petrogenesis of the elephant moraine a79001 meteorite multiple magma pulses on the shergottite parent body abstract the eeta 79001 achondrite consists of two distinct igneous lithologies joined along a planar non brecciated contact both are basaltic rocks composed primarily of pigeonite augite and maskelynite but one contains zoned megacrysts of olivine orthopyroxene and chromite that represent disaggregated xenoliths of harzburzite both lithologies probably formed from successive volcanic flows or multiple injections of magma into a small shallow chamber many similarities between the two virtually synchronous magmas suggest that they are related possible mechanisms to explain their differences involve varying degrees of assimilation fractionation from similar parental magmas or partial melting of a similar source peridotite of these assimilation of the observed megacryst assemblage seems most plausible however some isotopic contamination may be required in any of these petrogenetic models the meteorite has suffered extensive shock metamorphism and localized melting during a large impact event that probably excavated and liberated it from its parent body both basaltic lithologies and the inferred ultramafic protolith of the megacryst assemblage are petrologically similar to other members of the shergottite group and all may have been derived from a volcanic plutonic complex on a planetary body

petrogenesis of the zagami meteorite inferences from synchrotron x ray sxrf microprobe and electron microprobe analyses of pyroxenes abstract the zagami meteorite is a shergottite a basalt rich in pigeonite augite and maskelynite glass shocked plagioclase it is one of the snc meteorites which are inferred to have formed on mars major and minor element abundances in pyroxene principally pigeonite from newly available unm samples were determined by electron microprobe abundances of trace elements ni cu zn and ga were obtained by synchrotron x ray sxrf microprobe many pigeonite grains have normally zoned rims and reversely zoned cores so that the most magnesian pigeonite forms rings or annuli in thin section view in three dimensions the most magnesian pigeonite may form tubes or hourglass shapes the cores commonly contain magmatic inclusions other pigeonites have irregular or multiple magnesian regions with magmatic inclusions among and between them abundances of ni and cr in the magnesian zones and cores are partially decoupled from abundances of other compatible mg and incompatible fe ti elements the textures chemical zoning and element decouplings in the pigeonites are consistent with igneous crystal growth at moderate cooling rates not isothermally or near equilibrium from element partitioning and mass balance the unm zagami is estimated to contain 20 5 or less cumulus pyroxene half augite and half pigeonite half the proportion suggested by equilibrium distribution of fe mg between pigeonite and melt this inconsistency may be resolved if all pigeonite melt element partitioning was affected by the rapid growth of the pigeonites the composition of the parental magma for the unm zagami calculated by mass balance on bulk and pyroxene compositions is similar to the 1170 c melt of the bmnh zagami not the 1140 c melt inferred as parental by other workers the inference that the pyroxene zoning patterns reflect only magmatic events is consistent with an igneous age of 180 m y for zagami

39ar 40ar dating of the zagami martian shergottite and implications for magma origin of excess 40ar the zagami shergottite experienced a complex petrogenetic formation history mccoy et al 1992 1999 like several shergottites zagami contains excess 40ar relative to its formation age to understand the origin of this excess 40ar we made 39ar 40ar analyses on plagioclase and pyroxene minerals from two phases representing different stages in the magma evolution surprisingly all these separates show similar concentrations of excess 40ar 1 x 10 6 cm 3 g we present arguments against this excess 40ar having been introduced from the martian atmosphere as impact glass we also present evidence against excess 40ar being a partially degassed residue from a basalt that actually formed 4 gyr ago we utilize our experimental data on ar diffusion in zagami and evidence that it was shock heated to only 70 c and we assume this heating occurred during an ejection from mars 3 myr ago with these constraints thermal considerations necessitates either that its ejected mass was impossibly large or that its shock heating temperature was an order of magnitude higher than that measured we suggest that this excess 40ar was inherited from the zagami magma and that it was introduced into the magma either by degassing of a larger volume of material or by early assimilation of old k rich crustal material similar concentrations of excess 40ar in the analyzed separates imply that this magma maintained a relatively constant 40ar concentration throughout its crystallization this likely occurred through volatile degassing as the magma rose toward the surface and lithostatic pressure was released these concepts have implications for excess 40ar in other shergottites

the rocks of mars from far and near award of the meteoritical society 2001 h y mcsween jr the leonard medal address the rocks of mars from far and near

vesiculation of basaltic magma during eruption vesicle size distributions in vent lavas from the pu9u9o9o kupaianaha eruption of kilauea volcano are used to estimate nucleation and growth rates of h 2 o rich gas bubbles in basaltic magma nearing the earth9s surface 120 m depth by using well constrained estimates for the depth of volatile exsolution and magma ascent rate nucleation rates of 35 9 events cm 3 s 1 and growth rates of 3 2 x 10 4 cm s are determined directly from size distribution data the results are consistent with diffusion controlled growth as predicted by a parabolic growth law this empirical approach is not subject to the limitations inherent in classical nucleation and growth theory and provides the first direct measurement of vesiculation kinetics in natural settings in addition perturbations in the measured size distributions are used to examine bubble escape accumulation and coalescence prior to the eruption of magma

shock implantation of martian atmospheric argon in four basaltic shergottites a laser probe 40ar 39ar investigation spatially resolved argon isotope measurements have been performed on neutron irradiated samples of two martian basalts los angeles and zagami and two martian olivine phyric basalts dar al gani dag 476 and north west africa nwa 1068 with a similar to 50 mu m diameter focused infrared laser beam it has been possible to distinguish between argon isotopic signatures from host rock matrix minerals and localized shock melt products pockets and veins the concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured j values ar 40 ar 39 ratios and k2o wt in each phase melt pockets contain on average 10 times more gas 7 24 ppb ar 40 than shock veins and matrix minerals 0 3 3 ppb ar 40 the ar 40 ar 36 ratio of the martian atmosphere estimated from melt pocket argon extractions corrected for cosmogenie ar 36 is los angeles similar to 1852 zagami similar to 1744 and nwa 1068 similar to 1403 in addition los angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs 1 martian atmosphere in melt pockets and 2 a trapped component possibly martian interior ar 40 ar 16 480 490 in matrix minerals average apparent ar 40 ar 39 ages determined for matrix minerals in the four analyzed meteorites are 1290 ma los angeles 692 ma zagami 515 ma nwa 1068 and 1427 ma dag 476 these ar 40 ar 39 apparent ages are substantially older than the similar to 170 0474 ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40ar cosmic ray exposure cre ages were measured using spallogenic ar 16 and ar 38 production los angeles 3 1 0 2 ma zagami 2 9 0 4 ma and nwa 1068 2 0 0 5 ma yielded ages within the range of previous determinations dag 476 however yielded a young cre age 0 7 0 25 ma attributed to terrestrial alteration the high spatial variation of argon indicates that the incorporation of martian atmospheric argon into near surface rocks is controlled by localized glass bearing melts produced by shock processes in particular the larger mm size melt pockets contain near end member martian atmospheric argon based on petrography composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection three processes may have led to atmosphere incorporation 1 argon implantation due to atmospheric shock front collision with the martian surface 2 transformation of an atmosphere filled cavity into a localized melt zone and 3 shock implantation of atmosphere trapped in cracks pores and fissures

petrogenesis of shergottite meteorites inferred from minor and trace element microdistributions abstract as an extension of our previous work on the lherzolitic shergottites alha77005 and lew88516 ion microprobe measurements of rees and other selected trace and minor elements were made in individual minerals of the basaltic shergottites shergotty zagami and eeta79001 whole rock ree abundances in these achondrites are dominated by whitlockite which is usually present only in trace amounts one of the most significant features is the extensive zoning of trace and minor elements in pyroxenes this zonation which provides information on the crystallization histories of these meteorites is of primary magmatic origin and not the result of secondary processes such as diffusive reequilibration and or metasomatic infiltration coherent trends in element element plots for low ca pyroxenes indicate that each shergottite formed essentially by progressive closed system fractional crystallization the calculated ree patterns of the parent melts of all shergottites are lree depleted and parallel to their whole rock ree patterns also indicative of closed system fractional crystallization on the basis of observations reported in this study as well as previously determined petrologic and geochemical including isotopic characteristics we present a model that suggests possible relationships among all five known shergottites

trampled under foot a quartz and chert human trampling experiment at the cova del parco rock shelter spain abstract an assemblage of 500 385 quartz and 115 chert experimentally knapped artefacts underwent human trampling over the course of two weeks by a team of 10 12 excavators wearing soft soled shoes used while excavating in the rock shelter two zones of trampling were used zone 1 a high artefact density low trampling intensity and lower soil compaction and rockiness and zone 2 a low artefact density high trampling intensity higher soil compaction and rockiness the primary questions were to understand the difference in fracture rates and types between the chert and quartz and the difference between the zones of artefact density trampling intensity and soil density the results have shown that significantly more quartz fractured and were damaged compared to chert and for both materials there was significantly more breakage in the zone with the higher trampling intensity and higher soil compaction and rockiness there was no discerned association between the original artefacts size or weight and the occurrence of breakage of the quartz and chert artefacts except for a very weak association for narrower and thinner quartz artefacts being more likely to break overall there was relatively little horizontal or vertical movement of the artefacts over the two weeks with the largest size ranges moving the most but with no significant difference in the movement of artefacts amongst the smaller size ranges

que94201 shergottite crystallization of a martian basaltic magma abstract basaltic shergottites are commonly described as having crystallized from magmas that contained entrained or cumulus pyroxenes complexly zoned clinopyroxenes in que94201 a newly recovered antarctic shergottite are similar to those formed during continuous growth in some lunar basaltic melts pyroxene cores have nuclei of magnesian pigeonite mantled by augite in turn rimmed by strongly zoned ferroan pigeonite with outer rims of pyroxferroite fe ti oxide compositions indicate that this meteorite crystallized under relatively reducing conditions more like lunar basalts than other shergottites ree and other trace element abundances in zoned pyroxenes maskelynite and phosphates are consistent with continuous pyroxene crystallization from an evolving magma punctuated by the appearance of new phases on the liquidus the higher proportion of maskelynite in que94201 suggests that its composition may be closer to multiple saturation with pyroxenes plagioclase than other shergottites and all pyroxene zones show negative eu anomalies thus this meteorite may represent the first example of a martian magma unaffected by crystal accumulation the fe fe mg ratio and proportions of incompatible elements reflected in 4 modal whitlockite are high implying that this is a fractionated melt the calculated bulk ree pattern shows extreme lree depletion suggesting either less contamination with enriched crustal materials or a more lree depleted source than other basaltic shergottites

an impact melt origin for lithology a of martian meteorite elephant moraine a79001 mixing models using major and trace elements show that the bulk composition of lithology a xenocryst bearing magnesian basalt of elephant moraine a79001 eeta79001 can be reasonably approximated as a simple mixture of 44 eeta79001 lithology b ferroan basalt and 56 of allan hills a77005 alha7705 light lithology incompatible element poor lherzolite micro instrumental neutron activation analysis inaa data on xenocryst free groundmass samples of lithology a show that about 20 25 of the melt phase could be dissolved lherzolite the bulk and groundmass samples of lithology a have excesses in au which indicates either meteoritic contamination or addition by some unknown martian geochemical process previous workers have suggested that lithology a was formed by either assimilation of cumulates like alha77005 by a basalt like lithology b or by mixing of basaltic and lherzolitic magmas the former scenario is energetically improbable and unlikely to explain the normal fe mg zonation in lithology a groundmass pyroxenes whereas the latter scenario is unlikely to satisfy the constraints of the mixing model indicating the ultramafic component is poor in incompatible elements we suggest rather that eeta79001 lithology a is an impact melt composed dominantly of basalt like lithology b and lherzolitic cumulates like the trace element poor fraction of alha77005 or y 793605 this model can satisfy the energetic petrologic and geochemical constraints imposed by the samples if eeta79001 lithology a is an impact melt this would have considerable consequences for current models of martian petrologic evolution it would call into question the generally accepted age of magmatism of martian basalts and preclude the use of lithology a groundmass as a primary martian basalt composition in experimental studies regardless the latter is required because lithology a groundmass is a hybrid composition

the zaklodzie enstatite meteorite mineralogy petrology origin and classification the zaklodzie meteorite was found in september 1998 about 40 km west of zamo c in southeast poland macroscopic and microscopic observations in transmitted and reflected light microprobe analyses cathodoluminescence images and x ray diffraction data show that the meteorite is composed of clino and orthoenstatite two generations of feldspars relict olivine forsterite a polymorph of sio2 apparently cristobalite and opaque minerals fe ni alloy kamacite and taenite troilite schreibersite graphite and sulfide mg mn fe s which is probably keilite the texture is fine to inequigranular of cumulate type locally intergranular the mgs fes thermometer indicates that the sulfides crystallized at 580 600 oc thus the zaklodzie meteorite formed by the nearly complete melting of an enstatite chondrite protolith probably at 4 4 ga the process was likely caused by the decay of the 26al nuclide in the planetesimal interior the second stage of its evolution which could have happened at 2 1 ga involved partial re melting of most fusible components probably due to collision with another body the structure composition and origin of the meteorite and its relation to the parent rock indicate that zaklodzie may represent a primitive enstatite achondrite

mineralogy of antarctic basaltic shergottite queen alexandra range 94201 similarities to elephant moraine a79001 lithology b martian meteorite antarctic meteorite que 94201 is a new basaltic shergottite that is mainly composed of subequal amounts of maskelynite and pyroxenes pigeonite and augite plus abundant merrillite and accessory phases it also contains impact melt complex zoning patterns in que 94201 pyroxenes revealed by elemental map analyses using an electron microprobe suggest a crystallization sequence from mg rich pigeonite en62fss30wog to extremely fe rich pigeonite en5fs81wo14 via 110 mg rich augite bands en44fs20wo36 in a single crystal these textures along with the abundant plagioclase maskelynite indicates single stage rapid cooling 5 c year of this rock from a supercooled magma transition from mg rich augite to fe rich pigeonite reflects the onset of plagioclase crystallization enrichment of late stage phases in que 94201 implies crystallization from an evolved magma and suggests a different parent magma composition from the other basaltic shergottites lithology b of eeta79001 basaltic shergottite contains pyroxenes that show complex zoning with augite bands similar to those in que 94201 pyroxene which suggests similar one stage rapid cooling lithology b of eeta79001 also resembles que 94201 in its coarse grained texture of silicates and its high abundance of maskelynite although que 94201 probably crystallized from a more fractionated magma we also note that some apollo lunar mare basalts e g 12020 and 12021 have similar mineralogy and petrology to que 94201 especially in pyroxene zoning all these basaltic rocks with complex pyroxene zoning suggest rapid metastable crystallization from supercooled magmas

re evaluation of intercumulus liquid composition and oxidation state for the shergotty meteorite abstract the shergotty martian meteorite contains pigeonite and augite with homogeneous magnesian cores thought to be cumulus crystals using elemental maps of thin sections we estimate the abundance of cumulus pyroxene cores at 13 vol this estimate is roughly half that determined from prior melting experiments 28 and coincides with the observation of a similar discrepancy between the proportions of cumulus pyroxenes estimated from modal observations and melting experiments in the zagami shergottite an intercumulus liquid composition calculated using this new cumulus pyroxene estimate is virtually identical to that calculated for zagami our results suggest the possibility that melting experiments may not have been carried out under conditions appropriate for crystallization of the shergottites we consider two possible explanations crystallization of pyroxene cores at depth or under different redox conditions prior arguments for polybaric crystallization of shergottites based on pyroxene intergrowths and kaersutite inclusions are inconclusive using the melts program we have explored crystallization at higher pressure a greater proportion of pyroxene crystallizes at higher pressures rather than a smaller amount as necessary in this model even in calculations at 1 bar pigeonite crystallizes before augite and the temperature interval between the appearance of pigeonite and augite expands with pressure both pigeonite and augite appear to be in equilibrium with the same liquid composition and thus are thought to have co crystallized in shergotty so it seems likely that melts may not correctly predict pyroxene crystallization behavior in this fe rich magma fe ti oxide compositions in shergotty were previously used as evidence that the magma crystallized at redox conditions near qfm however ghosal et al 1998 recently suggested that the oxides record subsolidus reducing conditions of qfm 3 at 602 c and our analyses support that conclusion if a new solution model for oxides is used they also proposed that fe3 in shergotty pyroxenes indicates magmatic crystallization at approximately qfm 3 and our estimates of fe3 calculated from pyroxene stoichiometry in microprobe analyses match their reported values however we doubt that stoichiometric calculations provide accurate fe3 determinations other indicators suggest that magmatic conditions were oxidizing the feo content of shergotty maskelynite is greater and the negative eu anomaly in pyroxene cores is shallower than in another basaltic shergottite que94201 which is thought to have crystallized under reducing conditions near qfm 4 also the presence of silica fayalite and magnetite in mesostasis indicates oxiding conditions at least at a late magmatic stage thus we argue that shergotty crystallized under redox conditions near qfm

lateral spreading processes in mountain ranges insights from an analogue modelling experiment abstract the results of a study on gravitational induced lateral spreading phenomena are here reported the lateral spreading processes are widely represented in the italian apennines due to the widespread overlapping of stiff rock masses on more ductile ones the stress strain evolution of these processes was investigated by combining an analogical and a numerical modelling approach the analogue modelling reproduced the evolution of a carbonate ridge thrust over a clayey flysch with reference to the case study of mt roccatagliata mt sant angelo ridge the rock mass juxtaposition was reproduced in a laboratory experiment by overlapping a brittle concrete on a viscous ductile material these materials were selected with rheological properties physically scaled with respect to the natural rock mass prototypes the spreading phenomenon was modelled by opening horizontal panels in sequential stages and monitoring the resulting stresses within the ductile material during the experiment a stress strain modelling was also performed by an fdm numerical solution this modelling replied the laboratory experiment by testing the use of different rheological constitutive laws the resulting stresses and morphological evolutions are comparable with the analogical laboratory experiment only if a time dependent rheological behaviour is assumed for the ductile material the results show that lateral spreading processes can be properly investigated by combining analogue and numerical modelling techniques which take into account the viscous plastic behaviour of the used materials

petrography and mineralogy of two basalts and olivine pyroxene spinel fragments in achondrite eeta79001 three lithologies occur in shocked achondrite eeta79001 a1 exotic irregular crystals and clusters of coarse grained olivine fo81 55 cao 0 2 cr2o3 0 06 nio 0 5 wt low ca pyroxene en82wo2 en58wo12 and low ti chromite cr2o3 59 feo 27 tio2 0 7 wt a2 basaltic host to a1 containing pyroxene mainly pigeonite en67wo12 en52wo13 maskelynite an61 55or0 7 high ti chromite some rimming low ti chromite whitlockite minor cl apatite ilmenite pyrrhotite and mesostasis and b a homogeneous basalt containing augite laths en40 50wo25 35 in a matrix of pigeohite augite maskelynite an62 56 or1 2 ulvospinelilmenite intergrowth whitlockite cl apatite and mesostasis mineral compositions indicate an oxidation state similar to that of shergottites lithology a1 represents an unmetamorphosed cumulate that was disrupted and partly resorbed by a basaltic magma incorporation of low ca magnesian pyroxenes and olivine increased the mg fe ratio of the basaltic magma responsible for a2 and resulted in abundant crystallization of pigeonite lithology b was generated from a distinct magma lacking fragments of a1 the parental spinel harzburgite of a1 was not annealed and is probably a near surface igneous cumulate from a shergottite type basalt ringwood and majorite were found in shock veins

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

geochemistry of martian basalts with constraints on magma genesis abstract over the past decade our knowledge of the geochemical and mineralogical diversity of martian rocks has greatly increased rocks on the surface of mars are older than most martian meteorites and have a wider range in bulk chemistry higher sio 2 higher total alkalis and lower mgo contents recent martian meteorite discoveries nwa 7034 nwa 7635 and nwa 8159 are also significantly older than the shergottites and nwa 7034 contains clasts which have a similar range in chemistry of the surface basalts in this invited review i summarize what is known about the bulk chemistry major elements of martian igneous rocks and use the chemistry to constrain the formation conditions in the interior and how these conditions have changed through time the basalts at gale crater are consistent with forming from a metasomatized mantle source which affected not only the chemistry of the basalts but also the formation conditions basalts at gusev crater bounce rock at meridiani planum and those basalts not affected by metasomatism like those at gale crater have a mantle potential temperature of 1450 c for the noachian conversely martian shergottites have a much higher mantle potential temperature 1745 c which has always been problematic to explain for such young rocks new formation conditions calculations are consistent with previous models of formation by a hot mantle plume but solve the pressure of formation paradox from the previous model combining the calculations for mantle potential temperature with previous estimates for grs measurements of surface volcanics from the hesperian and amazonian i calculate a baseline cooling curve and mantle melting conditions for the martian interior through time

crystallization of the basaltic shergottites insights from crystal size distribution csd analysis of pyroxenes quantitative petrographic analysis using the crystal size distribution csd method provides a novel approach for examining the crystallization histories of basaltic shergottites grain number densities at different sizes are plotted against grain size and the resulting curve relates to the geologic processes involved with the crystallization of the grain population most basaltic shergottites are dominated by pigeonite and augite and because plagioclase is primarily interstitial and therefore constrained in its growth by the surrounding pyroxenes we limited our size measurements to the pyroxene phases the groundmasses of elephant moraine eet a79001 lithology a and dar al gani dag 476 are fine grained with cumulus pyroxene and interstitial plagioclase glass their simple linear csd plots record a single stage of pyroxene crystallization under steady state conditions of continuous nucleation and growth the textures of queen alexandra range que 94201 and eeta79001 lithology b are quite different from the other shergottites with intergrown pyroxene and plagioclase likewise their csd plots are also distinct with curved trends that suggest a lack of large grains most likely because of interference between simultaneously growing silicate phases however the csd plot shapes are smooth also implying a single stage of growth shergotty and zagami with coarser cumulus textures display csd plots that are generally linear over most grain sizes this implies that conditions of nucleation and growth were dominant during formation of the pyroxene populations both plots however also display kinks implying multiple stages of growth a similar kink is also visible in a csd plot of only the mg rich cores of shergotty pyroxenes which suggests the feature represents changes in conditions during core crystallization rather than an event coincident with the change in composition to the fe rich rims the plot may be interpreted as representing two stages of core growth with an intervening short hiatus of nucleation with continued crystallization associated with ascent of the magma eruption onto the surface probably triggered the compositional change to fe rich rims the csd analysis of products from a controlled crystallization study agree with experimental and petrologic estimates that cooling rates for zagami were on the order of a few tenths of a degree per hour growth rates derived from these cooling rates suggest crystallization of shergotty and zagami pyroxenes occurred over a period of a few weeks to months

a 3d reconstruction of plagioclase crystals in a synthetic basalt the three dimensional shapes of plagioclase crystals in an experimentally cooled basaltic liquid have been reconstructed with the aim of 1 better understanding crystal growth processes and the diversity of crystal shapes produced during cooling and 2 to assess the validity of crystal size distributions csds derived from 2d sections the experimental charge was cooled from above the liquidus 1175 c at a rate of 0 2 c h it contained 40 crystals at the quenching temperature of 1120 c to quantify the crystals in 3d photographs of a series of 2d polished sections were taken under an optical microscope using reflected light interpolation and 3d reconstruction of 261 individual crystals was performed using the gocad geomodeling software and their short s intermediate i and long l dimensions were measured plagioclase crystals are generally tabular with a nearly constant i l ratio on the other hand s i and s l shape factors are more variable although both are found to be correlated with length of the s axis these observations are believed to result either from crystal agglomeration and attachment preferentially along 010 faces or from varying thermodynamic or kinetic conditions during cooling growth rates along the s i and l axes have been calculated from the size of the largest crystals and vary from 1 5 10 10 to 5 1 10 10 and 7 2 10 10 m s respectively the csds for the maximal length and short axes of 3d crystals are presented and compared with csds obtained from 2d sections published corrections for cutting effects are found to be generally very satisfactory

petrology and shock metamorphism of the olivine phyric shergottite yamato 980459 evidence for a two stage cooling and a single stage ejection history abstract the basaltic martian meteorite yamato 980459 consists of large olivine phenocrysts and often prismatic pyroxenes set into a fine grained groundmass of smaller more fe rich olivine chromite and an interstitial residual material displaying quenching textures of dendritic olivine chain like augite and sulfide droplets in a glassy matrix yamato 980459 is thus the only martian meteorite without plagioclase maskelynite olivine is compositionally zoned from a mg rich core to a fe rich rim with the outer few micrometers being especially rich in iron with fo84 the cores are the most magnesian olivines found in martian meteorites so far pyroxenes are also mostly composite crystals of large orthopyroxene cores and thin ca rich overgrowths separate pigeonite and augites are rare on basis of the mineral compositions the cooling rates determined from crystal morphologies and crystal grain size distributions it is deduced that the parent magma of yamato 980459 initially cooled under near equilibrium conditions e g in a magma chamber allowing chromite and the mg rich silicates to form as cumulus phases fractional crystallization at higher cooling rates and a low degree of undercooling let to the formation of the ca al and fe rich overgrowths on olivine and orthopyroxene while the magma was ascending towards the martian surface finally and before plagioclase and also phosphates could precipitate the magma was very quickly erupted quenching the remaining melt to glass dendritic silicates and sulfide droplets the shape preferred orientation of olivine and pyroxene suggests a quick thin outflow of lava according to the shock effects found in the minerals of yamato 980459 the meteorite experienced an equilibration shock pressure of about 20 25 gpa its near surface position allowed the ejection from the planet s surface already by a single impact event and at relatively low shock pressures

zagami product of a two stage magmatic history abstract large specimens of the zagami shergottite show highly varied grain sizes and mineral abundances on a cm scale and preferred alignment of pyroxene laths we document the presence of whitlockite and of melt veins and pockets of shock origin pyroxene crystals have homogeneous mg rich pigeonite and augite cores overgrown by fe rich zoned pyroxene mostly pigeonite rims amphibole bearing magmatic melt inclusions occur exclusively in the cores we conclude that zagami experienced a twostage crystallization history the first stage occurred in a deep seated slowly cooling magma chamber there the homogeneous mg rich cores of the pyroxenes crystallized during relatively slow cooling the deep seated origin of the cores is also indicated by the presence of amphibole within them which requires pressures of formation equivalent to crystallization at depths 7 5 km on mars modest abundance of homogeneous mg rich cores 15 20 indicates that crystal settling did not play a significant role in this part of the magma chamber during the second stage the mg rich pyroxenes were entrained into a magma that either intruded to the near surface and cooled in a relatively thin dike or sill or extruded to the surface and crystallized in a lava flow 10 m thick again without indications for crystal settling this scenario is suggested by estimates of cooling rates of 0 1 0 5 c h based on sizes of the plagioclase maskelynite crystals and of 0 02 c h based on the width of pyroxene exsolution lamellae brearley 1991 irregular shapes of the mg rich cores result from resorption and possible solid state diffusion and the shapes and sizes of the pyroxenes after crystallization of fe rich pigeonite rims onto the cores were strongly controlled by the shapes and sizes of the cores the finer grained areas of the rock inherited smaller and more numerous mg rich pyroxene cores from the first stage than did the coarser grained areas crystallization of both augite and pigeonite cores at depth but mostly pigeonite rims in the nearsurface environment may be the result of a phase boundary shift and expansion of the pigeonite stability field at lower pressures the estimated depth of the magma chamber for zagami of 7 5 km and thickness of the putative lava flow of 10 m are consistent with calculations and observations of volcanic constructs and flows in the tharsis region of mars