A petrogenetic model for the origin and compositional variation of the martian basaltic meteorites

Abstract— The major element, trace element, and isotopic compositional ranges of the martian basaltic meteorite source regions have been modeled assuming that planetary differentiation resulted from crystallization of a magma ocean. The models are based on low to high pressure phase relationships estimated from experimental runs and estimates of the composition of silicate Mars from the literature. These models attempt to constrain the mechanisms by which the martian meteorites obtained their superchondritic CaO/Al₂O₃ ratios and their source regions obtained their parent/daughter (⁸⁷Rb/⁸⁶Sr, ¹⁴⁷Sm/¹⁴⁴Nd, and ¹⁷⁶Lu/¹⁷⁷Hf) ratios calculated from the initial Sr, Nd, and Hf isotopic compositions of the meteorites. High pressure experiments suggest that majoritic garnet is the liquidus phase for Mars relevant compositions at or above 12 GPa. Early crystallization of this phase from a martian magma ocean yields a liquid characterized by an elevated CaO/Al₂O₃ ratio and a high Mg#. Olivine‐pyroxene‐garnet‐dominated cumulates that crystallize subsequently will also be characterized by superchondritic CaO/Al₂O₃ ratios. Melting of these cumulates yields liquids with major element compositions that are similar to calculated parental melts of the martian meteorites. Furthermore, crystallization models demonstrate that some of these cumulates have parent/daughter ratios that are similar to those calculated for the most incompatible‐element‐depleted source region (i.e., that of the meteorite Queen Alexandra [QUE] 94201). The incompatible‐element abundances of the most depleted (QUE 94201‐like) source region have also been calculated and provide an estimate of the composition of depleted martian mantle. The incompatible‐element pattern of depleted martian mantle calculated here is very similar to the pattern estimated for depleted Earth's mantle. Melting the depleted martian mantle composition reproduces the abundances of many incompatible elements in the parental melt of QUE 94201 (e.g., Ba, Th, K, P, Hf, Zr, and heavy rare earth elements) fairly well but does not reproduce the abundances of Rb, U, Ta and light rare earth elements. The source regions for meteorites such as Shergotty are successfully modeled as mixtures of depleted martian mantle and a late stage liquid trapped in the magma ocean cumulate pile. Melting of this hybrid source yields liquids with major element abundances and incompatible‐element patterns that are very similar to the Shergotty bulk rock.     

Origin of lunar high‐titanium ultramafic glasses: Constraints from phase relations and dissolution kinetics of clinopyroxene‐ilmenite cumulates

Abstract— Phase equilibrium and dissolution kinetics experiments on synthetic late‐stage magma ocean cumulates are used to place constraints on hypotheses for the origin of lunar high‐Ti ultramafic glasses. Models for the production of high‐Ti lunar magmas have called for either (1) assimilation of late‐stage clinopyroxene‐ilmenite cumulates at shallow levels or (2) sinking of clinopyroxene‐ilmenite cumulates to form a hybrid mantle source. To satisfy the constraints of our experiments, we propose an alternative model that involves shallow‐level reaction and mixing of cumulates, followed by sinking of hybrid high‐Ti materials. This model can fulfill compositional requirements imposed by the pristine lunar glass suite that are difficult to satisfy in assimilation models. It also avoids difficulties that arise in overturn models from the low solidus temperatures of clinopyroxene‐ilmenite cumulates. Partially molten clinopyroxene‐ilmenite cumulates become gravitationally unstable with respect to underlying mafic cumulates only when they have cooled to within ～30°C of their solidus (～1125°C at 100 km depth). At these temperatures, the viscosity of mafic cumulates is too high to allow for growth and descent of clinopyroxene‐ilmenite diapirs on the appropriate time scale. Reaction and mixing between late‐stage liquids and mafic cumulates at shallow levels would produce a refractory hybrid material that is negatively buoyant at higher temperatures and could sink more efficiently to the depths inferred for production of high‐Ti ultramafic glasses.     

Basait generation at the Apollo 12 site,Part 2: Source heterogeneity,multiple melts,and crustal contamination

Abstract— The petrogenesis of Apollo 12 mare basalts has been examined with emphasis on trace-element ratios and abundances. Vitrophyric basalts were used as parental compositions for the modelling, and proportions of fractionating phases were determined using the MAGFOX program of Longhi (1991). Crystal fractionation processes within crustal and sub-crustal magma chambers are evaluated as a function of pressure. Knowledge of the fractionating phases allows trace-element variations to be considered as either source related or as a product of post-magma-generation processes. For the ilmenite and olivine basalts, trace-element variations are inherited from the source, but the pigeonite basalt data have been interpreted with open-system evolution processes through crustal assimilation. Three groups of basalts have been examined: (1) Pigeonite basalts — produced by the assimilation of lunar crustal material by a parental melt (up to 3% assimilation and 10% crystal fractionation, with an “r” value of 0.3). (2) Ilmenite basalts — produced by variable degrees of partial melting (4–8%) of a source of olivine, pigeonite, augite, and plagioclase, brought together by overturn of the Lunar Magma Ocean (LMO) cumulate pile. After generation, which did not exhaust any of the minerals in the source, these melts experienced closed-system crystal fractionation/accumulation. (3) Olivine basalts — produced by variable degrees of partial melting (5–10%) of a source of olivine, pigeonite, and augite. After generation, again without exhausting any of the minerals in the source, these melts evolved through crystal accumulation. The evolved liquid counterparts of these cumulates have not been sampled. The source compositions for the ilmenite and olivine basalts were calculated by assuming that the vitrophyric compositions were primary and the magmas were produced by non-modal batch melting. Although the magnitude is unclear, evaluation of these source regions indicates that both be composed of early- and late-stage Lunar Magma Ocean (LMO) cumulates, requiring an overturn of the cumulate pile.     

The origin of eucrites,diogenites, and olivine diogenites: Magma ocean crystallization and shallow magma chamber processes on Vesta

The asteroid 4 Vesta is one of the very few heavenly bodies to have been linked to samples on Earth: the howardite‐eucrite‐diogenite (HED) meteorite suite. This large and diverse suite of meteorites provides a detailed picture of Vesta's igneous and postigneous history. We have used the range of igneous rock types and compositions in the HED suite to test a series of chemical models for solidification processes following peak melting (magma ocean) conditions on Vesta. Fractional crystallization cannot have been a dominant early process in the magma ocean because it leads to excessive Fe‐enrichment in the melt. Models that are dominated by equilibrium crystallization cannot produce orthopyroxene cumulates (diogenites). Our best models invoke 60–70% equilibrium crystallization of a magma ocean followed by continuous extraction of the residual melt into shallow magma chambers. Fractional crystallization in these magma chambers combined with continuous or periodic addition of more melt from the slowly compacting crystal mush (magmatic recharge) can produce all of the igneous HED lithologies (noncumulate and cumulate eucrites, diogenites, dunites, harzburgites, and olivine diogenites). Magmatic recharge can also explain the narrow range in eucrite compositions and the variability of incompatible trace element concentrations in diogenites. We predict an internal structure for Vesta that permits excavation of the HEDs during the formation of the Rheasilvia basin, while remaining consistent with observations from the Dawn mission and most impact models.     

The oxygen fugacity of olivine‐phyric martian basalts and the components within the mantle and crust of Mars

Abstract— The oxygen fugacity of olivine‐phyric martian basalts is estimated using olivine‐pyroxene‐spinel equilibria, supported by detailed petrography. Results are plotted, along with previous oxygen fugacity estimates, against La/Yb, which is used as a proxy for long‐term incompatible‐element depletion or enrichment in martian basalt reservoirs. In general, the correlation between oxygen fugacity and La/Yb observed by Herd et al. (2002a) holds for the olivine‐phyric basalts. The implications of the correlation are re‐evaluated in light of work by Borg et al. (Forthcoming), which indicates that the variations in radiogenic isotopic composition can be modeled by mixing of mantle sources established by 4.5 Ga through crystallization of a magma ocean in lieu of assimilation of crustal material. The results demonstrate that the crust‐like component, interpreted as trapped liquid in a magma ocean cumulate pile, must be oxidized to explain the oxygen fugacity of the martian basalts. Consequently, the pre‐eruptive water contents of the more oxidized basalts are expected to be higher, although water is not called upon as the cause of the oxidation. Unmixing of mantle components provides an important context for the interpretation of oxygen isotopes, demonstrated here, and of samples returned from the martian surface.     

Constraints on the structure of the martian interior determined from the chemical and isotopic systematics of SNC meteorites

Abstract— The crystallization ages of martian (SNC) meteorites give evidence that martian volcanism has continued until recent times‐perhaps until the present. These meteorites also indicate that the mantle source regions of this volcanism are modestly to extremely depleted by terrestrial standards. These 2 observations produce a conundrum. How is it that such depleted source regions have produced basaltic magma for such a long time? This contribution attempts to quantify the radiogenic heat production in 2 distinct martian mantle source regions: those of the shergottites and nakhlites. Compared to the depleted upper mantle of the Earth (MORB), the nakhlite source region is depleted by about a factor of 2, and the shergottite source region is depleted by a factor of 6. According to current geophysical models, the nakhlite source contains the minimum amount of radioactive heat production to sustain whole‐mantle convection and basalt generation over geologic time. A corollary of this conclusion is that the shergottite source contains much too little radioactivity to produce recent (<200 Ma) basalts. A model martian interior with a deep nakhlite mantle that is insulated by a shallow shergottite mantle may allow basalt production from both source regions if the divide between the nakhlite‐shergottite mantles acts as a thermal boundary layer. Similarities between lunar and martian isotopic reservoirs indicate that the Moon and Mars may have experienced similar styles of differentiation.     

A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites

Abstract— Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. During cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.     

What we have learned about Mars from SNC meteorites

Abstract— The SNC meteorites are thought to be igneous martian rocks, based on their young crystallization ages and a close match between the composition of gases implanted in them during shock and the atmosphere of Mars. A related meteorite, ALH84001, may be older and thus may represent ancient martian crust. These petrologically diverse basalts and ultramafic rocks are mostly cumulates, but their parent magmas share geochemical and radiogenic isotopic characteristics that suggest they may have formed by remelting the same mantle source region at different times. Information and inferences about martian geology drawn from these samples include the following: Planetary differentiation occurred early at ～4.5 Ga, probably concurrently with accretion. The martian mantle contains different abundances of moderately volatile and siderophile elements and is more Fe-rich than that of the Earth, which has implications for its mineralogy, density, and origin. The estimated core composition has a S abundance near the threshold value for inner core solidification. The former presence of a core dynamo may be suggested by remanent magnetization in SNC meteorites, although these rocks may have been magnetized during shock. The mineralogy of martian surface units, inferred from reflectance spectra, matches that of basaltic shergottites, but SNC lithologies thought to have crystallized in the subsurface are not presently recognized. The rheological properties of martian magmas are more accurately derived from these meteorites than from observations of martian flow morphology, although the sampled range of magma compositions is limited. Estimates of planetary water abundance and the amount of outgassed water based on these meteorites are contradictory but overlap estimates based on geological observations and atmospheric measurements. Stable isotope measurements indicate that the martian hydrosphere experienced only limited exchange with the lithosphere, but it is in isotopic equilibrium with the atmosphere and has been since 1.3 Ga. The isotopically heavy atmosphere/hydrosphere composition deduced from these rocks reflects a loss process more severe than current atmospheric evolution models, and the occurrence of carbonates in SNC meteorites suggests that they, rather than scapolite or hydrous carbonates, are the major crustal sink for CO₂. Weathering products in SNC meteorites support the idea of limited alteration of the lithosphere by small volumes of saline, CO₂-bearing water. Atmospheric composition and evolution are further constrained by noble gases in these meteorites, although Xe and Kr isotopes suggest different origins for the atmosphere. Planetary ejection of these rocks has promoted an advance in the understanding of impact physics, which has been accomplished by a model involving spallation during large cratering events. Ejection of all the SNC meteorites (except ALH84001) in one or two events may provide a plausible solution to most constraints imposed by chronology, geochemistry, and cosmic ray exposure, although problems remain with this scenario; ALH84001 may represent older martian crust sampled during a separate impact.     

The evolution of a global magma ocean with olivine flotation

We investigate the thermal evolution of a global magma ocean divided by a dunite septum into a shallow and a lower part with help of simple stability estimations and cooling time calculations. It comes out that the septum is not stable against thermal convection so that this mechanism for a separated chemical evolution of the two mantle regions does not work.     

An impact-melt origin for lithology A of martian meteorite Elephant Moraine A79001

Abstract— Mixing models using major and trace elements show that the bulk composition of lithology A (xenocryst-bearing magnesian basalt) of Elephant Moraine A79001 (EETA79001) can be reasonably approximated as a simple mixture of ～44% EETA79001 lithology B (ferroan basalt) and ～56% of Allan Hills A77005 (ALHA7705) light lithology (incompatible element-poor lherzolite). Micro-instrumental neutron activation analysis (INAA) data on xenocryst-free groundmass samples of lithology A show that about 20–25% of the melt phase could be dissolved lherzolite. The bulk and groundmass samples of lithology A have excesses in Au, which indicates either meteoritic contamination or addition by some unknown martian geochemical process. Previous workers have suggested that lithology A was formed by either assimilation of cumulates (like ALHA77005), by a basalt (like lithology B), or by mixing of basaltic and lherzolitic magmas. The former scenario is energetically improbable and unlikely to explain the normal Fe/Mg zonation in lithology A groundmass pyroxenes, whereas the latter scenario is unlikely to satisfy the constraints of the mixing model indicating the ultramafic component is poor in incompatible elements. We suggest rather that EETA79001 lithology A is an impact melt composed dominantly of basalt like lithology B and lherzolitic cumulates like the trace-element-poor fraction of ALHA77005 or Y-793605. This model can satisfy the energetic, petrologic, and geochemical constraints imposed by the samples. If EETA79001 lithology A is an impact melt, this would have considerable consequences for current models of martian petrologic evolution. It would call into question the generally accepted age of magmatism of martian basalts and preclude the use of lithology A groundmass as a primary martian basalt composition in experimental studies. Regardless, the latter is required because lithology A groundmass is a hybrid composition.     

The titanium contents of lunar mare basalts

Abstract— Lunar mare basalt sample data suggest that there is a bimodal distribution of TiO₂ concentrations. Using a refined technique for remote determination of TiO₂, we find that the maria actually vary continuously from low to high values. The reason for the discrepancy is that the nine lunar sample return missions were not situated near intermediate basalt regions. Moreover, maria with 2–4 wt% TiO₂ are most abundant, and abundance decreases with increasing TiO₂. Maria surfaces with TiO₂ >5 wt% constitute only 20% of the maria. Although impact mixing of basalts with differing Ti concentrations may smear out the distribution and decrease the abundance of high‐Ti basalts, the distribution of basalt Ti contents probably reflects both the relative abundances of ilmenite‐free and ilmenite‐bearing mantle sources. This distribution is consistent with models of the formation of mare source regions as cumulates from the lunar magma ocean.     

Samarium-neodymium and rubidium-strontium systematics of nakhlite Governador Valadares

Abstract— The Sm-Nd systematics of whole-rock and mineral separate samples from nakhlite Governador Valadares define a good ¹⁴⁷Sm-¹⁴³Nd mineral isochron age of 1.37 ± 0.02 Ga. This age is in excellent agreement with the ³⁹Ar-⁴⁰Ar and Rb-Sr ages obtained previously for this meteorite. However, the Rb-Sr isotopic data for our sample show that the isotopic system is disturbed. The lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases. The whole-rock and acid-washed mineral data yield a Rb-Sr age of 1.20 ± 0.05 Ga, which probably represents a lower limit to the crystallization age of the rock. The petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill (McSween, 1994). Thus, the Sm-Nd isotopic age probably represents the age of such a magmatic event. The initial ε¹⁴³Nd value determined for the rock at 1.37 Ga is +17 ± 1, indicating that the parent magma of the rock came from a light-rare-earth-element-depleted source of ¹⁴⁷Sm/¹⁴⁴Nd = ～0.237 based on a simple two-stage evolution model. Results of the same model calculation for the initial ⁸⁷Sr/⁸⁶Sr ratio of the rock suggest that its source material was depleted in ⁸⁷Rb/⁸⁶Sr by ～50% relative to the estimated martian value at 1.37 Ga. Both the high Sm/Nd and low Rb/Sr values support a clinopyroxene-rich cumulate source for the genesis of the nakhlite Governador Valadares. Furthermore, our Sm-Nd age and ε¹⁴³Nd data and the previously published ε¹⁴²Nd datum for the rock (Harper et al., 1995) are consistent with early differentiation of the parent planet, formation of cumulate sources ～4.56 Ga ago, and late melting of the sources and formation of the rock ～1.37 Ga ago. The good agreement of isotopic ages and petrographic features among Governador Valadares, Nakhla, and Lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories. The nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to ～1.37 Ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history.     

Water in the Martian interior—The geodynamical perspective

Petrological analysis of the Martian meteorites suggests that rheologically significant amounts of water are present in the Martian mantle. A bulk mantle water content of at least a few tens of ppm is thus expected to be present despite the potentially efficient degassing during accretion, magma ocean solidification, and subsequent volcanism. We examine the dynamical consequences of different thermochemical evolution scenarios testing whether they can lead to the formation and preservation of mantle reservoirs, and compare model predictions with available data. First, the simplest scenario of a homogenous mantle that emerges when ignoring density changes caused by the extraction of partial melt is found to be inconsistent with the isotopic evidence for distinct reservoirs provided by the analysis of the Martian meteorites. In a second scenario, reservoirs can form as a result of partial melting that induces a density change in the depleted mantle with respect to its primordial composition. However, efficient mantle mixing prevents these reservoirs from being preserved until present unless they are located in the stagnant lid. Finally, reservoirs could be formed during fractional crystallization of a magma ocean. In this case, however, the mantle would likely end up being stably stratified as a result of the global overturn expected to accompany the fractional crystallization. Depending on the assumed density contrast, little secondary crust would be produced and the lithosphere would be extremely cool and dry, in contrast to observations. In summary, it is very challenging to obtain a self‐consistent evolution scenario that satisfies all available constraints.     

Fractional crystallization of the lunar magma ocean: Updating the dominant paradigm

We report results of systematic experimental simulation of fractional crystallization of a lunar magma ocean (LMO) with the Lunar Primitive Upper Mantle bulk composition. These results complement prior work that simulated equilibrium crystallization. In contrast to previous numerical models for investigating magma ocean solidification processes and implications, our combined program simulates these processes directly using petrologic experimentation. Our experiments mimic LMO crystallization that is fractional throughout the process, rather than switching from initially equilibrium to fractional crystallization partway through. To do this, we adopted an iterative approach in which the starting material for each run is synthesized using the composition of the melt phase from the prior run. We compare our results to those from long-standing numerical models of LMO crystallization and show that although some features of those models are broadly reproduced, there are key differences in liquid lines of descent and the cumulate lithologies generated. Our results can be used to estimate the possible thickness of a primordial lunar crust formed from flotation of plagioclase during magma ocean solidification. Our estimate is greater than that from the recent Gravity Recovery and Interior Laboratory (GRAIL) mission, but consistent with the criteria on which the starting bulk composition was originally calculated. It assumes perfectly efficient separation of all plagioclase formed from the crystallizing magma ocean, which is likely not the case. We also demonstrate that a non-chondritic bulk composition, with respect to trace elements, is not required in order to generate a KREEP (potassium, rare earth elements, and phosphorus) signature from magma ocean crystallization.     

Magma source transition of lunar mare volcanism at 2.3 Ga

Mare basalts provide insights into the composition and thermal history of the lunar mantle. The ages of mare basalts suggest a first peak of magma activity at 3.2–3.8 Ga and a second peak at ~2 Ga. In this study, we reassess the correlation between the titanium contents and the eruption ages of mare basalt units using the compositional and chronological data updated by SELENE (Kaguya). Using morphological and geological criteria, we calculated the titanium content of 261 mare units across a representative area of each mare unit. In the Procellarum KREEP Terrane, where the latest eruptions are located, an increase in the mean titanium content is observed during the Eratosthenian period, as reported by previous studies. We found that the increase in the mean titanium content occurred within a relatively short period near approximately 2.3 Ga, suggesting that the magma source of the mare basalts changed at this particular age. Moreover, the high‐titanium basaltic eruptions are correlated with a second peak in volcanic activity near ~2 Ga. The high‐titanium basaltic eruptions occurring during the last volcanic activity period can be explained by the three possible scenarios (1) the ilmenite‐bearing cumulate rich layer in the core‐mantle boundary formed after the mantle overturn, (2) the basaltic material layers beneath the lunar crust formed through upwelling magmas, and (3) ilmenite‐bearing cumulate blocks remained in the upper mantle after the mantle overturn.     

Settling of metal droplets in a terrestrial magma ocean: On the correction of the Stokes velocity

The formation of magma oceans on at least the major terrestrial planets is widely assumed even if the full accretion history and early evolution of terrestrial planets is discussed rather controversial. Various processes occur within these magma oceans, among them the settling of small iron drops and their chemical equilibration with the silicate environment. Different models were proposed to explain the differentiation within a magma ocean. In order to model a magma ocean and give constraints about settling time of iron droplets, a constant velocity is used in most models, which is calculated using Stokes’ famous formula. According to the other model parameters this assumption is invalid, since Stokes’ assumption of a creeping flow is violated. However, former models investigated the behaviour of a large number of droplets, which makes it is impossible to solve the flow around each single iron drop. We introduce a model, in which the flow around iron drops in a molten silicate environment is computed using the appropriate hydrodynamic equations. We investigate the terminal velocity of a single drop descending through a magma ocean and additionally study the effects of the presence of other obstacles in the neighbourhood. We determine a new mean velocity, which may serve as an input parameter for the existent models of magma oceans. Although we used a full fluid-dynamical approach, the velocity computed with our model is not very different from the simple Stokes case and thus proves that the former assumptions were legitimate and can be used in future as well as the new velocity presented here.     

A petrographic history of martian meteorite ALH84001: Two shocks and an ancient age

Abstract— ALH84001 is an igneous meteorite, an orthopyroxenite of martian origin. It contains petrographic evidence of two shock metamorphic events, separated by thermal and chemical events. The evidence for two shock events suggests that ALH84001 is ancient and perhaps a sample of the martian highlands. From petrography and mineral chemistry, the history of ALH84001 must include: crystallization from magma, a first shock (impact) metamorphism, thermal metamorphism, low-temperature chemical alteration, and a second shock (impact) metamorphism. Originally, ALH84001 was igneous, an orthopyroxene-chromite cumulate. In the first shock event, the igneous rock was cut by melt-breccia or cataclastic veinlets, now bands of equigranular fine-grained pyroxene and other minerals (crush zones). Intact fragments of the cumulate were fractured and strained (now converted to polygonized zones). The subsequent thermal metamorphism (possibly related to the first shock) annealed the melt-breccia or cataclastic veinlets to their present granoblastic texture and permitted chemical homogenization of all mineral species present. The temperature of metamorphism was at least 875 °C, based on mineral thermometers. Next, Mg-Fe-Ca carbonates and pyrite replaced plagioclase in both clasts and granular bands, producing ellipsoidal carbonate globules with sub-micron scale compositional stratigraphy, repeated identically in all globules. The second shock event produced microfault offsets of carbonate stratigraphy and other mineral contacts, radial fractures around chromite and maskelynite, and strain birefringence in pyroxene. Maskelynite could not have been preserved from the first shock event, because it would have crystallized back to plagioclase. The martian source area for ALH84001 must permit this complex, multiple impact history. Very few craters on young igneous surfaces are on or near earlier impact features. It is more likely that ALH84001 was ejected from an old igneous unit (Hesperian or Noachian age), pocked by numerous impact craters over its long exposure at the martian surface.     

Fluid dynamics of local martian magma oceans

The evolution of a melt region produced by a large impact during Mars formation is addressed. While some impact induced melt is redistributed during crater excavation, sufficiently large impacts (much larger than basin forming impacts) generate an intact melt region which is retained beneath the excavation zone, i.e., a local magma ocean. Local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet, mechanism of crystallization, and melt region size. Within the uncertainties of various parameters, two scenarios are possible. For sufficiently weak rheology or large melt region size, evolution is characterized by rapid extrusion and formation of a global magma ocean. For sufficiently strong rheology or small melt region size, in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment. Subsequent to in situ crystallization, local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling. For large melt regions, lateral spreading occurs via plastic deformation and results in an asymmetric, global, partial melt layer. For small melt region size, viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer. A hypothesis for the origin of the hemispherical crustal dichotomy and Tharsis rise is suggested. The dichotomy is associated with a global partial melt layer produced by evolution of a large, local magma ocean. After dichotomy formation, evolution of a second, smaller, local magma ocean is related to Tharsis development.     

Geochemistry of diogenites: Still more diversity in their parental melts

Abstract— We report on the major and trace element abundances of 18 diogenites, and O‐isotopes for 3 of them. Our analyses extend significantly the diogenite compositional range, both in respect of Mg‐rich (e.g., Meteorite Hills [MET] 00425, MgO = 31.5 wt%) and Mg‐poor varieties (e.g., Dhofar 700, MgO = 23 wt%). The wide ranges of siderophile and chalcophile element abundances are well explained by the presence of inhomogeneously distributed sulfide or metal grains within the analyzed chips. The behavior of incompatible elements in diogenites is more complex, as exemplified by the diversity of their REE patterns. Apart from a few diogenite samples that contain minute amounts of phosphate, and whose incompatible element abundances are unlike the orthopyroxene ones, the range of incompatible element abundances, and particularly the range of Dy/Yb ratios in diogenites is best explained by the diversity of their parental melts. We estimate that the FeO/MgO ratios of the diogenite parental melts range from about 1.4 to 3.5 and therefore largely overlap the values obtained for non‐cumulate eucrites. Our results rule out the often accepted view that all the diogenites formed from parental melts more primitive than eucrites during the crystallization of a magma ocean. Instead, they point to a more complex history, and suggest that diogenites were derived from liquids produced by the remelting of cumulates formed from the magma ocean.     

The parent magmas of the cumulate eucrites: A mass balance approach

Abstract— The cumulate eucrite meteorites are gabbros that are related to the eucrite basalt meteorites. The eucrite basalts are relatively primitive (nearly flat REE patterns with La ～ 8–30 × CI), but the parent magmas of the cumulate eucrites have been inferred as extremely evolved (La to > 100 × CI). This inference has been based on mineral/magma partitioning, and on mass balance considering the cumulate eucrites as adcumulates of plagioclase + pigeonite only; both approaches have been criticized as inappropriate. Here, mass balance including magma + equilibrium pigeonite + equilibrium plagioclase is used to test a simple model for the cumulate eucrites: that they formed from known eucritic magma types, that they consisted only of magma + crystals in chemical equilibrium with the magma, and that they were closed to chemical exchange after the accumulation of crystals. This model is tested for major and rare earth elements (REE). The cumulate eucrites Serra de Magé and Moore County are consistent, in both REE and major elements, with formation by this simple model from a eucrite magma with a composition similar to the Nuevo Laredo meteorite: Serra de Magé as 14% magma, 47.5% pigeonite, and 38.5% plagioclase; Moore County as 35% magma, 37.5% pigeonite, and 27.5% plagioclase. These results are insensitive to the choice of mineral/magma partition coefficients. Results for the Moama cumulate eucrite are strongly dependent on choice of partition coefficients; for one reasonable choice, Moama's composition can be modeled as 4% Nuevo Laredo magma, 60% pigeonite, and 36% plagioclase. Selection of parent magma composition relies heavily on major elements; the REE cannot uniquely indicate a parent magma among the eucrite basalts. The major element composition of Y-791195 can be fit adequately as a simple cumulate from any basaltic eucrite composition. However, Y-791195 has LREE abundances and La/Lu too low to be accommodated within the model using any basaltic eucrite composition and any reasonable partition coefficients. Postcumulus loss of incompatible elements seems possible. It is intriguing that Serra de Magé, Moore County, and Moama are consistent with the same parental magma; could they be from the same igneous body on the eucrite parent asteroid (4 Vesta)?