Experimental constraints on ureilite petrogenesis

This experimental study explores the petrogenesis of ureilites by a partial melting/smelting process. Experiments have been performed over temperature (1150-1280 °C), pressure (5-12.5 MPa), and low oxygen fugacity (graphite-CO gas) conditions appropriate for a hypothetical ureilite parent body ∼200 km in size. Experimental and modeling results indicate that a partial melting/smelting model of ureilite petrogenesis can explain many of the unique characteristics displayed by this meteorite group. Compositional information preserved in the pigeonite-olivine ureilites was used to estimate the composition of melts in equilibrium with the ureilites. The results of 20 experiments saturated with olivine, pyroxene, metal, and liquid with appropriate ureilite compositions are used to calibrate the phase coefficients and pressure-temperature dependence of the smelting reaction. The calibrated coefficients are used to model the behavior of a hypothetical residue that is experiencing fractional smelting. The residue is initially olivine-rich and smelting progressively depletes the olivine content and enriches the pyroxene and metal contents of the residues. The modeled residue composition at 1260 °C best reproduces the trend of ureilite bulk compositions. The model results also indicate that as a ureilite residue undergoes isothermal decompression smelting over a range of temperatures, Ca/Al values and Cr₂O₃ contents are enriched at lower temperatures (below ∼1240 °C) and tend to decrease at higher temperatures. Therefore, fractional smelting can account for the high Ca/Al and Cr₂O₃ wt% values observed in ureilites. We propose that ureilites were generated from an olivine-rich, cpx-bearing residue. Smelting began when the residue was partially melted and contained liquid, olivine, and carbon. These residues experienced varying degrees of fractional smelting to produce the compositional variability observed within the pigeonite-bearing ureilites. Variations in mineral composition, modal proportions, and isotopic signatures are best described by heterogeneous accretion of the ureilite parent body followed by minimal and variable degrees of igneous processing.     

Impact features of enstatite-rich meteorites

Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any ²⁶Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.     

The Almahata Sitta polymict breccia and the late accretion of asteroid 2008 TC3

On October 7, 2008, a small asteroid named 2008 TC₃ was detected in space about 19 h prior to its impact on Earth. Numerous world-wide observations of the object while still in space allowed a very precise determination of its impact area: the Nubian Desert of northern Sudan, Africa. The asteroid had a pre-atmospheric diameter of ∼4 m; its weight is reported with values between ∼8 and 83 t, and the bulk density with ∼2–3 g/cm³, translating into a bulk porosity in the range of ∼20–50%. Several dedicated field campaigns in the predicted strewn field resulted in the recovery of more than 700 (monolithological) meteorite fragments with a total weight of ∼10.5 kg. These meteorites were collectively named “Almahata Sitta”, after the nearby train station 6, and initially classified as an anomalous polymict ureilite. Further work, however, showed that Almahata Sitta is not only a ureilite but a complex polymict breccia containing chemically and texturally highly variable meteorite fragments, including different ureilites, a ureilite-related andesite, metal-sulfide assemblages related to ureilites, and various chondrite classes (enstatite, ordinary, carbonaceous, Rumuruti-like). It was shown that that chondrites and ureilites derive from one parent body, i.e., asteroid 2008 TC₃, making this object, in combination with the remotely sensed physical parameters, a loosely aggregated, rubble-pile-like object. Detailed examinations have been conducted and mineral-chemical data for 110 samples have been collected, but more work on the remaining samples is mandatory.     

Evidence from polymict ureilite meteorites for a disrupted and re-accreted single ureilite parent asteroid gardened by several distinct impactors

Ureilites are ultramafic achondrites that exhibit heterogeneity in mg# and oxygen isotope ratios between different meteorites. Polymict ureilites represent near-surface material of the ureilite parent asteroid(s). Electron microprobe analyses of >500 olivine and pyroxene clasts in several polymict ureilites reveal a statistically identical range of compositions to that shown by unbrecciated ureilites, suggesting derivation from a single parent asteroid. Many ureilitic clasts have identical compositions to the anomalously high Mn/Mg olivines and pyroxenes from the Hughes 009 unbrecciated ureilite (here termed the “Hughes cluster”). Some polymict samples also contain lithic clasts derived from oxidized impactors. The presence of several common distinctive lithologies within polymict ureilites is additional evidence that ureilites were derived from a single parent asteroid.In situ oxygen three isotope analyses were made on individual ureilite minerals and lithic clasts, using a secondary ion mass spectrometer (SIMS) with precision typically better than 0.2-0.4‰ (2SD) for δ¹⁸O and δ¹⁷O. Oxygen isotope ratios of ureilitic clasts fall on a narrow trend along the CCAM line, covering the range for unbrecciated ureilites, and show a good anti-correlation with mineral mg#. SIMS analysis identifies one ferroan lithic clast as an R-chondrite, while a second ferroan clast is unlike any known meteorite. An exotic enstatite grain is derived from an enstatite chondrite or aubrite, and another pyroxene grain with Δ¹⁷O of −0.4 ± 0.2‰ is unrelated to any known meteorite type.Ureilitic olivine clasts with mg#s < 85 are much more common than those with mg# > 85 which include the melt-inclusion-bearing “Hughes cluster” ureilites. Thus melt was present in regions of the parent ureilite asteroid with a bulk mg# > 85 when the asteroid was disrupted by impact, giving rise to two types of ureilites: common ferroan ones that were residual after melting and less common magnesian ones that were still partially molten when disruption occurred. One or more daughter asteroids re-accreted from the remnants of the mantle of the proto-ureilite asteroid. Polymict ureilite meteorites represent regolith that subsequently formed on the surface of a daughter asteroid, including impact-derived material from at least six different meteoritic sources.     

Fractional melting and smelting on the ureilite parent body

We investigate petrologic and physical aspects of melt extraction on the parent asteroid of the ureilite meteorites (UPB). We first develop a petrologic model for simultaneous melting and smelting (reduction of FeO by C) at various depths. For a model starting composition, determined from petrologic constraints to have been CV-like except for elevated Ca/Al (2.5 × CI), we determine (1) degree of melting, (2) the evolution of mg, (3) production of CO + CO₂ gas and (4) the evolution of mineralogy in the residue as a function of temperature and pressure. We then use these relationships to examine implications of fractional vs. batch melt extraction.In the shallowest source regions (∼30 bars), melting and smelting begin simultaneously at ∼1050 °C, so that mg and the abundance of low-Ca pyroxene (initially pigeonite, ultimately pigeonite + orthopyroxene) begin to increase immediately. However, in the deepest source regions (∼100 bars), smelting does not begin until ∼1200 °C, so that mg begins to increase and low-Ca pyroxene (pigeonite) appears only after ∼21% melting. The final residues in these two cases, obtained just after the demise of augite, match the end-members of the ureilite mg range (∼94-76) in pyroxene abundance and type. In all source regions, production of CO + CO₂ by smelting varies over the course of melting. The onset of smelting results in a burst of gas production and very high incremental gas/melt ratios (up to ∼2.5 by mass); after a few % (s)melting, however, these values drastically decline (to <0.05 in the final increments).Physical modelling based on these relationships indicates that melts would begin to migrate upwards after only ∼1-2% melting, and thereafter would migrate continuously (fractionally) and rapidly (reaching the surface in < a year) in a network of veins/dikes. All melts produced during the smelting stage in each source region have gas contents sufficient to cause them to erupt explosively and be lost. However, since in all but the shallowest source regions part of the melting sequence occurs without smelting, fractional melting implies that a significant fraction of UPB melts may have erupted more placidly to form a thin crust (∼3.3 km thick for a 100 km radius body).Our calculations suggest that melt extraction was so rapid that equilibrium trace element partitioning may not have been attained. We present a model for disequilibrium fractional melting (in which REE partitioning is limited by diffusion) on the UPB, and demonstrate that it produces a good match to the ureilite data. The disequilibrium model may also apply to trace siderophile elements, and might help explain the “overabundance” of these elements in ureilites relative to predictions from the smelting model.Our results suggest that melt extraction on the UPB was a rapid, fractional process, which can explain the preservation of a primitive oxygen isotopic signature on the UPB.     

Siderophile geochemistry of ureilites: A record of early stages of planetesimal core formation

New bulk-compositional data, including trace siderophile elements such as Ir, Os, Au, and Ni, are presented for 25 ureilites. Without exception, ureilites have siderophile abundances too high to plausibly have formed as cumulates. Ureilites undoubtedly underwent a variety of “smelting,” by which C was oxidized to CO gas while olivine FeO was reduced to Fe-metal. However, pressure-buffered equilibrium smelting is not a plausible model for engendering the wide range (75-96 mol%) of mafic-silicate core mg among ureilites. The smelting reaction produces too much CO gas. Even supposing a disequilibrium process with the smelt-gas leaking out of the mantle, none of the ureilites, least of all the ureilite with the most “reduced” (highest) olivine-core mg (ALH84136), has the high Fe-metal abundance predicted by the smelted-cores model. In principle, the Fe-metal generated by smelting could have been subsequently lost, but siderophile data show that ureilites never underwent efficient depletion of Fe-metal. Ureilites display strong correlations among siderophile ratios such as Au/Ir, Ni/Ir, Co/Ir, As/Ir, Se/Ir, and Sb/Ir. Ureilite siderophile depletion patterns loosely resemble siderophile fractionations, presumably nebular in origin, among carbonaceous chondrites. However, Zn, for an element of moderate volatility, is anomalously high in ureilites. A tight correlation between Au and Ni extrapolates to the low-Ni/Au side of the compositional range of carbonaceous chondrites. From this mismatch, mild but nonetheless significant depletions of refractory siderophile elements such as Ir and Os, and moderate depletions of strongly siderophile, weakly chalcophile elements such as Ni and Au, we infer that the ureilite siderophile fractionations are largely the result of a non-nebular process, i.e., removal of S-rich metallic melt, possibly with minor entrainment of Fe-metal. Several lines of trace-element evidence indicate that melt porosity during ureilite anatexis was at least moderate. The ureilite pattern of very mild depletions of extremely siderophile elements, but much deeper depletions of moderately siderophile, chalcophile elements, suggests that asteroidal core formation probably occurs in two discrete stages. In general, separation of a considerable proportion (several wt%) of S-rich metallic melt probably occurs long before, and at a far lower temperature than, separation of the remaining S-poor Fe-metal. Apart from the Fe-metal itself, only extremely siderophile elements wait until the second stage to sequester mainly into the core.     

Petrogenesis of olivine-phyric shergottite Larkman Nunatak 06319: Implications for enriched components in martian basalts

We report on the petrography and geochemistry of the newly discovered olivine-phyric shergottite Larkman Nunatak (LAR) 06319. The meteorite is porphyritic, consisting of megacrysts of olivine (?2.5 mm in length, Fo_77-52) and prismatic zoned pyroxene crystals with Wo₃En₇₁ in the cores to Wo_8-30En_23-45 at the rims. The groundmass is composed of finer grained olivine (<0.25 mm, Fo_62-46), Fe-rich augite and pigeonite, maskelynite and minor quantities of chromite, ulvöspinel, magnetite, ilmenite, phosphates, sulfides and glass. Oxygen fugacity estimates, derived from the olivine-pyroxene-spinel geo-barometer, indicate that LAR 06319 formed under more oxidizing conditions (QFM -1.7) than for depleted shergottites. The whole-rock composition of LAR 06319 is also enriched in incompatible trace elements relative to depleted shergottites, with a trace-element pattern that is nearly identical to that of olivine-phyric shergottite NWA 1068. The oxygen isotope composition of LAR 06319 (Δ¹⁷O = 0.29 ±0.03) confirms its martian origin.Olivine megacrysts in LAR 06319 are phenocrystic, with the most Mg-rich megacryst olivine being close to equilibrium with the bulk rock. A notable feature of LAR 06319 is that its olivine megacryst grains contain abundant melt inclusions hosted within the forsterite cores. These early-trapped melt inclusions have similar trace element abundances and patterns to that of the whole-rock, providing powerful evidence for closed-system magmatic behavior for LAR 06319. Calculation of the parental melt trace element composition indicates a whole-rock composition for LAR 06319 that was controlled by pigeonite and augite during the earliest stages of crystallization and by apatite in the latest stages. Crystal size distribution and spatial distribution pattern analyses of olivine indicate at least two different crystal populations. This is most simply interpreted as crystallization of megacryst olivine in magma conduits, followed by eruption and subsequent crystallization of groundmass olivine.LAR 06319 shows close affinity in mineral and whole-rock chemistry to olivine-phyric shergottite, NWA 1068 and the basaltic shergottite NWA 4468. The remarkable features of these meteorites are that they have relatively similar quantities of mafic minerals compared with olivine-phyric shergottites (e.g., Y-980459, Dho 019), but flat and elevated rare earth element patterns more consistent with the LREE-enriched basaltic shergottites (e.g., Shergotty, Los Angeles). This relationship can be interpreted as arising from partial melting of an enriched mantle source and subsequent crystal-liquid fractionation to form the enriched olivine-phyric and basaltic shergottites, or by assimilation of incompatible-element enriched martian crust. The similarity in the composition of early-trapped melt inclusions and the whole-rock for LAR 06319 indicates that any crustal assimilation must have occurred prior to crystallization of megacryst olivine, restricting such processes to the deeper portions of the crust. Thus, we favor LAR06319 forming from partial melting of an “enriched” and oxidized mantle reservoir, with fractional crystallization of the parent melt upon leaving the mantle.     

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo_76-55 from core to rim), pyroxene (from core to rim En₇₀Fs₂₅Wo₅, En₅₀Fs₂₅Wo₂₅, and En₄₅Fs₄₅Wo₁₀), and Cr-rich spinel in a matrix of maskelynite (An₅₂Ab₄₅), pyroxene (En_30-40Fs_40-55Wo_10-25,), olivine (Fo₅₀), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo_>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.     

Martian regolith in Elephant Moraine 79001 shock melts? Evidence from major element composition and sulfur speciation

Shock veins and melt pockets in Lithology A of Martian meteorite Elephant Moraine (EETA) 79001 have been investigated using electron microprobe (EM) analysis, petrography and X-ray Absorption Near Edge Structure (XANES) spectroscopy to determine elemental abundances and sulfur speciation (S²⁻ versus S⁶⁺). The results constrain the materials that melted to form the shock glasses and identify the source of their high sulfur abundances. The XANES spectra for EETA79001 glasses show a sharp peak at 2.471 keV characteristic of crystalline sulfides and a broad peak centered at 2.477 keV similar to that obtained for sulfide-saturated glass standards analyzed in this study. Sulfate peaks at 2.482 keV were not observed. Bulk compositions of EETA79001 shock melts were estimated by averaging defocused EM analyses. Vein and melt pocket glasses are enriched in Al, Ca, Na and S, and depleted in Fe, Mg and Cr compared to the whole rock. Petrographic observations show preferential melting and mobilization of plagioclase and pyrrhotite associated with melt pocket and vein margins, contributing to the enrichments. Estimates of shock melt bulk compositions obtained from glass analyses are biased towards Fe- and Mg- depletions because, in general, basaltic melts produced from groundmass minerals (plagioclase and clinopyroxene) will quench to a glass, whereas ultramafic melts produced from olivine and low-Ca pyroxene megacrysts crystallize during the quench. We also note that the bulk composition of the shock melt pocket cannot be determined from the average composition of the glass but must also include the crystals that grew from the melt - pyroxene (En_72-75Fs_20-21Wo_5-7) and olivine (Fo_75-80). Reconstruction of glass + crystal analyses gives a bulk composition for the melt pocket that approaches that of lithology A of the meteorite, reflecting bulk melting of everything except xenolith chromite.Our results show that EETA79001 shock veins and melt pockets represent local mineral melts formed by shock impedance contrasts, which can account for the observed compositional anomalies compared to the whole rock sample. The observation that melts produced during shock commonly deviate from the bulk composition of the host rock has been well documented from chondrites, rocks from terrestrial impact structures and other Martian meteorites. The bulk composition of shock melts reflects the proportions of minerals melted; large melt pockets encompass more minerals and approach the whole rock whereas small melt pockets and thin veins reflect local mineralogy. In the latter, the modal abundance of sulfide globules may reach up to 15 vol%. We conclude the shock melt pockets in EETA79001 lithology A contain no significant proportion of Martian regolith.     

Ureilitic breccias: clues to the petrologic structure and impact disruption of the ureilite parent asteroid

The majority of the 143 ureilite meteorites are monomict (unbrecciated) ultramafic rocks, which represent the mantle (olivine+low-Ca pyroxene residues and less abundant cumulates) of a partially melted (25–30%), carbon-rich asteroid 125 km in radius. Accumulated petrologic and geochemical studies of these meteorites have led to a picture of a ureilite parent body (UPB) that was stratified in mg#, pyroxene abundance and pyroxene type, due to the pressure dependence of carbon redox control, and which preserved a pre-magmatic heterogeneity in Δ¹⁷O. The absence, however, of ureilitic crustal rocks (i.e. basalts) in the meteorite record, leads to significant gaps in our knowledge of the geologic history of the UPB.

Ureilitic breccias provide considerable information that cannot be obtained from the monomict samples, and help to fill in those gaps. Fourteen ureilites are polymict breccias (at least three of which contain solar wind gases) that formed in a regolith. They contain a variety of clast types representing indigenous ureilitic lithologies not known among the monomict samples, as well as several types of non-indigenous impactor materials. In addition, one ureilite (FRO 93008) is a dimict breccia, consisting of two ultramafic lithologies that could not have formed in close proximity on the UPB.

Several feldspathic lithologies representing melts complementary to the monomict ureilite residues or cumulates have been recognized in polymict ureilites. From these lithologies we infer that melt extraction on the UPB was a rapid, fractional process in which trace element and oxygen isotopic equilibrium was not achieved. The majority of melts that reached the surface erupted explosively (due to high contents of CO/CO₂) and were lost into space. Thus, it is likely that the UPB never had an extensive basaltic crust. Melts generated at the shallowest depths and late fractionates, in which carbon had largely been consumed by reduction, were the most likely to have been preserved. Our sample of the UPB is limited to depths equivalent to 100 bars pressure or less, but minor augite-bearing feldspathic lithologies and related cumulates may represent melts derived from deeper.

In addition, we infer that the UPB was catastrophically disrupted, while still hot, by an impacting projectile. Meter-sized ejecta from this impact reaccreted into one or more daughter bodies, on which the brecciated ureilites formed. Ureilite meteorites are derived from these offspring, rather than from the UPB. The remnant of the original UPB may consist largely of olivine plus augite, and thus not resemble the majority of ureilites.     

Petrogenesis of augite-bearing ureilites Hughes 009 and FRO 90054/93008 inferred from melt inclusions in olivine, augite and orthopyroxene

Melt inclusions in ureilites occur only in the small augite- and orthopyroxene-bearing subgroups. Previously [Goodrich C.A., Fioretti A.M., Tribaudino M. and Molin G. (2001) Primary trapped melt inclusions in olivine in the olivine-augite-orthopyroxene ureilite Hughes 009. Geochim. Cosmochim. Acta65, 621-652] we described melt inclusions in olivine in the olivine-augite-orthopyroxene ureilite Hughes 009 (Hughes). FRO 90054/93008 (FRO) is a near-twin of Hughes, and has abundant melt inclusions in all three primary silicates. We use these inclusions to reconstruct the major, minor and rare earth element composition of the Hughes/FRO parent magma and evaluate models for the petrogenesis of augite-bearing ureilites.Hughes and FRO consist of 23-47 vol % olivine (Fo 87.3 and 87.6, respectively), 7-52 vol % augite (mg 89.2, Wo 37.0 and mg 88.8, Wo 38.0, respectively), and 12-56 vol % orthopyroxene (mg 88.3, Wo 4.9 and mg 88.0, Wo 4.8, respectively). They have coarse-grained (?3 mm), highly-equilibrated textures, with poikilitic relationships indicating the crystallization sequence olivine → augite → orthopyroxene. FRO is more shocked than Hughes, experienced greater secondary reduction, and is more weathered. The two meteorites are probably derived from the same lithologic unit.Melt inclusions in olivine consist of glass ± daughter cpx ± metal-sulfide-phosphide spherules ± chromite, and have completely reequilibrated Fe/Mg with their hosts. We follow the method of Goodrich et al. (2001) for reconstructing the composition of the primary trapped liquid they represent (olPTL), but correct an error in our treatment of the effects of reequilibration. Inclusions in augite consist of glass, which shows only partial reequilibration of Fe/Mg. The composition of the primary trapped liquid they represent (augPTL) is reconstructed by reverse fractional crystallization of wall augite from the most ferroan glass. Inclusions in orthopyroxene consist of glass + 30-50 vol % daughter cpx. The cpx shows complete, but the glass only partial, reequilibration of Fe/Mg. A range of possible compositions for the primary trapped liquid they represent (opxPTL) is calculated by modal recombination of glass and cpx, followed by addition of wall orthopyroxene and adjustment of Fe/Mg for equilibrium with the primary orthopyroxene. Only a small subset of these compositions is plausible on the basis of being orthopyroxene-saturated.Results indicate that olPTL, assumed to represent the parent magma of these rocks, was saturated only with olivine and in equilibrium with Fo ∼ 83. AugPTL and opxPTL are very similar in composition; both are close to augite + orthopyroxene co-saturation and in equilibrium with Fo 87/8. We suggest that olPTL was reduced to Fo 87/8 due to smelting during ascent, and show that this produces a composition very similar to that of augPTL and opxPTL.REE data for each of the three primary silicates and the least evolved melt inclusions in olivine are used to calculate REE abundances in the Hughes/FRO parent magma. All four methods yield very similar results, indicating a REE pattern that is strongly LREE-depleted (Sm/La = 3.3-3.7), with a small negative Eu anomaly (Eu/Eu* = 0.82) and slight HREE-depletion (Gd/Lu = 1.4-1.6).The Hughes/FRO parent magma provides a robust constraint on models for the petrogenesis of augite-bearing ureilites. Its major, minor and rare earth element composition suggests derivation through mixing and/or assimilation processes, rather than as a primary melt on the ureilite parent body.     

Feldspathic clast populations in polymict ureilites: Stalking the missing basalts from the ureilite parent body

Polymict ureilites DaG 164/165, DaG 319, DaG 665, and EET 83309 are regolith breccias composed mainly of monomict ureilite-like material, but containing ∼2 vol% of feldspathic components. We characterized 171 feldspathic clasts in these meteorites in terms of texture, mineralogy, and mineral compositions. Based on this characterization we identified three populations of clasts, each of which appears to represent a common igneous (generally basaltic) lithology and whose mafic minerals show a normal igneous fractionation trend of near-constant Fe/Mn ratio over a range of Fe/Mg ratios that extend to much higher values than those in monomict ureilites. The melts represented by these populations are unlikely to be impact melts, because the ubiquitous presence of carbon in polymict ureilites (the regolith of the ureilite parent body) implies that impact melts would have crystallized under conditions of carbon redox control and therefore have highly magnesian mafic mineral compositions with constant Mn/Mg ratio. Therefore, these melts appear to be indigenous products of igneous differentiation on the ureilite parent body (UPB), complementary to the olivine-pigeonite residues represented by the majority of monomict ureilites.The most abundant population is characterized by albitic plagioclase in association with pyroxenes, phosphates, ilmenite, silica, and incompatible-element enriched glass. Model calculations suggest that it formed by extensive fractional crystallization of the earliest melt(s) of precursor materials from which the most magnesian (shallowest) olivine-pigeonite ureilites formed. A less abundant population, characterized by labradoritic plagioclase, may have formed from melts complementary to more ferroan olivine-pigeonite ureilites, and derived from deeper in the UPB. The third population, characterized by the presence of olivine and augite, could only have formed from melts produced at greater depths in the UPB than the olivine-pigeonite ureilites. Many other feldspathic clasts cannot be positively associated with any of these three populations, because their mafic mineral compositions exhibit carbon redox control. However, they may be products of early crystallization of basaltic melts produced on the UPB, before carbon was exhausted by reduction.Partial melting on the ureilite parent body was a fractional (or incremental) process. Melts were produced early in UPB history, and most likely extracted rapidly, thus preserving primitive chemical and oxygen isotopic signatures in the residues.     

Comparative petrology, geochemistry, and petrogenesis of evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica

New data is presented for five evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica, LAP 02205, LAP 02224, LAP 02226, LAP 02436, and LAP 03632. These basalts have nearly identical mineralogies, textures, and geochemical compositions, and are therefore considered to be paired. The LaPaz basalts contain olivine (Fo_64-2) and pyroxene (Fs₃₂Wo₈En₆₀ to Fs_84-86Wo₁₅En_2-0) crystals that record extreme chemical fractionation to Fe-enrichment at the rims, and evidence for silicate liquid immiscibility and incompatible element enrichment in the mesostasis. The basalts also contain FeNi metals with unusually high Co and Ni contents, similar to some Apollo 12 basalts, and a single-phase network of melt veins and fusion crusts. The fusion crust has similar chemical characteristics to the whole rock for the LaPaz basalts, whereas the melt veins represent localized melting of the basalt and have an endogenous origin. The crystallization conditions and evolved nature of the LaPaz basalts are consistent with fractionation of olivine and chromite from a parental liquid similar in composition to some olivine-phyric Apollo 12 and Apollo 15 basalts or lunar low-Ti pyroclastic glasses. However, the young reported ages for the LaPaz mare basalts (∼2.9 Ga) and their relative incompatible element enrichment compared to Apollo mare basalts and pyroclastic glasses indicate they cannot be directly related. Instead, the LaPaz mare basalts may represent fractionated melts from a magmatic system fed by similar degrees of partial melting of a mantle source similar to that of the low-Ti Apollo mare basalts or pyroclastic glasses, but which possessed greater incompatible element enrichment. Despite textural differences, the LaPaz basalts and mare basalt meteorite NWA 032 have similar ages and compositions and may originate from the same magmatic system on the Moon.     

FeMgMn relations of ureilite olivines and pyroxenes and the genesis of ureilites

The Fe/Mn-Fe/Mg relations of published microprobe analyses of ureilite olivine cores suggest that FeO reduction in the magmatic stage was important in the genesis of some of these meteorites. Consideration of Mg/Mn and Fe/Mn partitioning between olivine and pigeonite cores shows that these two phases are not in equilibrium. The data are consistent with combined fractional crystallization and FeO reduction being the major processes of ureilite genesis. Few, if any, of the analyzed ureilites are consistent with a partial melting residue model.     

The Nilpena ureilite,an unusual polymict breccia: implications for origin

Nilpena (173 g), a new ureilite find from the Parachilna area of South Australia, is an unusual polymict breccia containing polymineralic aggregates, mineral fragments and achondritic and chondritic lithic enclaves in a dark, C-rich matrix. The polymineralic aggregates consist of equigranular-textured olivine Fa₂₀ and pigeonite En₇₅Wo₉FS₁₆, and exhibit evidence of shock in the form of undulose extinction and kink-banding. Monomineralic fragments consist of olivine Fa_19–24 (with highly forsteritic rims up to Fa₃) and pigeonite, and appear to be derived by brecciation of the polymineralic aggregates. The enclave material consists of lithic granular olivine fragments, porphyritic enstatite fragments (either enstatite chondrite or aubrite), olivine-clinobronzite fragments resembling an H3 chondrite, and eucritelike lithic fragments composed of plagioclase An₉₈, salitic clinopyroxene Wo_48.5En_31.4Fs_20.1 and olivine Fa_49–53. The matrix contains kamacite (generally rich in P), schreibersite and troilite. The texture of Nilpena suggests formation by disruption of a olivine-pigeonite granular aggregate while the presence of the diverse chondritic and achondritic enclave material suggests an origin as a surface or near-surface breccia.Like other ureilites Nilpena is strongly differentiated with respect to cosmic abundances but is significantly enriched in Ba and LREE. A lack of correlation of lithophile elements with $\text{Fe}\text{(Fe + Mg)}$ ratio among ureilites suggests that the differentiation was not caused by varying degrees of partial melting of a homogeneous source. A cumulate origin therefore seems more plausible.     

The origin of shoshonites: new insights from the Tertiary high-potassium intrusions of eastern Tibet

The shoshonitic intrusions of eastern Tibet, which range in age from 33 to 41 Ma and in composition from ultramafic (SiO₂ = 42 %) to felsic (SiO₂ = 74 %), were produced during the collision of India with Eurasia. The mafic and ultramafic members of the suite are characterized by phenocrysts of phlogopite, olivine and clinopyroxene, low SiO₂, high MgO and Mg/Fe ratios, and olivine forsterite contents of Fo₈₇ to Fo₉₃, indicative of equilibrium with mantle olivine and orthopyroxene. Direct melting of the mantle, on the other hand, could not have produced the felsic members. They have a phenocryst assemblage of plagioclase, amphibole and quartz, high SiO₂ and low MgO, with Mg/Fe ratios well below the values expected for a melt in equilibrium with the mantle. Furthermore, the lack of decrease in Cr with increasing SiO₂ and decreasing MgO from ultramafic to felsic rocks precludes the possibility that the felsic members were derived by fractional crystallization from the mafic members. Similarly, magma mixing, crustal contamination and crystal accumulation can be excluded as important processes. Yet all members of the suite share similar incompatible element and radiogenic isotope ratios, which suggests a common origin and source. We propose that melting for all members of the shoshonite suite was initiated in continental crust that was thrust into the upper mantle at various points along the transpressional Red River-Ailao Shan-Batang-Lijiang fault system. The melt formed by high-degree, fluid-absent melting reactions at high-T and high-P and at the expense of biotite and phengite. The melts acquired their high concentrations of incompatible elements as a consequence of the complete dissolution of pre-existing accessory minerals. The melts produced were quartz-saturated and reacted with the overlying mantle to produce garnet and pyroxene during their ascent. The felsic magmas reacted little with the adjacent mantle and preserved the essential features of their original chemistry, including their high SiO₂, low Ni, Cr and MgO contents, and low Mg/Fe ratio, whereas the mafic and ultramafic magmas are the result of extensive reaction with the mantle. Although the mafic magmas preserved the incompatible element and radiogenic isotope ratios of their crustal source, buffering by olivine and orthopyroxene extensively modified their MgO, Ni, Cr, SiO₂ contents and Mg/Fe ratio to values dictated by equilibrium with the mantle.     

Stable isotopes and the noncarbonaceous derivation of ureilites, in common with nearly all differentiated planetary materials

The abundant, diverse ureilite meteorites are peridotitic asteroidal mantle restites that have remarkably high bulk carbon contents (average 3 wt%) and have long been linked to the so-called carbonaceous chondrites (although this term is potentially misleading, because the high petrologic type “carbonaceous” chondrites are, if anything, C-poor compared to ordinary chondrites). Ureilite oxygen isotopic compositions, i.e., diversely negative (CCAM-like) Δ¹⁷O, viewed in isolation, have long been viewed as confirming the carbonaceous-chondritic derivation hypothesis. However, a very different picture emerges through analysis of a compilation of recently published high-precision isotopic data for chromium, titanium and nickel for ureilites and various other planetary materials. Ureilites have lower ε⁶²Ni and far lower ε⁵⁰Ti and ε⁵⁴Cr than any known variety of carbonaceous chondrite. On a plot of ε⁵⁰Ti vs. ε⁵⁴Cr, and similarly Δ¹⁷O vs. ε⁵⁴Cr, ureilite compositions cluster far from and in a direction approximately orthogonal to a trend internal to the carbonaceous chondrites, and the carbonaceous chondrites are separated by a wide margin from all other planetary materials. I conclude that notwithstanding the impressive resemblance to carbonaceous chondrites in terms of diversely negative Δ¹⁷O, the ureilite precursors accreted from preponderantly noncarbonaceous (sensu stricto) materials. Despite total depletion of basaltic matter, the ureilites retain moderate pyroxene/olivine ratios; which is an expected outcome from simple partial melting of moderate-SiO₂/(FeO + MgO) noncarbonaceous chondritic material, but would imply an additional process of major reduction of FeO if the precursor material were carbonaceous-chondritic. The striking bimodality of planetary materials on the ε⁵⁰Ti vs. ε⁵⁴Cr and Δ¹⁷O vs. ε⁵⁴Cr diagrams may be an extreme manifestation of the effects of episodic accretion of early solids in the protoplanetary nebula. However, an alternative, admittedly speculative, explanation is that the bimodality corresponds to a division between materials that originally accreted in the outer solar system (carbonaceous) and materials that accreted in the inner solar system (noncarbonaceous, including the ureilites).     

Disequilibrium features in experimentally shocked mixtures of olivine plus silica glass powders

Seventeen shock-recovery experiments were performed on powder mixtures of one part (by weight) olivine (St. John's forsterite) plus two parts silica glass (pure vitreous silica) in order to characterize the physical and chemical interaction of two chemically incompatible components during shock. Powders of <45 m grain size were shocked by impact of projectiles launched from a 20 mm gun which created pressures ranging from 6.2 to 64.2 GPa (1 GPa= 10kbar).Petrographie features observed in thin section attest to mechanical and thermal metamorphism. Samples shocked to pressures from 6.2 to 39.3 GPa form compacted, mosaic, granular aggregates with fractured and strained grains. Samples shocked to pressures from 42.9 to 64.2 GPa form vesicular, mixed melts containing flow schlieren and relict olivine fragments. Petrographic disequilibrium is manifested in cataclastic textures showing deformational anisotropy and in thermal effects showing non-uniform intergranular melting. This disequilibrium is caused by an irregular pressure distribution resulting from the rapid collapse of pore spaces.The chemical composition of the shock melts are similar in each of six samples shocked to pressures of 42.9 to 64.2 GPa. Melt chemistry is bimodal in each sample. Colorless melts are 99.9% SiO₂ and represent pure silica glass melts; pale to dark green melts range in composition from 47% to 64% SiO₂ and represent a progressive mixture of olivine melt (41% SiO₂) with silica glass melt. Surprisingly, the compositions of the colored glasses are intermediate between the composition of pure olivine and the bulk composition of the original starting material (79% SiO₂) and are similar to enstatitic pyroxene compositions (50% to 57% SiO₂; 33% to 37% MgO). Although bulk compositions of shocked samples are unchanged, the creation of melts with pyroxene compositions instead of bulk sample compositions may indicate that an incipient eutectic-type fusion may have occurred in small olivine-normative domains surrounding individual olivine grains. Chemical disequilibrium is evidenced by the creation of these olivine-normative melts from a quartz-normative starting compositions and by the chemical heterogeneity in the melts.     

Melt inclusions in olivine and pyroxene phenocrysts from lamprophyres of Chhaktalao Area,Madhya Pradesh,India

Melt inclusions in olivine and pyroxene phenocrysts in kersantite and camptonite at Chhaktalao in Madhya Pradesh, India are mainly of the evolved type forming daughter minerals of olivine, pyroxene, plagioclase, spinel, mica, titanomagnetite and sulphides. Heating studies exhibit a temperature range from 1215° to 1245°C for the melt inclusions in olivine in camptonite and 1220–1245°C for olivine in kersantite. The temperature for melt inclusions in pyroxene ranged from 1000° to 1150°C in camptonite and 850–1100°C for pyroxene in kersantite. The bubble inside these melt inclusions is mainly CO₂. The T_h°C of CO₂ into liquid phase occurred between 26° and 31°C in olivine and 25–30°C in pyroxene from kersantite and camptonite. The maximum density estimated is 0.72 g/cm³ and the minimum is 0.45 g/cm³. The depth of entrapment of the melt inclusion is estimated between 10–15 km. The pressure of entrapment of melt inclusion in olvine is 4.6 kbar where as that in pyroxene is 3.7 kbar. The lamprophyres in the Chhaktalao area are considered to be derived from low depth and low pressure region, possibly within spinel lherzolite zone.     

Thin intergranular melt films and melt pockets in spinel peridotite xenoliths from the Rhön area (Germany): early stage of melt generation by grain boundary melting

Optical microscopy and transmission electron microscopy (TEM) on a porphyroclastic high temperature spinel peridotite from the Rhön area reveal fine, irregular glass layers and pockets along mineral interfaces, cracks in olivine, inside olivine crystals and in spongy rims of clinopyroxene. The chemical composition of the glass deviates significantly from the composition of the host basanite. Electron diffraction technique confirms the amorphous nature of the glass, thus classifying it as a former melt. Every grain or phase boundary shows amorphous intergranular glass layers of variable thickness and characteristic chemical composition with distinct chemical inhomogeneities. Olivine grain boundaries, as the most common type of interfaces, exhibit two different types of melt glasses: (1) Type I melt at olivine grain boundaries, which is characterized by low contents of SiO₂ (～37?wt%) and Al₂O₃ (～5?wt%) and elevated contents of MgO (～31?wt%) and FeO (～22?wt%), is supposed to have formed prior to or during the thermal overprint and the dynamic recrystallisation of the xenolith in the mantle. Melt inclusions inside olivine grains with an average composition of type I melt are suggested to be earlier melt droplets at olivine interfaces, overgrown by migrating olivine grain boundaries during recrystallization in the mantle prior to the uplift of the xenolith. (2) Type II melt, the most common type of melt in the xenolith, shows higher contents of SiO₂ (～48?wt%) and Al₂O₃ (～17?wt%) but lower contents of MgO (～20?wt%) and FeO (～11?wt%). The observation of different types of glass within a single xenolith indicates the development of different chemical melt equilibria at interfaces or triple junctions in the xenolith. The absence of geochemical trends in bivariate plots excludes a unifying process for the genesis of these glasses. Melt inclusions in the spongy rims of clinopyroxene are interpreted to be the product of a potassium-rich metasomatism. The formation of most amorphous intergranular melt layers and pockets at the mineral interfaces including type II melt at olivine grain boundaries is suggested to result from decompression melting during the uplift with the basalt magma. We suggest that these glasses were produced by grain boundary melting due to lattice mismatch and impurity segregation. The observed intergranular amorphous layers or melts represent the very beginning of mineral melting by grain boundary melting.