Northwest Africa 773: lunar mare breccia with a shallow-formed olivine-cumulate component, inferred very-low-Ti (VLT) heritage, and a KREEP connection

Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo₆₄ to Fo₇₀, average Fo₆₇), 27% pigeonite (En₆₀Fs₂₄Wo₁₆ to En₆₇Fs₂₇Wo₆), 11% augite (En₅₀Fs₁₇Wo₃₃ to En₄₇Fs₁₃Wo₄₀), 2% orthopyroxene (En₇₀Fs₂₆Wo₄), 11% plagioclase (An₈₀ to An₉₄), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.     

Mineralogy and petrology of the diabasic rocks in a differentiated olivine diabase sill complex,Sierra Ancha,Arizona

The Precambrian Sierra Ancha sill complex, more than 700 feet thick, is a multiple intrusion with a central layer of feldspathic olivine-rich diabase, and upper and lower layers of olivine diabase derived from a high-alumina basalt magma. Minor rock types include albite diabase and albite-diabase pegmatite. Deuteric alteration was extensive. Principal primary minerals are plagioclase (An₇₂ to An₁₆), augite (Wo₄₃En₄₄Fs₁₃ to Wo₄₀En₃₈Fs₂₂), olivine (Fo₇₄ to Fo₅₄), orthopyroxene (En₇₇ to En₄₄), magnetite (Mgt₆₆Usp₃₄ to Mgt₈₉Usp₁₁), and ilmenite (Ilm₈₆Hem₁₄ to Ilm₉₆Hem₄). Ilmenite formed by reaction-exsolution from magnetite_ss is consistently different in compositon from primary ilmenite. Primary ilmenite became enriched in Mn and depleted in Mg as crystallization proceded. A systematic Fe-Mg partition between contacting olivine and orthopyroxene suggests that equilibrium prevailed on an extremely local scale during crystallization. Albite-diabase pegmatite contains a mineral assemblage including augite, ferrosalite (Wo₄₉En₂₈Fs₂₃ to Wo₄₉En₁₄Fs₃₇), albite (An₂ to An₀), and iron-rich chlorite. Altered diabase and albite diabase also have unusually calcium-rich pyroxenes. The calcium-rich pyroxenes, which occur in assemblages like those characterizing some spilites, are richer in calcium and lower in aluminum and titanium than basaltic augite.Contribution No. 1712 of the Division of Geological Sciences, California Institute of Technology, Pasadena, California.     

Observation of reactions in synthetic Ca-poor pyroxene single crystals at elevated temperatures by X-ray diffraction

Diffuse streaks in diffraction patterns of synthetic pyroxene single crystals at elevated temperatures are used to determine which reactions are initiated and how they proceed. The samples investigated are a) a host orthopyroxene (Wo₄En₈₃Fs₁₃) containing oriented pigeonite (Wo₆En₇₈Fs₁₆) parallel to (100) and b) a pigeonite (Wo₈En₇₅Fs₁₇). The maximum temperatures were 820° C and 1,015° C, respectively. No partial melting occurs at these temperatures, all reactions are in the subsolidus. In case a) augite is formed parallel to the (001) plane of pigeonite, but the augite is not exsolved by the pigeonite. This is proved by the absence of the obligatory streaks between corresponding reflections in highly resolved precession photographs. Instead, there are streaks from augite to the corresponding reflections of the host orthopyroxene. Example b) demonstrates that the temperature of the high-low transformation of pigeonite is very sensitive to the Ca content and clearly depends on the exsolution of augite. This augite is oriented parallel to (100) of pigeonite, not to (001). Both the high and the low pigeonite are present over a range of ～150° C, while the exsolution of augite continues. Simultaneously, orthopyroxene is also formed sharing (100) of pigeonite. There seems to be an indication that only low pigeonite inverts to orthopyroxene.     

Mineralogy,petrology, and trace element geochemistry of the Johnstown meteorite: a brecciated orthopyroxenite with siderophile and REE-rich components

The Johnstown meteorite is a brecciated orthopyroxenite (diogenite) containing coarsegrained centimeter-sized clasts of cumulate origin that have undergone subsolidus recrystallization. The brecciated portion is dominated by subangular fragments of orthopyroxene (Wo_2–3En_72–74Fs_23–25) in a variably comminuted matrix of the same material. Minor and accessory phases include plagioclase (An_82–90Ab_10–18Or_0–1), diopside (Wo_44–45En_46–47Fs_9–10), olivine (Fo_71–72), tridymite, troilite, metallic Ni-Fe (~3% Ni), and chromite (Cm_71–80Hc_1–8Sp_11–19Mt_2–4Uv_1–3).The clastic component is parental to the brecciated matrix which contains no foreign lithic or mineralogic components. Siderophile trace element studies, however, reveal the presence of meteoritic (chondritic) contamination in the brecciated portion using unbrecciated clasts for indigenous values. Rare earth element abundances show a wide range of values for the light REE in different samples, although all samples exhibit a strong negative Eu anomaly, indicative of earlier plagioclase fractionation. Two pairs of adjacent brecciated and unbrecciated samples from different portions of the meteorite show, respectively, the most enriched and the most depleted light REE patterns. The variability in La content is over a factor of 100. However, in each case the REE pattern for the brecciated portion is very similar to that of the unbrecciated portion. These differences are attributed to sampling of variable amounts of residual, REE-enriched, trapped liquid. The most representative REE pattern for the bulk meteorite has an intermediate composition and was obtained from the largest sample. The data presented here indicate that Johnstown is a monomict breccia, in contrast to several other diogenites which may be considered to be polymict on the basis of their mineral compositions and/or clast populations.     

Geothermometry of exsolved augites from the Laramie Anorthosite Complex,Wyoming

Exsolved augite pyroxenes from the ferromonzonite border facies of the ferrosyenite in the Laramie Anorthosite Complex have been studied with the transmission electron microscope and the electron microprobe to determine their exsolution histories. The Lindsley and Andersen (1983) geothermometer gives initial crystallization temperatures of 1000° C for the bulk augite crystal (Wo₃₂ En₂₂ Fs₄₆). Exsolved lamellae are predominantly pigeonites with very low calcium contents (Wo_1–3 En_23–24 Fs_71–74) and have formation temperatures estimated to be in the range of 600 to 975° C. The uniform compositions of lamellae and hosts, despite the range in lamellar size and orientation, suggest that either 1) the ferromonzonite experienced an extended plateau in cooling or a reheating event at 600 to 650° C or 2) the pyroxenes recorded a blocking temperature. Two-feldspar geothermometry on exsolved feldspars also records 600° C and suggests that these low temperatures are not blocking temperatures.     

Petrology and geochemistry of the unbrecciated achondrite Northwest Africa 1240 (NWA 1240): an HED parent body impact melt

NWA 1240 is an unusual eucrite recently recovered in Morocco as a single stone of 98 g. It is an unbrecciated greenish-brown rock nearly devoid of fusion crust. It displays porphyritic texture consisting of skeletal hollow low-Ca pyroxene phenocrysts set in a variolitic (fan-spherulitic) mesostasis of fine elongate pyroxene and plagioclase crystals. Minor phases are skeletal chromite, iron, silica, troilite, ilmenite and minute amounts of phosphate and fayalite. Pyroxenes are unequilibrated and show one of the widest ranges of composition so far described for a eucrite, from En_76.0Wo_1.9Fs_22.1 to compositions nearly devoid of Mg (unusual ferrosilite and Fe-augite symplectites and possibly pyroxferroite). Plagioclase crystals contain significant amounts of Fe and Mg, which are possibly controlled by the Ca(Mg,Fe²⁺)Si₃O₈ plagioclase component.To discuss the potential effects of hot-desert weathering on NWA 1240, we have analyzed a series of Saharan eucrites (Agoult, Aoufous, Igdi, Smara, NWA 047 and NWA 049) and large aliquots (0.39 to 2.8 g) of eucrite falls (Bereba, Bouvante, Jonzac, Juvinas and Serra de Magé). These results indicate that among the elements we have determined, Pb, Ba and Sr are the most sensitive indicators of Saharan weathering.The bulk composition of NWA 1240 has been determined for 45 elements by ICP-AES and ICP-MS. The data show that the meteorite is not significantly weathered: its Pb concentration is very low; Ba and Sr concentrations are not anomalously high; the Th/U and Hf/Sm ratios are chondritic (Th/U = 3.65, Hf/Sm = 0.74). NWA 1240 is rich in MgO (10.4 wt%) and Cr₂O₃ (0.71 wt%), and displays striking similarities with cumulate eucrites, such as having similar incompatible trace element patterns and a significant positive Eu anomaly (Eu/Eu* = 1.37). The combination of fast cooling and cumulate eucrite-dominated composition suggests that NWA 1240 is not an igneous rock but rather an impact melt.     

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

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

Clinopyroxenes from mantle-related xenocrysts in alkaline basalts from Hannuoba (China): augite–pigeonite exsolutions and their thermal significance

Optically homogeneous augite xenocrysts, closely associated with spinel–peridotite nodules, occur in alkali basalts from Hannuoba (Hebei province, China). They were studied by electron and X-ray diffraction to define the occurrence and significance of pigeonite exsolution microtextures. Sub-calcic augite (Wo₃₄) exsolved into En_62–62Fs_25–21Wo_13–17 pigeonite and En_46–45Fs_14–14Wo_40–42 augite, as revealed by TEM through diffuse coarser (001) lamellae (100–300 Å) and only incipient (100) thinner ones (<70 Å). C2/c augite and P2₁/c pigeonite lattices, measured by CCD-XRD, relate through a(Aug)?a(Pgt), b(Aug)?b(Pgt), c(Aug)≠c(Pgt) [5.278(1) vs 5.189(1)Å] and β(Aug)≠β(Pgt) [106.55(1) vs 108.55(2)°]. Cell and site volumes strongly support the hypothesis that the augite xenocrysts crystallised at mantle depth from alkaline melts. After the augite xenocrysts entered the magma, (001) lamellae first formed by spinodal decomposition at a T_min of about 1,100 °C, and coarsened during very rapid transport to the surface; in a later phase, possibly on cooling, incipient (100) lamellae then formed.     

Lamellar exsolution systems in clinopyroxene

Exsolution systems in synthetic pyroxenes were studied by transmission electron microscopy. An iron free sample En₈₀Wo₂₀ was prepared by devitrifying glass at 1300°C. Samples with bulk composition En₅₀Fs₃₀Wo₂₀ and En₃₅Fs₃₈Wo₂₇ were given various but well-defined heat treatments. The exsolution systems observed cannot unambiguously be related to the heat treatment. Periodic lamellar exsolution was observed parallel to (001) and (100) with sharp satellite reflections in the diffraction diagram. In more complex exsolution systems coarse (100) lamellae were found together with fine lamellae parallel to (001) and (100). An unusual phenomenon occurs at a (100) twin boundary where both individuals display exsolution lamellae parallel to (001). Pigeonite lamellae in one twin meet augite lamellae of the other individual at the twin boundary and vice-versa. The precise matching is achieved by a change in width near the boundary. Smoothly curved phase boundaries are developed in the obtuse angle of crosshatched (100) and (001) pigeonite lamellae in augite, whereas the boundaries in the acute angle are straight with sharp edges. This is consistent with elastic energy constraints.     

A Combined Transmission Electron Microscope and Electron Microprobe Study of Bushveld Pyroxenes from the Bethal Area

Ca-rich and Ca-poor pyroxenes present in the Bushveld rocksof the Bethal area display well developed exsolution texturestypical of slowly cooled mafic intrusions. This gave rise topoor reproducibility in electron microprobe analyses of thesame pyroxene grain, as well as results which departed fromthe bulk composition of the original homogeneous mineral. EMMA-4was used together with the electron microprobe to establishthe composition of the constituent phases in exsolved pyroxenes.The data showed that microprobe analyses carried out with adefocused beam were equivalent to the bulk composition of thepyroxenes. Microprobe analyses obtained using a focused beamwere found to approach closely the bulk composition of pyroxenesonly when the exsolution density reached 90 lamellae per millimetre. Transmission electron microscope examination of microstructuresin ion-thinned samples of pyroxenes at 100 kV and 1000 kV showedthat the exsolution mechanism in Ca-rich and Ca-poor pyroxeneswas one of heterogeneous nucleation. Subsequent growth tookplace by means of the migration of ledges along the (100) plane.Pigeonite inversion was also shown to occur in iron-rich Ca-poorpyroxene exsolution lamellae in augite. Fractionation trends established for the Bethal pyroxenes frommicroprobe analyses indicated an overall range from Fs₁₄En₈₄Wo₂to Fs₆₀En₃₁Wo₉ in the Ca-poor pyroxene and Fs₇En₅₀Wo₄₃ to Fs₃₆En₂₇Wo₃₇in the Ca-rich pyroxene. Comparison of pyroxene fractionationtrends from the western, eastern and Bethal areas of the Bushveldsuggests that crystallization took place under different conditionsof pressure and temperature.     

Late paleozoic gabbroids of the Lesser Khingan terrane of the eastern Central-Asian fold belt: Age, geochemistry, and tectonic setting

This paper considers the results of mineralogical, geochemical, and geochronological studies of leucogabbroids of the Karagai Massif located within the Lesser Khingan superterrane of the Central Asian fold belt. The main features of the rock-forming minerals of the gabbroids are the high calcium content of the clinopyroxene corresponding in composition to diopside-salite (Wo_48–51En_33–39Fs_11–16) or augite (Wo_22–35En_46–47Fs_18–33), the high basicity of the plagioclase (An_90–92), and the presence of primary magmatic magnesian hornblende. The age of the massif was determined by U-Pb zircon dating as 257 ± 1 Ma. The compositional peculiarities of the gabbroids indicate that they were most probably formed in island-arc or active continental margin settings. With allowance for the existing geodynamic reconstructions and the data obtained in the present study, the formation of the Late Paleozoic gabbroids of the Karagai Massif was presumably related to the final stages of the subduction processes that preceded the collapse of the terranes of the eastern Central Asian fold belt.     

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

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

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

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

Mineralogy and partial melt textures within an ultramafic-mafic body,Greece

Ultramafic-mafic rocks from Makrirrakhi, Central Greece exhibit features of an original ophiolite sequence which contains depleted mantle material, ultramafic containing partial melt textures and possibly the mafic pluton which resulted from the coalescing of these partial melt segregations. Considerable mineralogical variation exists: unzoned olivine crystals range in composition from Fo_78–84 (mafics) to Fo_88–92 (ultramafics), plagioclases An_64–79 (mafics) to An_80–90 (ultramafics) and spinel varies from a chromian spinel (ultramafics) to a more aluminous-titaniferous spinel (mafics). Pyroxenes from the ultramafics display a limited range: En_89–92 Fs_9–8 Wo_0–2 (orthopyroxene) and En_48–54 Fs_1–10 Wo_38–50 (clinopyroxene). Mafic rocks display a greater range being richer in ferrosilite En_36–65 Fs_3–20 Wo_33–51. Pyroxenes from within the partial melt segregations have chemical affinities with those from the gabbrotroctolite series. A model of partial melt within the upper mantle, and, a set of criteria to distinguish partial melt textures from cumulate textures, are developed from analytical data and textural evidence.     

Pyroxenes in the Shaw (L-7) chondrite

The Shaw L-group chondrite differs from orthodox type 6 ordinary chondrites in ways which suggest that it experienced unusually high metamorphic temperatures and anatexis. Electron microprobe and single crystal X-ray diffraction data indicate that Shaw contains three pyroxenes: the augite (Fs_11.3Wo_38.2) and calcic orthopyroxene (Fs_19.4WO_4·5) reported by other workers and a second, Ca- and Al-poor orthopyroxene (Fs_16·8Wo_1·2) which we interpret as inverted protobronzite. Comparison of the Shaw assemblage with experimental data suggests that a two-phase (augite-protobronzite) assemblage developed at peak metamorphic temperatures of ~1250–1300°C, that partial reaction of augite and protobronzite produced calcic orthopyroxene and by-product spinel at temperatures approximately 150°C lower and that protobronzite inverted to bronzite free of stacking disorder during subsequent slow cooling. The intracrystalline distribution of Fe and Mg in the Ca-poor bronzite (K_E + 0·07; determined by crystal structure analysis) indicates an equilibration temperature of ~500°C.Shaw differs sufficiently in texture and mineralogy from type 6 ordinary chondrites to justify its assignment to a separate petrologic type: L-7.     

Phase Relations and Crystallization Sequence in a Contact-Metamorphosed Rock from the Gunflint Iron Formation, Minnesota

Portions of the Gunflint Iron Formation, originally a ferruginoussediment, were metamorphosed by the intrusion of the DuluthComplex to assemblages containing: pigeonite (Wo₁₀En₂₄Fs₆₆)+olivine(Fo₁₃Fa₃₇)+Fe-Ti oxide (Mt₆₂Usp₃₄Hc₄)+plagioclase (An₉₄Ab₆)+vapor+augite (Wo₄₀En₂₀Fs₄₀) or cummingtonite Fe/(Fe+Mg) {smalltilde} 0.69; quartz was present but probably was not in equilibriumwith olivine. Comparison with synthetic phase-equilibrium studiesindicate conditions of initial recrystallization of T 800 °C,P_total 2kb, fo₂ slightly below that of the pure fayalite-magnetite-quartzassemblage, and P_H2O < P_total. During the slow cooling process following initial recrystallization,the phases present underwent a complex series of exsolution,inversion, oxidation, and hydration reactions. Pigeonite initiallyexsolved augite along (001), then inverted to orthopyroxene,which then exsolved augite along (100). The augite exsolvedonly pigeonite on (001) during its cooling history. The Fe-Tioxide for the most part oxidized to an intergrowth of magnetiteand ilmenite, although unoxidized portions later exsolved ulvöspinel.Cummingtonite exsolved actinolite, forming irregular patchesof the latter. Olivine, orthopyroxene, and augite reacted withplagioclase to form retrograde amphiboles. Orthopyroxene had difficulty nucleating during this slow coolingprocess, forming only at widely spaced points in mosaics ofprimary pigeonite grains, and never nucleating within primaryaugite grains. The resulting orthopyroxene grains are much largerthan the original pigeonite grains.     

Concordant Rb-Sr, Sm-Nd, and Ar-Ar ages for Northwest Africa 1460: A 346 Ma old basaltic shergottite related to “lherzolitic” shergottites

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 ε_Nd 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” ε_Nd 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.     

Mineralogical variations in the Delakhari sill,Deccan trap intrusion,central India

The Delakhari sill (maximum thickness cf. 200 m) is the most extensive Deccan Trap instrusion which occurs in central India, between longitutdes 78°3835 to 78°2240 and latitudes 22°26 and 22°2230. Based on petrographic examination, the sill is divided, from bottom to top, into (1) the Lower Chilled Zone (LCZ), up to 8 m thick, marked by abundant interstitial glass and an overall fine grain size, (2) the Olivine-Rich Zone (ORZ), 27 m thick, enriched in olivine (relative to the other zones in the sill), (3) the Central Zone (CZ), 70 m thick, marked by depletion in olivine and overall coarse grain size, (4) the Upper Zone (UZ), 55 m thick, marked by the presence of two chemically and morphologically distinct olivine types and abundant interstitial granophyre, and (5) the Upper Chilled Zone (UCZ), 10–25m thick, marked by abundant interstitial glass.Compositions of the pyroxenes and olivines show an overall increase in Fe/Mg with crystallization, but extensive interzonal and intrazonal variations and overlaps exist. Olivine ranges from Fa₂₄ (ORZ) to Fa₉₅ (UZ). In the UZ and inner UCZ, an equant (Fa_44–50, called type-A olivine) and interstitial skeletal olivine (Fa_70–95, called type-B olivine) occur together. Compositions of the Ca-rich and Ca-poor pyroxenes fall in the range Wo₃₈En₃₄Fs₂₈ to Wo₃₃En₈Fs₅₉ and Wo₁₄En₄₁Fs₄₅ to Wo₁₆En₁₉Fs₆₅, respectively. Overall, the two pyroxene trends converge with Fe-enrichment except for one anomalous sample from the UZ which contains a Ca-rich (Wo₃₄En₈Fs₅₈) and a Ca-poor (Wo₁₀En₁₈Fs₇₂) pyroxene well within the Forbidden Zone of Smith (1972).Compositions of coexisting oxide minerals indicate that the sill crystallized at oxygen fugacities from 10^–10 atm (ORZ) to 10^–13 (UZ). The magma prior to intrusion appears to have been derived from a more primitive melt from which a considerable amount of olivine and plagioclase have fractionated out. A model of open, interrupted fractional crystallization in the sill is proposed to explain the compositional variations exhibited by the major mineral phases.A previous study (Crookshank 1936) concluded that the sill is actually a multiple intrusion and has given rise to the lowermost (flow I) and the topmost (flow III) lava flows in the neighboring area around Tamia (78°4015, 22°2035). The olivines of flows I and III have compositions Fo₈₇ and Fo₈₈ respectively, and are much more Mg-rich than the maximum Mg-rich olivine (Fo₇₆) of the Delakhari sill, refuting the possibility of the sill being the feeder of the lava flows I and III.Geosciences Department, University of Texas at Dallas Contribution No. 338     

Mineralogy,petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa_18.1–28.3, average Fa_24.9±3.2, PMD = 12.8; Cr₂O₃ = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs_3.2–18.7Wo_0.2–4.5; average Fs_14.7±3.7Wo_1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ¹⁷O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa₂₇) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ¹⁷O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ¹⁸O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.     

New constraints on early Solar System chronology from Al-Mg and U-Pb isotope systematics in the unique basaltic achondrite Northwest Africa 2976

We report elemental abundances and the isotopic systematics of the short-lived ²⁶Al-²⁶Mg (half-life of ∼0.73 Ma) and long-lived U-Pb radiochronometers in the ungrouped basaltic meteorite Northwest Africa (NWA) 2976. The bulk geochemical composition of NWA 2976 is clearly distinct from that of the eucrites and angrites, but shows broad similarities to that of the paired NWA 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples. The major and trace element abundances in NWA 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body. The Al-Mg and Pb-Pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early Solar System. The absolute Pb-Pb internal isochron age of NWA 2976, obtained from acid leaching residues of three whole-rock samples and two pyroxene separates, is 4562.89 ± 0.59 Ma (MSWD = 0.02). This Pb-Pb age is calculated using the measured ²³⁸U/²³⁵U ratio of a NWA 2976 whole-rock of 137.751 ± 0.018 (2σ) which was determined relative to the recently revised value of 137.840 ± 0.008 for the SRM 950a U isotope standard. The Al-Mg systematics reveal the presence of ²⁶Mg isotopic anomalies produced by the decay of ²⁶Al with an (²⁶Al/²⁷Al)₀ of (3.94 ± 0.16) × 10⁻⁷, and indicate a time of formation of 0.26 ± 0.18 Ma after the D’Orbigny angrite. Using the revised Pb-Pb age of 4563.36 ± 0.34 Ma for the D’Orbigny anchor (corrected for its U isotopic composition), we deduce an Al-Mg model age of 4563.10 ± 0.38 Ma for NWA 2976, which is consistent with its Pb-Pb internal isochron age.The concordance of the Pb-Pb and Al-Mg chronometers, when taking into account the differences in the U isotopic compositions of the D’Orbigny and NWA 2976 achondrites (whose parent bodies likely formed in distinct regions of early Solar System as indicated by their different oxygen isotopic compositions), implies that ²⁶Al was homogeneously distributed in the early Solar System. It also suggests that igneous processes on planetesimals, as represented by the formation of various basaltic meteorite groups that likely originated on distinct parent bodies (e.g., eucrites and angrites, as well as ungrouped achondrites), were widespread throughout the protoplanetary disk within the first ∼5 Ma of the history of the Solar System.