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1.
A study was undertaken to determine the chronology, petrogenesis and relationships among the shergottites, Shergotty and Zagami and the unique achondrite ALHA77005. These meteorites are the product of a variety of complex processes.Petrogenesis: Chondrite-normalized abundance patterns of Shergotty and Zagami are very similar and show pronounced depletions of both the light REE (La-Nd) and heavy REE (Dy-Lu) relative to Sm-Gd. These characteristic depletions are even more pronounced for ALHA77005. The light REE depletion is qualitatively consistent with the presence of cumulus pyroxene and/or olivine in these meteorites, but trace element models show that the parental magmas of all three meteorites were probably also light REE depleted. Both trace element model calculations and combined Rb-Sr and Sm-Nd isotopic systematics show that the meteorites could not have been co-magmatic nor can ALHA77005 be representative of the source material of the shergottites. Light REE depletion of the parental magmas also implies light REE depletion of the source material. The Sm-Nd systematics of the shergottites require a time-averaged sub-chondritic (light REE enriched) Sm-Nd ratio since 4.6 AE ago. The Sm-Nd systematics of ALHA77005 permit a time-averaged super-chondritic (light REE depleted) Sm/Nd ratio if its crystallization age is less than TICE = 0.72 AE.Chronology. Rb-Sr internal isochrons for all three meteorites and a Sm-Nd internal isochron for Zagami are concordant at ~ 180 Myr. 39Ar-40Ar plateau ages of Shergotty and Zagami maskelynite are ~250–260 Myr. These ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite. The concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity. The time of this event is most precisely given by the Rb-Sr age of 180 ± 4 Myr for Zagami.The crystallization ages of the meteorites were not precisely determined. Extreme upper limits are determined by Sm-Nd model ages relative to an eucrite initial 143Nd144Nd = 0.505835 at 4.6 AE ago. These model ages for Shergotty, Zagami and ALHA77005 are 3600, 3500 and 2850 Myr, respectively. The Sm-Nd whole rock age of 1340 ± 60 Myr for the three meteorites gives the crystallization age if the Sm/Nd ratios of the precursor materials were always the same. We consider this 1340 Myr age as a “best estimate” upper limit. “Best estimate” lower limits for Shergotty and Zagami are taken from the average 39Ar-40Ar ages of 1200 and 900 Myr of pyroxene separates. The average 39Ar-40Ar age of a whole rock sample of ALHA77005 was 1600 Myr and can be partitioned between a low temperature (feldspar) phase and a high temperature (olivine + pyroxene + inclusions) “phase”. The average apparent 39Ar-40Ar age of the low temperature phase is ~1050 Myr, which is chosen as the “best estimate” lower limit to the age. The crystallization ages of Shergotty, Zagami and ALHA77005 probably lie within the ranges of 1200–1300, 900–1300 and 1000–1300 Myr, respectively. The Rb-Sr whole rock age of 4400 ± 400 Myr and single-stage BABI model ages of ~4800–5100 Myr are interpreted as reflecting differentiation of the parent body at ~4600 Myr ago.The complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history.  相似文献   

2.
A New Martian Meteorite from Antarctica:Grove Mountains (GRV) 020090   总被引：15，自引：0，他引：15  
Reported in this paper are the petrology and mineral chemistry of GRV 020090, the second Martian meteorite collected from the Grove Mountains, Antarctica. This meteorite, with a mass of 7.54 g, is completely covered by a black and glazy fusion crust. It has two distinct textural regions. The interstitial region is composed of euhedral grains of olivine, pigeonite, and anhedral interstitial maskelynite, with minor chromite, augite, phosphates and troilite. The poikilitic region consists of three clasts of pyroxenes, each of which has a pigeonite core and an augite rim. A few grains of subhedral to rounded olivine and euhedral chromite are enclosed in the pyroxene oikocrysts. GRV 020090 is classified as a new member of lherzolitic shergottites based on the modal composition and mineral chemistry. This work will shed light on the composition of Martian crust and magmatism on the Mars.  相似文献   

3.
Zagami consists of a series of increasingly evolved magmatic lithologies. The bulk of the rock is a basaltic lithology dominated by pigeonite (Fs28.7–54.3), augite (Fs19.5–35.0) and maskelynite (Ab42–53). Approximately 20 vol.% of Zagami is a basaltic lithology containing FeO-enriched pyroxene (pigeonite, Fs27.0–80.8) and mm- to cm-sized late-stage melt pockets. The melt pockets are highly enriched in olivine-bearing intergrowths, mesostases, phosphates (both whitlockite and water-bearing apatite), Fe,Ti-oxides and sulfides. The systematic increases in abundances of late-stage phases, Fs and incompatible element (e.g., Y and the REEs) contents of pigeonite, Ab contents of maskelynite, and FeO concentrations of whitlockite all point to a fractional crystallization sequence.The crystallization order in Zagami and the formation of these various lithologies was controlled by the abundances of iron, phosphorus, and calcium. During fractional crystallization, iron and phosphorus enrichment occurred, ultimately forcing the crystallization of calcium phosphates and olivine-bearing intergrowths. The limited amount of calcium in the melt and its partitioning between phosphates and silicates controlled the crystallization of phosphates, plagioclase, pigeonite, and augite. The presence of these FeO-enriched, water-poor late-stage lithologies has important implications. Discrepancies between experimental and petrologic studies to infer the history of basaltic shergottites may be partially explained by the use of starting compositions which are too FeO-poor in the experimental studies. The water-poor nature of the late-stage melt pockets suggests crystallization from a very dry magma, although whether this magma was always dry or experienced significant near-surface degassing remains an open question. Finally, the presence of fractional crystallization products within Zagami suggests that this may be a relatively common process on Mars.  相似文献   

4.
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, Fo77-52) and prismatic zoned pyroxene crystals with Wo3En71 in the cores to Wo8-30En23-45 at the rims. The groundmass is composed of finer grained olivine (<0.25 mm, Fo62-46), 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.  相似文献   

5.
We report on the petrology and chemistry of North West Africa 1068 (NWA 1068), a shergottite recently recovered in Morocco. This meteorite has a total known mass of about 577 g and comprises 23 fragments. The largest fragment is a greenish-brown rock devoid of fusion crust. It displays a porphyritic texture consisting of a fine-grained groundmass and olivine grains. Excluding the impact melt pockets and the minor carbonate veins produced by terrestrial weathering, modal analyses indicate the following mineral proportions: 52 vol% pyroxenes, 22% maskelynite, 21% olivine, 2% phosphates (merrillite and chlorapatite), 2% opaque oxides (mainly ilmenite and chromite) and sulfides, and 1% K-rich mesostasis. Olivines with various habits occur as clusters often associated with chromite, or single crystals ranging in size from 50 μm to 2 millimeters (“megacrysts”). These crystals originate probably from disrupted cumulates with strong affinities with peridotitic shergottites.The bulk composition of NWA 1068 has been determined for 45 elements. It is an Al-poor ferroan basaltic rock, rich in MgO. Its major element composition is similar to those reported for other picritic shergottites, especially EETA79001A. Furthermore, key element ratios such as Fe/Mn (45), Al/Ti (6.6), Na/Ti (1.83), Ga/Al (4.4 × 10−4) and Na/Al (0.28) are typical of Martian meteorites. The trace elements demonstrate unambiguously that NWA 1068 is unpaired with any of the other hot desert finds: it is the first picritic shergottite with a REE pattern similar to those of Shergotty, Zagami, and Los Angeles.Incompatible element abundances indicate that NWA 1068 was not formed from a “primitive” shergottitic melt. It derived more likely from a basaltic shergottite, which has accumulated (and possibly partly digested) fragments of an olivine-rich lithology, in full agreement with major element abundances and petrographical interpretations.  相似文献   

6.
Lithium abundances and isotopic compositions were measured by ion microprobe in individual grains of pyroxene, and in a few maskelynites and Ca-phosphates grains, from the Martian meteorite Northwest Africa 480 (NWA 480).In pyroxenes Li abundances are nearly constant from core to rim with concentrations ranging between 3 and 4 μg/g. In contrast, a significant isotopic zoning is observed with δ7Li values increasing within single crystals from ∼ −17‰ in the core to ∼ +10‰ in the rim, most of the variability being observed in the core. Plagioclase (now maskelynite) and phosphate crystals, which co-crystallized with the pyroxene rims, display similar δ7Li values. Because of the incompatible behavior of Li, the present constancy of Li concentrations within zoned pyroxenes rules out any simple crystallization model in a closed system for Li. The large Li isotopic variations observed within pyroxenes support this conclusion. There is no evidence in support of secondary alteration of NWA 480 to explain the Li isotopic variations, which thus most likely reflect magmatic processes on Mars. Degassing might explain the Li systematics observed in NWA 480 pyroxenes. Because Li has a strong affinity with water-rich fluids, a significant loss of Li from NWA 480 parental melt can happen upon melt emplacement and cooling. Such a Li loss could compensate the effect of crystal fractionation and thus help to maintain constant the Li content of the melt. Li isotopic fractionation is anticipated to accompany this process, 7Li being depleted relative to 6Li in the volatile phase. The magnitude of the isotopic change of the fractionating melts is difficult to predict because it depends on the value of the Li isotopic fractionation and on the amount of Li loss, but at first glance it seems consistent with the increase of δ7Li values observed in NWA 480 pyroxenes with increasing fractionation. The present data suggest that degassing prevailed not only during the crystallization of shergottites like Zagami and Shergotty, but also during the crystallization of the other types of basaltic shergottites.  相似文献   

7.
Shergottites contain cumulus pigeonite and augite, probably without cumulus plagioclase and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to plagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, crystallization sequences and inferred source region compositions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components.The bulk composition of the Earth's upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basalts display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth's upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth's upper mantle which attribute its oxidation state, its siderophile element abundances and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.  相似文献   

8.
H isotope measurements of carbonate, phosphate, feldspathic and mafic glasses, and post-stishovite silica phase in the shergottites Zagami, Shergotty, SaU 005, DaG 476, ALHA 77005 and EETA 79001, as well as in Chassigny and ALH 84001, show that all these phases contain deuterium-enriched water of extraterrestrial origin. The minerals and glasses analyzed may contain an initial primary hydrogen component, but their isotopic composition was modified to varying degrees by three different processes: interaction with a fractionated exchangeable water reservoir on Mars, hydrogen devolatilization by impact melting, and terrestrial contamination. Positive correlations between δD and water abundance in feldspathic glass and post-stishovite silica in Zagami, Shergotty, and SaU 005 is indicative of mixing of a high δD component (3000-4000‰) and a less abundant, low δD component (∼0‰). The high δD component is primarily derived from the Martian exchangable reservoir, but may also have been influenced by isotopic fractionation associated with shock-induced hydrogen loss. The low δD component is either a terrestrial contaminant or a primary “magmatic” component. The negative correlation between δD and water abundances in mafic and feldspathic glasses in ALH 84001, ALHA 77005, and EETA 79001 is consistent with the addition of a low δD terrestrial contaminant to a less abundant high-deuterium Martian component. The low δD of magmatic glass in melt inclusions suggests that the δD of Martian parent magma was low and that the initial H isotope signature of Mars may be similar to that of Earth.  相似文献   

9.
李毅恒  汪在聪  何琦 《地质学报》2021,95(9):2889-2900
NWA 8545 是一块玄武质无球粒陨石,它与碳质陨石(CC)NWA 011 成对.CC 被认为是来自于外太阳系的一类物质,由于同位素异常,它们区别于来自内太阳系的非碳质陨石(NC).NWA 011 及其成对陨石作为CC中稀有的玄武质无球粒陨石,其记录的岩浆过程可以被用来研究外太阳系早期行星母体的岩浆活动.本文利用扫描电镜、电子探针和激光剥蚀电感耦合等离子质谱仪(LA-ICP-MS)对NWA 8545 中的辉石、斜长石和陨磷钙钠石进行岩相学以及原位主微量元素的分析,并根据矿物模式丰度计算全岩稀土元素含量.电子探针结果显示NWA 8545 与 HED族陨石Eucrite(钙长辉长无球粒陨石)具有相似的主量元素特征,同时其岩相学与5 型Eucrite类似.激光微量数据表明辉石、斜长石和陨磷钙钠石的稀土元素配分都表现出略微的Ce异常,但其辉石的Ba、Sr等元素并未出现明显的富集现象,即该陨石受地球风化作用影响不明显.利用辉石和斜长石的稀土元素含量,计算平衡熔体的成分,显示其平衡熔体的成分都与全岩的成分比较接近,可以认为两者是在封闭的体系下接近同时结晶.结合变质过程和母体岩浆的成分,本文认为NWA 8545 是由其母体岩浆在经历分离结晶过程后喷发到母体行星表面冷却形成的.  相似文献   

10.
Whether water has played a role in the petrogenesis of martian basalts remains a subject of significant debate. Estimates of pre-eruptive water concentrations in martian magmas are impeded by the effects of degassing and shock. However, zoning trends of light lithophile elements (LLE) in pyroxene have been interpreted as evidence for the degassing of magmatic water, on the basis of the soluble behavior of these elements in hydrothermal fluids. We provide ion microprobe analyses of LLE in pyroxene in the Zagami and Shergotty martian basalts, complemented by detailed electron microprobe analyses and major-element X-ray maps, to independently verify the zoning of LLE and its relationship to texture and major-element variations. Our results corroborate previous results; specifically, that Li concentrations are lower in rims than cores of Shergotty and Zagami pyroxene. In contrast, we see no evidence for a core-to-rim decrease in B. In the absence of further data, we interpret the decrease in Li as reflecting either loss after crystallization of pyroxene cores, consistent with magmatic degassing, or the diffusive preferential loss of Li from pyroxene rims, possibly as a result of shock. Because the partitioning behavior of Li between hydrous fluid, minerals, and melt under relevant conditions of pressure, temperature, and melt composition is unknown, the viability of the water degassing hypothesis depends on experiments establishing the compatibility of Li in hydrous fluid associated with martian basaltic melt and the incompatibility of Li in pyroxene at elevated pressures.  相似文献   

11.
Uranium-lead, Rb-Sr, and Sm-Nd isotopic analyses have been performed on the same whole-rock, mineral, and leachate fractions of the basaltic martian meteorite Zagami to better constrain the U-Pb isotopic systematics of martian materials. Although the Rb-Sr and Sm-Nd systems define concordant crystallization ages of 166 ± 6 Ma and 166 ± 12 Ma, respectively, the U-Pb isotopic system is disturbed. Nevertheless, an age of 156 ± 6 Ma is derived from the 238U-206Pb isotopic system from the purest mineral fractions (maskelynite and pyroxene). The concordance of these three ages suggest that the 238U-206Pb systematics of the purest Zagami mineral fractions have been minimally disturbed by alteration and impact processes, and can therefore be used to constrain the behavior of U and Pb in the Zagami source region. The μ value of the Zagami source region can be estimated, with some confidence from the 238U-206Pb isochron, to be 3.96 ± 0.02. Disturbance of the U-Pb isotopic systems means that this represents a minimum value. The μ value of the Zagami source is significantly lower than the μ values estimated for most basaltic magma sources from Earth and the Moon. This is surprising given the high initial 87Sr/86Sr ratio (0.721566 ± 82) and low initial εNd value (−7.23 ± 0.17) determined for Zagami that indicate that this sample is derived from one of the most highly fractionated reservoirs from any known planetary body. This suggests that Mars is characterized by a low bulk planet U/Pb ratio, a feature that is consistent with its relatively volatile-rich nature.The leachates contain terrestrial common Pb that was probably added to the meteorite during handling, curation, or sawing. The mineral fractions, particularly those with significant amounts of impact melt glass, contain a second contaminant. The presence of this contaminant results in Pb-Pb ages that are older than the crystallization age of Zagami, indicating that the contaminant is characterized by a high 207Pb/206Pb ratio. Such a contaminant could be produced by removal of single-stage Pb from a relatively high μ martian reservoir before ∼1.8 Ga, and therefore could be an ancient manifestation of hydrous alteration of martian surface material.  相似文献   

12.
A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1-2 GPa and 1350-1650 °C simulate the estimated pressure-temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb (DREE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure.We apply the experimentally obtained olivine-melt and orthopyroxene-melt DREE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε143Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε182W, ε142Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε143Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb-Sr and Sm-Nd ages of shergottites.  相似文献   

13.
The basaltic achondrite, shergottite, nakhlite, and chassignite meteorites appear to define a petrological and geochemical sequence. Assuming that they developed from basaltic liquids produced by low pressure partial melting of plagioclase peridotites, their petrological and chemical distinctions can be understood in terms of the compositional differences between their source periodites. The source regions of basaltic achondrite magmas were alkali-poor, metal-bearing peridotites in which pigeonite and/or orthopyroxene was the only pyroxene. By simultaneously increasing the ratio of high-Ca pyroxene to low-Ca pyroxene, the alkali content of the feldspar, the oxidation state, and the overall volatile content of the basaltic achondrite source peridotite, peridotites capable of yielding the parent liquids of the shergottites can be produced. Further increases can produce peridotites capable of yielding the parent liquids of the nakhlites and chassignites.Addition of a volatile-rich component to the volatile-poor type of peridotite required for the source regions of the eucrites appears to be capable of producing the required series of peridotites. Alternatively, progressive volatile-loss from a volatile-rich material, possibly of roughly cosmic composition, could have produced this sequence of peridotites. A simple two-component model of planetary compositions is, to a first approximation, consistent with the petrology and chemistry of these igneous meteorite groups.  相似文献   

14.
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 39Ar-40Ar 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 40Ar/36Ar versus 39Ar/36Ar, 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 40Ar/36Ar 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 40Ar/36Ar 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 40Ar in the shock melts.The shock features of shergottites are also reviewed in the context of what is known of the geologic history of the Martian surface through remote observation. The oldest, most heavily cratered surfaces of Mars are thought to be ?4.0 Ga; we contend that ancient rocks from Mars (Noachian >3.5 Ga) are likely to record multiple impact events reflecting megaregolith formation and the cumulative effects of erosion and aqueous alteration occurring during or since that era. Young rocks (Late Amazonian, <0.6 Ga) should record a relatively simple history of emplacement and ejection from the near surface. We show that although shergottites are strongly shocked, they are relatively pristine crystalline igneous rocks and not pervasively altered breccias. The petrography of shergottites is at odds with an ancient age interpretation. A model in which young coherent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible.  相似文献   

15.
Larkman Nunatak (LAR) 06319 is an olivine-phyric shergottite whose olivine crystals contain abundant crystallized melt inclusions. In this study, three types of melt inclusion were distinguished, based on their occurrence and the composition of their olivine host: Type-I inclusions occur in phenocryst cores (Fo77-73); Type-II inclusions occur in phenocryst mantles (Fo71-66); Type-III inclusions occur in phenocryst rims (Fo61-51) and within groundmass olivine. The sizes of the melt inclusions decrease significantly from Type-I (∼150-250 μm diameter) to Type-II (∼100 μm diameter) to Type-III (∼25-75 μm diameter). Present bulk compositions (PBC) of the crystallized melt inclusions were calculated for each of the three melt inclusion types based on average modal abundances and analyzed compositions of constituent phases. Primary trapped liquid compositions were then reconstructed by addition of olivine and adjustment of the Fe/Mg ratio to equilibrium with the host olivine (to account for crystallization of wall olivine and the effects of Fe/Mg re-equilibration). The present bulk composition of Type-I inclusions (PBC1) plots on a tie-line that passes through olivine and the LAR 06319 whole-rock composition. The parent magma composition can be reconstructed by addition of 29 mol% olivine to PBC1, and adjustment of Fe/Mg for equilibrium with olivine of Fo77 composition. The resulting parent magma composition has a predicted crystallization sequence that is consistent with that determined from petrographic observations, and differs significantly from the whole-rock only in an accumulated olivine component (∼10 wt%). This is consistent with a calculation indicating that ∼10 wt% magnesian (Fo77-73) olivine must be subtracted from the whole-rock to yield a melt in equilibrium with Fo77. Thus, two independent estimates indicate that LAR 06319 contains ∼10 wt% cumulate olivine.The rare earth element (REE) patterns of Type-I melt inclusions are similar to that of the LAR 06319 whole-rock. The REE patterns of Type-II and Type-III melt inclusions are also broadly parallel to that of the whole-rock, but at higher absolute abundances. These results are consistent with an LAR 06319 parent magma that crystallized as a closed-system, with its incompatible-element enrichment being inherited from its mantle source region. However, fractional crystallization of the reconstructed LAR 06319 parent magma cannot reproduce the major and trace element characteristics of all enriched basaltic shergottites, indicating local-to-large scale major- and trace-element variations in the mantle source of enriched shergottites. Therefore, LAR 06319 cannot be parental to the enriched basaltic shergottites.  相似文献   

16.
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 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial εNd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial εNd value of +8.2 ± 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 143Nd/144Nd 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 ± 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 ∼4.5 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.  相似文献   

17.
Combined 147Sm-143Nd and 176Lu-176Hf chronology of the martian meteorite Larkman Nunatak (LAR) 06319 indicates an igneous crystallization age of 193 ± 20 Ma (2σ weighted mean). The individual 147Sm-143Nd and 176Lu-176Hf internal isochron ages are 183 ± 12 Ma and 197 ± 29 Ma, respectively, and are concordant with two previously determined 147Sm-143Nd and 87Rb-87Sr internal isochron ages of 190 ± 26 Ma and 207 ± 14 Ma, respectively (Shih et al., 2009). With respect to the 147Sm-143Nd isotope systematics, maskelynite lies above the isochron defined by primary igneous phases and is therefore not in isotopic equilibrium with the other phases in the rock. Non-isochronous maskelynite is interpreted to result from shock-induced reaction between plagioclase and partial melts of pyroxene and phosphate during transformation to maskelynite, which resulted in it having unsupported 143Nd relative to its measured 147Sm/144Nd ratio. The rare earth element (REE) and high field strength element (HFSE) compositions of major constituent minerals can be modeled as the result of progressive crystallization of a single magma with no addition of secondary components. The concordant ages, combined with igneous textures, mineralogy, and trace element systematics indicate that the weighted average of the radiometric ages records the true crystallization age of this rock. The young igneous age for LAR 06319 and other shergottites are in conflict with models that advocate for circa 4.1 Ga crystallization ages of shergottites from Pb isotope compositions, however, they are consistent with updated crater counting statistics indicating that young volcanic activity on Mars is more widespread than previously realized (Neukum et al., 2010).  相似文献   

18.
The timescale of accretion and differentiation of asteroids and the terrestrial planets can be constrained using the extinct 182Hf-182W isotope system. We present new Hf-W data for seven carbonaceous chondrites, five eucrites, and three shergottites. The W isotope data for the carbonaceous chondrites agree with the previously revised 182W/184W of chondrites, and the combined chondrite data yield an improved ?W value for chondrites of −1.9 ± 0.1 relative to the terrestrial standard. New Hf-W data for the eucrites, in combination with published results, indicate that mantle differentiation in the eucrite parent body (Vesta) occurred at 4563.2 ± 1.4 Ma and suggest that core formation took place 0.9 ± 0.3 Myr before mantle differentiation. Core formation in asteroids within the first ∼5 Myr of the solar system is consistent with the timescales deduced from W isotope data of iron meteorites. New W isotope data for the three basaltic shergottites EETA 79001, DaG 476, and SAU 051, in combination with published 182W and 142Nd data for Martian meteorites reveal the preservation of three early formed mantle reservoirs in Mars. One reservoir (Shergottite group), represented by Zagami, ALH77005, Shergotty, EETA 79001, and possibly SAU 051, is characterized by chondritic 142Nd abundances and elevated ?W values of ∼0.4. The 182W excess of this mantle reservoir results from core formation. Another mantle reservoir (NC group) is sampled by Nakhla, Lafayette, and Chassigny and shows coupled 142Nd-182W excesses of 0.5-1 and 2-3 ? units, respectively. Formation of this mantle reservoir occurred 10-20 Myr after CAI condensation. Since the end of core formation is constrained to 7-15 Myr, a time difference between early silicate mantle differentiation and core formation is not resolvable for Mars. A third early formed mantle reservoir (DaG group) is represented by DaG 476 (and possibly SAU 051) and shows elevated 142Nd/144Nd ratios of 0.5-0.7 ? units and ?W values that are indistinguishable from the Shergottite group. The time of separation of this third reservoir can be constrained to 50-150 Myr after the start of the solar system. Preservation of these early formed mantle reservoirs indicates limited convective mixing in the Martian mantle as early as ∼15 Myr after CAI condensation and suggests that since this time no giant impact occurred on Mars that could have led to mantle homogenization. Given that core formation in planetesimals was completed within the first ∼5 Myr of the solar system, it is most likely that Mars and Earth accreted from pre-differentiated planetesimals. The metal cores of Mars and Earth, however, cannot have formed by simply combining cores from these pre-differentiated planetesimals. The 182W/184W ratios of the Martian and terrestrial mantles require late effective removal of radiogenic 182W, strongly suggesting the existence of magma oceans on both planets. Large impacts were probably the main heat source that generated magma oceans and led to the formation metallic cores in the terrestrial planets. In contrast, decay of short-lived 26Al and 60Fe were important heat sources for melting and core formation in asteroids.  相似文献   

19.
The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo76-55 from core to rim), pyroxene (from core to rim En70Fs25Wo5, En50Fs25Wo25, and En45Fs45Wo10), and Cr-rich spinel in a matrix of maskelynite (An52Ab45), pyroxene (En30-40Fs40-55Wo10-25,), 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.  相似文献   

20.
This paper presents new major and trace element data from 150 garnet xenocrysts from the V. Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province (ADP). Based on the concentrations of Cr2O3, CaO, TiO2 and rare earth elements (REE) the garnets were divided into seven groups: (1) lherzolitic “depleted” garnets (“Lz 1”), (2) lherzolitic garnets with normal REE patterns (“Lz 2”), (3) lherzolitic garnets with weakly sinusoidal REE patterns (“Lz 3”), (4) lherzolitic garnets with strongly sinusoidal REE patterns (“Lz 4”), (5) harzburgitic garnets with sinusoidal REE patterns (“Hz”), (6) wehrlitic garnets with weakly sinusoidal REE patterns (“W”), (7) garnets of megacryst paragenesis with normal REE patterns (“Meg”). Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts. Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle, which is evidenced by a nearly vertical CaO-Cr2O3 trend from harzburgitic (“Hz”) to lherzolitic (“Lz 4”) garnet composition. Harzburgitic garnets (“Hz”) have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues. At the second stage of metasomatism, garnets with weakly sinusoidal REE patterns (“Lz 3”, “W”) were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites. At the last stage, the garnets interacted with basaltic melts, which resulted in the decrease CaO-Cr2O3 trend of “Lz 2” garnet composition. Cr-poor garnets of megacryst paragenesis (“Meg”) could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites. P-T estimates of the garnet xenocrysts indicate that the interval of ～60–110 km of the lithospheric mantle beneath the V. Grib pipe was predominantly affected by the silicate melts, whereas the lithospheric mantle deeper than 150 km was influenced by the carbonatite melts.  相似文献   

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