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Knowing the distribution and origin of water in terrestrial planets is crucial to understand their formation, evolution and the source of their atmospheres and surface water. The nakhlites represent a suite of minimally shocked meteorites that likely originated from lava flows from a single volcano or from a shallow intrusion or sill complex on Mars. Measuring the water contents and D/H ratios of their igneous minerals allows identification of phases that have preserved their magmatic hydrogen, and therefrom permits estimation of the water content of their mantle source. Pyroxene, olivine, melt inclusions and mesostasis of five nakhlites (NWA 998, Nakhla, Y 000593, MIL 03346 and NWA 6148) were analyzed in situ for water contents and H isotopes, and major and trace element contents. No water was detected in olivine grains except in Y 000593. The water content of pyroxenes is highly heterogeneous within individual grains and between grains within a single meteorite. Water concentrations in pyroxene ( After ruling out significant influence from spallation, exchange with the martian atmosphere, shock, surface alteration, and hydrothermal processes, the H data of the pyroxenes can be explained by degassing and crystallization processes. Degassing is consistent with a decrease of water content from pyroxene interior to edge. Fractionation of H isotopes during degassing results in increases of δD during H loss from pyroxene but in decreases in δD during H2O-OH loss from a melt. Consequently, the low-water content, high-δD of most pyroxenes is best explained by degassing after the pyroxenes had crystallized. All melt and plagioclase inclusions analyzed are located in degassed pyroxenes and are also degassed. The lower δD of the mesostasis (24 ± 131‰) compared to that of the least-degassed pyroxenes (430 ± 172‰) is likely the result of melt degassing and interaction with hydrothermal fluids. Magmatic H, however, has been preserved in each nakhlite in some pyroxenes that are characterized by >15 ppm H2O and δD

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and (187)Re–(187)Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En(55–57)Wo(3–4)Fs(40–42); Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa(50–53); Fe/Mn = 1.1–2.1), chromite (Cr# ~ 0.94), awaruite, sulfides, plagioclase (Ab(92)An(5)Or(3)) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1–10(2) °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high supersaturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re–Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O–Ti isotope systematics (Δ(17)O = −1.052 ± 0.004, 2 SD; ε(50)Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = −2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

Northwest Africa (NWA) 6704/6693 are medium- to coarse-grained achondrites with unique petrologic and geochemical traits that are distinct from the currently established meteorite groups. Here, we report on the extinct 26Al-26Mg and 53Mn-53Cr systems to establish fine-scale chronology of its formation and Cr and Ti isotopic anomalies to constrain the composition of the source reservoir of NWA 6704/6693. Excesses in the neutron-rich 54Cr and 50Ti isotopes, due to nucleosynthetic anomalies, separate NWA 6704/6693 from the vast majority of established achondrites and instead resemble the excesses seen among the carbonaceous chondrites; specifically, the CR-type chondrites. The excesses in these isotopes indicate a common feeding zone during accretion in the protoplanetary disk between the source of NWA 6704/6693 and that of the carbonaceous chondrites. The 26Al-26Mg data for pyroxene and plagioclase from NWA 6704 yield a (26Al/27Al)0 = (3.15 ± 0.38)×10−7 (MSWD = 0.49) and an initial δ26Mg∗ = −0.004 ± 0.005 at the time of isotopic closure. This initial (26Al/27Al)0 translates to an absolute age of 4563.14 ± 0.38 Ma, relative to the D’Orbigny angrite. However, given the potential heterogeneity of 26Al, the D’Orbigny angrite might not be a good age anchor for the purpose of calculating 26Al-26Mg ages. The 26Al-26Mg age relative to another carbonaceous achondrite, NWA 2976, is 4562.66 ± 0.60 Ma. The 53Mn-53Cr systematics of NWA 6704/6693 indicate a (53Mn/55Mn)0 of (2.59 ± 0.34) × 10−6 (MSWD = 1.2) with an evolved initial e53Cr of +0.14 ± 0.03. The (53Mn/55Mn)0 yields an 53Mn-53Cr age of 4562.17 ± 0.76 Ma relative to the D’Orbigny angrite. Concordant ages determined using the short-lived 26Al-26Mg and 53Mn-53Cr systems and extant 207Pb-206Pb system (4562.60 ± 0.30 Ma for NWA 6704/6693; Amelin et al., 2019) indicate rapid cooling and nearly contemporaneous closing of multiple isotope systems. The ancient crystallization ages and positive 54Cr and 50Ti anomalies of NWA 6704/6693 indicate widespread melting and differentiation processes occurring in both non-carbonaceous (NC) and carbonaceous chondrite (CC) regions of the protoplanetary disk. Additionally, we report the Cr and Ti isotopic composition for a petrologic range of CR-type materials (CR2, CR6, and achondrites). The additional Cr and Ti isotopic data for these CR-type materials indicates a range in isotopic composition not previously observed based on CR2 chondrites alone.

We report a petrography, mineral chemistry, oxygen and chromium isotopic study of Grove Mountains (GRV) 020043 together with a subset of other acapulcoites and lodranites. GRV 020043 is a petrologic type 4 chondrite, with chondrules of diverse types and sizes, and is composed of low-Ca pyroxene (40 vol.%), olivine (24 vol.%), diopside (8 vol.%), plagioclase (10 vol.%), Fe-Ni metal (kamacite and taenite), troilite and some accessory minerals (chromite and apatite). The olivine in GRV 020043 has an average fayalite content (Fa) of 10.7 mol.% with the low-Ca pyroxene having an average ferrosilite content (Fs) of 10.8 mol.%. The whole rock oxygen isotopic composition of GRV 020043 is +3.226 ± 0.267‰, +0.797 ± 0.131‰, and −0.927 ± 0.017‰ for δ18O, δ17O, and Δ17O, respectively, with a bulk chromium isotopic compositions of e54Cr = −0.48 ± 0.10. These characteristics of GRV 020043 are different from all established or ungrouped chondrites but agree with those of the acapulcoite-lodranite clan. We therefore suggest that GRV 020043 represents the chondritic precursor of acapulcoite-lodranite parent body. The similarity of bulk oxygen and chromium isotopic compositions among GRV 020043, Acapulco, Northwest Africa (NWA) 468 (metal-rich lodranite), NWA 8118 (lodranite), NWA 8287 (acapulcoite), and NWA 8422 (lodranite) indicates that they originated from a common oxygen and chromium reservoir in the protoplanetary disk or may have derived from a parent body with a differentiated multilayer structure.

Mars probably accreted within the first 10 million years of Solar System formation and likely underwent magma ocean crystallization and crust formation soon thereafter. To assess the nature and timescales of these large-scale mantle differentiation processes we applied the short-lived 182 Hf– 182 W and 146 Sm– 142 Nd chronometers to a comprehensive suite of martian meteorites, including several shergottites, augite basalt NWA 8159, orthopyroxenite ALH 84001 and polymict breccia NWA 7034. Compared to previous studies the 182 W data are significantly more precise and have been obtained for a more diverse suite of martian meteorites, ranging from samples from highly depleted to highly enriched mantle and crustal sources. Our results show that martian meteorites exhibit widespread 182 W/ 184 W variations that are broadly correlated with 142 Nd/ 144 Nd, implying that silicate differentiation (and not core formation) is the main cause of the observed 182 W/ 184 W differences. The combined 182 W– 142 Nd systematics are best explained by magma ocean crystallization on Mars within ∼20–25 million years after Solar System formation, followed by crust formation ∼15 million years later. These ages are indistinguishable from the I–Pu–Xe age for the formation of Mars' atmosphere, indicating that the major differentiation of Mars into mantle, crust, and atmosphere occurred between 20 and 40 million years after Solar System formation and, hence, earlier than previously inferred based on Sm–Nd chronometry alone.

The Northwest Africa (NWA) 773 clan of meteorites is a group of paired and/or petrogenetically related stones that contain at least six different lithologies: magnesian gabbro, ferroan gabbro, anorthositic gabbro, olivine phyric basalt, regolith breccia, and polymict breccia. Uranium-lead dates of baddeleyite in the magnesian gabbro, ferroan gabbro, and components within breccia lithologies of paired lunar meteorites NWA 773, NWA 3170, NWA 6950, and NWA 7007 indicate a chronologic link among the meteorites and their components. A total of 50 baddeleyite grains were analyzed and yielded weighted average 207Pb-206Pb dates of 3119.4 ± 9.4 (n = 27), 3108 ± 20 (n = 13), and 3113 ± 15 (n = 10) Ma for the magnesian gabbro, ferroan gabbro, and polymict breccia lithologies, respectively. A weighted average date of 3115.6 ± 6.8 Ma (n = 47/50) was calculated from the baddeleyite dates for all lithologies. A single large zircon grain found in a lithic clast in the polymict breccia of NWA 773 yielded a U-Pb concordia date of 3953 ± 18 Ma, indicating a much more ancient source for some of the components within the breccia. A U-Pb concordia date of apatite and merrillite grains from the magnesian gabbro and polymict breccia lithologies in NWA 773 is 3112 ± 33 Ma, identical to the baddeleyite dates. Magnesian and ferroan gabbros, as well as the dated baddeleyite and Ca-phosphate-bearing detritus in the breccia lithologies, formed during the same igneous event at about 3115 Ma. These data also strengthen proposed petrogenetic connections between magnesian and ferroan gabbro lithologies, which represent some of the youngest igneous rocks known from the Moon.

The abundances of highly siderophile elements (HSE; including Re, Os, Ir, Ru, Pt, and Pd) and (187)Re-(187)Os isotopic systematics were determined for two fragments from ungrouped achondrite NWA 7325. Rhenium-Os systematics are consistent with closed-system behavior since formation or soon after. The abundances of the HSE were therefore largely unaffected by late-stage secondary processes such as shock or terrestrial weathering. As an olivine gabbro cumulate, this meteorite has a bulk composition consistent with derivation from a body that produced a core, mantle and crust. Also consistent with derivation from a body that produced a core, both fragments of NWA 7325 have HSE abundances that are highly depleted compared to bulk chondrites. One fragment has ~0.002 × CI chondrite Ir and relative HSE abundances similar to bulk chondrites. The other fragment has ~0.0002 × CI chondrite Ir, and relative HSE abundances that are fractionated compared to bulk chondrites. The chondritic relative HSE abundances of the fragment characterized by higher HSE abundances most likely reflect the addition of exogenous chondritic material during or after crystallization by surface impacts. The HSE in the other fragment is likely more representative of the parent body crust. One formation model that can broadly account for the HSE abundances in this fragment is multiple episodes of low-pressure metal-silicate equilibration, followed by limited late accretion and mantle homogenization. Given the different HSE compositions of the two adjoining fragments, this meteorite provides an example of the overprint of global processes (differentiation and late accretion) by localized impact contamination.

Northwest Africa (NWA) 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with a possible link to the planet Mercury. In order to understand the origin of this meteorite and the nature of its parent body, we have determined its crystallisation age using the long-lived U–Pb and short-lived Al–Mg chronometers. An internal Pb–Pb isochron defined by six acid leached pyroxene fractions yields an age of 4563.4 ± 2.6 Ma, assuming that the 238U/235U ratio for NWA 7325 is identical to the bulk Earth and Solar System value of 137.794. The Al–Mg isotope analyses of seven fractions (four plagioclase, one pyroxene, one olivine and one whole rock) define a regression line corresponding to 26Al/27Al0 = (3.03 ± 0.14) × 10−7 and an initial δ26Mg∗ of 0.093 ± 0.004‰. When anchored to the D’Orbigny angrite, this initial 26Al/27Al yields an age of 4563.09 ± 0.26 Ma. The Pb–Pb age of 4563.4 ± 2.6 Ma and Al–Mg age of 4563.09 ± 0.26 Ma are in complete agreement, but the low U concentrations of NWA 7325 resulted in a relatively low precision Pb–Pb age. The observed excess in initial δ26Mg∗ can be explained by 27Al/24Mg fractionation and subsequent Mg isotopic evolution after planetary differentiation. Furthermore, the parental magma of NWA 7325 most likely formed within 1.72 Ma after calcium-aluminium rich inclusion (CAI) formation. NWA 7325 formed near simultaneously with quenched angrites and a number of ungrouped achondrites at ∼4563 Ma, suggesting that a multitude of planetary bodies had formed and differentiated by ∼4–5 Myr after CAI formation. This ancient age may be interpreted as an argument against NWA 7325 originating from Mercury, however it does not completely rule it out.

Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites. Apparent 40Ar/39Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of laser-probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40Ar/39Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40Ar/36Ar ratios of 1600-1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein. We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37Ar/39Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0 ± 0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38Ar, approximates the timing of shock melting in this meteorite.

– Northwest Africa (NWA) 4797 is an ultramafic Martian meteorite composed of olivine (40.3 vol%), pigeonite (22.2%), augite (11.9%), plagioclase (9.1%), vesicles (1.6%), and a shock vein (10.3%). Minor phases include chromite (3.4%), merrillite (0.8%), and magmatic inclusions (0.4%). Olivine and pyroxene compositions range from Fo66–72,En58–74Fs19–28Wo6–15, and En46–60Fs14–22Wo34–40, respectively. The rock is texturally similar to “lherzolitic” shergottites. The oxygen fugacity was QFM−2.9 near the liquidus, increasing to QFM−1.7 as crystallization proceeded. Shock effects in olivine and pyroxene include strong mosaicism, grain boundary melting, local recrystallization, and pervasive fracturing. Shock heating has completely melted and vesiculated igneous plagioclase, which upon cooling has quench-crystallized plagioclase microlites in glass. A mm-size shock melt vein transects the rock, containing phosphoran olivine (Fo69–79), pyroxene (En44–51Fs14–18Wo30–42), and chromite in a groundmass of alkali-rich glass containing iron sulfide spheres. Trace element analysis reveals that (1) REE in plagioclase and the shock melt vein mimics the whole rock pattern; and (2) the reconstructed NWA 4797 whole rock is slightly enriched in LREE relative to other intermediate ultramafic shergottites, attributable to local mobilization of melt by shock. The shock melt vein represents bulk melting of NWA 4797 injected during pressure release. Calculated oxygen fugacity for NWA 4797 indicates that oxygen fugacity is decoupled from incompatible element concentrations. This is attributed to subsolidus re-equilibration. We propose an alternative nomenclature for “lherzolitic” shergottites that removes genetic connotations. NWA 4797 is classified as an ultramafic poikilitic shergottite with intermediate trace element characteristics.

– Northwest Africa (NWA) 5298 is an evolved basaltic shergottite that has bulk characteristics and mineral compositions consistent with derivation from an oxidized reservoir in Mars. Chemically zoned clinopyroxene (64.5%, augite and pigeonite), with interstitial lath-shaped plagioclase (29.4%, An40 to An55), constitutes the bulk of this meteorite. The plagioclase has been converted by shock to both isotropic maskelynite and spherulitic, birefringent feldspar representing a quenched vesicular melt. The remainder of the rock consists of minor amounts of Fe-Ti oxides (ilmenite and titanomagnetite), phosphates (merrillite and apatite), silica polymorph, fayalite, pyrrhotite, baddeleyite, and minor hot desert weathering products (calcite and barite). Oxygen fugacity derived from Fe-Ti oxide thermobarometry is close to the quartz-fayalite-magnetite (QFM) buffer indicating that the late stage evolution of this magma occurred under more oxidizing condition than those recorded in most other shergottites. Merrillite contains the largest abundances of rare earth elements (REE) of all phases, thereby controlling the REE budget in NWA 5298. The calculated bulk rock REE pattern normalized to CI chondrite is relatively flat. The evolution of the normalized REE patterns of the bulk rock, clinopyroxene, plagioclase, and phosphate in NWA 5298 is consistent with closed-system chemical behavior with no evidence of crustal contamination or postcrystallization disturbance of the REE contents of these phases.

The shergottites exhibit a range of major and trace element compositions, crystallization ages, and initial Sr, Nd, Hf, and Pb isotopic compositions. To constrain the physical mechanisms by which shergottites obtain their compositional characteristics, we examined the major and trace element record preserved in olivine in the more primitive shergottites. Based on such characteristics as the Mg#, V zoning, calculated DNi,Co, the olivine in Y-980459 are most likely phenocrysts. Many of these same characteristics indicate that the olivines in other shergottites are not in equilibrium with the adjacent melt. However, in most cases they are not xenocrystic, but additions of olivine from the same basaltic system. Elephant Moraine (EET) A79001 may be an exception with the olivine data suggesting that it is xenocrystic. In this case, the olivine crystallized from a reduced and LREEdepleted melt and was incorporated into an oxidized and enriched basalt. Vanadium and CaO in olivine appear to record the appearance of spinel and pyroxene on the liquidus of most of the shergottites. Most of the olivine shergottites represent basalts produced by melting of reduced (IW to IW + 1), depleted mantle sources. Olivine data indicate that many of the primary melts derived from this source had similar Ni, Co, and Mn. Shergottites such as Northwest Africa (NWA) 1110/1068 and perhaps Roberts Massif (RBT) 04261 that appear to be derived from more enriched sources have distinctly different olivine. In the case of NWA 1110/1068, the olivine data suggests that the enriched component was added to system prior to olivine crystallization.

Nakhlite Northwest Africa (NWA) 998 is an augite-rich cumulate igneous rock with mineral compositions and oxygen isotopic composition consistent with an origin on Mars. This 456- gram, partially fusion-crusted meteorite consists of (by volume) ~75% augite (core composition Wo39En39Fs22), ~9% olivine (Fo35), ~7% plagioclase (Ab61An35) as anhedra among augite and olivine, ~3.5% low-calcium pyroxenes (pigeonite and orthopyroxene) replacing or forming overgrowths on olivine and augite, ~1% titanomagnetite, and other phases including potassium feldspar, apatite, pyrrhotite, chalcopyrite, ilmenite, and fine-grained mesostasis material. Minor secondary alteration materials include iddingsite associated with olivine (probably Martian), calcite crack fillings, and iron oxide/hydroxide staining (both probably terrestrial). Shock effects are limited to minor cataclasis and twinning in augite. In comparison to other nakhlites, NWA 998 contains more low-calcium pyroxenes and its plagioclase crystals are blockier. The large size of the intercumulus feldspars and the chemical homogeneity of the olivine imply relatively slow cooling and chemical equilibration in the late- and post-igneous history of this specimen, and mineral thermometers give subsolidus temperatures near 730 °C. Oxidation state was near that of the QFM buffer, from about QFM-2 in earliest crystallization to near QFM in late crystallization, and to about QFM + 1.5 in some magmatic inclusions. The replacement or overgrowth of olivine by pigeonite and orthopyroxene (with or without titanomagnetite), and the marginal replacement of augite by pigeonite, are interpreted to result from late-stage reactions with residual melts (consistent with experimental phase equilibrium relationships). Apatite is concentrated in planar zones separating apatite-free domains, which suggests that residual magma (rich in P and REE) was concentrated in planar (fracture?) zones and possibly migrated through them. Loss of late magma through these zones is consistent with the low bulk REE content of NWA 998 compared with the calculated REE content of its parent magma.

Samarium-neodymium isotopic analyses of unleached and acid-leached mineral fractions from the recently identified olivine-bearing shergottite Northwest Africa 1195 yield a crystallization age of 347 ± 13 Ma and an eNd143 value of +40.1 ± 0.9. Maskelynite fractions do not lie on the Sm–Nd isochron and appear to contain a martian surface component with low 147Sm/144Nd and 143Nd/144Nd ratios that was added during shock. The Rb–Sr system is disturbed and does not yield an isochron. Terrestrial Sr appears to have affected all of the mineral fractions, although a maximum initial 87Sr/86Sr ratio of 0.7016 is estimated by passing a 347 Ma reference line through the maskelynite fraction that is least affected by contamination. The high initial eNd143 value and the low initial 87Sr/86Sr ratio, combined with the geologically young crystallization age, indicate that Northwest Africa 1195 is derived from a source region characterized by a long-term incompatible-element depletion. The age and initial Sr and Nd isotopic compositions of Northwest Africa 1195 are very similar to those of Queen Alexandra Range 94201, indicating these samples were derived from source regions with similar Sr–Nd isotopic systematics. These similarities suggest that these two meteorites share a close petrogenetic relationship and might have been erupted from a common volcano. The meteorites Yamato 980459, Dar al Gani 476, Sayh al Uhaymir 005/008, and Dhofar 019 also have relatively old ages between 474 and 575 Ma and trace element and/or isotopic systematics that are indicative of derivation from incompatible-element-depleted sources. This suggests that the oldest group of meteorites is more closely related to one another than they are to the younger meteorites that are derived from less incompatible-element-depleted sources. Closed-system fractional crystallization of this suite of meteorites is modeled with the MELTS algorithm using the bulk composition of Yamato 980459 as a parent. These models reproduce many of the major element and mineralogical variations observed in the suite. In addition, the rare earth element systematics of these meteorites are reproduced by fractional crystallization using the proportions of phases and extents of crystallization that are calculated by MELTS. Other shergottites that demonstrate enrichments in incompatible-elements and have evolved Sr and Nd isotopic systematics have some geochemical systematics that are similar to those observed in the depleted group. Most notably, although they exhibit a very limited range of incompatible trace element and isotopic compositions, they have highly variable major element compositions. This is also consistent with evolution from a common mantle source region by variable amounts of fractional crystallization. If this scenario is correct, it suggests that the combined effects of source composition and fractional crystallization are likely to account for the major element, trace element, and isotopic diversity of all shergottites.

0.0008 (level of confidence 95%) was defined on the basis of 117 spinel-bearing xenoliths from this work and data from the literature, including data for massif peridotites. The 187 Os/ 188 Os ratio of the PUM is similar to the range of compositions defined by ordinary and enstatite chondrites, not carbonaceous chondrites. Spinel-bearing mantle peridotites sampled by volcanism and peridotite massifs appear to have been extracted from a common fertile source (PUM) between 1 and 2 Ga ago. These peridotites now form part of the subcontinental lithospheric mantle underlying continental crust of similar or greater formation age. Copyright © 2001 Elsevier Science Ltd Xenoliths derived from the mantle and brought to the surface via volcanism are among the few types of materials that permit direct study of the chemical composition of the Earth’s upper mantle. By examining the isotopic composition of mantle samples, a time dimension is added to the estimate of the chemical composition of domains within the mantle. This is important for understanding the chemical evolution of the terrestrial mantle from a primitive composition, created during Earth’s early history, to the present differentiated states of the mantle and crust. Because of these aspects, mantle xenoliths have been vigorously studied for major and trace element compositions, mineral compositions, and lithophile isotope systems such as Rb-Sr, U-Pb, and Sm-Nd (Menzies et al., 1987; McDonough and Frey, 1989). The addition of the Re-Os isotope system ( 187 Re3 187 Os 1 b 2 ; l 5 1.666 z 10 211 a 21 ; Smoliar et al.

Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO2), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of fH 2 O variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu*=0.84–0.50) are modeled by melting with fH 2 O

Potassic volcanics and intrusives of Eocene age (52±1 Ma) in the Highwood Mountains of north-central Montana provide evidence for interaction of asthenospheric magmas with Archean mantle lithosphere of the Wyoming Craton. Diverse rock types were produced by shallow level degassing, fractional crystallization and magma mixing within two separate magma series whose parental liquids were latite and olivine minette. Halogen systematics of apatite microphenocrysts and other evidence have established that shallow degassing of the phlogopite-diopside-phyric minettes to yield heteromorphic leucite-salite-phyric mafic phonolites and shonkinites was an important process. Geochemical and mineralogical data suggest that fractional crystallization of olivine, clinopyroxene, mica and leucite, accompanied by widespread magma mixing, produced a spectrum of more evolved magmas such as leucite phonolites, malignites, alkali syenites and trachytes along with mica clinopyroxenite cumulates. Sr, Nd and Pb isotopic compositions and trace element data for the primitive olivine minette magmas are explicable by a multistage model involving three source components. One component is ancient Ba-LREE-enriched, U-Th-HFSE-depleted subcontinental lithospheric mantle, which has been recognized in other alkalic rocks of the region, including those from the Crazy Mountains and Smoky Butte. Glimmerite-veined harzburgite and phlogopite dunite xenoliths (one with -eNd of −33 at 52 Ma) found in the most primitive Highwood olivine minette are probably samples of this material. The other two components are asthenospheric mantle with isotopic composition near that of bulk earth (and dominant in Montana alnoitic diatremes) and a young subduction-related component (probably Eocene, but possibly as old as late Cretaceous), which is required to explain the Rb/Sr-87Sr/86Sr systematics of the Highwood rocks. A consistent model for the petrogenesis of the Highwood parental mafic magmas involves partial melting of asthenospheric mantle wedge triggered by infiltration of melts released from the metasomatized carapace above a low-angle subducted slab of Farallon Plate lithosphere, followed by assimilative interaction of these melts with ancient, phlogopite-rich, metasome veins upon ascent through the Wyoming Craton mantle keel.

BASALTS, basaltic andesites and andesites in subduction-related magmatic arcs are all depleted in high-field-strength elements (such as Ti, V, Zr, Nb, Hf and Ta) relative to mid-ocean-ridge basalt (MORB). Here we show that these depletions can be produced in liquids by interaction with depleted mantle periodotite, as would occur during the ascent of an arc magma through the mantle lithosphere of the overriding plate. This process involves extensive reaction between liquid, olivine, orthopyroxene and spinel. High-field-strength element depletions are produced because olivine, orthopyroxene and spinel have higher crystal/liquid distribution coefficients for these elements than for other incompatible trace elements. Liquids modified by mantle–magma interaction will also be depleted in heavy rare-earth elements, Cr and Ni, and enriched in light rare-earth and large-ion lithophile elements, relative to MORB. These characteristics are all common in mafic magmas at convergent plate margins1–7.

Experimental techniques for examining trace-element behavior are reviewed. Applications of these techniques are described for a variety of partition coefficients including: olivine/liquid, subcalcic pyroxene/liquid, calcic pyroxene/liquid, amphibole/liquid, plagioclase/liquid, alkali feldspar/liquid, garnet/liquid, apatite/liquid and whitlockite/liquid, ilmenite/liquid, armalcolite/liquid and pseudobrookite/liquid, magnetite/liquid and spinel/liquid, perovskite/liquid, and melilite/liquid partition coefficients. Partition coefficients between immiscible liquids are discussed and suggestions for further work are noted. A detailed bibliography is included.

Rare earth and other trace element abundances are determined in megacrysts of clinopyroxene, orthopyroxene, amphibole, mica, anorthoclase, apatite and zircon, as well as their host basalts, in an effort to gather data on mineral/melt trace element partitioning during the high pressure petrogenesis of basic rocks. Phase equilibria, major element partitioning and isotopic ratio considerations indicate that while most of the pyroxene and amphibole megacrysts may have been in equilibrium with their host magmas at high pressures, mica, anorthoclase, apatite, and zircon megacrysts are unlikely to have formed in equilibrium with their host basalts. It is instead concluded that they were precipitated from more evolved magmas, and have been mixed into their present hosts.

The major and trace element chemistry of phonolites containing spinel Iherzolite xenoliths from Bokkos (Nigeria), Phonolite Hill (northeastern Australia) and Heldburg (East Germany) is consistent with an origin by fractional crystallization of basanitic magmas at upper mantle pressures (10–15 kbar). At Bokkos, spatially associated lavas ranging from hawaiitic nepheline mugearite to nepheline benmoreite can be modeled very well by fractional crystallization of kaersutitic amphibole + olivine + Fe-Ti-spinel + apatite, a crystal extract consistent with experimentally-determined near-liquidus phase relationships for mugearitic liquids. Further fractional crystallization of aluminous clinopyroxene + mica + apatite will yield the phonolites. A similar model relating the unusual Iherzolite-bearing mafic nepheline benmoreite from Pigroot (New Zealand) to basanitic lavas of the East Otago province is not supported by major and trace element data. The Pigroot lava is possibly the product of melting of a mantle source region previously enriched in Sr and light rare earth elements, with subsequent minor fractional crystallization of olivine + kaersutite. Dynamic flow crystallization processes operating within conduit systems from mantle pressures are capable of yielding large volumes of evolved phonolitic liquids from primary basanitic liquids, if magma flow rates are appropriate. This mechanism may provide an explanation for the volumetric bias towards salic differentiates in some alkalic provinces.

Crystalline pargasite-rich spinel peridotite mylonite from St. Paul's Rocks containing 5.7% water bound in hydrous minerals was reacted in sealed platinum capsules in a piston-cylinder apparatus from 10 - 30 kb. At 10 kb the subsolidus assemblage is amphibole, olivine, orthopyroxene, clinopyroxene and spinel, an amphibole lherzolite; with increasing pressure garnet appears at 18 kb, spinel and amphibole disappear at about 25 kb; the resulting high pressure assemblage is that of a garnet lherzolite. The solidus was located in the presence of water-rich vapor, but vapor dissolves completely in the liquid at higher temperatures, and the liquid becomes water-undersaturated. Stability limits in the melting interval were determined for amphibole, garnet, spinel, clinopyroxene, orthopyroxene, and olivine (the liquidus mineral). The results are consistent with a published conclusion that St. Paul's Rocks is a diapiric, solid-state mantle intrusion initially mobilized at a depth between 45 km and 70 km near 1,000 - 1,050°C. An estimate of the solidus of peridotite with 0.2% water is presented and compared with other studies. At intermediate pressures this solidus is determined by the breakdown of amphibole. Discrepancies among results of the various studies probably arise at least in part from experimental problems involved in the complex systems.

Strontium isotopic data for megacrysts and lavas from six eruptive centers within the Newer Basalts province of southeastern Australia show that megacrysts of clinopyroxene are in isotopic equilibrium with associated basalts, but that megacrysts of kaersutite, ferrokaersutite, orthopyroxene and anorthoclase may exhibit slight disequilibrium with their host basalts. Furthermore, the anorthoclase megacrysts may be either more or less radiogenic than their hosts. The 87Sr/86Sr ratios for 14 basalts from throughout the province vary from 0.7035 to 0.7045 and it is proposed that anorthoclase, amphibole and orthopyroxene megacrysts which crystallized in isotopic equilibrium with one magma may have been caught up in a pulse of a later magma of a different isotopic composition. The variations in the 87Sr/86Sr ratios for the basalts are attributed to variations in the isotopic composition of their source regions. Such isotopic heterogeneity is supported by published data for ultramafic xenoliths which occur in the Newer Basalts lavas.

We report U-Pb, 87Rb-87Sr, 40Ar-39Ar, and 238U/235U isotopic data for paired ungrouped achondrites NWA 6704 and NWA 6693 that were derived from a highly oxidised parent body with broadly chondritic composition (Warren et al., 2013; Hibiya et al., 2019). Pb-isotopic ages derived from isochrons for multiple acid-leached pyroxene fractions are 4562.76 + 0.22/−0.30 Ma for NWA 6704 and 4562.63 + 0.29/−0.21 Ma for NWA 6693, calculated using 238U/235U ratio of 137.7784 ± 0.0097 measured in NWA 6704. The Rb-Sr mineral isochron age of 4543 ± 46 Ma (initial 87Sr/86Sr = 0.699013 ± 0.000055) is consistent with the Pb-isotopic age. Together with 187Re-187Os isochron age of 4576 ± 250 Ma for NWA 6704 (Hibiya et al., 2019), and 26Al-26Mg and 53Mn-53Cr ages calculated using the rapidly crystallized angrite D’Orbigny as a time anchor are also consistent with the Pb-isotopic age ( Sanborn et al., 2019 ), these data indicate that the parent rocks of NWA 6693 and NWA 6704 remained closed to migration of both lithophile and siderophile elements since crystallisation and initial cooling. The whole rock 40Ar-39Ar age of 4199 ± 32 Ma suggests a complete resetting of the K-Ar system approximately 360 Ma after crystallisation. A later event at ≤2.12 Ga partially reset the K-Ar system as shown by the low temperature heating steps. Both meteorites have high 87Rb/86Sr ratios (up to 7.0 in NWA 6693 pyroxene) and very radiogenic 87Sr/86Sr up to 1.15. Together with the absence of secondary disturbance in the Rb-Sr and U-Pb systems, this makes them suitable for cross-calibration of the isotopic chronometers. These meteorites are also promising candidates to serve as age reference samples for the early Solar System chronology, as an alternative or complement to angrites of the early generation (D’Orbigny, Sahara 99555) that are currently used for this purpose. Plagioclase in NWA 6704 has a sufficiently low Rb/Sr ratio to define precise initial 87Sr/86Sr of 0.698997 ± 0.000027, which corresponds to the time of separation of the parent body precursor material from the solar nebula of 1.5 ± 2.1 Ma. This value suggests that the parent asteroid accreted within 3.6 Ma after CAI formation, or before 4563.7 Ma using the CAI age of 4567.3 Ma (Connelly et al., 2012).