to the magmas may be responsible for the evolutionary trend from ne normative to hy normative magmas and was probably facilitated by intensified convection resulting from mixing a model is presented whereby primitive magma undergoes fractionation in an intracrustal magma chamber to yield more evolved liquids influx of hot primitive magma into the base of the chamber facilitates assimilation but eventually mixing yields the hy trachybasalts and finally the ne trachybasalts are erupted

plagioclase xenocrysts and mafic magmatic enclaves in some granitoids of the sierra nevada batholith california plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the sierra nevada batholith are complexly zoned and commonly divided into three neatly distinct parts an oscillatory locally patchy zoned andesine or more calcic core a ring with dusty calcic plagioclase and a normally zoned rim of sodic plagioclase although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts cores of both phenocrysts and large plagioclase crystals show similar zoning and composition the andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma respectively and been incorporated into the coeval magma when the two magmas mixed introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution development of abundant patchy zoning and coating with dusty calcic plagioclase in both granitoids and mafic magmatic enclaves composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves this hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc alkaline granitoids and mafic magmatic enclaves of the sierra nevada batholith plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons

tectonothermal evolution of the continental crust beneath the yakutian diamondiferous province siberian craton u pb and hf isotopic evidence on zircons from crustal xenoliths of kimberlite pipes abstract u pb age and hf isotope data were collected on zircons from crustal xenoliths from the late devonian kimberlite pipes of markha terrane the xenoliths include mafic garnet granulites gt pl cpx opx amp bt scp and garnet biotite gneisses grt bt pl kfs qtz scp the data from this study demonstrate that the crust of the markha terrane experienced several tectonothermal events zircons with archean hf model ages tdm 3 13 2 5 ga are predominant in all xenoliths with one exception these data imply that most of the lower and middle crust beneath the markha terrane was produced in the archean time later this paleoarchean crust was significantly reworked in several tectonothermal events including a neoarchean stage 2 9 2 5 ga and several paleoproterozoic metamorphic stages 1 98 1 9 and 1 8 ga there is very little evidence for generation of juvenile crust associated with these metamorphic events importantly crustal xenoliths from kimberlite pipes of the markha terrane record all the same events that occurred in the adjacent daldyn and magan terranes

thermohaline variability in the upper ocean the main goal of this exploratory study is to determine how the temperature salinity relationship changes with horizontal length scale and depth in the ocean temperature and salinity were measured on a range of scales from 4 m to 1000 km towing a seasoar along isobars and isopycnals in the subtropical gyre of the north pacific during the winter of 1997 the wavelet transform technique is used to compute the horizontal density ratio and thermohaline variability as a function of scale and location measurements along an isobar in the mixed layer show that the horizontal density ratio is 1 at all scales observed that is horizontal temperature and salinity gradients tend to cancel each other in their effect on density thermohaline variability at small scales is intermittent and clusters around large scale thermohaline anomalies below the base of the mixed layer horizontal gradients of temperature are only partially opposed by salinity and the density ratio is close to 2 in the thermocline the distribution of thermohaline variability is uniform along isobars but intermittent and colocated at different scales along isopycnals density compensated variability ubiquitous in the mixed layer is reduced along deeper isopycnals compensation of horizontal temperature and salinity gradients supports recent theoretical ideas that mixing in the winter mixed layer depends on horizontal density gradients

thermohaline structure in the california current system observations and modeling of spice variance 1 upper ocean thermohaline structure in the california current system is investigated using sustained observations from autonomous underwater gliders and a numerical state estimate both observations and the state estimate show layers distinguished by the temperature and salinity variability along isopycnals i e spice variance mesoscale and submesoscale spice variance is largest in the remnant mixed layer decreases to a minimum below the pycnocline near 26 3 kg m 3 and then increases again near 26 6 kg m 3 layers of high low meso and submesoscale spice variance are found on isopycnals where large scale spice gradients are large small consistent with stirring of large scale gradients to produce smaller scale thermohaline structure passive tracer adjoint calculations in the state estimate are used to investigate possible mechanisms for the formation of the layers of spice variance layers of high spice variance are found to have distinct origins and to be associated with named water masses high spice variance water in the remnant mixed layer has northerly origin and is identified as pacific subarctic water while the water in the deeper high spice variance layer has southerly origin and is identified as equatorial pacific water the layer of low spice variance near 26 3 kg m 3 lies between the named water masses and does not have a clear origin both effective horizontal diffusivity h and effective diapycnal diffusivity v are elevated relative to the diffusion coefficients set in the numerical simulation but changes in h and v with depth are not sufficient to explain the observed layering of thermohaline structure

a flower like glomerophyric diorite porphyry from central china constraints on the unusual texture abstract an unusual texture with plagioclase phenocrysts clustered as flower like glomerocrysts has been discovered in the wulong diorite porphyry at the southern margin of north china craton the rock consists mainly of plagioclase and amphibole with the former as dominant phenocryst two groups of the plagioclase phenocrysts have been distinguished flower like glomerocrysts fg type and single isolated phenocrysts sp type chemical compositions of both fg and sp type plagioclases are similar in anorthite contents an35 45 which are slightly higher than the matrix an25 35 calculation of the plagioclase crystallization time based on crystal size distribution csd program shows that the fg type crystals have been formed within a longer timescale 226 2782 years than the sp type crystals 98 1910 years the concave down csds and lack of small crystals for fg type plagioclases suggest that coarsening may have been involved in contrast a concave up csd within the size of 3 10 mm plagioclase indicating that a rejuvenation kinetic process after coarsening a possible formation history for the unusual flower like plagioclase glomerocrysts was proposed as follows 1 initial nucleation and growth of single crystals in a confined environment 2 ascent of early crystals with magma upwelling and radial coarsening at the roof layer of magma chamber 3 remobilization of dense crystal mush and final emplacement at shallow level of crust 4 rapid cooling and extensive nucleation and growth of matrix 5 post magmatic alteration and metamorphism it is likely that the morphologic diversity of plagioclase glomerocrysts are largely influenced by variation of undercooling degrees the second step is the most important for the formation of the flower like glomerophyric texture which requires an environments of relatively low undercooling degree

a mafic intrusion of arc affinity in a post orogenic extensional setting a case study from ganluogou gabbro in the northern yidun arc belt eastern tibetan plateau abstract generally arc related or subduction related mafic magmas are formed during or slightly postdate subduction and characterized by depletion in high field strength elements hfses relative to the large ion lithophile elements liles and light rare earth elements lrees combining with mineral chemistry and sr nd isotopes these geochemical characteristics were usually used to define an arc setting especially for some ancient arcs that had been strongly modified by later tectonic activities however we report an exceptional case from the northern part of the triassic yidun arc belt eastern tibetan plateau the ganluogou gabbro 152 ma occurs as several intrusive bodies its mineral assemblage is olivine chrysolite plagioclase anorthite clinopyroxene diopside amphibole edenite and pargasite and phlogopite whole rock geochemistry shows low sio 2 42 87 46 99 wt total rear earth elements ree 22 8 28 4 ppm na 2 o k 2 o 0 92 1 34 wt and high al 2 o 3 mgo and feo contents it has small variations of initial 87 sr 86 sr 87 sr 86 sr i 0 7053 0 7055 and e nd t values 4 8 to 1 8 all the samples exhibit enrichment in liles including th and u but strongly depleted in hfses including nb ta zr and hf for the mineral chemistry there are two type amphiboles amp i show higher v sc cr sr nb and zr contents but lower th and u contents than those of amp ii their ree patterns range from convex shape without eu anomaly to lree enriched pattern with weak positive eu anomaly we suggest that amp i was crystallized from a liquid that was mainly buffered by olivine clinopyroxene and plagioclase while amp ii crystallized from later melt that was mainly buffered by olivine based on clinopyroxene chemistry compositions of coexisting olivine and plagioclase and whole rock sr nd isotopes the parental magma of the ganluogou gabbro is interpreted as a tholeiitic arc affinity magma which might be derived from an n morb mantle that had been metasomatised by slab derived melts in the late triassic 237 206 ma thus the ganluogou gabbro provides an example that magmas exhibiting arc affinity could in fact be formed in a post orogenic extensional setting

cumulate gabbros from the southwest indian ridge 54 s 7 16 e implications for magmatic processes at a slow spreading ridge a diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the southwest indian ridge 54 s 7 16 e between the islas orcadas and shaka fracture zones the cumulus nature of the gabbroic rocks in the suite is indicated by phase modal and cryptic layering igneous lamination and low incompatible element abundances we present a mass balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk rock major element compositions the model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid calculated proportions of trapped liquid range from 3 15 values that are characteristic of adcumulates to mesocumulates models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids thus explaining the high tio2 contents observed in magnesian clinopyroxenes cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers but disequilibrium plagioclase compositions suggest some crystallization at greater depth furthermore basalt compositions projected onto the olivine clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures from less than 10 to greater than 20 kb as well as polybaric fractional crystallization we suggest that the southwest indian ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros the adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large but ephemeral magma chambers below segments of the southwest indian ridge

palaeomagnetic stratigraphy of pliocene continental deposits of the bolivian altiplano abstract fluvio lacustrine post miocene deposits of the bolivian altiplano have been sampled in 400 m of section in the la paz and avo avo basins in order to provide a magnetostratigraphic scheme for chronostratigraphic interpretations stable characteristics components were isolated after stepwise af or thermal demagnetizations the directions of cleaned magnetization were compared to the normal and reverse dipole field directions at the site the plot of the mean directions calculated for each stratigraphic level define successive magnetozones which can be identified on the standard geomagnetic polarity scale with the help of the k ar ages obtained on tuff layers the deposition of the middle and upper part of the la paz formation covers the gauss epoch the gauss matuyama 2 48 ma and the gilbert gauss 3 4 ma limits are located in the top and the middle of the formation the occurrence of the first glacial interglacial oscillation can be placed in the early matuyama ca 2 2 ma in agreement with the oxygen isotopic record from the equatorial ocean

along isopycnal variability of spice in the north pacific two hydrographic surveys in the gulf of alaska and the north pacific subtropical gyre are presented both surveys are roughly perpendicular to lateral temperature gradients and were collected in the summer when there was a shallow mixed layer and a seasonal thermocline isopycnal displacements and horizontal velocities are dominated by internal waves spice anomalies along isopycnals are examined to diagnose lateral stirring mechanisms the spectra of spice anomaly gradients along near surface isopycnals roughly follow power laws of kx0 6 0 2 variance spectra power laws of kx 1 4 0 2 and in most cases the spectra become redder at depth the near surface spectra are possibly consistent with the predictions of quasi geostrophic turbulence theory when surface buoyancy effects are accounted for but the spectra at depth are inconsistent with any quasi geostrophic theory probability distributions of spice gradients exhibit a large peak at low gradients and long tails for large gradients symptomatic of fronts vertical coherence of the spice signal falls off with a decorrelation depth scale that has a maximum of about 80 m at 100 km wavelengths and depends on horizontal wavelength with a power law of approximately kx 1 2 lateral decorrelation length scales are 20 40 km close to the baroclinic rossby radius lateral stirring occurs over large scales with average lateral displacements of about 200 km in the upper 75 m decreasing to 100 km at greater depths the depth variation of the statistics indicates that time history of tracer stirring on each isopycnal is important or that there are unconsidered depth dependent stirring mechanisms

submesoscale water mass spectra in the sargasso sea submesoscale stirring contributes to the cascade of tracer variance from large to small scales multiple nested surveys in the summer sargasso sea with tow yo and autonomous platforms captured submesoscale water mass variability in the seasonal pycnocline at 20 60 m depths to filter out internal waves that dominate dynamic signals on these scales spectra for salinity anomalies on isopycnals were formed salinitygradient spectra are approximately flat with slopes of 20 2 6 0 2 over horizontal wavelengths of 0 03 10km while the two to three realizations presented here might be biased more representative measurements in the literature are consistent with a nearly flat submesoscale passive tracer gradient spectrum for horizontal wavelengths in excess of 1km a review of mechanisms that could be responsible for a flat passive tracer gradient spectrum rules out i quasigeostrophic eddy stirring ii atmospheric forcing through a relict submesoscale winter mixed layer structure or nocturnal mixed layer deepening iii a downscale vortical mode cascade and iv horizontal diffusion because of shear dispersion of diapycnal mixing internal wave horizontal strain appears to be able to explain horizontal wavenumbers of 0 1 7 cycles per kilometer cpkm but not the highest resolved wavenumbers 7 30cpkm submesoscale subduction cannot be ruled out at these depths though previous observations observe a flat spectrum well below subduction depths so this seems unlikely primitive equation numerical modeling suggests that nonquasigeostrophic subinertial horizontal stirring can produce a flat spectrum the last need not be limited to mode one interior or surface rossby wavenumbers of quasigeostrophic theory but may have a broaderband spectrum extending to smaller horizontal scales associated with frontogenesis and frontal instabilities as well as internal waves

subthermocline eddies over the washington continental slope as observed by seagliders 2003 09 abstractin the california current system subthermocline lenslike anticyclonic eddies generated within the california undercurrent cu are one mechanism for lateral transport of the warm saline waters of the cu garfield et al established the name cuddies for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of cu water this study presents observations of subthermocline eddies collected from a time series of seaglider surveys in the northern california current system gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5 5 yr close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones water properties at the core of anticyclonic eddies were similar to those in the core of the cu over the continental slope these anticyclones are examples of cuddies anticyclonic cyclonic eddies had average radii of 20 4 20 6 km peak azimuthal current speeds of 0 25 0 23

the spatial distribution and annual cycle of upper ocean thermohaline structure 1 observations of the spatial distribution and persistence of thermohaline structure are presented and show how advection and diffusion affect a passive tracer more than two years of underwater glider observations in the central subtropical north pacific showed thermohaline variability over horizontal scales from 5 to 1300 km thermohaline fluctuations along isopycnals spice fluctuations were elevated in layers throughout the water column with the largest fluctuations near the surface and subtropical frontal regions fluctuations were uncorrelated between the layers but stirred by the same velocity field spice variance had local extrema in the vertical because of differences in source water properties and the influence of neighboring water masses spice variance spanned about three orders of magnitude along deeper isopycnals with larger variance where different water masses met and where velocity and vorticity variance were elevated horizontal wave number spectra of spice had slopes of 2 everywhere in the upper 1000 m submesoscale spice fluctuations had slopes in physical space near the ratio of the coriolis parameter to the buoyancy frequency f n consistent with predictions of quasi geostrophic theory in the mixed layer thermohaline structure had a significant annual cycle with smaller interannual differences thermohaline fluctuations left behind during restratification and isolated from the mixed layer decayed with time because of diffusion along isopycnals horizontal diffusivity estimates in the remnant mixed layer were 0 4 m2 s 1 at 15 28 km wavelengths and 0 9 m2 s 1 at 35 45 km wavelengths

the flow of the undercurrent over the continental borderland off southern california 1 iniroduction 2 background the bottom topography off southern california has been the cc a slow had equatornard flow is the eastern limb of termed a continental borderland to distinguish it from the ordinary the large scale anticyclonic gyre of the temperate north pacific shelf slope morphology found along most continental margins ocean the cc carries relatively cool low salinity waters of shepard and emuy 19411 islands shallow banks basins and subarctic origin to lower latitudes on average the core of the cc troughs form a complex bathymetric region extending from the has velocities of 0 10 to 0 15 m s and traces a path roughly coast to 200 lon offshore the southern california coastline has a parallel to the california baja california coastline localized concave curvature between the headland at point conception to measurements of flow can exceed 0 5 m s davis 19851 an the u s mexican border as a whole this region is referred to as instantaneous realization of the cc includes a tangle of meanders the california bight the geographic bathymetric dguration eddies and narrow jetlike flows and counterllows the cc lies at of the bight exm a pronounced influence upon the local circula varying distances off the coast and at the latitude of the tion patterns and heme upon the distributions of physical e u s mexican border there is a strong shoreward component to sverdrup and fleming 19411 and biological quantities e g the flow in most seasons especially summa the flow splits one allen 1945 owen 19801 during 1985 we conducted an oceano branch turns and flows northward into the california bight graphic survey off southern california the sampling grid was becoming the eastern limb of a cyclonic eddy centered about the designed to reveal the mumce of the geographic bathymetric offshore banks seasonally fall and winter a weak inshore features upon the circulation of the california current system countercurrent ic develops at the surface along various portion6 ccs especially the flow of the california undercurrent cu of the coastline reid et d 19581 at depth the cu cdes the survey grid also includes station lines mss the core of the waters of subtropical origin poleward along the continental slope california current cc and transects an offshore dipole eddy these waters are relatively warm and saline and have low mm simpson and lym this issue the station grid of this study trations of dissolved oxygen the cu also demonstrates a welldiffers considerably from that used by the california cooperative defined seasonality and at times opposes the overlying dace oceanic fisheries investigations calcofi surveys in both its flow maps and sections of the long term seasonal means of relaspatial resolution and depth of sampling the objectives of thic tive geostrophic velocity and distributions of ocean characteristics report are 1 to examine the spatial continuity of the cu through for the ccs are available in calcofi atlas 30 lym et d 19821 the complex bathymeny of the bight 2 to trace the poleward for a comprehensive review of the long term seasonal large scale paths and offshore spread of waters of southern origin transported description of the ccs the readex is referred to recent papas by by the cu 3 to contrast this subsurface flow with the surface hickey 1979 chelton 19841 and lynn andslnpsm 19871 and flow and 4 to compare these results with the long term calcofi to important references of pioneering work given therein mean conditions

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

the petrogenesis of plagioclase phyric basalts at mid ocean ridges 1 plagioclase ultraphyric basalts pubs have been sampled along most mid ocean ridges with ultraslow to intermediate spreading rates over the past 40 years the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation however when the global population of pub lavas is examined this hypothesis becomes less compelling pub host lavas demonstrate a large range of compositions and densities similar to aphyric glasses from the same ridge segments most importantly the majority of pub host liquids are less dense than their phenocryst cargo meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment furthermore pub lavas have never been sampled on axis at fast spreading centers or from locations with noted contemporaneous axial magma chambers where pubs should be abundant if plagioclase is buoyant in mid ocean ridge basalt morb instead we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust possibly followed by loss of olivine during magma ascent pubs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts which precludes long crustal residence times for these magmas in addition to being a proxy for lower spreading rates our findings also suggest that pub eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber

underwater gliders reveal rapid arrival of el nino effects off california s coast 1 the 2009 2010 el nino marked the first occurrence of this climate phenomenon since the initiation of sustained autonomous glider surveillance in the california current system ccs spray glider observations reveal the subsurface effects of el nino in the ccs with spatial and temporal resolutions that could not have been obtained practically with any other observational method glider observations show that upper ocean waters in the ccs were unusually warm and isopycnals were abnormally deep during the el nino event but indicate no anomalous water masses in the region observed oceanic anomalies in the ccs are nearly in phase with an equatorial el nino index and local anomalies of atmospheric forcing these observations point toward an atmospheric teleconnection as an important mechanism for the 2009 2010 el nino s remote effect on the midlatitude ccs

observations of sound speed fluctuations in the western philippine sea in the spring of 2009 as an aid to understanding long range acoustic propagation in the philippine sea statistical and phenomenological descriptions of sound speed variations were developed two moorings of oceanographic sensors located in the western philippine sea in the spring of 2009 were used to track constant potential density surfaces isopycnals and constant potential temperature surfaces isotherms in the depth range 120 2000 m the vertical displacements of these surfaces are used to estimate sound speed fluctuations from internal waves while temperature salinity variability along isopycnals are used to estimate sound speed fluctuations from intrusive structure often termed spice frequency spectra and vertical covariance functions are used to describe the space time scales of the displacements and spiciness internal wave contributions from diurnal and semi diurnal internal tides and the diffuse internal wave field related to the garrett munk gm spectrum are found to dominate the sound speed variability spi

zonal evolution of alaskan stream structure and transport quantified with argo data the alaskan stream as flows west southwestward along the south side of alaska and the aleutian island arc a western boundary current at the northern edge of the north pacific subpolar gyre the argo float array has improved sampling of the gulf of alaska allowing quantification of the as s zonal evolution from 140 w to 175 w geostrophic alongshore transport of the as in the upper 1000 dbar referenced to an assumed level of no motion at 1000 dbar shows little change from east to west however alongshore absolute geostrophic transports in the top 2000 dbar obtained by combining mean absolute 1000 dbar velocities from float displacements with the geostrophic velocity fields generally increase to the west we estimate full depth transports by fitting a barotropic and the first two baroclinic modes calculated from a climatology to the absolute geostrophic velocities in the upper 2000 dbar and applying the velocities from these fits from 2000 dbar to the seafloor flowing west from its formation region at 140 w 145 w the full depth as becomes stronger more barotropic and also narrower once it reaches 160 w with along shore transports increasing from 16 4 4 9 sv 1 sv 106 m3 s 1 at 140 w to 32 6 5 2 sv at 175 w mean concentrations of relatively warm salty oxygen poor and nutrient rich pacific equatorial water pew in the as decrease from 17 8 0 3 to 8 5 0 5 between 140 w and 175 w however the volume transport of pew by the as exhibits little change over the pew density range between these longitudes

experimental study of plagioclase rim growth around anorthite seed crystals in rhyodacitic melt the purpose of this study was to replicate experimentally the growth of new rims around highly anorthitic plagioclase core phenocrysts analogous to the incorporation of xenocrysts into a silicic magma body through magma mixing aniakchak rhyodacite forms the bulk starting composition and phase equilibria experiments constrained the pre eruption magma conditions to be 110 mpa and 870 880 c the experimental runs were seeded with great sitkin anorthite an 9 1 9 5 crystals new rim growth of an 2 8 3 8 plagioclase occurred at rates between 3 5 0 3 x 10 1 0 to 60 6 20 0 x 10 1 0 cm s at pressures and temperatures from 50 to 150 mpa and 825 to 880 c the values in parentheses are 1 standard deviation microlite crystallization an 2 7 4 1 occurred in all experiments within the plagioclase stability field and their growth rates varied from 4 4 1 3 x 10 1 0 to 65 7 10 1 x 10 1 0 cm s the rim and microlite growth rates are similar to one another within each experiment and microlite number density n v is correlated approximately inversely with rim growth rates microlite crystallinities increased from 4 2 to 49 7 vol as a function of increasing t e f f up to 95 c the results indicate growth dominated crystallization at low t e f f and nucleation dominated crystallization at high t e f f in agreement with previous studies assuming the experiments apply to nature the rim growth rates can provide a minimum estimate on how fast magma mixing can occur rims that are 10 to 100 m wide can grow in 10 days to 4 months recording fast mixing timescales as long as eruption occurs shortly after mixing the growth rate estimates presented here generally agree with those derived from sodic rims growing around anorthite cores after mixing between basalt and andesite prior to the 1996 eruption of karymsky volcano kamchatka

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

bimodal tholeiitic and mildly alkalic basalts from bhir area central deccan volcanic province india geochemistry and petrogenesis abstract bimodal tholeiitic and mildly alkalic basalts occur near bhir in the central part of deccan volcanic province dvp major and trace element concentrations show that of the ten flows nine are tholeiitic and one is an alkalic basalt the bhir basalts have a wide range of chemical composition geochemical variations in the stratigraphic section define three distinct phases of evolution zones 1 to 3 crystal fractionation of plagioclase clinopyroxene olivine and fe ti oxide expanded the compositional range low mg s 39 55 low concentrations of ni and cr and high zr suggest the evolved nature of the bhir basalts fractionation modeling suggests about 42 fractional crystallization in spite of the dominant role of fractional crystallization in the evolution of bhir basalts some other processes must be sought to explain the chemical variations crustal contamination magma mixing and degree of partial melting are suggested to explain the observed chemical variations resorption reverse zoning and compositional bimodality in plagioclase phenocrysts indicate magma mixing samples of flows one and four suspected of being contaminated all have enriched sio2 and lile k rb and ba contents and depletion in ti and p believed to be due to granitic crustal contamination as compared to tholeiitic basalts the alkalic basalts are characterized by low sio2 and high tio2 na2o k2o and p2o5 alkalic basalts are richer in lile rb and ba hfse nb zr and y and ree than the tholeiitic basalts the alkalic basalt occurrence is important from a petrogenetic point of view and also suggests that the sources of alkalic basalt magmas may be of variable ages under different parts of the dvp based on major trace and rare earth element distributions it is suggested that asthenospheric mantle having affinities with the source of oib was the source material of the magmas and the range in the composition of tholeiitic and alkalic basalts was probably controlled by different degrees of melting and or inhomogeneities in the mantle source