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1.
Hydrous alteration of olivine macrocrysts in a Martian olivine phyric basalt, NWA 10416, and a terrestrial basalt from southern Colorado are examined using SEM, EPMA, TEM, and µXRD techniques. The olivines in the meteorite contain linear nanotubes of hydrous material, amorphous areas, and fluid dissolution textures quite distinct from alteration identified in other Martian meteorites. Instead, they bear resemblance to terrestrial deuteric alteration features. The presence of the hydrous alteration phase Mg‐laihunite within the olivines has been confirmed by µXRD analysis. The cores of the olivines in both Martian and terrestrial samples are overgrown by unaltered rims whose compositions match those of a separate population of groundmass olivines, suggesting that the core olivines are xenocrysts whose alteration preceded crystallization of the groundmass. The terrestrial sample is linked to deep crustal metasomatism and the “ignimbrite flare‐up” of the Oligocene epoch. The comparison of the two samples suggests the existence of an analogous relatively water‐rich magmatic reservoir on Mars.  相似文献   

2.
Abstract— Dhofar 019 is a new martian meteorite found in the desert of Oman. In texture, mineralogy, and major and trace element chemistry, this meteorite is classified as a basaltic shergottite. Olivine megacrysts are set within a groundmass composed of finer grained olivine, pyroxene (pigeonite and augite), and maskelynite. Minor phases are chromite‐ulvöspinel, ilmenite, silica, K‐rich feldspar, merrillite, chlorapatite, and pyrrhotite. Secondary phases of terrestrial origin include calcite, gypsum, celestite, Fe hydroxides, and smectite. Dhofar 019 is most similar to the Elephant Moraine (EETA) 79001 lithology A and Dar al Gani (DaG) 476/489 shergottites. The main features that distinguish Dhofar 019 from other shergottites are lack of orthopyroxene; lower Ni contents of olivine; the heaviest oxygen‐isotopic bulk composition; and larger compositional ranges for olivine, maskelynite, and spinel, as well as a wide range for pyroxenes. The large compositional ranges of the minerals are indicative of relatively rapid crystallization. Modeling of olivine chemical zonations yield minimum cooling rates of 0.5‐0.8 °C/h. Spinel chemistry suggests that crystallization took place under one of the most reduced conditions for martian meteorites, at an fO2 3 log units below the quartz‐fayalite‐magnetite (QFM) buffer. The olivine megacrysts are heterogeneously distributed in the rock. Crystal size distribution analysis suggests that they constitute a population formed under steady‐state conditions of nucleation and growth, although a few grains may be cumulates. The parent melt is thought to have been derived from partial melting of a light rare earth element‐ and platinum group element‐depleted mantle source. Shergottites, EETA79001 lithology A, DaG 476/489, and Dhofar 019, although of different ages, comprise a particular type of martian rocks. Such rocks could have formed from chemically similar source(s) and parent melt(s), with their bulk compositions affected by olivine accumulation.  相似文献   

3.
Abstract— We examine the occurrences, textures, and compositional patterns of spinels in the olivine‐phyric shergottites Sayh al Uhaymir (SaU) 005, lithology A of Elephant Moraine A79001 (EET‐A), Dhofar 019, and Northwest Africa (NWA) 1110, as well as the Iherzolitic shergottite Allan Hills (ALH) A77005, in order to identify spinel‐olivine‐pyroxene assemblages for the determination of oxygen fugacity (using the oxybarometer of Wood [1991]) at several stages of crystallization. In all of these basaltic martian rocks, chromite was the earliest phase and crystallized along a trend of strict Cr‐Al variation. Spinel (chromite) crystallization was terminated by the appearance of pyroxene but resumed later with the appearance of ulvöspinel. Ulvöspinel formed overgrowths on early chromites (except those shielded as inclusions in olivine or pyroxene), retaining the evidence of the spinel stability gap in the form of a sharp core/rim boundary (except in ALH A77005, where subsolidus reequilibration diffused this boundary). Secondary effects seen in chromites include reaction with melt before ulvöspinel overgrowth, reaction with melt inclusions, reaction with olivine hosts (in ALH A77005), and exsolution of ulvöspinel or ilmenite. All chromites experienced subsolidus Fe/Mg reequilibration. Spinel‐olivine‐pyroxene assemblages representing the earliest stages of crystallization in each rock essentially consist of the highest‐Cr#, lowest‐fe# chromites not showing secondary effects plus the most magnesian olivine and equilibrium low‐Ca pyroxene. Assemblages representing the onset of ulvöspinel crystallization consist of the lowest‐Ti ulvöspinel, the most magnesian olivine in which ulvöspinel occurs as inclusions, and equilibrium low‐Ca pyroxene. The results show that, for early crystallization conditions, oxygen fugacity (fO2) increases from SaU 005 and Dhofar 019 (?QFM ‐3.8), to EET‐A (QFM ‐2.8) and ALH A77005 (QFM ‐2.6), to NWA 1110 (QFM ‐1.7). Estimates for later conditions indicate that in SaU 005 and Dhofar 019 oxidation state did not change during crystallization. In EET‐A, there was an increase in fO2 that may have been due to mixing of reduced material with a more oxidized magma. In NWA 1110, there was a dramatic increase, indicating a non‐buffered system, possibly related to its high oxidation state. Differences in fO2 among shergottites are not primarily due to igneous fractionation but, rather, to derivation from (and possibly mixing of) different reservoirs.  相似文献   

4.
The fall and recovery of the Tissint meteorite in 2011 created a rare opportunity to examine a Martian sample with a known, short residence time on Earth. Tissint is an olivine‐phyric shergottite that accumulated olivine antecrysts within a single magmatic system. Coarse olivine grains with nearly homogeneous cores of Mg# >80 suggest slow re‐equilibration. Many macroscopic features of this sample resemble those of LAR 06319, including the olivine crystal size distribution and the presence of evolved oxide and olivine compositions. Unlike LAR 06319, however, no magmatic hydrous phases were found in the analyzed samples of Tissint. Minor and trace element compositions indicate that the meteorite is the product of closed‐system crystallization from a parent melt derived from a depleted source, with no obvious addition of a LREE‐rich (crustal?) component prior to or during crystallization. The whole‐rock REE pattern is similar to that of intermediate olivine‐phyric shergottite EETA 79001 lithology A, and could also be approximated by a more olivine‐rich version of depleted basaltic shergottite QUE 94201. Magmatic oxygen fugacities are at the low end of the shergottite range, with log fO2 of QFM‐3.5 to ‐4.0 estimated based on early‐crystallized minerals and QFM‐2.4 estimated based on the Eu in pyroxene oxybarometer. These values are similarly comparable to other depleted shergottites, including SaU 005 and QUE 94201. Tissint occupies a previously unsampled niche in shergottite chemistry: containing olivines with Mg# >80, resembling the enriched olivine‐phyric shergottite LAR 06319 in its crystallization path, and comparable to intermediate olivine‐phyric shergottite EETA 79001A, depleted olivine‐phyric shergottite DaG 476, and depleted basaltic shergottite QUE 94201 in its trace element abundances and oxygen fugacity. The apparent absence of evidence for terrestrial alteration in Tissint (particularly in trace element abundances in the whole‐rock and individual minerals) confirms that exposure to the arid desert environment results in only minimal weathering of samples, provided the exposure times are brief.  相似文献   

5.
Abstract— Elephant Moraine (EET) A79001 is the only Martian meteorite that consists of both an olivine‐phyric shergottite (lithology A) and a basaltic shergottite (lithology B). The presence of these lithologies in one rock has previously been ascribed to mixing processes (either magmatic or impact‐induced). Here we present data regarding phase changes across the contact between the lithologies. These data show that the contact is gradational and suggest that it is a primary igneous feature consistent with crystallization of a single cooling magma. We present a model to establish a petrogenetic connection between an olivine‐phyric and a basaltic shergottite through differentiation. The model involves the shallow or surface emplacement of a magma that contained pre‐eruptive solids (phenocrysts and minor xenocrysts). Subsequent differentiation via crystal settling and in situ crystallization (Langmuir 1989) resulted in a layered sequence of lithology A overlain by lithology B, with gradations in modal abundance of maskelynite (increasing from A to B) and pigeonite/maskelynite (decreasing from A to B), and a gradational change in pattern of pyroxene zonation (zones of magnesian augite separating magnesian and ferroan pigeonite appear and thicken into B) across the contact. A pigeonite phenocryst‐bearing zone near the contact in lithology B appears to be intermediate between lithology A and the bulk of lithology B (which resembles basaltic shergottite Queen Alexandra Range [QUE] 94201). Re‐examination of Sr isotopic compositions in lithology A and across the contact is required to test and constrain the model.  相似文献   

6.
Abstract— Five basaltic meteorites from the LaPaz ice field are paired on the basis of their mineralogy and texture, and represent a unique basalt type distinct from those in the Apollo or Luna sample collections. LaPaz Icefield (LAP) 02205, LAP 02224, LAP 02226, LAP 02436 and LAP 03632 all contain plagioclase, pyroxene, ilmenite, spinel, olivine, and minor troilite, metal, phosphate, baddeleyite and silica (cristobalite). Brown glassy melt veins are ubiquitous and cross the primary igneous texture. Plagioclase, the major mineral and occurring as laths in a subophitic texture, is of narrow compositional range, from An85–89. Pyroxene, also a major mineral, is strongly zoned, from augite and pigeonite cores to very iron‐rich rims. Ilmenite laths comprise approximately 3–5% of the basalts. Spinels show a large compositional range, comparable to that documented in Apollo 15 basalts, indicating an early chromite‐rich stage followed by an intermediate to late stage with Cr‐rich ulvöspinel. Relatively large, subhedral to skeletal olivine crystals (Fo46–62) are sparse, and are too Forich to be in equilibrium with the bulk rock, indicating that these are xenocrysts rather than phenocrysts. The presence of melt veins with a similar composition to the bulk rock, maskelynitized plagioclase feldspar, and metastable cristobalite indicate that these rocks underwent significant shock, between 30 and 50 GPa. Calculated oxygen fugacity, using spinel‐ilmenite‐iron metal equilibria, is within the range defined by previous studies of lunar materials. The bulk composition (low MgO) and low calculated temperatures, together with modelling calculations, indicate an origin by fractional crystallization of a more primitive low TiO2 parent liquid similar to Apollo 12 olivine basalt.  相似文献   

7.
Petrology of Martian meteorite Northwest Africa 998   总被引：1，自引：0，他引：1  
Abstract— Nakhlite Northwest Africa (NWA) 998 is an augite-rich cumulate igneous rock with mineral compositions and oxygen isotopic composition consistent with an origin on Mars. This 456-gram, partially fusion-crusted meteorite consists of (by volume) ∼75% augite (core composition Wo39En39Fs22), ∼9% olivine (Fo35), ∼7% plagioclase (Ab61An35) as anhedra among augite and olivine, ∼3.5% low-calcium pyroxenes (pigeonite and orthopyroxene) replacing or forming overgrowths on olivine and augite, ∼1% titanomagnetite, and other phases including potassium feldspar, apatite, pyrrhotite, chalcopyrite, ilmenite, and fine-grained mesostasis material. Minor secondary alteration materials include “iddingsite” associated with olivine (probably Martian), calcite crack fillings, and iron oxide/hydroxide staining (both probably terrestrial). Shock effects are limited to minor cataclasis and twinning in augite. In comparison to other nakhlites, NWA 998 contains more low-calcium pyroxenes and its plagioclase crystals are blockier. The large size of the intercumulus feldspars and the chemical homogeneity of the olivine imply relatively slow cooling and chemical equilibration in the late- and post-igneous history of this specimen, and mineral thermometers give subsolidus temperatures near 730 °C. Oxidation state was near that of the QFM buffer, from about QFM-2 in earliest crystallization to near QFM in late crystallization, and to about QFM + 1.5 in some magmatic inclusions. The replacement or overgrowth of olivine by pigeonite and orthopyroxene (with or without titanomagnetite), and the marginal replacement of augite by pigeonite, are interpreted to result from late-stage reactions with residual melts (consistent with experimental phase equilibrium relationships). Apatite is concentrated in planar zones separating apatite-free domains, which suggests that residual magma (rich in P and REE) was concentrated in planar (fracture?) zones and possibly migrated through them. Loss of late magma through these zones is consistent with the low bulk REE content of NWA 998 compared with the calculated REE content of its parent magma.  相似文献   

8.
Abstract— Northwest Africa (NWA) 1670, contains olivines of up to 5 mm in size representing about 30% of the studied section. With subordinate clinopyroxene and chrome‐spinel microphenocrysts (0.2‐0.5 mm), they represent a xenocrystic association. Phenocrysts are surrounded by a groundmass, predominantly comprising bundles of plagioclase and clinopyroxene (typically 20 × 200 μm crystals). Olivine and kirschsteinite are present in the groundmass in lesser amounts. The olivine xenocrysts (Fo90) are significantly fractured and show mosaicism for their major part, the remaining showing faint undulatory extinction. They are surrounded with a rim of 100–200 μm zoned down to Fo80 and overgrown with serrated olivine, Fo80 to Fo60 (about 100 μm). Olivine in the groundmass is zoned from Mg# 0.55 to 0.15; its CaO content ranges 2.0 to 8.4%. Subcalcic kirschsteinite is zoned from Mg# 0.13 to 0.03, CaO increasing from 15.8 to 21.3%. Pyroxenes xenocrysts (Mg# = 0.77) are superseded in the groundmass by less magnesian pyroxenes, Mg# 0.61 to 0.17, with an average FeO/ MnO of 98. Their compositions range from En30 Fs22 Wo27 Al‐Ts28 Ti‐Ts2 to En2 Fs37 Wo22 Al‐Ts40 Ti‐Ts1. Anorthite microcrysts (An99‐100) are restricted to the groundmass. Accessories are pyrrhotite, kamacite, Ca‐phosphate, titanomagnetite, hercynite and Ca‐carbonate. The bulk chemical composition confirms that NWA 1670 corresponds to a normal angrite melt that incorporated olivine. High Mg olivine xenocrysts and the associated mineralogy are typical of angrites. We suggest that it is an impact melt with relict phenocrysts. The strong silica undersaturation, the presence of Fo90 olivine xenocrysts and carbonate support their derivation as melilite‐like melts in the presence of carbonate.  相似文献   

9.
Abstract— Experiments on a Martian basalt composition show that Dv augite/melt is greater than Dv pigeonite/melt in samples equilibrated under the same fO2 conditions. This increase is due to the increased availability of elements for coupled substitution with the V3+ or V4+ ions, namely A1 and Na. For this bulk composition, both A1 and Na are higher in concentration in augite compared with pigeonite; therefore more V can enter augite than pigeonite. Direct valence state determination by XANES shows that the V3+ and V4+ are the main V species in the melt at fO2 conditions of IW‐1 to IW+3.5, whereas pyroxene grains at IW‐1, IW, and IW+1 contain mostly V3+. This confirms the idea that V3+ is more compatible in pyroxene than V4+. The XANES data also indicates that a small percentage of V2+ may exist in melt and pyroxene at IW‐1. The similar valence of V in glass and pyroxene at IW‐1 suggests that V2+ and V3+ may have similar compatibilities in pyroxene.  相似文献   

10.
Abstract— In 1998, Dar al Gani (DaG) 476 was found in the Libyan desert. The meteorite is classified as a basaltic shergottite and is only the 13th martian meteorite known to date. It has a porphyritic texture consisting of a fine‐grained groundmass and larger olivines. The groundmass consists of pyroxene and feldspathic glass. Minor phases are oxides and sulfides as well as phosphates. The presence of olivine, orthopyroxene, and chromite is a feature that DaG 476 has in common with lithology A of Elephant Moraine (EET) A79001. However, in DaG 476, these phases appear to be early phenocrysts rather than xenocrysts. Shock features, such as twinning, mosaicism, and impact‐melt pockets, are ubiquitous. Terrestrial weathering was severe and led to formation of carbonate veins following grain boundaries and cracks. With a molar MgO/(MgO + FeO) of 0.68, DaG 476 is the most magnesian member among the basaltic shergottites. Compositions of augite and pigeonite and some of the bulk element concentrations are intermediate between those of lherzolitic and basaltic shergottites. However, major elements, such as Fe and Ti, as well as LREE concentrations are considerably lower than in other shergottites. Noble gas concentrations are low and dominated by the mantle component previously found in Chassigny. A component, similar to that representing martian atmosphere, is virtually absent. The ejection age of 1.35 ± 0.10 Ma is older than that of EETA79001 and could possibly mark a distinct ejection. Dar al Gani 476 is classified as a basaltic shergottite based on its mineralogy. It has a fine‐grained groundmass consisting of clinopyroxene, pigeonite and augite, feldspathic glass and chromite, Ti‐chromite, ilmenite, sulfides, and whitlockite. Isolated olivine and single chromite grains occur in the groundmass. Orthopyroxene forms cores of some pigeonite grains. Shock‐features, such as shock‐twinning, mosaicism, cracks, and impact‐melt pockets, are abundant. Severe weathering in the Sahara led to significant formation of carbonate veins crosscutting the entire meteorite. Dar al Gani 476 is distinct from other known shergottites. Chemically, it is the most magnesian member among known basaltic shergottites and intermediate in composition for most trace and major elements between Iherzolitic and basaltic shergottites. Unique are the very low bulk REE element abundances. The CI‐normalized abundances of LREEs are even lower than those of Iherzolitic shergottites. The overall abundance pattern, however, is similar to that of QUE 94201. Textural evidence indicates that orthopyroxene, as well as olivine and chromite, crystallized as phenocrysts from a magma similar in composition to that of bulk DaG 476. Whether such a magma composition can be a shergottite parent melt or was formed by impact melting needs to be explored further. At this time, it cannot entirely be ruled out that these phases represent relics of disaggregated xenoliths that were incorporated and partially assimilated by a basaltic melt, although the texture does not support this possibility. Trapped noble gas concentrations are low and dominated by a Chassigny‐like mantle component. Virtually no martian atmosphere was trapped in DaG 476 whole‐rock splits. The exposure age of 1.26 ± 0.09 Ma is younger than that of most shergottites and closer to that of EETA79001. The ejection age of 1.35 ± 0.1 Ma could mark another distinct impact event.  相似文献   

11.
Abstract— Mineralogy, major element compositions of minerals, and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite Northwest Africa (NWA) 032, an unbrecciated basalt found in October 1999. The rock consists predominantly of olivine, pyroxene and chromite phenocrysts, set in a crystalline groundmass of feldspar, pyroxene, ilmenite, troilite and trace metal. Whole‐rock shock veins comprise a minor, but ubiquitous portion of the rock. Undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro‐faults in groundmass and phenocrysts also are attributed to shock. Several geochemical signatures taken together indicate unambiguously that NWA 032 originated from the Moon. The most diagnostic criteria include whole‐rock oxygen isotopic composition and ratios of Fe/Mn in the whole rock, olivine, and pyroxene. A lunar origin is documented further by the presence of Fe‐metal, troilite, and ilmenite; zoning to extremely Fe‐rich compositions in pyroxene; the ferrous oxidation state of all Fe in pyroxene; and the rare earth element (REE) pattern with a well‐defined negative europium anomaly. This rock is similar in major element chemistry to basalts from Apollo 12 and 15, but is enriched in light REE and has an unusually high Th/Sm ratio. Some Apollo 14 basalts yield a closer match to NWA 032 in REE patterns, but have higher concentrations of Al2O3. Ar‐Ar step release results are complex, but yield a whole‐rock age of ?2.8 Ga, suggesting that NWA 032 was extruded at 2.8 Ga or earlier. This rock may be the youngest sample of mare basalt collected to date. Noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites. In short, the geochemical and petrographic features of NWA 032 are not matched by Apollo or Luna samples, nor by previously identified lunar meteorites, indicating that it originates from a previously unsampled mare deposit. Detailed assessment of petrographic features, olivine zoning, and thermodynamic modelling indicate a relatively simple cooling and crystallization history for NWA 032. Chromite‐spinel, olivine, and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 °C/h based on the polyhedral morphology of olivine. Comparison of olivine size with crystal growth rates and preserved Fe‐Mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days. Plumose textures in groundmass pyroxene, feldspar, and ilmenite, and Fe‐rich rims on the phenocrysts formed during rapid crystallization (cooling rates ?20 to 60 °C/h) after eruption.  相似文献   

12.
Abstract— A petrologic and TEM study of a remarkable dark inclusion (DI) in the Ningqiang CV3 chondrite reveals that it is a mixture of highly primitive solar nebula materials. The DI contains two lithologies. The first, lithology A, contains micron‐sized olivine and pyroxene grains rimmed by amorphous materials with compositions similar to the underlying crystalline grains. The second, lithology B, appears to preserve the mineralogy of lithology A before formation of the amorphous rims. Overall, the Ningqiang DI appears to record the following processes: 1) formation (condensation and Fe‐enrichment) of olivine crystals in the nebula with compositions of Fo42–62; 2) irradiation, resulting in amorphitization of the olivine and pyroxene to varying degrees; 3) partial annealing, resulting in formation of fairly large, euhedral olivine and pyroxene grains with remnant amorphous sharply‐bounded rims; 4) in some cases, prolonged annealing, resulting in the formation of microcrystalline olivine or pyroxene rims. The latter annealing would have been a natural consequence of irradiation near the critical temperature for olivine; and 5) mixture of the above materials (lithology A) with nebular condensate high‐Ca pyroxene and olivine, which escaped nebular processing, to become lithology B. We suggest that the amorphous rims in lithology A formed in an energetic solar event such as a bi‐polar outflow or FU‐orionis flare.  相似文献   

13.
Abstract– Northwest Africa (NWA) 4797 is an ultramafic Martian meteorite composed of olivine (40.3 vol%), pigeonite (22.2%), augite (11.9%), plagioclase (9.1%), vesicles (1.6%), and a shock vein (10.3%). Minor phases include chromite (3.4%), merrillite (0.8%), and magmatic inclusions (0.4%). Olivine and pyroxene compositions range from Fo66–72,En58–74Fs19–28Wo6–15, and En46–60Fs14–22Wo34–40, respectively. The rock is texturally similar to “lherzolitic” shergottites. The oxygen fugacity was QFM?2.9 near the liquidus, increasing to QFM?1.7 as crystallization proceeded. Shock effects in olivine and pyroxene include strong mosaicism, grain boundary melting, local recrystallization, and pervasive fracturing. Shock heating has completely melted and vesiculated igneous plagioclase, which upon cooling has quench‐crystallized plagioclase microlites in glass. A mm‐size shock melt vein transects the rock, containing phosphoran olivine (Fo69–79), pyroxene (En44–51Fs14–18Wo30–42), and chromite in a groundmass of alkali‐rich glass containing iron sulfide spheres. Trace element analysis reveals that (1) REE in plagioclase and the shock melt vein mimics the whole rock pattern; and (2) the reconstructed NWA 4797 whole rock is slightly enriched in LREE relative to other intermediate ultramafic shergottites, attributable to local mobilization of melt by shock. The shock melt vein represents bulk melting of NWA 4797 injected during pressure release. Calculated oxygen fugacity for NWA 4797 indicates that oxygen fugacity is decoupled from incompatible element concentrations. This is attributed to subsolidus re‐equilibration. We propose an alternative nomenclature for “lherzolitic” shergottites that removes genetic connotations. NWA 4797 is classified as an ultramafic poikilitic shergottite with intermediate trace element characteristics.  相似文献   

14.
Abstract— New data are reported from five previously unanalyzed Apollo 12 mare basalts that are incorporated into an evaluation of previous petrogenetic models and classification schemes for these basalts. This paper proposes a classification for Apollo 12 mare basalts on the basis of whole-rock Mg# [molar 100*(Mg/(Mg+Fe))] and Rb/Sr ratio (analyzed by isotope dilution), whereby the ilmenite, olivine, and pigeonite basalt groups are readily distinguished from each other. Scrutiny of the Apollo 12 feldspathic “suite” demonstrates that two of the three basalts previously assigned to this group (12031, 12038, 12072) can be reclassified: 12031 is a plagioclase-rich pigeonite basalt (Nyquist et al, 1979); and 12072 is an olivine basalt Only basalt 12038 stands out as a unique sample (Nyquist et al., 1981) to the Apollo 12 she, but whether this represents a single sample from another flow at the Apollo 12 site or is exotic to this site is equivocal. The question of whether the olivine and pigeonite basalt suites are co-magmatic is addressed by incompatible trace-element chemistry: the trends defined by these two suites when Co/Sm and Sm/Eu ratios are plotted against Rb/Sr ratio demonstrate that these two basaltic types cannot be co-magmatic. Crystal fractionation/accumulation paths have been calculated and show that neither the pigeonite, olivine, or ilmenite basalts are related by this process. Each suite requires a distinct and separate source region. This study also examines sample heterogeneity and the degree to which whole-rock analyses are representative, which is critical when petrogenetic interpretation is undertaken. Sample heterogeneity has been investigated petrographically (inhomogeneous mineral distribution) with consideration of duplicate analyses, and whether a specific sample (using average data) plots consistently upon a fractionation trend when a number of different compositional parameters are considered. Using these criteria, four basalts have been identified where reported analyses are not representative of the whole-rock composition: 12005, an ilmenite basalt; 12006 and 12036, olivine basalts; and 12031 previously classified as a feldspathic basalt, but reclassified as part of the pigeonite suite (Nyquist et al., 1979).  相似文献   

15.
Northwest Africa (NWA) 7397 is a newly discovered, enriched, lherzolitic shergottite, the third described example of this group. This meteorite consists of two distinct textural lithologies (1) poikilitic—comprised of zoned pyroxene oikocrysts, with chadacrysts of chromite and olivine, and (2) nonpoikilitic—comprised of olivine, low‐Ca and high‐Ca pyroxene, maskelynite, and minor abundances of merrillite, spinel, ilmenite, and pyrrhotite. The constant Ti/Al ratios of pyroxene oikocrysts suggests initial crystallization of the poikilitic lithology at depth (equivalent to pressures of approximately 10 kbar), followed by crystallization of the nonpoikilitic lithology at shallower levels. Oxygen fugacity conditions become more oxidizing during crystallization ranging from fO2 conditions of approximately QFM‐2 to QFM‐0.7. Magma calculated to be in equilibrium with the major rock‐forming minerals is LREE‐enriched relative to depleted or intermediate shergottites and has flat overall profiles. Therefore, we suggest that the parental magma for NWA 7397 had sampled an enriched, oxidized, Martian geochemical source, similar to that of other enriched basaltic and olivine‐phyric shergottites. We present a polybaric formation model for the lherzolitic shergottite NWA 7397, to account for the petrologic constraints. Three successive stages in the development of NWA 7397 are discussed (1) formation of a REE‐enriched parental magma from a distinct Martian mantle reservoir; (2) magma ponding and development of a staging chamber concomitant with initial crystallization of the poikilitic lithology; and (3) magma ascent to the near surface, with entrainment of cumulates from the staging chamber and subsequent crystallization of the nonpoikilitic lithology en route to the surface.  相似文献   

16.
New data from a petrological and geochemical examination of 12 coarse basaltic fines from the Apollo 12 soil sample 12023,155 provide evidence of additional geochemical diversity at the landing site. In addition to the bulk chemical composition, major, minor, and trace element analyses of mineral phases are employed to ascertain how these samples relate to the Apollo 12 lithological basalt groups, thereby overcoming the problems of representativeness of small samples. All of the samples studied are low‐Ti basalts (0.9–5.7 wt% TiO2), and many fall into the established olivine, pigeonite, and ilmenite classification of Apollo 12 basaltic suites. There are five exceptions: sample 12023,155_1A is mineralogically and compositionally distinct from other Apollo 12 basalt types, with low pigeonite REE concentrations and low Ni (41–55 ppm) and Mn (2400–2556 ppm) concentrations in olivine. Sample 12023,155_11A is also unique, with Fe‐rich mineral compositions and low bulk Mg# (=100 × atomic Mg/[Mg+Fe]) of 21.6. Sample 12023,155_7A has different plagioclase chemistry and crystallization trends as well as a wider range of olivine Mg# (34–55) compared with other Apollo 12 basalts, and shows greater similarities to Apollo 14 high‐Al basalts. Two other samples (12023,155_4A, and _5A) are similar to the Apollo 12 feldspathic basalt 12038, providing additional evidence that feldspathic basalts represent a lava flow proximal to the Apollo 12 site rather than material introduced by impacts. We suggest that at least one parent magma, and possibly as many as four separate parent magmas, are required in addition to the previously identified olivine, pigeonite, and ilmenite basaltic suites to account for the observed chemical diversity of basalts found in this study.  相似文献   

17.
Tissint is an olivine‐phyric shergottite from an incompatible element depleted Martian mantle source. Oxythermobarometry applied to Tissint mineral phases demonstrates that the Tissint magma underwent an increase in oxygen fugacity, from ~3.5 log units below the quarz‐fayalite‐magnetite (QFM ) buffer during the early stages of crystallization, to QFM ?1.4 during the latter stages. This is the first time that such an oxidation event has been observed in a depleted shergottite. The reason for the oxidation event is unclear; however, calculations using the MELTS thermodynamic model suggest that auto‐oxidation is insufficient to cause more than ~1 log unit of oxidation, and therefore an external oxidation mechanism—such as oxidation by degassing—is required. If volatiles are responsible for the oxidation, then it indicates that volatiles are not exclusively tied to the enriched Martian mantle reservoir. A series of experiments using the Tissint parental magma were carried out under fixed (isothermal) or variable (cooling rate) temperature control, and at either reducing (QFM ?3.2) or oxidizing (QFM ?1) redox conditions. The observed liquid line of descent supports a potential genetic relationship between basaltic shergottites and olivine‐phyric shergottites. A peritectic relation where olivine is resorbed to form pyroxene is favored by increased oxygen fugacity; if oxidation during crystallization is more common than presently believed, it may explain why olivine is typically anhedral in olivine‐phyric shergottites. Results from a cooling‐rate experiment in which the oxygen fugacity was changed during the latter stages of crystallization resulted in olivine with a Cr compositional profile consistent with oxidized isothermal experiments, despite forming primarily under reducing conditions. A similar profile is observed in Tissint olivines, consistent with its redox history. Our results provide insights into the potential influence of oxidation events on the compositional zoning of minor or trace elements in olivine in olivine‐phyric basalts.  相似文献   

18.
Abstract— Mixing models using major and trace elements show that the bulk composition of lithology A (xenocryst-bearing magnesian basalt) of Elephant Moraine A79001 (EETA79001) can be reasonably approximated as a simple mixture of ～44% EETA79001 lithology B (ferroan basalt) and ～56% of Allan Hills A77005 (ALHA7705) light lithology (incompatible element-poor lherzolite). Micro-instrumental neutron activation analysis (INAA) data on xenocryst-free groundmass samples of lithology A show that about 20–25% of the melt phase could be dissolved lherzolite. The bulk and groundmass samples of lithology A have excesses in Au, which indicates either meteoritic contamination or addition by some unknown martian geochemical process. Previous workers have suggested that lithology A was formed by either assimilation of cumulates (like ALHA77005), by a basalt (like lithology B), or by mixing of basaltic and lherzolitic magmas. The former scenario is energetically improbable and unlikely to explain the normal Fe/Mg zonation in lithology A groundmass pyroxenes, whereas the latter scenario is unlikely to satisfy the constraints of the mixing model indicating the ultramafic component is poor in incompatible elements. We suggest rather that EETA79001 lithology A is an impact melt composed dominantly of basalt like lithology B and lherzolitic cumulates like the trace-element-poor fraction of ALHA77005 or Y-793605. This model can satisfy the energetic, petrologic, and geochemical constraints imposed by the samples. If EETA79001 lithology A is an impact melt, this would have considerable consequences for current models of martian petrologic evolution. It would call into question the generally accepted age of magmatism of martian basalts and preclude the use of lithology A groundmass as a primary martian basalt composition in experimental studies. Regardless, the latter is required because lithology A groundmass is a hybrid composition.  相似文献   

19.
Remotely sensed observations from recent missions (e.g., GRAIL, Kaguya, Chandrayaan‐1) have been interpreted as indicating that the deep crust and upper mantle are close to or at the lunar surface in many large impact basins (e.g., Crisium, Apollo, Moscoviense). If this is correct, the capability of either impact or volcanic processes to transport mantle lithologies to the lunar surface should be enhanced in these regions. Somewhat problematic to these observations and interpretations is that examples of mantle lithologies in the lunar sample collection (Apollo Program, Luna Program, lunar meteorites) are at best ambiguous. Dunite xenoliths in high‐Ti mare basalt 74275 are one of these ambiguous examples. In this high‐Ti mare basalt, olivine occurs in three generations: olivine associated with dunite xenoliths, olivine megacrysts, and olivine microphenocrysts. The dunite xenoliths are anhedral in shape and are generally greater than 800 μm in diameter. The interior of the xenoliths are fairly homogeneous with regard to many divalent cations. For example, the Mg# (Mg/Mg + Fe × 100) ranges from 82 to 83 in their interiors and decreases from 82 to 68 over the 10–30 μm wide outer rim. Titanium and phosphorus X‐ray maps of the xenolith illustrate that these slow diffusing elements preserve primary cumulate zoning textures. These textures indicate that the xenoliths consist of many individual olivine grains approximately 150–200 μm in diameter with low Ti, Al, and P cores. These highly incompatible elements are enriched in the outer Fe‐rich rims of the xenoliths and slightly enriched in the rims of the individual olivine grains. Highly compatible elements in olivine such as Ni exhibit a decrease in the rim surrounding the xenolith, an increase in the incompatible element depleted cores of the individual olivine grains, and a slight decrease in the “interior rims” of the individual olivine grains. Inferred melt composition, liquid lines of descent, and zoning profiles enable the reconstruction of the petrogenesis of the dunite xenoliths. Preservation of primary magmatic zoning (Ti, P, Al) and lack of textures similar to high‐pressure mineral assemblages exhibited by the Mg‐suite (Shearer et al. 2015) indicate that these xenoliths do not represent deep crustal or shallow mantle lithologies. Further, they are chemically and mineralogically distinct from Mg‐suite dunites identified from the Apollo 17 site. More likely, they represent olivine cumulates that crystallized from a low‐Ti mare basalt at intermediate to shallow crustal levels. The parent basalt to the dunite xenolith lithology was more primitive than low‐Ti basalts thus far returned from the Moon. Furthermore, this parental magma and its more evolved daughter magmas are not represented in the basalt sample suite returned from the Taurus‐Littrow Valley by the Apollo 17 mission. The dunite xenolith records several episodes of crystallization and re‐equilibration. During the last episode of re‐equilibration, the dunite cumulate was sampled by the 74275 high‐Ti basalt and transported over a period of 30–70 days to the lunar surface.  相似文献   

20.
We evaluate the relationship between the intensity of remanent magnetization and fO2 in natural and synthetic Mars meteorites. The olivine‐phyric shergottite meteorite Yamato 980459 (Y‐980459) and a sulfur‐free synthetic analog (Y‐98*) of identical major element composition were analyzed to explore the rock magnetic and remanence properties of a basalt crystallized from a primitive melt, and to explore the role of magmatic and alteration environment fO2 on Mars crustal anomalies. The reducing conditions under which Y‐980459 is estimated to have formed (QFM‐2.5; Shearer et al. 2006) were replicated during the synthesis of Y‐98*. Y‐980459 contains pyrrhotite and chromite. Chromite is the only magnetic phase in Y‐98*. The remanence‐carrying capacity of Y‐980459 is comparable to other shergottites that formed in the fO2 range of QFM‐3 to QFM‐1. The remanence‐carrying capacity of these low fO2 basalts is 1–2 orders of magnitude too weak to account for the intense crustal anomalies observed in Mars's southern cratered highlands. Moderately oxidizing conditions of >QFM‐1, which are more commonly observed in nakhlites and Noachian breccias, are key to generating either a primary igneous assemblage or secondary alteration assemblage capable of acquiring an intense remanent magnetization, regardless of the basalt character or thermal history. This suggests that if igneous rocks are responsible for the intensely magnetized crust, these oxidizing conditions must have existed in the magmatic plumbing systems of early Mars or must have existed in the crust during secondary processes that led to acquisition of a chemical remanent magnetization.  相似文献   

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