trace element geochemistry of k rich impact spherules from howardites abstract the howardite eucrite diogenite hed achondrites are a group of meteorites that probably originate from the asteroid vesta howardites are complex polymict breccias that sometimes contain in addition to various rock debris impact melt glasses which show an impressive range of compositions in this paper we report on the geochemistry and o isotopes of a series of 6 saharan polymict breccias 4 howardites and 2 polymict eucrites and on the trace element abundances of high k impact spherules found in two of them northwest africa nwa 1664 and 1769 which are likely paired the high k impact spherules found in the howardites nwa 1664 and nwa 1769 display remarkable trace element patterns compared to eucrites or howardites they all show prominent enrichments in cs rb k li and ba strong depletion in na while the ree and other refractory elements are unfractionated these features could not have been generated during impact melting of their host howardites nor other normal hed target materials the involvement of na poor rocks and possibly rocks of granitic composition appears likely although these lithologies cannot be well constrained at present our results demonstrate that the surface of vesta is certainly more diverse than previously thought indeed despite the large number of available hed meteorites about 1000 different meteorites the latter are probably not sufficient to describe the whole surface of their parent body

magmatic history of red sea rifting perspective from the central saudi arabian coastal plain a newly recognized tertiary dike complex and comagmatic volcanic rocks exposed on the central saudi arabian coastal plain record early stages of magmatism related to red sea rifting intrusive and stratigraphic relationships and new potassium argon dating indicate episodic magmatism from about 30 ma to the present additional stratigraphic and radiometric evidence suggests that limited rift related magmatism began as early as about 50 ma an early phase of crustal extension in the region was accompanied by faulting and graben formation and by dike swarm intrusion the style of extension and intrusion changed at 20 ma localized volcanism and sheeted dike injection ceased and were replaced by the intrusion of thick gabbro dikes this change may mark the onset of sea floor spreading in the central red sea the dikes and volcanic rocks consist of a bimodal mafic felsic suite with transitional subalkaline to alkaline chemistry although no unique petrogenetic model can be developed for the suite the following observations and conclusions have been drawn from the available reconnaissance geochemistry 1 strontium isotopic initial ratios overlap in the range 0 7031 to 0 7047 and are not clearly related to alkalinity or silica content the bimodality and alkalinity of the suite are therefore not products of contamination by the radiogenic precambrian upper crustal granitic rocks exposed in the region 2 several of the evolved rocks rhyolites and comendite show large to extreme negative europium anomalies and heavy rare earth element hree enrichment features that cannot be explained by low pressure crystal fractionation of potential parent magmas an intriguing possibility is that hree enrichments and large negative eu anomalies of some peraluminous rhyolites and granites may result from the complete melting of garnet from middle or lower crustal rocks 3 most of the basalts are light rare earth element lree enriched consistent with derivation from fertile mantle in a continental rift setting however interaction of the mafic melts with lower continental crust cannot be ruled out a model of poly baric mantle melt derivation producing several alkaline subalkaline cycles best explains magmatism in the red sea region differences in the depths and dynamics of mantle melt extraction and transport brought about through changes in crust and mantle structure as the rift and paar developed may account for the transition from mixed alkaline subalkaline bimodal magmatism of the pre 20 ma rift basin to exclusively subalkaline tholeiitic magmatism at the red sea spreading axis and to predominantly alkali basalt volcanism inland within the arabian shield

magma systems related to the red sea opening abstract four distinct magma environments can be recognized in association with the red sea basin 1 1 mid ocean ridge basalts within the present actively spreading axial trough 2 2 basaltic dikes and layered gabbros within the extended precambrian crust on the arabian margin 3 3 granites and rhyolites developed in the extended precambrian crust and 4 4 plateau basalts asymmetrically distributed on the arabian flanks of the red sea these magmas developed as the result of the initial break up of africa and the establishment of an oceanic spreading center within the red sea original asymmetric continental extension occurred on east dipping detachment faults in the oligocene and caused morb like magmas to invade an extending precambrian crust contemporaneous small scale mantle convection initiated limited partial melting under the arabian plate to form the undersaturated alkali olivine plateau basalts slow extension through the miocene allowed an evaporite basin to develop in the red sea while small volumes of rift and plateau basalts formed on the arabian plate a small change in plate motion in the pliocene initiated the seafloor spreading in the axial trough where morb magmas formed oceanic crust rise of the asthenosphere produced small scale thermal perturbations under the arabian plate controlled by an east dipping master detachment fault small scale mantle convection extended partial melting 2 5 eastward under the arabian plate producing more undersaturated diverse alkali olivine plateau basalts distinct chemical and isotopic signatures of these four magma types lead to obvious actualistic correlations with the geodynamic setting

descriptive nomenclature and classification of pyroclastic deposits and fragments recommendations of the iugs subcommission on the systematics of igneous rocks

the arabian continental alkali basalt province part i evolution of harrat rahat kingdom of saudi arabia the harrat rahat lava field forms a major component of an extensive but poorly known continental alkali basalt province extending from yemen in the south through saudi arabia and jordan to syria in the north the continental intraplate volcanism which produced the arabian lava fields harrats was contemporaneous with opening of the red sea collision of the arabian and eurasian plates and uplift of the afro arabian dome harrat rabat has evolved over the past 10 m y it contains two prominent lateritic disconformities which separate three stratigraphic units the shawahit 10 2 5 ma hammah 2 5 1 7 ma and madinah 1 7 recent basalts comprising 68 19 and 13 of the total harrat volume respectively the shawahit basalt is composed of coarse grained dictytaxitic olivine transitional basalt otb and minor 10 alkali olivine basalt aob which erupted through scoria cones and shield volcanoes in contrast the hammah basalt is dominated by equal volumes of fine grained intergranular to intersertal aob and hawaiite which erupted mainly through scoria cones and relatively few shield volcanoes sparse domes and flows of mugearite and benmoreite first appear in the hammah basalt the madinah basalt spans the full compositional range of harrat rahat 103 analyzed samples comprise otb 8 aob 47 hawaiite 32 mugearite 4 benmoreite 8 and trachyte 2 here scoria cones dominate whereas shield volcanoes are sparse and occur only in the lower madinah basalt domes and associated pyroclastic deposits are common the final events of harrat volcanism include 11 post neolithic eruptions 4 500 yr old and two historical eruptions in 641 and 1256 a d arabian harrat magmatism may have been initiated by partial melting of garnet perido tite in the asthenosphere at a depth 100 km accumulation of this partial melt at the crust mantle boundary 37 44 km may have resulted in miocene uplift of the afro arabian dome harrat rahat volcanism began shortly after significant initial uplift with the voluminous extrusion of shawahit otb lava chemical and petrographic data suggest that these otb magmas ascended rapidly with little crustal residence time the data support a model of open system fractionation for aob and hawaiite lavas of the younger hammah and madinah basalts in this model some of the potential shawahit otb was trapped in crustal chambers during magma ascent crystal fractionation in these chambers was accompanied by their periodic replenishment by rising otb magma this resulted in the mixing of magma types and the development of aob and hawaiite magmas which are anomalously enriched in highly incompatible elements the mugearite benmoreite and trachyte lavas are the products of advanced fractionation in these crustal chambers with little or no magma replenishment

the timing of uplift volcanism and rifting peripheral to the red sea a case for passive rifting prior to the formation of the red sea the northeastern afro arabian continent had low relief and was largely below sea level from the late cretaceous to the early oligocene the events leading to the formation of the red sea followed the sequence 1 alkaline volcanism and rifting beginning about 30 32 ma affecting a narrow linear zone in the continent 2 rotational block faulting and detachment faulting well underway by 25 ma 3 gabbro and diorite magmatism andesite to rhyolite volcanism and fine grained nonmarine sedimentation in the rift between 20 and 25 ma 4 fine grained marine sedimentation in the rift as the early shelves started to subside in the middle miocene and 5 uplift of the adjacent continents about 3 km and subsidence of the shelves about 4 km between 13 8 and 5 ma the youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the proterozoic arabian shield that range in age from 13 8 to 568 ma the youngest of these ages coupled with the present high relief along the arabian escarpment and published heat flow measurements indicate that 2 5 4 km uplift has occurred in the last 13 8 m y the sequence volcanism rifting followed by uplift leads to our adoption of a passive mantle model for rift origin models that require uplift to create the rift are rejected because of the late uplift we advocate a model of lithospheric extension caused by two dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere it is acknowledged that traction models could explain the observed data but they imply a rigid static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions neither requirement is necessary in the extension model the rift starts with mechanical extension in a narrow zone of lithosphere between 25 32 ma in our model the thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis once partially melted rocks formerly part of the continental lithosphere join the upwelling asthenosphere resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge the resulting density decrease explains the uplift we think that the red sea began as a consequence of changing plate geometries resulting from the collision of india and eurasia after the collision the segment of the owens fracture zone north of the carlsberg ridge became locked forcing the northeast corner of afro arabia to rotate with the indian plate away from the rest of africa

volatile degassing of basaltic achondrite parent bodies evidence from alkali elements and phosphorus abstract the volatile alkali element na k rb cs and p abundances in eucrites diogenites and basaltic clasts from polymict eucrites howardites and mesosiderites were compared with an average of highly incompatible refractory ahir elements ba la ce nd sm hf th u normalized to cosmic abundances ratios of e g rb ahir remain constant during normal magmatic differentiation because the very low and approximately equal mineral melt partition coefficients for these elements are inefficient in causing fractionation in the melt the na ahir ratio in eucrites is essentially constant at 0 050 cosmic basaltic eucrites and basaltic clasts show a positive correlation between k rb and cs and alkali element ahir ratios the averages for basaltic eucrites are k ahir 0 043 rb ahir 0 0088 cs ahir 0 0040 cosmic the positive correlations between alkali elements and alkali ahir are due to progressive volatile loss of the alkali elements from the basalts and not to source region heterogeneities this process probably had a minor effect on the parent body inventory of volatile elements in diogenites the alkali ahir ratios are systematically greater than in basaltic eucrites and are more variable the lower values are 1 4 2 the means for basaltic eucrites while the higher values are similar to those of cv and co chondrites for k rb and cs the highest alkali ahir ratios were obtained on the most incompatible poor of the diogenites and indicate minimum ratios for the parent body of na ahir 0 24 k ahir 0 49 rb ahir 0 55 and cs ahir 0 14 cosmic there are well defined negative correlations between k rb cs and alkali ahir ratio in the diogenites the data are consistent with progressive alkali loss through volatile outgassing during crystallization of one or more parent magmas this process resulted in 90 loss of the volatile element inventory from the parent body phosphorus also shows evidence for volatile loss from the basaltic eucrites and elevated p ahir ratios in diogenites the low p ahir ratio in eucrites is largely controlled by volatile loss rather than through equilibration with a metallic liquid as previously suggested the parent body of the basaltic achondrites appears to have initially contained alkali element and presumably other volatile element abundances comparable to those in co and cv chondrites and perhaps higher

three dimensional spectral analysis of compositional heterogeneity at arruntia crater on 4 vesta using dawn fc abstract we introduce an innovative three dimensional spectral approach three band parameter space with polyhedrons that can be used for both qualitative and quantitative analyzes improving the characterization of surface compositional heterogeneity of 4 vesta it is an advanced and more robust methodology compared to the standard two dimensional spectral approach two band parameter space the dawn framing camera fc color data obtained during high altitude mapping orbit resolution 60 m pixel is used the main focus is on the howardite eucrite diogenite hed lithologies containing carbonaceous chondritic material olivine and impact melt the archived spectra of heds and their mixtures from relab hoserlab and usgs databases as well as our laboratory measured spectra are used for this study three dimensional convex polyhedrons are defined using computed band parameter values of laboratory spectra polyhedrons based on the parameters of band tilt r0 92 m r0 96 m mid ratio r0 75 m r0 83 m r0 83 m r0 92 m and reflectance at 0 55 m r0 55 m are chosen for the present analysis an algorithm in idl programming language is employed to assign fc data points to the respective polyhedrons the arruntia region in the northern hemisphere of vesta is selected for a case study because of its geological and mineralogical importance we observe that this region is eucrite dominated howarditic in composition the extent of olivine rich exposures within an area of 2 5 crater radii is 12 larger than the previous finding thangjam g et al 2014 meteorit planet sci 49 1831 1850 lithologies of nearly pure cm2 chondrite olivine glass and diogenite are not found in this region although there are no unambiguous spectral features of impact melt the investigation of morphological features using fc clear filter data from low altitude mapping orbit resolution 18 m pixel suggests potential impact melt features inside and outside of the crater our spectral approach can be extended to the entire vestan surface to study the heterogeneous surface composition and its geology

trace element fractionation by impact induced volatilization sims study of lunar hasp samples impact is a dominant process acting on the surfaces of planetary bodies and is the major process that modifies the physical and chemical nature of planetary materials on atmosphere free bodies once volcanic activity has ceased elements with relatively high volatilities are readily mobilized during impact hasp high alumina silica poor compositions are thought to be generated by impact induced volatilization processes on the moon the high al si ratio of hasp is believed to be the result of preferential loss of sio 2 relative to al 2 o 3 here we report sims analyses for hasp glass beads and devitrified glasses from the apollo 14 regolith to assess the behavior of li be b ree sr y and zr during impact events we compare the behavior of these trace elements to the behavior of major and minor elements analyzed by the electron microprobe the hasp samples we studied fall into two compositional groups with one group apparently derived from kreepy lunar lithologies and the other from anorthosite the kreepy hasp beads have an estimated mass loss of 28 compared to their kreepy protolith we estimate that 44 of sio 2 and 35 of feo were lost during impact induced volatilization the mass loss estimate for the anorthositic protolith to produce anorthositic hasp is more uncertain because of the uncertainty of the protolith composition however an estimate of approximately 25 appears reasonable with 38 loss of silica clearly impact processes in the early stages of planetary evolution are important in the modification of geochemical signatures and ordinarily geochemically coherent elements may be decoupled because of different volatilities

timing of mantle upwelling evidence for a passive origin for the red sea rift mantle xenoliths from western saudi arabia some 220 km east of the red sea margin indicate temperatures as high as 900 c in the mantle at about 40 km depth beneath an area of the arabian shield where surface heat flow is lower than the world average this suggests that there has been a large increase in the mantle temperature that has not had time to equilibrate at the surface the amount of uplift and the difference between the observed xenolith temperature and that obtained by projecting the surface heat flow to depth suggests that the lithosphere has been thinned by 30 to 100 km to its present calculated thickness of about 80 km calculations of the time necessary for thermal reequilibration indicate that the lithospheric thinning probably occurred less than 20 m y ago so it postdates rifting which started about 30 ma if there was a prolonged upwelling of deep mantle material resulting in a thinner lithosphere prior to rifting there should have been enough time for some reequilibration at the surface and the observed heat flow should be higher over the broad area of uplift adjacent to the red sea our calculations are consistent with a postrift age of uplift adjacent to the red sea as determined from regional geologic evidence and fission track ages on apatites from shield rocks the red sea appears to have begun with alkaline volcanism on the arabian shield followed by rifting uplift and lithospheric thinning adjacent to the red sea rift began 5 10 m y after rifting this suggests that the red sea is a passive rift that formed because of two dimensional plate motions rather than as a consequence of mantle upwelling a possible cause for the stress in the plates might have been the change of plate motions in the indian ocean that resulted from the collision of india with eurasia

siderophile and other geochemical constraints on mixing relationships among hed meteoritic breccias we have used neutron activation and electron probe fused bead techniques to analyze the bulk major and trace element compositions of 104 named hed meteorites about 100 102 distinct meteorites depending upon pairings including 32 polymict eucrites 30 howardites and six diogenites most were not previously analyzed for siderophile trace elements many not even for major elements our typical sample was 350 mg and in some cases two separate chips were analyzed as a test of meteorite heterogeneity meteorites with extraordinary compositions include bluewing 001 an unequilibrated eucrite that is rich in ti sm and other incompatible elements y 791192 a cumulate dominated polymict eucrite and lew 87002 an oddly sm rich howardite dominated by a ferroan variety of diogenite the eucrite diogenite mixing ratio is the single most important factor determining the compositions of polymict heds but wide ranges in eucrite incompatible element contents in diogenite cr and v contents and in sc contents of both eucrites and diogenites make for diversity among the polymict heds as our new siderophile data help to show the common practice of describing the entire class of howardites as regolith breccias is erroneous most howardites are fragmental breccias showing no sign of origin from true in the lunar sense i e soil like near surface regolith howardites are highly diverse in ni content often remarkably ni poor compared to lunar regolith breccias however the few 8 howardites with between 300 and 1200 lg g ni consistently show some combination of other traits suggestive of regolith origin most importantly all four cases or five if we include malvern which appears to have been altered by annealing of howardites known to have enrichments in solar wind noble gases belong to the 300 lg g ni group in many cases an abundance of glasses particularly in spheroidal or turbid brown form provides additional evidence for regolith origin we propose that the important subset of howardites that are regolith breccias be formally distinguished by the designation regolithic howardite apart from high siderophile levels the regolithic howardites are compositionally distinctive in having al2o3 consistently near 8 9 wt corresponding to a eucrite diogenite mixing ratio of precisely 2 1 assuming the heds are reasonably representative of the ancient i e pre vestoid launch surface of vesta this clustering of regolith composition is difficult to explain unless most of the ancient diogenite component was brought to the surface in a single early episode i e probably a single great impact after which smaller scale cratering with no further major excavations of diogenite until the vestoid forming event efficiently homogenized the surface such a single excavation model may also help to explain why diogenites in marked contrast with eucrites are seldom polymict and why al2o3 poor diogenite dominated howardites consistently lack major siderophile enrichments the low siderophile contents of polymict eucrites are most enigmatic possibly in the hedasteroidal context low collision velocities etc only materials blended by multiple impacts consistently acquire major enrichments in siderophile elements 2009 elsevier ltd all rights reserved

metasomatic products of the lunar magma ocean the role of kreep dissemination the origin of the incompatible element rich lunar component kreep is in the lunar magma ocean lmo the fractionated residual melt after crystallization of the lmo represents urkreep after warren and wasson 1979 the percentage of this residual melt is low enough to be within the realm of silicate liquid immiscibility sli this process has the ability to split the kreep signature into k and reep fractions which are manifest as lunar granite k and phosphate phases present in highland lithologies or as quartz ferrotroctolite in lunar soils reep we envisage this as a localized but significant process since only a small portion of urkreep undergoes sli norms of experimental and apollo 15 basaltic immiscible glasses suggest that thevreep fraction found in the lunar highlands has undergone post sli fractionation of at least fayalite this significantly reduces the density of the reep fraction and coupled with its low viscosity 10 15 poise it can percolate upward metasomatizing the lunar crust the higher viscosity of the granitic melt 30000 poise inhibits its mobility and it forms pods in the lower rust as required for vhk basalt petrogenesis with the identification of kreepy components the composition of urkreep can be calculated using experimental evidence the kreep components may be recombined to give the pre sli composition or liquid liquid kd s can be used to calculate a pre sli composition from lunar granite both calculated urkreep compositions are lower in mgo and al2o3 and higher in feo and p2o5 than reported low and high k kreep compositions the calculated ree abundances are higher and the ree profiles are slightly more lree enriched than the previously reported kreep compositions however the ree profile calculated using liquid liquid kd s is concave upwards compared to the lree enriched profile produced by recombining the kreep components pre sli whitlockite apatite fractionation occurred prior to immiscibility in order to generate the concave upwards profile the lree enriched profile represents the pre sli magma prior to phosphate fractionation the presence of superkreepy rock types can be accounted for by k and reep fraction assimilation and the compositions of olivine vitrophyres can now be modeled using analyzed lunar rock types without the inference of a mythical high mg component other more widespread kreepy rocks e g apollo 14 breccias apollo 15 kreep basalts lkfm basalts are produced by incorporation of the more widespread pre sli urkreep component the splitting of kreep into identifiable lithological components allows the petrogenesis and role of kreep in lunar evolution to be better understood

tertiary anorogenic granites of the western border of the yemen plateau north yemen y a r was affected from 30 to 20 ma ago by intense magmatic activity trap series contemporary with the main tensional tectonic phases of the afro arabian plate in the trap activity two main phases have been recognized the former mainly basaltic between 30 and 26 ma and the second mainly acidic between 23 and 20 ma during the latter period of activity a basaltic dyke swarm and stock like bodies and dykes granophyres were emplaced along the escarpment and coastal plain some granitic bodies intruded the trap series towards the end of the first phase and during the second one on the basis of geochemical and petrological data the tertiary granites of yemen belong to alkaline or peralkaline associations of a type the textural characteristics indicate that the granites are either hypersolvus or subsolvus the mafic mineral assemblages allow three main types of granites to be distinguished by their different environments of crystallization as regards the origin of these kinds of granites several hypotheses account for the peculiarities observed the lack of crustal xenoliths the preliminary data on the sr isotopic ratio the data on major and trace elements point to a direct origin by fractional crystallization without substantial crustal interaction this agrees well with the strong tensile tectonics linked with the uplift of the afro arabian dome and with the lithosphere thinning which provide a minimal opportunity for interaction of the magmas with the crust

the moon a taylor perspective we address several current lunar problems the data suggest that the moon likely possesses an fe rich metallic core a few percent of lunar volume although definitive proof is lacking refractory elements appear to be enriched relative both to the composition of the primordial solar nebula ci and the earth very volatile elements appear to be depleted uniformly at high levels we adopt the single impact hypothesis for lunar origin which formed a high temperature silicate vapor disk mostly of metal poor silicate material from an impactor theia that was already depleted in volatiles we speculate that the curious lunar bulk composition resulted from condensation from high temperature vapor at around a few earth radii this could produce an enriched refractory element composition that cut off below 1000 k producing a uniform depletion in very volatile elements

grosvenor mountains 95 howardite pairing group insights into the surface regolith of asteroid 4 vesta regolithic howardites are analogs for the surface materials of asteroid 4 vesta recently mapped by the dawn spacecraft rigorously evaluating pairing of howardites recovered in 1995 in the grosvenor mountains gro 95 antarctica enables an examination of a larger more representative regolith sample previous work on two of the howardites studied here concluded that gro 95602 and gro 95535 are solar wind rich surface regolith samples and that they are not paired with each other leading to uncertainty regarding pairing relationships between the other gro 95 howardites based on petrology cosmic ray exposure history and terrestrial age four gro 95 howardites are paired the paired howardites gro 95534 95535 95574 95581 were from a meteoroid with radius of 10 15 cm a preatmospheric size comparable to that of kapoeta the largest known regolithic howardite the paired gro 95 howardites contain clasts of at least 18 separate hed lithologies providing evidence they were assembled from diverse source materials the total eucrite diogenite mixing ratio ratio of all eucrite lithologies to all diogenite lithologies in the paired gro 95 howardites is 2 1 petrographically determined basaltic eucrite cumulate eucrite ratios in regolithic howardites studied here and previously vary more widely than total eucrite diogenite ratios relative to eucritic pyroxene plagioclase is depleted in these howardites which provides evidence that plagioclase is preferentially comminuted in the vestan regolith the extent of plagioclase depletion could be an indicator of regolith maturity

comment on topographic and volcanic asymmetry around the red sea constraints on rift models by t h dixon e r ivins and j f brenda

consequences of explosive eruptions on small solar system bodies the case of the missing basalts on the aubrite parent body aubrite meteorites are composed of constituents which are almost certainly of igneous origin if they were generated by the melting and fractionation of enstatite chondrite like parental material as seems very likely then plagioclase rich basaltic complements to the aubrites should have formed however such materials are not known as individual meteorites and the compositions of two plagioclase silica clasts and one albite silica diopside anorthite clast probably an impact melt in the norton county aubrite breccia suggest that they are not the putative enstatite plagioclase basalts we propose a new mechanism that explains the absence of these materials showing that the expansion of even very small amounts of volatiles present in a melt approaching the surface of a small low gravity body will be enough to disrupt the melt into a spray of droplets moving faster than the local escape velocity this explosive volcanic process of melt removal requires larger melt volatile contents on larger bodies and data on the solubility of volatiles in basaltic melts suggest that the process was limited to bodies smaller than about 100 km in radius

pristine lunar glasses criteria data and implications major element analyses of several thousand glasses from all of the apollo landing sites have resulted in the identification of 25 groups of pristine i e volcanic glass the nickel in these pristine glasses is shown to be indigenous not meteoritic contamination and to be correlated with mg the chemical data indicate that these glasses are consistently better candidates for primary magmas than the majority of crystalline mare basalts the pristine glasses support the view that assimilative processes ringwood and kesson 1976 involving two cumulate systems in the differentiated mantle operated during mare petrogenesis the reality of those assimilative interactions is evident by the occurrence of two linear arrays among the chemistries of the glasses data suggest that these cumulate components in the differentiated mantle persist for lateral distances of at least 1000 km and therefore appear to be products of a vast magma ocean that existed early in lunar history alternative hypotheses claiming that the highlands crust and mare source regions were produced by serial magmatism without a magma ocean have not yet considered all of the constraints derived from the pristine glasses

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

post crystallization reheating and partial melting of eucrite eet90020 by impact into the hot crust of asteroid 4vesta 4 50 ga ago we performed petrologic radiometric ar ar sm nd and mn cr ages and ion microprobe studies of the basaltic eucrite eet90020 this is one of the few rare basaltic eucrites whose 39ar 40ar age has not been reset during impact bombardment on the hed parent body 4 ga ago and thus should provide a unique opportunity to study the nature of the early thermal events on its parent body presumably asteroid 4vesta hand specimen inspection shows that the rock consists of a fine grained and a coarse grained lithology microscopy indicates that the fine grained lithology has a granulitic texture with a coarser grained area and a large opaque assemblage embedded in the granulitic matrix the coarse grained lithology has an igneous subophitic texture the rock has pyroxenes similar to those in type 5 eucrites type 5 pyroxene and experienced prolonged thermal metamorphism after rapid crystallization from a near surface melt however minor mineral assemblages are unusual and suggest a complex thermal history tridymite occurs as large laths irregular crystals 1 5 gpa after the reheating event eet90020 seems to have experienced the following thermal history 1 crystallization during rapid cooling near the surface 2 some brecciation by impact 3 thermal metamorphism that produced type 5 pyroxene and 4 short reheating that caused partial melting and rapid cooling 39ar 40ar measurements show a relatively flat pattern and an age of 4 49 0 01 ga which is consistent with rapid cooling from high temperature event 4 resetting of the sm nd ages at 4 51 0 04 ga appears to be closely related to the remelting of ca phosphates rb sr data suggest rb loss from tridymite during partial melting the resetting of the mn cr age may have been related to the formation of cr ulvospinels event 4 we suggest that all these ages were reset by partial melting event 4 we further suggest that the partial melting event event 4 that reset the ages 4 50 ga ago was caused by an impact into eet90020 which was part of the hot crust of 4vesta and resulted in an increase in the temperature from the ambient temperature of 870 c to above the subsolidus temperature of eucrites of 1060 c

spectroscopic characterization of mineralogy and its diversity across vesta the mineralogy of vesta based on data obtained by the dawn spacecraft s visible and infrared spectrometer is consistent with howardite eucrite diogenite meteorites there are considerable regional and local variations across the asteroid spectrally distinct regions include the south polar rheasilvia basin which displays a higher diogenitic component and equatorial regions which show a higher eucritic component the lithologic distribution indicates a deeper diogenitic crust exposed after excavation by the impact that formed rheasilvia and an upper eucritic crust evidence for mineralogical stratigraphic layering is observed on crater walls and in ejecta this is broadly consistent with magma ocean models but spectral variability highlights local variations which suggests that the crust can be a complex assemblage of eucritic basalts and pyroxene cumulates overall vesta mineralogy indicates a complex magmatic evolution that led to a differentiated crust and mantle

chemical composition and origin of apollo 15 impact melts to better understand the apennine front we have analyzed 14 apollo 15 impact melt rocks including hand samples rake samples and coarse fines for major fused bead microprobe and trace element inaa abundances the melt rocks show a remarkable diversity of chemical composition although all but one have typical kreep incompatible element patterns they appear to form five chemical groups all but the most kreep rich of which may represent single impacts three groups are of low k fra mauro composition none correspond to very high alumina vha compositions the compositions are unlike nearly all other impact melts and regoliths from highland sites indicating a distinct provenance for the apollo 15 samples serenitatis basin impact melt compositions apollo 17 poikilitic boulders are not duplicated although one apollo 17 aphanitic melt sample 73215 is similar to one of the apollo 15 groups the apollo 15 group consisting only of 15445 and 15455 remains the best candidate for imbrium basin impact melt while the most kreep rich group probably consists of melts produced in varied post imbrium impacts into the apennine bench formation apollo 15 kreep basalt flows the most populated group was previously unrecognized and could be either pre or post imbrium basin it is possible that it not 15445 15455 represents imbrium melt one unique sample has a non kreep incompatible element pattern with low abundances and a positive eu anomaly its target was rather primitive and would not appear to be of regional extent or present in the area in post imbrium times it is either a pre imbrian melt or exotic the apennine front does not consist of a monolithic lkfm composition but shows a much wider diversity of compositions than was previously recognized

evidence for k rich terranes on vesta from impact spherules the howardite eucrite diogenite hed clan is a group of meteorites that probably originate from the asteroid vesta some of them are complex breccias that contain impact glasses whose compositions mirror that of their source regions some k rich impact glasses up to 2 wt k2o suggest that in addition to basalts and ultramafic cumulates k rich rocks are exposed on vesta s surface one k rich glass up to 6 wt k2o with a felsic composition provides the first evidence of highly differentiated k rich rocks on a large asteroid they can be compared to the rare lunar granites and suggest that magmas generated in a large asteroid are more diverse than previously thought

elemental mapping by dawn reveals exogenic h in vesta s regolith using dawn s gamma ray and neutron detector we tested models of vesta s evolution based on studies of howardite eucrite and diogenite hed meteorites global fe o and fe si ratios are consistent with hed compositions neutron measurements confirm that a thick diogenitic lower crust is exposed in the rheasilvia basin which is consistent with global magmatic differentiation vesta s regolith contains substantial amounts of hydrogen the highest hydrogen concentrations coincide with older low albedo regions near the equator where water ice is unstable the young rheasilvia basin contains the lowest concentrations these observations are consistent with gradual accumulation of hydrogen by infall of carbonaceous chondrites observed as clasts in some howardites and subsequent removal or burial of this material by large impacts

the yemen trap series genesis and evolution of a continental flood basalt province abstract the yemen basaltic plateau trap series was built up by fissure eruptions related to tensional tectonics in the afro arabian plate two main phases of activity have been identified and dated between 30 and 26 and 23 and 19 ma respectively the first phase was dominated by eruptions of alkaline to transitional basalts with a few rhyolitic ignimbrites during the second phase rhyolites with minor transitional basalts were erupted in the southern and southwestern part of the area geochemical and isotopic data indicate that the basalts are the result of combined processes of crystal fractionation and crustal contamination on mantle derived parental liquids the degree of crustal contamination generally decreases from south to north

geological evolution of the afro arabian dome abstract the afro arabian dome includes the elevated continental regions enclosing the red sea gulf of aden and the ethiopian rift system and extends northwards as far as jordan it is more than an order of magnitude larger than other african uplifts both the structures and the igneous rocks of the dome appear to be products of the superimposition of two perhaps three semi independent generating systems initiated at different times but all still active a strain pattern dominated by nw trending basins and rifts first became established early in the cretaceous by the end of the oligocene much of the extensional strain had been taken up along the red sea and gulf of aden axes which subsequently developed into an ocean palaeogene trap volcanism of mildly alkaline to transitional character was related to this horizontal extension rather than to doming further west the east sahara swell has a history of intermittent alkaline volcanicity which began in the mesozoic and was independent of magmatism in the afro arabian dome volcanicity specifically related to doming began in the miocene along a n s zone of uplift extending from ethiopia to syria this elongated swell forms the northern termination of the east african system of domes and rifts characterized by episodic vertical uplift but very little extension superimposition of epeirogenic uplift upon structures formed by horizontal extension took place in the neogene volcanicity related to vertical tectonics is mildly alkaline in character whereas transitional and tholeiitic magmas are found along the spreading axes

the arabian continental alkali basalt province part ii evolution of harrats khaybar ithnayn and kura kingdom of saudi arabia three coalesced basaltic lava fields harrats khaybar ithnayn and kura constitute the largest contiguous area of cenozoic basalt in saudi arabia similar in extent 20 564 km2 and volume 1 850 km3 to harrat rahat which is situated only 25 km to the south harrat kura with an age range from about 11 to 5 ma is the oldest of these harrats it contains a single stratigraphic unit the kura basalt primarily composed of alkali olivine basalt aob with subordinant basanite and hawaiite and a single cluster of four phonolite domes in contrast harrats khaybar 5 ma to present and ithnayn 3 ma to present are composed of more mildly alkaline basalt types the two oldest units on harrat khaybar the jarad and mukrash basalts are dominated by dictytaxitic to ophitic olivine transitional basalt otb flows which were extruded primarily from major arterial lava tubes to form unusual morphological features here designated whaleback lava flows the lava tubes emanate from a 50 km long north trending fault scarp along the western edge of the central vent zone which marks the site of voluminous outwelling of otb lava the fault scarp ceased to be a major site of extrusion at the end of mukrash time immediately to the east of the fault scarp a thin 0 7 to 1 0 km uptilted slab of crust formed the roof of a high level magma chamber 60 x 25 km that dominated the eruptive history of the abyad basalt the youngest stratigraphic unit on harrat khaybar the abyad basalt occurs as seven stratigraphic subunits the two youngest of which qb6 and qb7 are post neolithic and historical in age in contrast to the jarad and mukrash basalts the abyad basalt is dominated by aob and hawaiite with only minor otb no abyad differentiated rocks occur on harrat ithnayn however fractional crystallization in the high level magma chamber beneath harrat khaybar has resulted in an extreme compositional range of rock types from mugearite to benmoreite trachyte and comendite the vent distribution at harrat khaybar shows a distinct zonation indicative of a high level compositionally stratified magma chamber whole rock chemistry suggests that even the most primitive basalts are not primary magmas but rather fractionated melts derived from primary magmas of variable melt fractions generated at differing mantle depths the more undersaturated harrat kura basalts were derived from primary magmas representing the smallest degrees of partial melting at the greatest depth crystal fractionation of most primary magmas appears to have taken place in reservoirs at the crust mantle boundary chemical and petrographic data suggest that open system magma recharge was an important process in the high level magma chamber beneath harrat khaybar the chemical and mineralogical heterogeneity of the harrat khaybar comendites their sr isotope values and their zr nb ratios all indicate that they were derived by varying degrees of crystal fractionation 95 crustal melting and halogen complexing the north trending volcanic axes of harrats rahat khaybar and ithnayn form a single 600 km long linear vent system designated the makkah madinah nafud mmn volcanic line the three transitional to mildly alkaline harrats extruded from this volcanic line differ from most arabian harrats which like harrat kura to the west are significantly more undersaturated the relatively large degrees of partial melting evident in harrats rahat khaybar and ithnayn suggest that the mmn volcanic line may overlie a north south thermal lineation in the mantle the petrochemical stratigraphic sequence in these three harrats is nearly identical early voluminous extrusions of otb were followed by later less extensive extrusions of minor otb and abundant aob hawaiite and differentiated lavas as this petrochemical stratigraphy evolved it progressed northward along the mmn trend with time beginning on harrat rahat about 10 ma on harrat khaybar about 5 ma and on harrat ithnayn about 3 ma the interplay of open system magmatic processes beneath this migrating volcanic line has resulted in a classic example of a strongly fractionated mildly alkaline basalt series

carbon in howardite eucrite and diogenite basaltic achondrites the c contents and isotopic compositions of four eucrites four diogenites and two howardites have been determined stepped heating in an o atmosphere was employed to convert selectively different carbonaceous materials to co2 gas at various temperatures this technique successfully distinguishes between terrestrial contaminants and indigenous c with the exception of the kapoeta howardite the howardite eucrite and diogenite hed meteorites contain 10 30 ppm indigenous c with 13c between 29 and 19 kapoeta a regolith breccia has an elevated c content and 13c due to the presence of 13c enriched carbonate minerals 13c 20 in cm2 or cr2 like clasts the range in 13c displayed by hed samples is similar to that of other solar system basalts such as lunar rocks and martian meteorites but distinctly different from that of the terrestrial mantle the diogenites have a slightly lower total c yield and higher 13c than the eucrites which is a result of degassing of trapped co cc2 co2 3 from the silicate lattice during metamorphism or annealing however three out of the four diogenites studied appear to contain a discrete component possibly of graphitic c coating silicate grains that is seemingly unaffected by the extended annealing period experienced by the diogenites it is possible that this component might host the indigenous primitive xe recently identified in diogenites

dawn the vesta hed connection and the geologic context for eucrites diogenites and howardites the dawn mission has provided new evidence strengthening the identification of asteroid vesta as the parent body of the howardite eucrite and diogenite hed meteorites the evidence includes vesta s petrologic complexity detailed spectroscopic characteristics unique space weathering diagnostic geochemical abundances and neutron absorption characteristics chronology of surface units and impact history occurrence of exogenous carbonaceous chondritic materials in the regolith and dimensions of the core all of which are consistent with hed observations and constraints global mapping of the distributions of hed lithologies by dawn cameras and spectrometers provides the missing geologic context for these meteorites thereby allowing tests of petrogenetic models and increasing their scientific value

the fate of pyroclasts produced in explosive eruptions on the asteroid 4 vesta we evaluate the consequences of explosive activity having taken place during volcanic eruptions on the differentiated asteroid 4 vesta which is the likely parent body of the howardite eucrite diogenite hed meteorites for a wide range of magma volatile contents we calculate the eruption speeds and subsequent trajectories of the pyroclastic magma droplets produced by considering the size distribution and eruption speeds of the droplets and the mass fluxes at which they are ejected we show that under all realistic circumstances the droplets will have formed lava fountains that were extremely optically dense as a result virtually all of the droplets will have reached the surface having suffered a negligible amount of radiative cooling and will have coalesced into lava ponds feeding lava flows typically 1 of the pyroclasts will have undergone enough cooling to allow them to accumulate into a recognizable fall deposit consisting of unwelded or partly welded volcanic glass beads this result is consistent with the apparent absence of identifiable pyroclastic material in the hed and other types of differentiated meteorites

the volatile content of vesta clues from apatite in eucrites apatite was analyzed by electron microprobe in 3 cumulate and 10 basaltic eucrites eucritic apatite is fluorine rich with minor chlorine and hydroxyl calculated by difference we confirmed the hydroxyl content by measuring hydroxyl directly in apatites from three representative eucrites using secondary ionization mass spectroscopy overall most eucritic apatites resemble fluorine rich lunar mare apatites but intriguing oh and cl rich apatites suggest a role for water and or hydrothermal fluids in the vestan interior or on other related differentiated asteroids most late stage apatite found in mesostasis has little hydroxyl or chlorine and is thought to have crystallized from a degassed magma however several apatites exhibit atypical compositions and or textural characteristics for example the isotopically anomalous basaltic eucrite pasamonte has apatite in the mesostasis with significant oh apatites in juvinas also have significant oh and occur as veinlets crosscutting silicates euhedral apatites in the moore county cumulate eucrite occur as inclusions in pyroxene and are also hydroxyl rich 0 62 wt oh the oh was confirmed by sims analysis and this apatite clearly points to the presence of water at least locally in the vestan interior portions of elephant moraine eet 90020 have large and abundant apatites which may be the product of apatite accumulation in a zone of melt rock reaction relatively chlorine rich apatites occur in basaltic eucrite graves nunataks gra 98098 approximately 1 wt cl particularly striking is the compositional similarity between apatite in gra 98098 and apatites in lunar kreep which may indicate the presence of residual magmas from an asteroid wide magma ocean on vesta

the relation of mesozoic cainozoic volcanism to tectonics in the afro arabian dome abstract the mesozoic cainozoic volcanism of ne africa and arabia is described in terms of four major magmatic provinces these are dominantly basaltic and vary in general composition from strongly alkaline to tholeiitic with some overlap between provinces most but not all of this volcanism has taken place within the confines of the afro arabian dome and its attendant rifts but the magmatism is not explicable in terms of a mantle plume beneath afar because such a plume explains neither the spatial nor temporal distribution of volcanicity instead i propose a multi stage model in which basaltic magmas of transitional composition were generated in paleogene time as a response to regional extension with subsequent evolution of the red sea and gulf of aden into proto oceans these magmas took on the character of oceanic tholeiites in the neogene alkalic activity related to epeirogenic doming was superimposed on this extension related volcanism a third independent magma generating mechanism appears to have operated nw of the afro arabian dome where small volumes of alkaline basalt magma have been erupted intermittently since early in the cretaceous this activity may result from periodic tapping of deep asthenosphere by tensile fracturing and upwarping of the northward drifting african plate

the quest for regolithic howardites part 2 surface origins highlighted by noble gases we report noble gas data of helium he neon ne argon ar krypton kr and xenon xe cosmic ray exposure cre ages and nominal gas retention k ar u th he ages for seven howardites cre 01400 eet 87513 eet 87518 eet 99400 gro 95535 gro 95602 san 03472 in continuing research to identify regolithic samples and better understand the vestan regolith in our previous work we found little correlation between suggested regolith parameters of ni 300 g g al 2o 3 8 9 wt and eucrite diogenite e d ratio of 2 1 warren et al 2009 and trapped solar wind sw fractionated solar wind fsw or planetary noble gas components from impacted material noble gas indicators of a regolithic origin here we have expanded our data set to include samples outside of these parameters to further explore composition and the differences in ni content as indicators for the presence of non vesta material in addition our sample set includes two potentially paired meteorites from the gro suite finally in our petrographic studies the samples selected showed no evidence for carbonaceous chondrite fragments which should reduce the effect of contamination by planetary noble gas components and will allow us to better identify sw fsw components where present of the samples studied here three howardites gro 95535 gro 95602 and eet 87513 show evidence for a regolithic origin with both isotopic and element noble gas ratios clearly pointing to the presence of trapped components similar to sw fsw or planetary the two gro howardites gro 95535 and gro 95602 show similar noble gas ratios to our previously defined sw fsw dominated regolithic group lew 85313 and met 00423 suggesting a surface origin for these samples however interestingly the gro samples show vastly different cosmogenic noble gas abundances and thus different cre ages which suggests that they are not paired for howardite eet 87513 the data hint to the presence of cm material with a neon release pattern similar to our defined planetary fsw dominated regolithic group cm rich samples pra 04401 sco 06040 our petrological investigations found no evidence for cm fragments within eet 87513 though a single clast was reported previously buchanan et al 1993 aside from the ne release pattern the remaining noble gas data show more similarity with sw fsw components the remaining four howardites cre 01400 eet 87518 eet 99400 and san 03472 and are dominated by cosmogenic noble gases and show no evidence for a regolithic origin our data suggest that a cm composition is likely present in all samples to some degree but that this can be overprinted by sw components or cosmogenic components obtained in situ on the vestan surface or during transit to earth respectively the presence of cm material is an important parameter for understanding the evolution of vesta s surface while we have uncovered three further regolithic howardites 13 regolithic total of 41 analysed further noble gas analysis of hed meteorites is needed to not only determine regolithic origins but to better characterise the abundance of carbonaceous chondrite material and its effect on the noble gas signatures of such samples

pyroclast loss or retention during explosive volcanism on asteroids influence of asteroid size and gas content of melt we review the conditions under which explosive volcanism took place on early forming differentiated asteroids the pressure dependent solubility of typical asteroid volatiles in melts implies that the gas driving explosive volcanism on asteroids less than approximately 100 km in diameter was probably present mainly as a free phase capable of accumulating into large gas bodies and thus causing slug flow in melts approaching the surface in contrast in asteroids larger than approximately 100 km the gas was probably present largely as a dispersion of small bubbles we show that these gas distributions have implications for the size distribution of the pyroclastic droplets produced in explosive eruptions at the surface all pyroclastic melt droplets are accelerated by the expanding gases but their speeds lag the gas speed by a finite amount that is a function of the droplet size and density and the asteroid size and hence acceleration due to gravity we compute pyroclast speeds and by comparing them with escape velocities we identify the critical pyroclast diameter on a given size asteroid that distinguishes droplets lost to space from droplets that return to the surface identification of asteroidal pyroclasts and measurements of their sizes could throw light on the amounts of gas driving eruptions

glasses in howardites impact melts or pyroclasts we have analyzed glasses in eight howardites with the aim of distinguishing their origins as impact melts or pyroclasts although theoretical calculations predict that pyroclastic eruptions could have taken place on vesta the occurrence of pyroclastic glasses in hed meteorites has not been documented this study involved petrographic examination of textures electron microprobe analysis of major and minor elements and la icp ms analysis for selected trace elements previously documented textural and compositional differences between lunar impact melt and pyroclastic glasses partly guided this study this work yielded no positive identification of pyroclastic glasses the most likely explanation is that pyroclastic glasses never formed either because vesta contains insufficient volatiles to have powered explosive eruptions or because eruptive conditions produced optically dense fire fountains that allowed melt drops to collect as lava ponds the impact melt glasses were grouped low alkali ca rich and k rich based on compositions we suggest that these glasses are the result of impacts onto known hed lithologies the low alkali glasses are impact melts of bulk hed lithologies we hypothesize that the ca rich and k rich glasses result from oversampling of plagioclase and of mesostasis that experienced liquid immiscibility respectively during micrometeorite impacts into eucrite targets

global crustal metamorphism of the eucrite parent body abstract we conducted a petrologic study of 18 equilibrated eucrites with the goal of understanding the formation and evolution of the crust of asteroid 4 vesta the likely source of eucrite meteorites their textures indicate that eucrites were basaltic igneous rocks but modified by postcrystallization brecciation recrystallization and impact melting recrystallized rocks are the most common among clasts in the eucrites impact melt breccias are rare equilibration temperatures of the metamorphic clasts range from 700 to 1000 c with most in the 800 900 c range complex exsolution and recrystallization textures in pyroxenes suggest that eucrites cooled very slowly from high peak temperatures after reheating this extended period of thermal metamorphism requires a high interior temperature in a vesta sized body this metamorphism could have taken place during and shortly after crust formation we suggest the crust of vesta evolved through the eruption rapid cooling and subsequent burial and reheating of lava flows as the crust grew in thickness the depth of burial increased causing temperatures to rise and leading to global metamorphism this resulted in only a small percentage of unequilibrated eucrites only the last few percent of basalts erupted would not be metamorphosed and a wide range of metamorphic temperatures and durations for the equilibrated eucrites

basaltic magmatism on the moon a perspective from volcanic picritic glass beads abstract it is widely accepted that basaltic magmas are products of partial fusion of periodotite within planetary mantles as such they provide valuable insights into the composition structure and processes of planetary interiors those compositions which approach primary melt compositions provide the most direct information about planetary interiors and serve as a starting point to understand basaltic evolution within the collection of lunar samples returned by the apollo and luna missions are homogeneous picritic glass beads of volcanic origin these picritic glasses are our closest approximations to primary magmas as such these glass beads provide a unique perspective concerning the origin of mare basalts the characteristics of the lunar interior and processes in the early differentiation of the moon we have obtained trace element data for these picritic glasses using sims techniques these data and literature isotopic and experimental data on the picritic glasses are placed within the framework of mare basaltic magmatism the volcanic glasses are very diverse in their trace element characteristics for example they have a wide range of ree pattern shapes and concentrations like the crystalline mare basalts all picritic glasses have a negative eu anomaly unlike the crystalline mare basalts there is little correlation between the size of the eu anomaly and overall ree concentrations trace element differences among the various glasses suggests that a kreep component was incorporated into their mantle source this implies large scale mixing of the lunar magma ocean derived cumulate pile subtle differences among glasses suggest that local mixing of sources may also have been an important process preservation of subtle chemical differences in the picritic glasses and crystalline basalts may be interpreted as indicating that they were produced by small to moderate degrees of partial melting and that the lunar mantle did not experience extensive melting during episodes of mare volcanism several lines of evidence are consistent with the view that the picritic glasses were derived from mantle sources that were compositionally distinct from the sources for crystalline mare basalts these are parallel but no common liquid lines of descent chemical differences between picritic glasses and the more primitive crystalline mare basalts experimental studies indicating that the picritic glasses are multiply saturated at depths greater than that of the mare basalts differences in lead isotopic data and the mode of eruption i e fire fountaining for glass beads these data also provide circumstantial evidence that suggests that the picritic glasses were derived from a source somewhat more volatile rich than that of the mare basalts several petrogenetic models are suggested by the trace element characteristics of the picritic glasses 1 1 partial melting of heterogeneous lunar mantle at depths greater than 300 km to produce the parental magmas picritic for both the mare basalts and picritic glasses picritic magmas represented by glass beads were erupted to the surface with small degrees of fractional crystallization while mare basalts were produced by larger degrees of fractional crystallization 15 30 of similar but not identical picritic magmas 2 2 picritic magmas represented by the glass beads were generated at depths greater than 400 km in a volatile enriched relative to the mare basalt source heterogeneous mantle while mare basalts are fractional crystallization products of picritic magmas generated at depths of less than 400 km 3 3 the picritic magmas represented by the glass beads represent polybaric melting that initiated at depths of at least 1000 km a primitive mantle component or less processed cumulate mantle components may have been involved in the generation of the picritic glasses in any of these models

the borehole fluid effect in electrical resistivity imaging fluid that fills boreholes in crosswell electrical resistivity investigations provides the necessary electrical contact between the electrodes and the rock formation but it is also the source of image artifacts in standard inversions that do not account for the effects of the boreholes the image distortions can be severe for large resistivity contrasts between the rock formation and borehole fluid and for large borehole diameters we have carried out 3d finite element modeling using an unstructured grid approach to quantify the magnitude of borehole effects for different resistivity contrasts borehole diameters and electrode configurations relatively common resistivity contrasts of 100 1 and borehole diameters of 10 and 20 cm yielded for a bipole length of 5 m apparent resistivity underestimates of approximately 12 and 32 when using ab mn configurations and apparent resistivity overestimates of approximately 24 and 95 when usingam bn configurations effects are generally more severe at shorter

composition and petrology of hed polymict breccias the regolith of 4 vesta the polymict breccias of the howardite eucrite and diogenite hed clan of meteorites preserve records of regolith processes that occur on vesta their putative home world these breccias howardites polymict eucrites and polymict diogenites are impact engendered mixtures of diogenites and eucrites the compositions of polymict breccias can be used to constrain the lithologic diversity of the vestan crust and the excavation depths of these materials we have done petrological and compositional studies of multiple samples of 5 polymict eucrites and 28 howardites to investigate these issues older analyses were done on samples of approx 0 5 gram mass by inaa newer analyses on samples of approx 5 gram mass by xrf and icp ms we estimate the percentage of eucritic material poem of polymict breccias by comparing their al and or ca contents to those of average basaltic eucrite and diogenite our samples have poem ranging from 28 to 98 adding two polymict diogenites from extends the range to poem 10 one hypothesis is that ancient well mixed vestan regolith has poem approx 67 and has a higher content of admixed impactor material several of our howardites have poem of 59 74 al and or ca contents 10 of poem 67 about a third have ni contents 300 micro g g suggesting they contain 2 chondritic material cm and or cr these may be regolithic howardites only one lew 85313 contains ne dominated by a solar wind sw component pca 02066 is dominated by impact melt material of polymict parentage and petrologically appears to be a mature regolith breccia yet it does not contain sw ne gro 95602 falls within the poem window contains sw ne yet has a ni content of 193 micro g g its petrologic characteristics suggest it was formed from immature regolith no polymict breccia clasts no glass trace element characteristics of the polymict breccias demonstrate the dominance of main group eucrites as the basaltic component mixing diagrams of zr nb ba hf and ta with al show no evidence for a significant contribution from stannern trend eucrites an exception is polymict eucrite lew 86001 poem 92 which is dominated by stannern trend basaltic debris howardite lap 04838 poem 84 has higher incompatible trace concentrations than other polymict breccias excluding lew 86001 and either contains a stannern trend basaltic component or has a significant contributions from evolved eucrites like nuevo laredo

ferromagnetic resonance intensity a rapid method for determining lunar glass bead origin ferromagnetic resonance intensity is was tested as a criterion for use as a nondestructive reliable and rapid screening method for distinguishing volcanic from impact glass spherules in the lunar soils this method relies on the absence or presence of single domain fe formed during impact melting by autoreduction of fe2 in the melt extensive scanning electron microscope sem inspection of individual glass bead surfaces provided reliable evidence of the mode of glass genesis ferromagnetic resonance intensity was tested against the type apollo 15 and apollo 17 volcanics additional volcanics were also found during the course of the study ninety four percent of the volcanic glasses had is values less than one implying the absence of single domain fe seventy five percent of impact glasses had elevated is values the remaining impact beads had is values less than one these samples lack single domain fe and could represent impact melts of bedrock with little or no regolith contribution the ability to discriminate bedrock derived impact melts as opposed to impact melts partially or completely derived from regolith and the primary discrimination of volcanic glasses from impact glasses suggests the fmr technique can be important in determining the origin of lunar glass spheres

the brecciated texture of polymict eucrites petrographic investigations of unequilibrated meteorites from the antarctic yamato collection

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites

magnesian basalt clasts from the eet 92014 and kapoeta howardites and a discussion of alleged primary magnesian hed basalts we have performed petrologic and geochemical studies on pyroxene phyric magnesian basalt clasts from the howardites eet 92014 and kapoeta pyroxenes in the eet 92014 clast are strongly zoned from wo4 5en68 9fs26 6 cores to wo22 2en18 5fs59 2 rims the pyroxene has also suffered later feo metasomatism along healed fractures that has resulted in compositions with atypically high feo mno ratios metasomatized pyroxenes reach compositions as ferroan as wo3 2en37 0fs59 8 pyroxenes in the kapoeta clast have cores that are more magnesian wo1 8en80 2fs18 0 and are less strongly zoned to wo3 6en51 0fs45 4 atypically high feo mno in the most ferroan pyroxenes in this clast coupled with late ferroan olivine up to fa64 reflect late stage addition of feo to the melt rather than igneous fractionation the eet 92014 clast is enriched in the refractory incompatible lithophile elements and shows a refractory lithophile element pattern similar to stannem and bouvante the clast is enriched in cr2o3 and mgo and depleted in al2o3 and cao compared to main group or stannern trend eucrites based on bulk major and trace lithophile element composition this pyroxene phyric clast cannot be directly related to basaltic eucrites this clast is either an impact melt of a trace element rich polymict eucrite or howardite target or a primary melt of a more magnesian source region than that which produced basaltic eucrites we favor the former interpretation low siderophile element abundances within the ranges for monomict eucrites indicate 0 5 chondritic contamination in the clast however the kapoeta clast has a major element composition within the range of howardites and likely is an impact melt of such a target lithology comparison with other reported magnesian basalt clasts from howardites indicates that in all likelihood all heretofore alleged primary magnesian basalt clasts are impact melts of polymict targets two magnesian eucrites alha81001 and pomozdino and magnesian eucrite clasts from the polymict eucrite petersburg may possibly be primary melts of a magnesian source region not directly related to basaltic eucrites

k th ti systematics and new three component mixing model of hed meteorites prospective study for interpretation of gamma ray and neutron spectra for the dawn mission abstract the dawn spacecraft carries a gamma ray and neutron detector grand which will measure and map the abundances of selected elements on the surface of asteroid 4 vesta we compare the variability of moderately volatile refractory incompatible element ratios k th and k ti in howardite eucrite and diogenite hed meteorites with those in other achondrite suites that represent asteroidal crusts because these ratios may be accurately measured by grand and likely reflect initial chemical compositions of the hed parent body the k th and k ti variations can differentiate hed meteorites from angrites and some unique eucrite like lithologies the results suggest that k th and ti abundances determined from grand data could not only confirm that vesta is the parent body of hed meteorites but might also allow recognition of as yet unsampled compositional terranes on vesta besides the k th ti systematics study we propose a new three component mixing model for interpretation of grand spectra required because the spatial resolution of grand is coarser than the spectral compositional heterogeneity of vesta s surface the mixing model uses abundances of k ti fe and mg that will be analyzed more accurately than other prospective grand analyzed elements we examine propagated errors due to grand analytical uncertainties and intrinsic errors that stem from an assumption introduced into the mixing model the error investigation suggests that the mixing model can adequately estimate not only the diogenite eucrite mixing ratio but also the abundances of most major and minor elements within the grand propagated errors