Petrogenesis of a vitrophyre in the martian meteorite breccia NWA 7034 (original) (raw)
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Geochimica et Cosmochimica Acta, 2019
In part 1 of our examination of Martian meteorite Northwest Africa 8159 (NWA 8159) we illustrated many interesting mineralogical and textural attributes that make this martian basalt unique. Unlike the shergottites that illustrate a clear relationship between the extent of trace element and isotopic characteristics and oxygen fugacity (reduced, depleted magmas; oxidized, enriched magmas), NWA 8159 illustrates a decoupling of this relationship as it has oxidized and depleted signatures. In part 2, using a series of new observations and measurements (Cl isotopes, XANES, TEM, empirical modeling) we use NWA 8159 to explore the interaction between mantle-derived magmas and the martian crust. The magnetite-orthopyroxene intergrowths associated with olivine are a product of a martian subsolidus oxidation event near the QFM buffer and not a magmatic reaction in an oxidizing magma (>QFM+3). This subsolidus event is further supported by Cr valence in the olivine, alteration of P-rich olivine, and end-member magnetite in the matrix of the meteorite. Although this subsolidus alteration makes it extremely difficult to determine the original fO 2 of the parental magma for NWA 8159, there is evidence that during the initial stages of crystallization the fO 2 was modestly reducing (~IW+1). Potential manifestations of more reducing magmatic conditions include P-rich cores in the olivine and low Fe 3+ in silicates (plagioclase, pyroxene). Further, if analogous to all other depleted shergottites, NWA 8159 initially crystallized under reducing conditions. This decoupling between oxygen fugacity and isotopic-trace element characteristics suggests that basalts derived from the martian mantle interacted with the crust in ways that significantly influenced redox history and volatile element isotopic composition (Cl, S), without dramatically modifying many of its radiogenic isotope and trace element mantle fingerprints.
Isotopic and petrographic evidence for young Martian basalts
Geochimica et Cosmochimica Acta, 2008
Radiometric age data for shergottites yield ages of 4.0 Ga and 180-575 Ma; the interpretation of these ages has been, and remains, a subject of debate. Here, we present new 39 Ar-40 Ar laser probe data on lherzolitic shergottites Allan Hills (ALH) 77005 and Northwest Africa (NWA) 1950. These two meteorites are genetically related, but display very different degrees of shock damage. On a plot of 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar, the more strongly shocked ALH 77005 (45-55 GPa) does not yield an array of values indicating an isochron, but the data are highly scattered with the shock melts yielding 40 Ar/ 36 Ar ratios of 1600-2026. Apparent ages calculated from these extractions range from 374-8183 Ma, with 50% of the data, particularly from the shock melts, yielding impossibly old ages (>4.567 Ga). On the same plot, extractions from igneous minerals in the less shocked NWA 1950 (30-44 GPa) yield a fitted age of 382 ± 36 Ma. Argon extractions from the shock melts are well distinguished from minerals, with the melts exhibiting the highest 40 Ar/ 36 Ar ratios (1260-1488) and the oldest apparent ages. Laser step heating was also performed on maskelynite separates from NWA 1950 yielding ages of 1000 Ma at the lowest release temperatures, and ages of 360 and 362 Ma at higher temperature steps. Stepped heating data from previous studies have yielded ages of 500 and 700 Ma to 1.7 Ga for ALH 77005 maskelynite separates. If the ages obtained from igneous minerals represent undegassed argon from an ancient (4.0 Ga) rock, then the ages are expected to anticorrelate with the degree of shock heating. The data do not support this inference. Our data support young crystallization ages for minerals and Martian atmosphere as the origin of excess 40 Ar in the shock melts.
Geochimica et Cosmochimica Acta, 2002
Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 Ϯ 11 Ma and a Sm-Nd age of 173 Ϯ 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 Ϯ 10 and 166 Ϯ 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 Ϯ 4, and the initial Nd value is ϩ11.1 Ϯ 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 Ϯ 4 and an initial Nd value of ϩ8.2 Ϯ 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low 143 Nd/ 144 Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 Ϯ 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental. Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ϳ4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.
Meteoritics & Planetary Science, 2012
Northwest Africa (NWA) 4797 is an ultramafic Martian meteorite composed of olivine (40.3 vol%), pigeonite (22.2%), augite (11.9%), plagioclase (9.1%), vesicles (1.6%), and a shock vein (10.3%). Minor phases include chromite (3.4%), merrillite (0.8%), and magmatic inclusions (0.4%). Olivine and pyroxene compositions range from Fo 66-72 ,En 58-74 Fs 19-28 Wo 6-15 , and En 46-60 Fs 14-22 Wo 34-40 , respectively. The rock is texturally similar to ''lherzolitic'' shergottites. The oxygen fugacity was QFM)2.9 near the liquidus, increasing to QFM)1.7 as crystallization proceeded. Shock effects in olivine and pyroxene include strong mosaicism, grain boundary melting, local recrystallization, and pervasive fracturing. Shock heating has completely melted and vesiculated igneous plagioclase, which upon cooling has quenchcrystallized plagioclase microlites in glass. A mm-size shock melt vein transects the rock, containing phosphoran olivine (Fo 69-79 ), pyroxene (En 44-51 Fs 14-18 Wo 30-42 ), and chromite in a groundmass of alkali-rich glass containing iron sulfide spheres. Trace element analysis reveals that (1) REE in plagioclase and the shock melt vein mimics the whole rock pattern; and (2) the reconstructed NWA 4797 whole rock is slightly enriched in LREE relative to other intermediate ultramafic shergottites, attributable to local mobilization of melt by shock. The shock melt vein represents bulk melting of NWA 4797 injected during pressure release. Calculated oxygen fugacity for NWA 4797 indicates that oxygen fugacity is decoupled from incompatible element concentrations. This is attributed to subsolidus re-equilibration. We propose an alternative nomenclature for ''lherzolitic'' shergottites that removes genetic connotations. NWA 4797 is classified as an ultramafic poikilitic shergottite with intermediate trace element characteristics.
Geochimica Et Cosmochimica Acta, 2009
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 (62.5 mm in length, Fo 77-52 ) and prismatic zoned pyroxene crystals with Wo 3 En 71 in the cores to Wo 8-30 En 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, ulvö spinel, 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 wholerock composition of LAR 06319 is also enriched in incompatible trace elements relative to depleted shergottites, with a traceelement pattern that is nearly identical to that of olivine-phyric shergottite NWA 1068. The oxygen isotope composition of LAR 06319 (D 17 O = 0.29 ±0.03) confirms its martian origin.
Geochimica et Cosmochimica Acta, 2010
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 En 70 Fs 25 Wo 5 , En 50 Fs 25 Wo 25 , and En 45 Fs 45 Wo 10 ), and Cr-rich spinel in a matrix of maskelynite (An 52 Ab 45 ), pyroxene (En 30-40 Fs 40-55 Wo 10-25 ,), olivine (Fo 50 ), 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.
Geochimica et Cosmochimica Acta, 2003
Samarium-neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 Ϯ 11 Ma and an initial Nd 143 value of ϩ36.6 Ϯ 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial 87 Sr/ 86 Sr ratio of 0.701249 Ϯ 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace elementisotopic systematics observed in the martian meteorite suite.
Shergottite Northwest Africa 6963: A Pyroxene-Cumulate Martian Gabbro
Journal of Geophysical Research: Planets
Northwest Africa (NWA) 6963 was found in Guelmim-Es-Semara, Morocco, and based on its bulk chemistry and oxygen isotopes, it was classified as a Martian meteorite. On the basis of a preliminary study of the textures and crystal sizes, it was resubclassified as a gabbroic shergottite because of the similarity with terrestrial and lunar gabbros. However, the previous work was not a quantitative investigation of NWA 6963; to supplement the original resubclassification and enable full comparison between this and other Martian samples; here we investigate the mineralogy, petrology, geochemistry, quantitative textural analyses, and spectral properties of gabbroic shergottite NWA 6963 to constrain its petrogenesis, including the depth of emplacement (i.e., base of a flow versus crustal intrusion). NWA 6963 is an enriched shergottite with similar mineralogy to the basaltic shergottites but importantly does not contain any fine-grained mesostasis. Consistent with the mineralogy, the reflectance (visible/near-infrared and thermal infrared) spectrum of powdered NWA 6963 is similar to other shergottites because they are all dominated by pyroxene, but its reflectance is distinct in terms of albedo and spectral contrast due to its gabbroic texture. NWA 6963 represents a partial cumulate gabbro that is associated with the basaltic shergottites. Therefore, NWA 6963 could represent a hypabyssal intrusive feeder dike system for the basaltic shergottites that erupted on the surface. Plain Language Summary This study investigates a new meteorite from Mars, which has different properties than previous Martian meteorites. Specifically, this rock has large crystals that likely formed as the magma ponded in the crust instead of erupting as a lava flow. On Earth, 10 times more magma gets stuck in the crust than erupts on the surface; therefore, we would expect something similar on Mars-yet this rock is the first example of an intrusive magma on Mars. This work shows that this meteorite possibly represents the feeder dike system that fed the lava flow represented by the other shergottite meteorites.
Crystallization conditions of Los Angeles, a basaltic Martian meteorite
Geochimica Et Cosmochimica Acta, 2002
The texture of Los Angeles (stone 1) is dominated by relatively large (0.5-2.0 mm) anhedral to subhedral grains of pyroxene, and generally subhedral to euhedral shocked plagioclase feldspar (maskelynite). Minor phases include subhedral titanomagnetite and ilmenite, Fe-rich olivine, olivine+augite-dominated symplectites [some of which include a Si-rich phase and some which do not], pyrrhotite, phosphate(s), and an impact shock-related alkali- and silica-rich glass closely associated with anhedral to euhedral silica grains. Observations and model calculations indicate that the initial crystallization of Mg-rich pigeonitic pyroxenes at ≤1150 °C, probably concomitantly with plagioclase, was followed by pigeonitic and augitic compositions between 1100 and 1050 °C whereas between 1050 and 920 to 905 °C pyroxene of single composition crystallized. Below 920 to 905 °C, single composition Fe-rich clinopyroxene exsolved to augite and pigeonite. Initial appearance of titanomagnetite probably occurred near 990 °C and FMQ-1.5 whereas at and below 990 °C and ≥FMQ-1.5 titanomagnetite and single composition Fe-rich clinopyroxene may have started to react, producing ilmenite and olivine. However, judging from the most common titanomagnetite compositions, we infer that most of this reaction likely occurred between 950 and 900 °C at FMQ-1.0±0.2 and nearly simultaneously with pyroxene exsolution, thus producing assemblages of pigeonite, titanomagnetite, olivine, ilmenite, and augite. We deem this reaction as the most plausible explanation for the formation of the olivine+augite-dominated symplectites in Los Angeles. But we cannot preclude possible contributions to the symplectites from the shock-related alkali- and silica-rich glass or shocked plagioclase, and the breakdown of Fe-rich pigeonite compositions to olivine+augite+silica below 900 °C. Reactions between Fe-Ti oxides and silicate minerals in Los Angeles and other similar basaltic Martian meteorites can control the T- fO 2 equilibration path during cooling, which may better explain the relative differences in fO 2 among the basaltic Martian meteorites.