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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.

Lunar granulitic meteorites provide new constraints on the composition and evolution of the lunar crust as they are potentially derived from outside the Apollo and Luna landing sites. Northwest Africa (NWA) 3163, the focus of this study, and its paired stones NWA 4881 and NWA 4483, are shocked granulitic noritic anorthosites. They are petrographically and compositionally distinct from the Apollo granulites and noritic anorthosites. Northwest Africa 3163 is REE-depleted by an order of magnitude compared to Apollo granulites and is one of the most trace element depleted lunar samples studied to date. New in-situ mineral compositional data and Rb–Sr, Ar–Ar isotopic systematics are used to evaluate the petrogenetic history of NWA 3163 (and its paired stones) within the context of early lunar evolution and the bulk composition of the lunar highlands crust. The NWA 3163 protolith was the likely product of reworked lunar crust with a previous history of heavy REE depletion. The bulk feldspathic and pyroxene-rich fragments have 87Sr/86Sr that are indistinguishable and average 0.699282 ± 0.000007 (2σ). A calculated source model Sr TRD age of 4.340 ± 0.057 Ga is consistent with (1) the recently determined young FAS (Ferroan Anorthosite) age of 4.360 ± 0.003 Ga for FAS 60025, (2) 142Nd model ages for the closure of the Sm–Nd system for the mantle source reservoirs of the Apollo mare basalts (4.355–4.314 Ga) and (3) a prominent age peak in the Apollo lunar zircon record (c. 4.345 Ga). These ages are ∼100 Myr younger than predicted timescales for complete LMO crystallization (∼10 Myrs after Moon formation, Elkins-Tanton et al., 2011). This supports a later, major event during lunar evolution associated with crustal reworking due to magma ocean cumulate overturn, serial magmatism, or a large impact event leading to localized or global crustal melting and/or exhumation. The Ar–Ar isotopic systematics on aliquots of paired stone NWA 4881 are consistent with an impact event at ⩾ 3.5 Ga. This is inferred to record the event that induced granularization of NWA 3163 (and paired rocks). A later event is also recorded at ∼2 Ga by Ar–Ar isotopes is consistent with an increase in the number of impacts on the lunar surface at this time (Fernandes et al., 2013). Northwest Africa 3163 and its paired stones therefore record a c. 2.4 Gyr record of lunar crustal production, metamorphism, brecciation, impacts and eventual ejection from the lunar surface.

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.

Determining the timescales of the accretion and chemical differentiation of meteorite parent bodies provides some of the most direct constraints on the formation of planetesimals and the earliest stages of planet formation. We present high-precision Hf–W isotope data for a comprehensive set of ureilites, ultramafic mantle restites derived from a partially melted and incompletely differentiated asteroid. All samples are characterized by strong 182W deficits, indicating that silicate melt extraction on the ureilite parent body at 3.3 ± 0.7 Ma after CAI formation postdated core formation in iron meteorite parent bodies by ∼2–3 Ma. Thermal modeling of planetesimal heating by 26Al-decay combined with the new Hf–W data indicates that the ureilite parent body accreted at ∼1.6 Ma after CAI formation and, therefore, more than ∼1 Ma later than iron meteorite parent bodies, but more than ∼0.5 Ma earlier that most chondrite parent bodies. Due to its relatively ‘late’ accretion, the ureilite parent body contained too little 26Al to cause complete melting and, therefore, would have probably remained incompletely differentiated even without exhaustion of 26Al by silicate melt segregation. Our results show that both in terms of degree of differentiation and accretion timescale the ureilite parent body is intermediate between fully differentiated and undifferentiated bodies, implying that there is an inverse correlation between extent of melting and metal–silicate separation versus time of accretion and differentiation.

International audience; Paleomagnetic studies of meteorites have shown that the solar nebula was likely magnetized and that many early planetary bodies generated dynamo magnetic fields in their advecting metallic cores. The surface fields on these bodies were recorded by a diversity of chondrites and achondrites, ranging in intensity from several a to several hundred mu T. In fact, an achondrite parent body without evidence for paleomagnetic fields has yet to be confidently identified, hinting that early solar system field generation and the dynamo process in particular may have been common. Here we present paleomagnetic measurements of the ungrouped achondrite NWA 7325 indicating that it last cooled in a near-zero field (

This study explores the petrogenesis of Shisr 161, an immature lunar regolith breccia meteorite with low abundances of incompatible elements, a feldspathic affinity, and a significant magnesian component. Our approach was to identify all clasts >0.5 mm in size in a thin section, characterize their mineral and melt components, and reconstruct their bulk major and minor element compositions. Trace element concentrations in representative clasts of different textural and compositional types indicate that the clast inventory of Shisr 161 is dominated by impact melts that include slowly cooled cumulate melt rocks with mafic magnesian mineral assemblages. Minor exotic components are incompatible-element-rich melt spherules and glass fragments, and a gas-associated spheroidal precipitate. Our hypothesis for the petrologic setting of Shisr 161 is that the crystallized melt clasts originate from the upper ~1 km of the melt sheet of a 300 to 500 km diameter lunar impact basin in the Moon’s feldspathic highlands. This hypothesis is based on size requirements for cumulate impact melts and the incorporation of magnesian components that we interpret to be mantle-derived. The glassy melts likely formed during the excavation of the melt sheet assemblage, by an impact that produced a >15 km diameter crater. The assembly of Shisr 161 occurred in a proximal ejecta deposit of this excavation event. A later impact into this ejecta deposit then launched Shisr 161 from the Moon. Our geochemical modeling of remote sensing data combined with the petrographic and chemical characterization of Shisr 161 reveals a preferred provenance on the Moon’s surface that is close to pre-Nectarian Riemann-Fabry basin.

New major- and trace-element abundances, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances, and oxygen and rhenium–osmium isotope data are reported for oligoclase-rich meteorites Graves Nunataks 06128 and 06129 (GRA 06128/9), six brachinites (Brachina; Elephant Morraine 99402/7; Northwest Africa (NWA) 1500; NWA 3151; NWA 4872; NWA 4882) and three olivine-rich achondrites, which are referred to here as brachinite-like achondrites (NWA 5400; NWA 6077; Zag (b)). GRA 06128/9 represent examples of felsic and highly-sodic melt products from an asteroid that may provide a differentiation complement to brachinites and/or brachinite-like achondrites. The new data, together with our petrological observations, are consistent with derivation of GRA 06128/9, brachinites and the three brachinite-like achondrites from nominally volatile-rich and oxidised ‘chondritic’ precursor sources within their respective parent bodies. Furthermore, the range of Δ17O values (∼0‰ to −0.3‰) among the meteorites indicates generation from isotopically heterogeneous sources that never completely melted, or isotopically homogenised. It is possible to generate major- and trace-element compositions similar to brachinites and the three studied brachinite-like achondrites as residues of moderate degrees (13–30%) of partial melting of primitive chondritic sources. This process was coupled with inefficient removal of silica-saturated, high Fe/Mg felsic melts with compositions similar to GRA 06128/9. Melting of the parent bodies of GRA 06128/9, brachinites and brachinite-like achondrites halted well before extensive differentiation, possibly due to the exhaustion of the short-lived radionuclide 26Al by felsic melt segregation. This mechanism provides a potential explanation for the cessation of run-away melting in asteroids to preserve achondrites such as GRA 06128/9, brachinites, brachinite-like achondrites, acapulcoite-lodranites, ureilites and aubrites. Moderate degrees of partial melting of chondritic material and generation of Fe–Ni–S-bearing melts are generally consistent with HSE abundances that are within factors of ∼2–10 × CI-chondrite abundances for GRA 06128/9, brachinites and the three brachinite-like achondrites. However, in detail, brachinite-like achondrites NWA 5400, NWA 6077 and Zag (b) are interpreted to have witnessed single-stage S-rich metal segregation, whereas HSE in GRA 06128/9 and brachinites have more complex heritages. The HSE compositions of GRA 06128/9 and brachinites require either: (1) multiple phases in the residue (e.g., metal and sulphide); (2) fractionation after generation of an initial melt, again involving multiple phases; (3) fractional fusion, or; (4) a parent body with non-chondritic relative HSE abundances. Petrological and geochemical observations permit genetic links (i.e., same parent body) between GRA 06128/9 and brachinites and similar formation mechanisms for brachinites and brachinite-like achondrites.

Shergottite meteorites are a suite of mafic to ultramafic igneous rocks whose parental magmas probably derived from the martian mantle. In this study, a suite of 23 shergottites, spanning their known range in bulk compositions, Rb–Sr, Sm–Nd, and Lu–Hf isotopes, were measured for 187Re–187Os isotopic systematics and highly siderophile element abundances (HSE: including Os, Ir, Ru, Pt, Pd, Re). The chief objective was to gain new insight on the chemical evolution of the martian mantle by unraveling the long-term HSE budget of its derivative melts. Possible effects upon HSEs related to crustal contamination, as well as terrestrial and/or martian surface alteration are also examined. Some of the shergottites are hot arid-desert finds. Their respective acetic acid leachates and residues show that both Re and Os display open-system behavior during sample residence at or near the martian and/or terrestrial surfaces. In some meteorites, the alteration effects can be circumvented by analysis of the leached residues. For those shergottites believed to record robust Re–Os isotopic systematics, calculated initial 187Os/188Os are well correlated with the initial 143Nd/144Nd. Shergottites from mantle sources with long-term melt-depleted characteristics (initial e143Nd of +36 to +40) have chondritic initial γ187Os ranging from −0.5 to +2.5. Shergottites with intermediate initial e143Nd of +8 to +17 have a range in initial γ187Os of −0.6 to +2.3, which overlaps the range for depleted shergottites. Shergottites from long-term enriched sources, with initial e143Nd of ∼−7, are characterized by suprachondritic γ187Os values of +5 to +15. The initial γ187Os variations for the shergottites do not show a correlation with indices of magmatic differentiation, such as MgO, or any systematic differences between hot arid-desert finds, Antarctic finds, or observed falls. The strong correlation between the initial e143Nd and γ187Os in shergottites from approximately +40 and 0 to −7 and +15, respectively, is assessed in models for mixing depleted mantle-derived melts with ancient crust (modeled to be similar to evolved shergottite in composition), and with assimilation-fractional crystallization. These models show that the correlation is unlikely to result from participation of martian crust. More likely, this correlation relates to contributions from depleted and enriched reservoirs formed in a martian magma ocean at ca. 4.5 Ga. These models indicate that the shergottite endmember sources were generated by mixing between residual melts and cumulates that formed at variable stages during solidification of a magma ocean. The expanded database for the HSE abundances in shergottites suggests that their martian mantle sources have similar HSE abundances to the terrestrial mantle, consistent with prior studies. The relatively high HSE abundances in both planetary mantles likely cannot be accounted for by high pressure–temperature metal–silicate partitioning at the bases of magma oceans, as has been suggested for Earth. If the HSE were instead supplied by late accretion, this event must have occurred prior to the crystallization of the last martian magma ocean.

Northwest Africa (NWA) 4472 is a polymict lunar regolith meteorite. The sample is KREEP-rich (high concentrations of potassium, rare earth elements and phosphorus) and comprises a heterogeneous array of lithic and mineral fragments. These clasts and mineral fragments were sourced from a range of lunar rock types including the lunar High Magnesian Suite, the High Alkali Suite, KREEP basalts, mare basalts and a variety of impact crater environments. The KREEP-rich nature of NWA 4472 indicates that the sample was ejected from regolith on the nearside of the Moon in the Procellarum KREEP Terrane and we have used Lunar Prospector gamma-ray remote sensing data to show that the meteorite is most similar to (and most likely sourced from) regoliths adjacent to the Imbrium impact basin. U–Pb and Pb–Pb age dates of NWA 4472 phosphate phases reveal that the breccia has sampled Pre-Nectarian (4.35 Ga) rocks related to early episodes of KREEP driven magmatism. Some younger phosphate U–Pb and Pb–Pb age dates are likely indicative of impact resetting events at 3.9–4 Ga, consistent with the suggested timing of basin formation on the Moon. Our study also shows that NWA 4472 has sampled impact melts and glass with an alkali-depleted, incompatible trace element-rich (high Sc, low Rb/Th ratios, low K) compositional signature not related to typical Apollo high-K KREEP, or that sampled by KREEPy lunar meteorite Sayh al Uhaymir (SaU) 169. This provides evidence that there are numerous sources of KREEP-rich protoliths on the Moon.

Multiple lines of evidence show that the Rb–Sr, Sm–Nd, and Ar–Ar isotopic systems individually give robust crystallization ages for basaltic (or diabasic) shergottite Northwest Africa (NWA) 1460. In contrast to other shergottites, NWA 1460 exhibits minimal evidence of excess 40Ar, thus allowing an unambiguous determination of its Ar–Ar age. The concordant Rb–Sr, Sm–Nd, and Ar–Ar results for NWA 1460 define its crystallization age to be 346 ± 17 Ma (2σ). In combination with petrographic and trace element data for this specimen and paired meteorite NWA 480, these results strongly refute the suggestion by others that the shergottites are ∼4.1 Ga old. Current crystallization and cosmic-ray exposure (CRE) age data permit identification of a maximum of nine ejection events for Martian meteorites (numbering more than 50 unpaired specimens as of 2008) and plausibly as few as five such events. Although recent high resolution imaging of the Martian surface has identified limited areas of sparsely cratered terrains, the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come, or that the cratering chronology needs recalibration. Time-averaged 87Rb/86Sr = 0.16 for the mantle source of the parent magma of NWA 1460/480 over the ∼4.56 Ga age of the planet is consistent with previously estimated values for bulk silicate Mars in the range 0.13–0.16, and similar to values of ∼0.18 for the “lherzolitic” shergottites. Initial eNd for NWA 1460/480 at 350 ± 16 Ma ago was +10.6 ± 0.5, which implies a time-averaged 147Sm/144Nd of 0.217 in the Martian mantle prior to mafic melt extraction, similar to values of 0.211–0.216 for the “lherzolitic” shergottites. These time-averaged values do not imply a simple two-stage mantle/melt evolution, but must result from multiple episodes of melt extractions from the source regions. Much higher “late-stage” eNd values for the depleted shergottites imply similar processes carried to a greater degree. Thus, NWA 1460/480, the “lherzolitic” shergottites and perhaps EET 79001 give the best (albeit imperfect) estimate of the Sr- and Nd-isotopic characteristics of bulk silicate Mars.

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.

Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm–Nd and Lu–Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic—a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths. The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous–early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm–Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger “ages” suggesting that the metasomatism occurred during the Laramide. Highly variable Rb–Sr and Lu–Hf mineral “ages” for these same samples suggest that the Homestead peridotites did not achieve intermineral equilibrium during this metasomatism. This indicates that the metasomatic overprint likely was introduced shortly before kimberlite eruption through interaction of the peridotites with the host kimberlite, or petrogenetically similar magmas, in the Wyoming Craton lithosphere.

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.

International audience; Angrites formed by some of the earliest igneous activity in the solar system and provide insights into the early stages of planetary melting and differentiation. Moreover, they are pivotal reference points for early solar system chronology. In order to study the processes and timescales of metal segregation in early protoplanets and to assess the distribution of short-lived radionuclides in the early solar system, the Hf-182-W-182 system was applied to a comprehensive suite of angrites. Hf-182-W-182 isochron ages for angrites are in excellent agreement with previously reported Pb-207-Pb-206 and Mn-53-Cr-53 results but are similar to 1 Myr older than ages obtained from Al-26-Mg-26 chronometry. These inconsistencies probably reflect a disturbance of the Al-Mg system in the angrite feldspars, but could alternatively be explained by a heterogeneous distribution of Al-26 in the early solar system. Based on the Hf-W results four texturally and temporally resolved groups of angrites can be identified that were derived from at least two distinct mantle sources. These mantle sources are the result of separate events of core formation, both of which took place within similar to 2 Myr of CAI formation. Thus, core formation in the angrite parent body did not occur as a single event of metal segregation from a global magma ocean but rather took place under varying conditions by several more local events. The disparate Hf-W systematics of the two distinct angrite source regions indicate that convection in the magma ocean was inefficient in homogenizing the composition of the mantle, possibly as a result of a continuous bombardment with small planetesimals during ongoing core formation. Such impacts could have constantly removed primordial and earlier formed crust, facilitating rapid cooling of the magma ocean, which solidified as early as 3.6 +/- 0.7 Myr after CAI formation

Elemental and isotopic compositions of spinel peridotite, pyroxenite and glimmerite xenoliths in Eocene minette dikes from the Highwood Mountains and Eagle Buttes, Montana, U.S.A., reveal a prolonged, yet episodic, history of melt removal and addition within the shallow lithospheric mantle of the Archean Wyoming Craton or its modified margin. Ancient, but highly variable, enrichment in incompatible elements is indicated by extreme Sr, Nd and Pb isotopic compositions ( 87 Sr 86 Sr = 0.705 to 1.02 ; ϵ Nd = −9 to −43; 206 Pb 204 Pb = 15.8 to 23.2 ). Very low 187 Os 188 Os (0.110 or less), corresponding to Re depletion model ages ( T RD ) of 2.7 to 2.9 Ga in some of the peridotites reflects melt removal during the Archean. At least one product of Archean melt migrating through this mantle section is preserved as a websterite xenolith that gives 2.7 Ga model ages for both the SmNd and ReOs isotopic systems. The majority of xenoliths, however, define PbPb and SmNd isochrons of mid-Proterozoic age and have ReOs T RD ages of 2 Ga or less. The mid-Proterozoic age could reflect either the time of formation of these peridotites in the shallow mantle or a time of severe overprinting of the incompatible element budget of pre-existing material by interaction with migrating fluids and/or melts. Glimmerite veins within one harzburgite sample yield 1.8 Ga monazite UPb ages and probably represent the products of crystallization of the fluid/melt responsible for the incompatible element enrichment. The material introduced in the Proterozoic was derived from much older, presumably Archean, crustal materials as shown by marked negative Eu anomalies in many samples and highly evolved initial Sr and Nd isotopic compositions. The data highlight the complex chemical evolution experienced by mantle lithosphere and suggest a coupling between the timing of processes affecting the lithospheric mantle and those recorded in the overlying crustal section.

Two types of stromatic leucosomes are identified in metasedimentary rocks from the Skagit migmatite complex, North Cascades, Washington state, U.S.A. Both types are trondhjemitic and appear similar in outcrop, but, although both contain low abundances of REE, one type consists of leucosomes that are relatively REE-enriched compared to the other, and contains (1) small ( 700°C, water-rich fluid present), inferences about the origin of the above-listed mineralogical and fluid inclusion features, and modeling of leucosome trace element abundances. The second type of leucosome is interpreted to have formed entirely by subsolidus processes (e.g., metamorphic differentiation) because these leucosomes lack features consistent with an origin by partial melting. K-poor (tonalitic/trondhjemitic) leucosomes associated with metasedimentary (biotite-bearing) source rocks may form by water-saturated partial melting or by subsolidus processes. Both general leucosome-forming mechanisms may operate at different times during upper amphibolite facies regional metamorphism. Partial melting may be initiated by syn-metamorphic magmatic activity if crystallizing plutons serve as external sources of the water-rich fluid necessary for ultrametamorphism in the middle crust during orogenesis. Large-scale migmatite complexes such as the Skagit migmatites may form at least in part in response to contact effects of plutonism associated with high-grade metamorphism, so, although migmatite complexes are a volumetrically substantial part of many orogenic belts, they may not themselves represent a significant original source of magma for larger-scale igneous bodies.

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

Stratigraphically controlled sequences of in situ lavas were collected from Loihi Seamount using the Alvin submersible to evaluate the volcano's temporal geochemical evolution. Three sections with up to 370 m of relief were sampled from the two pit craters at the summit of Loihi. All of the analyses were done on glass separates. Our results indicate that tholeiitic and alkalic volcanism at the summit of Loihi has been coeval. The tholeiitic and alkalic lavas have similar incompatible element patterns and O, Pb, Sr, and Nd isotope ratios but are distinct in some incompatible element ratios. These results are consistent with the different Loihi rock types being derived by variable degrees of melting from a common source. The crossing and light-rare-earth-enriched rare earth element patterns and variable Sc/Yb ratios of the tholeiites indicate that their source was a garnet lherzolite. The relatively low δ18O values (∼4.9 ‰) for Loihi lavas are interpreted to be characteristic of the Hawaiian plume.

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.

Angrites are amongst the oldest basalts in the solar system and their origins are enigmatic, some even proposing the planet Mercury as the parent body (APB). Whatever their exact provenance their chronometry provides insights into early stages of planetary melting and differentiation. We present the first high-precision internal 182Hf- 182W isochrons for such early differentiated objects. Angrites Sahara 99555, D'Orbigny, and Northwest Africa 2999 define ages of 5.1 ± 1.3 Ma, 4.7 ± 1.3 Ma and 9.5 ± 3.3 Ma respectively after formation of calcium-aluminum-rich refractory inclusions (CAIs). These data are in good agreement with 26Al- 26Mg, 53Mn- 53Cr and most 207Pb- 206Pb ages for other angrites and provide evidence for two texturally and temporally well-resolved groups. The quenched angrites (SAH 99555, D'Orbigny and five others) have a weighted mean age of 4562.1 ± 0.4 Ma and are the products of igneous crystallization on the APB ∼ 5 Ma after the formation of CAIs, whereas the more slowly cooled angrites (NWA 2999, Angra dos Reis, LEW 86010, average age: 4557.7 ±0.2 Ma) reflect metamorphic closure ∼ 5 Ma later following second reheating process or a complex cooling history. The concordance obtained between various short-lived chronometers provides evidence that 26Al, 53Mn and 182Hf were homogeneously distributed in the solar nebula, although we cannot rule out the possibility of local small heterogeneities. Contrary to recent proposals, the data are also consistent with the previously determined age of the solar system based on 207Pb- 206Pb systematics of CAIs. The Hf-W data are discussed in the context of two endmember models for the early differentiation of the angrite parent body. In the first model, core formation occurred at 3-4 Ma after CAIs and both groups of angrites formed by two distinct partial melting events from the bulk mantle of the angrite parent body. In the second model, the angrite parent body underwent progressive core formation with an increasing degree of W-depletion over time. In this model, the two groups of angrites derive from distinct reservoirs. The heat sources responsible for such late melting and core formation are unclear. Quenched angrites are coeval with non-magmatic IAB iron meteorites and CB chondrules at ∼ 4562 Ma. However, demonstration of a genetic link between angrite melting and impact events must await the acquisition of still higher resolution chronometry. © 2007 Elsevier B.V. All rights reserved.

Carbon abundances have been determined in mantle xenoliths from alkalic basalts and kimberlites and interpreted in terms of the nature and distribution of the C-rich phases. Anhydrous Cr-diopside Group I spinel lherzolites from basalts typically contain 15–50 ppm C, and amphibole-bearing ones have only marginally higher concentrations (40–100 ppm). Carbon abundances in Al-augite Group II pyroxenites are not significantly different from those of the Group I rocks. Although most LREE-depleted lherzolite xenoliths contain less C than enriched samples, there is no clear relationship between abundances of C and the incompatible trace elements. In the suite of deformed cumulate peridotite and dunite xenoliths of the 1801 Kaupulehu flow of the Hualalai volcano, Hawaii, C abundances are clearly related to texture, modal composition, and style of deformation. The most C-rich rocks are wehrlites in which the clinopyroxenes deformed more brittly and thus possess higher fluid inclusion and crack densities than the surrounding olivines. Regardless of their lithology, all xenoliths from kimberlites (including both peridotites and eclogites) are C-rich compared to those from basalts. Most of the C in these xenoliths exists as calcite or carbonaceous matter associated with serpentine veins and was thus probably contributed by the kimberlite host. Primary carbonates are extremely rare in all xenoliths, although occasionally they have been observed as daughter products in fluid inclusions. Although most C exists as inclusions of CO2-rich vapor, condensed carbonaceous matter also appears to occur in all rocks as discrete platy grains and as a film on natural surfaces such as grain boundaries and cracks.

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.

Clinopyroxenes (cpx) separated from discrete spinel lherzolite xenoliths from Kilbourne Hole, New Mexico, are compositionally and isotopically heterogeneous. On a Nd-Sr isotope correlation diagram, the cpx plot largely within the mantle array, from near Bulk Earth to depleted MORB values. None of the bulk xenoliths are equivalent to primitive mantle; all have undergone one or more depletion events and some have been enriched in incompatible elements. The lherzolites as well as the cpx show significant variations in incompatible element ratios including Sm/ Nd and Sr/Nd; moreover, 147 Sm 144 Nd and 143 Nd 144 Nd ratios of the cpx are positively correlated and suggestive of an 0.4 Ga fractionation event. Cpx from spinel pyroxenite dikes and lherzolite wallrocks of composite xenoliths are relatively homogeneous (isotopically) and similar to OIB and some MORB. The wallrocks are isotopically equilibrated with the pyroxenites or nearly so, and have negative Nd model ages; we infer that the pyroxenite-forming event caused enrichment in incompatible elements in the contiguous wallrock. The pyroxenite parent magma was probably a primitive basanite characterized by low Hf/Sm and Ti/Sm ratios relative to primitive mantle as a consequence of residual garnet. Our data confirm the presence of a MORB-related component in the mantle beneath Kilbourne Hole. This component is fertile with respect to basaltic constituents, is relatively LREE-depleted, and is isotopically similar to MORB; it is probably derived from asthenosphere. The isotopic heterogeneity of the discrete lherzolites requires a second, enriched component characterized by relatively low 143 Nd 144 Nd and high 87 Sr 86 Sr ratios and unsupported 143 Nd 144 Nd ratios. Old but disparate Sr and Nd model ages require that MORB-related spinel lherzolites have had a complicated history and differentiated from primitive mantle more than 1 b.y. ago. Two plausible models, one involving more than one depletion event and the second involving mixing of mantle components depleted at distinct times, can explain the common observation that Sr model ages are older than Nd model ages.

Abundances of rare earth elements, Hf, Sc, Co, Cr and Th in garnet megacrysts and their volcanic hosts or matrices are used to estimate garnet/liquid partition coefficients for these elements. Samples include pyropes from kimberlite and highly alkalic basalts, almandines from basalt andesite, dacite, rhyodacite and rhyolite and a spessartine-almandine from alaskite. The pyrope/host partition coefficients are fairly uniform and agree with experimental data within a factor of 2. The almandine/matrix data show more scatter (due in part to impurities in the garnet separates) but the partition coefficients tend to increase with increasing Si O ratio of the matrix. The almandine/matrix partition coefficients are up to a factor of 10 higher than the pyrope/host partition coefficients. The spessartine-almandine is strongly enriched in heavy rare earths (~ 5000 times chondrites), Y, Sc and Co. The wide variation in garnet/liquid partition coefficients from kimberlites to rhyolites cannot be explained as an effect of temperature and we conclude that a major factor is the composition of the melt from which the garnet crystallized.

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.

The melting curves of CaCO_3 and MgCO_3 have been extended to pressures of 36 kb by experiments in piston-cylinder apparatus. At 30 kb, the melting temperatures of calcite and magnesite are 1610°C and 1585°C, respectively. New data for the magnesite dissociation reaction permit the location of an invariant point for the assemblage magnesite + periclase + liquid + vapor near 26 kb-1550°C. New data are also presented for the calcite-aragonite transition at 800°C, 950°C and 1100°C. At pressures above 36–50 kb, calcite and magnesite melt at temperatures lower than the solidus of dry mantle peridotite. Natural and experimental evidence suggests that carbon dioxide in the Earth's mantle could be present in a variety of forms: (a) a free vapor phase, (b) vapor dissolved in silicate magma, (c) crystalline carbonate, (d) carbonatite liquid, (e) carbon-bearing silicate analogs, or (f) carbonato-silicates (such as scapolite, spurrite, tilleyite, and related compounds).

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).