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目前火星岩石样品的唯一来源是火星陨石,是研究火星物质组成和成分演化、物理化学性质、壳幔分异过程、岩浆过程等的重要窗口.火星角砾岩的成分与探测器和火星车获得的火星表面平均成分及部分表面岩石成分相似,代表了来源于火星壳的富集型样品.其他大部分火星陨石通常为基性岩浆岩,形成于幔源岩浆上涌后喷发至表面或侵入为岩墙或岩床,携带了火星幔的物质组成信息,不同的微量元素和同位素组成特征指示其来源于火星幔不同的源区.部分岩浆在上涌过程中经历了火星壳的混染作用,形成的陨石记录了火星壳的部分物质组成.火星陨石的研究已经获得了大量火星表面和深部的物质组成和演化信息,主要认识包括:① 火星壳氧化程度较高,富集稀土元素,238 U/204 Pb比值较高,ε142 Nd和ε182 W较低;② 火星幔部的氧逸度和轻稀土元素富集程度高度不均一,同位素组成特征指示至少存在六个不同的源区(低147 Sm/144 Nd 和176 Lu/177 Hf、高147 Sm/144 Nd 和176 Lu/177 Hf、高147 Sm/144 Nd低176 Lu/177 Hf的三类辉玻无球粒陨石源区;较低147 Sm/144 Nd 和176 Lu/177 Hf、较高ε182 W的辉橄与纯橄无球粒陨石源区;NWA 8159 的源区;NWA 7034 中部分组分的源区);③幔部不均一的特征可能形成于火星分异早期.目前发现的火星样品数量约300 余块,大部分样品的结晶年龄小于 650 Ma,但是火星表面的地质单元主要为诺亚纪或西方纪产物.另外火星探测数据获得的表面岩石成分比火星陨石更富硅和碱性元素,且岩石种类更丰富,除玄武质岩石之外还有大量中性和酸性岩石.因此对火星物质组成和演化更全面、深入的研究还需新的探测任务(包括样品返回)的数据补充. 相似文献
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在通过对Allende(CV3)碳质球粒陨石的4个光薄片中发现的5个特殊蠕虫状橄榄石集合体(AOA)的岩石学和矿物化学特征研究,证实它们的矿物组合以富橄榄石和霞石为特征,可见少量的金属硫化物。橄榄石颗粒Fa(Fe/(Fe Mg)原子百分比)值范围在34.1 mol%～42.2 mol%,百分标准平均方差(PMD)值为6.1,表明这些颗粒达到了一定的热力学平衡。AOA可能属于星云直接凝聚形成,AOA和细粒CAI(FTA和富尖晶石-辉石型CAI)可能代表了太阳星云从高温到低温连续凝聚的产物。认为AOA的蚀变作用发生在太阳星云中,霞石和铁橄榄石可能是后期水化蚀变的产物,霞石可能替代了AOA中原始的矿物——钙长石和黄长石等。AOA可能在星云中还经历了热蚀变作用的过程,橄榄石和霞石颗粒均具有高的FeO质量分数(29.1%～34.4%,4.04%～9.70%),表明蚀变反应发生在高逸氧度的星云环境下。 相似文献
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矿物-微生物交互作用广泛参与地球表层系统物质循环与能量流动过程,深刻地影响着一系列重要的地表生物地球化学进程。近年来地表半导体矿物的相关研究,为矿物-微生物交互作用提供了崭新研究方向,揭示地表“日光-半导体矿物-微生物”系统电子传递过程及其环境效应,是地质微生物学交叉领域研究的核心科学问题之一。本研究从地表不同生境“矿物膜”出发,以光电化学技术证实喀斯特、红壤、岩石漆“矿物膜”在1 000 min长时间循环实验中平均光电流值约为5.4、3.4、3.2 μA/cm2,证实“矿物膜”良好日光响应特性且铁锰氧化物矿物在其中发挥核心作用。基于笔者前期研究所发现的“矿物膜”电活性菌富集且与半导体矿物分布呈正相关性这一现象,本文进一步构建模拟光电子红壤细菌群落系统,20天后细菌群落α多样性显著提升,研究证实细菌群落具有模拟光电子响应活性,且电极与溶液群落均具有演化方向性;16S rRNA测序分析表明模拟光电子作用下Shewanella、Pseudomonas、Streptococcus、Lactobacillus、Acinetobacter等电活性菌显著富集。综上,本文研究结果间接证实地表半导体矿物光电子可有效调控微生物群落结构并促进电活性菌在“矿物膜”中富集。 相似文献
4.
副矿物在岩石中的含量多在1%以下.它分布广,种类多,生成于地质作用的整个过程,具有重要研究意义.根据副矿物的组合、种类、数量、晶形、晶面特征、颜色、透明度、粒度、微量元素、包裹体、双晶、环带构造、光性异常和蚀变或次生变化等特征,能为解决某些成岩、成矿地质问题提供 相似文献
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造山带背景铜镍矿床以富水为主要特征,但水(流体)在成矿中的作用以及其对岩石的改造过程仍不明确。本文以黄山南铜镍矿床为例,通过辉橄岩和橄辉岩等超镁铁岩的蚀变矿物组合和H-O同位素变化规律,限定蚀变过程与流体性质及来源。黄山南超镁铁岩原生矿物主要有尖晶石、橄榄石、斜方辉石、单斜辉石和少量填隙状角闪石、云母,蚀变矿物有角闪石(浅闪石、阳起石、透闪石、普通角闪石以及镁闪石)、滑石、绿泥石和蛇纹石等。根据岩石结构与蚀变矿物比例,将超镁铁岩分为弱蚀变、中等蚀变和强蚀变3类。蚀变矿物组合与角闪石成分指示超镁铁岩经历了高温蚀变阶段(>700℃),形成了镁质闪石+滑石+绿泥石;中温蚀变阶段(700～550℃)和低温蚀变阶段(<550℃),分别形成了钙质闪石+滑石+绿泥石+蛇纹石与滑石+碳酸盐+蛇纹石的矿物组合。蚀变岩石普遍以中低温蚀变为主,可能与中低温阶段的叠加-改造作用相关。岩石随蚀变程度增加,Si、Na、K、Mn等主量元素和Rb、Ba等微量元素呈现明显降低趋势,表明大多数元素在流体改造过程中从岩石中迁出,说明蚀变过程为开放体系。中-强蚀变岩石中,硫化物矿物边部的形态呈锯齿状,但主体仍为磁... 相似文献
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本文通过野外地质调查、显微鉴定分析、EPMA测试和 X射线分析 ,确定香港坪洲岛坪洲组地层中的方沸石、锥辉石和次生方解石等矿物部分是交代原始盐类矿物钙芒硝形成的 ,它们多以集合体方式构成钙芒硝假晶 ,具有钙芒硝的棱面体形态 ,并可以和区域上含钙芒硝的盐系地层进行对比。本文认为 ,原始钙芒硝为上述矿物的形成提供了 Na2 O和 Ca O,热液活动带来了部分 Si O2 ,通过交代蚀变作用形成了丰富的钙芒硝假晶。这一研究成果表明 ,坪洲组是一盐系地层 ,大鹏湾盆地在晚白垩世—古近纪时期是一演化程度较高的硫酸盐型蒸发岩盆地。 相似文献
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结合矿物物理化学特征解释了沉积岩中常见的稳定重砂矿物石榴石、锆石、金红石、磷灰石、电气石和榍石的碎屑形态和稳定性;借助扫描电镜下呈现出来的形貌特征,如溶蚀、裂开、圆化程度等,分析了控制稳定重砂矿物蚀变作用的物理化学因素;认为稳定重砂矿物之间的相对稳定性受沉积环境和成岩条件的控制.在同生和成岩过程中,有机酸参与的流体可以改变部分稳定重砂矿物(如磷灰石、石榴石、电气石)的稳定性,并导致其分解,因此稳定重砂矿物的蚀变特征一方面可以辅助解释重砂矿物组合的空间变化,另一方面还可以揭示沉积过程中的气候条件、植被发育程度,从而指示沉积相,并约束成岩过程中流体的化学性质;此外,导致稳定重砂矿物溶蚀的流体对于其他造岩矿物的影响以及对次生孔隙的贡献程度值得进一步研究. 相似文献
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荷叶塘为一块我国降落的原始3型普通球粒陨石,因此具有重要研究意义。本文对荷叶塘陨石光薄片及粉末样品的岩石学、矿物学和全岩组成地球化学特征进行研究,为这块陨石的深入研究提供重要基础数据。研究表明荷叶塘陨石具L3型陨石岩石学特征,具典型的球粒陨石结构,球粒清晰,球粒结构类型多,基质重结晶程度低,组成模式为:球粒80vol%,金属和硫化物含量为5vol%,基质15vol%。矿物化学成分表明,该陨石球粒以Ⅰ型(贫铁型)球粒为主,橄榄石Fa0.41-34.1(PMD=51),低钙辉石Fs1.82-27.2(PMD=88),Wo0.18-3.13(PMD=103),铁纹石中Co含量平均0.62%(PMD=20),矿物成份不均一程度高,橄榄石矿物结晶颗粒内部化学成分变化大,呈正环带分布,与岩浆型结晶顺序一致,球粒与基质及间隙物成分明显不同,表现为不同物质来源。化学成分全岩分析结果显示,荷叶塘陨石亲石、亲铁元素含量均为L型陨石特征。依据以上岩石矿物学和化学组成特征,依照陨石亚分类参数,将其类型划分为L3.4型普通球粒陨石。冲击变质程度S2,风化程度W1。研究结果表明荷叶塘陨石为一块受后期水、热蚀变和风化影响较少的原始类型陨石。组成矿物成分极不均一,在矿物晶体内部,球粒内部及球粒与基质间均有明显变化。 相似文献
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Oxygen isotope ratios of merrillite and chlorapatite in the Martian meteorites ALH84001 and Los Angeles have been measured by ion microprobe in multicollector mode. δ18O values of phosphate minerals measured in situ range from ∼3 to 6‰, and are similar to Martian meteorite whole-rock values, as well as the δ18O of igneous phosphate on Earth. These results suggest that the primary, abiotic, igneous phosphate reservoir on Mars is similar in oxygen isotopic composition to the basaltic phosphate reservoir on Earth. This is an important first step in the characterization of Martian phosphate reservoirs for the use of δ18O of phosphate minerals as a biomarker for life on Mars. Cumulative textural, major-element, and isotopic evidence presented here suggest a primary, igneous origin for the phosphates in Los Angeles and ALH84001; textural and chemical evidence suggests that phosphates in ALH84001 were subsequently shock-melted in a later event. 相似文献
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William S. Cassata David L. Shuster Benjamin P. Weiss 《Geochimica et cosmochimica acta》2010,74(23):6900-6920
The thermal histories of Martian meteorite are important for the interpretation of petrologic, geochemical, geochronological, and paleomagnetic constraints that they provide on the evolution of Mars. In this paper, we quantify 40Ar/39Ar ages and Ar diffusion kinetics of Martian meteorites Allan Hills (ALH) 84001, Nakhla, and Miller Range (MIL) 03346. We constrain the thermal history of each meteorite and discuss the resulting implications for their petrology, paleomagnetism, and geochronology. Maskelynite in ALH 84001 yields a 40Ar/39Ar isochron age of 4163 ± 35 Ma, which is indistinguishable from recent Pb-Pb (Bouvier et al., 2009a) and Lu-Hf ages (Lapen et al., 2010). The high precision of this result arises from clear resolution of a reproducible trapped 40Ar/36Ar component in maskelynite in ALH 84001 (40Ar/36Ar = 632 ± 90). The maskelynite 40Ar/39Ar age predates the Late Heavy Bombardment and likely represents the time at which the original natural remanent magnetization (NRM) component observed in ALH 84001 was acquired. Nakhla and MIL 03346 yield 40Ar/39Ar isochron ages of 1332 ± 24 and 1339 ± 8 Ma, respectively, which we interpret to date crystallization. Multi-phase, multi-domain diffusion models constrained by the observed Ar diffusion kinetics and 40Ar/39Ar age spectra suggest that localized regions within both ALH 84001 and Nakhla were intensely heated for brief durations during shock events at 1158 ± 110 and 913 ± 9 Ma, respectively. These ages may date the marginal melting of pyroxene in each rock, mobilization of carbonates and maskelynite in ALH 84001, and NRM overprints observed in ALH 84001. The inferred peak temperatures of the shock heating events (>1400 °C) are sufficient to mobilize Ar, Sr, and Pb in constituent minerals, which may explain some of the dispersion observed in 40Ar/39Ar, Rb-Sr, and U-Th-Pb data toward ages younger than ∼4.1 Ga. The data also place conservative upper bounds on the long-duration residence temperatures of the ALH 84001 and Nakhla protolith to be °C and °C over the last ∼4.16 Ga and ∼1.35 Ga, respectively. MIL 03346 has apparently not experienced significant shock-heating since it crystallized, consistent with the fact that various chronometers yield concordant ages. 相似文献
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J. Filiberto 《Geochimica et cosmochimica acta》2008,72(2):690-701
Most magmas proposed as parental to the Martian SNC meteorites are high in iron and low in alumina. Yet, experiments at low pressures on such liquids have not produced the cumulate or melt-inclusion assemblages seen in the chassignite meteorites. Therefore, elevated pressure experiments under anhydrous and hydrous (water-undersaturated) conditions were conducted on a high-Fe, low-Al liquid proposed to be parental to the Chassigny meteorite. These experiments failed to produce the most magnesian cumulate phases, as well as the olivine hosted kaersutite-bearing melt-inclusion assemblage, of the chassignites. These results suggest that the parental liquid to the chassignite meteorites is both more magnesium and aluminum-rich than the previously considered composition (A∗; Johnson et al., 1991). The proposed composition is similar to the Martian Adirondack class Gusev basalt Humphrey and suggests a link between the Chassigny meteorite and rocks on the surface of Mars. 相似文献
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Allan H. Treiman Donald S. Musselwhite Charles K. Shearer Jr. 《Geochimica et cosmochimica acta》2006,70(11):2919-2934
Zoning patterns of light lithophile elements (the LLE: Li, Be, and B) in pyroxenes of some Martian basaltic meteorites have been used to suggest that the parent basalts were saturated in water and exsolved an aqueous fluid phase. Here, we examine LLE zoning in the augites of a quickly cooled Martian basalt that was not water-saturated—the Northwest Africa (NWA) 817 nakhlite. Analyses for LLE were by secondary ion mass spectrometry (SIMS), supported by EMP analyses of major and minor elements. In NWA 817, zoning of Be and B is consistent with igneous fractionations while Li abundances are effectively constant across wide ranges in abundance of other incompatible elements (Be, B, Ti, and Fe*). The lack of strong zoning in Li can be ascribed to intracrystalline diffusion, despite the rapid cooling of NWA 817. Most other nakhlites, notably Nakhla and Lafayette, cooled more slowly than did NWA 817 [Treiman, A.H., 2005. The nakhlite Martian meteorites: augite-rich igneous rock from Mars. Chem. Erde65, 203-270]. In them Li abundances are constant across augite, as are abundances of other elements. In Nakhla pyroxenes, all the LLE have effectively constant abundances across significant ranges in Fe* and Ti abundance. Lafayette is more equilibrated still, and shows constant abundances of LLE and nearly constant Fe*. A pyroxene in the NWA480 shergottite has constant Li abundances, and was interpreted to represent mineral fractionation coupled with exsolution of aqueous fluid. A simple quantitative model of this process requires that the partitioning of Li between basalt and aqueous fluid, LiDaq/bas, be 15 times larger than its experimentally determined value. Thus, its seems unlikely that the Li zoning pattern in NWA480 augite represents exsolution of aqueous fluid. Late igneous or sub-solidus diffusion seems more likely as is suggested by Li isotopic studies [Beck, P., Chaussidon, M., Barrat, J.-A., Gillet, Ph., Bohn, M., 2005. An ion-microprobe study of lithium isotopes behavior in nakhlites. Meteorit. Planet. Sci.40, Abstract #5118; Beck, P., Chaussidon, M., Barrat, J.-A., Gillet, Ph., Bohn, M., 2006. Diffusion induced Li isotopic fractionation during the cooling of magmatic rocks: the case of pyroxene phenocrysts from nakhlite meteorites. Geochim. Cosmochim. Acta70, in press]. Pyroxenes of the Shergotty and Zagami meteorites have nearly constant abundances of B, and Li that decreases core-to-rim. Applying the quantitative model to the constant B in these pyroxenes requires that BDaq/bas be 25 times larger than experimentally constrained values. Li abundances in pigeonite can be fit by the model of crystal fractionation and fluid loss, but only if LiDaq/bas is 30 times the experimentally constrained value. The Li abundance pattern in augite cannot be modeled by simple fractionation, suggesting some strong crystal-composition effects. Thus, Li and B distributions in Shergotty and Zagami pyroxenes cannot be explained by igneous fractionation and exsolution of aqueous vapor. Intracrystalline diffusion, complete for B and incomplete for Li, seems more consistent with the observed zoning patterns. 相似文献
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《Geochimica et cosmochimica acta》1999,63(11-12):1865-1875
Noble metals, Mo, W, and 24 other elements were determined in six SNC meteorites of presumably Martian origin. Based on element correlations, representative siderophile element concentrations for the silicate mantle of Mars were inferred. From a comparison with experimentally determined metal/silicate partition coefficients of the moderately siderophile elements: Fe, Ni, Co, W, Mo, and Ga, it is concluded that equilibrium between core forming metal and silicates in Mars has occurred at high temperatures (around 2200°C) and low pressures (<1 GPa). This suggests that metal segregation occurred concurrently with rapid accretion of Mars, which is consistent with the inference from excess 182W in Martian meteorites (Lee and Halliday, 1997). Concentrations of Ir, Os, Ru, Pt, and Au in the analyzed Martian meteorites, except ALH84001, are at a level of approximately 10−2–10−3 × CI. The comparatively high abundances of noble metals in Martian meteorites require the addition of chondritic material after core formation. The similarity in Au/La and Pt/Ca ratios between ALH84001 and the other Martian meteorites suggests crystallization of ALH84001 after complete accretion of Mars. 相似文献
16.
Lars E. Borg Larry E. NyquistHenry Wiesmann Young Reese 《Geochimica et cosmochimica acta》2002,66(11):2037-2053
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.
Allan H. Treiman 《Chemie der Erde / Geochemistry》2005,65(3):203-270
The seven nakhlite meteorites are augite-rich igneous rocks that formed in flows or shallow intrusions of basaltic magma on Mars. They consist of euhedral to subhedral crystals of augite and olivine (to 1 cm long) in fine-grained mesostases. The augite crystals have homogeneous cores of Mg′=63% and rims that are normally zoned to iron enrichment. The core-rim zoning is cut by iron-enriched zones along fractures and is replaced locally by ferroan low-Ca pyroxene. The core compositions of the olivines vary inversely with the steepness of their rim zoning - sharp rim zoning goes with the most magnesian cores (Mg′=42%), homogeneous olivines are the most ferroan. The olivine and augite crystals contain multiphase inclusions representing trapped magma. Among the olivine and augite crystals is mesostasis, composed principally of plagioclase and/or glass, with euhedra of titanomagnetite and many minor minerals. Olivine and mesostasis glass are partially replaced by veinlets and patches of iddingsite, a mixture of smectite clays, iron oxy-hydroxides and carbonate minerals. In the mesostasis are rare patches of a salt alteration assemblage: halite, siderite, and anhydrite/gypsum. The nakhlites are little shocked, but have been affected chemically and biologically by their residence on Earth.Differences among the chemical compositions of the nakhlites can be ascribed mostly to different proportions of augite, olivine, and mesostasis. Compared to common basalts, they are rich in Ca, strongly depleted in Al, and enriched in magmaphile (incompatible) elements, including the LREE. Nakhlites contain little pre-terrestrial organic matter. Oxygen isotope ratios are not terrestrial, and are different in anhydrous silicates and in iddingsite. The alteration assemblages all have heavy oxygen and heavy carbon, while D/H values are extreme and scattered. Igneous sulfur had a solar-system isotopic ratio, but in most minerals was altered to higher and lower values. High precision analyses show mass-independent fractionations of S isotopes. Nitrogen and noble gases are complex and represent three components: two mantle sources (Chas-E and Chas-S), and fractionated Martian atmosphere.The nakhlites are igneous cumulate rocks, formed from basaltic magma at ∼1.3 Ga, containing excess crystals over what would form from pure magma. After accumulation of their augite and olivine crystals, they were affected (to various degrees) by crystallization of the magma, element diffusion among minerals and magma, chemical reactions among minerals and magma, magma movement among the crystals, and post-igneous chemical equilibration. The extent of these modifications varies, from least to greatest, in the order: MIL03346, NWA817, Y000593, Nakhla=Governador Valadares, Lafayette, and NWA998.Chemical, isotopic, and chronologic data confirm that the nakhlites formed on Mars, most likely in thick lava flows or shallow intrusions. Their crystallization ages, referenced to crater count chronologies for Mars, suggest that the nakhlites formed on the large volcanic constructs of Tharsis, Elysium, or Syrtis Major. The nakhlites were suffused with liquid water, probably at ∼620 ma. This water dissolved olivine and mesostasis glass, and deposited iddingsite and salt minerals in their places. The nakhlites were ejected from Mars at ∼10.75 Ma by an asteroid impact and fell to Earth within the last 10,000 years.Although the nakhlites are enriched in incompatible elements, their source mantle was strongly depleted. This depletion event was ancient, as the nakhlites’ source mantle was fractionated while short-lived radionuclides (e.g., ) were still active. This differentiation event may have been core formation coupled with a magma ocean, as is inferred for the moon. 相似文献
18.
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
19.
A New Martian Meteorite from Antarctica:Grove Mountains (GRV) 020090 总被引：15，自引：0，他引：15
MIAOBingkui OUYANGZiyuan WANGDaode JUYitai WANGGuiqin LINYangting 《《地质学报》英文版》2004,78(5):1034-1041
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
20.
地外有机化合物 总被引：1，自引：0，他引：1
球粒陨石中的有机化合物起源于星际介质,是构成太阳星云的初始组分,并与其他物质一起吸积形成小行星和行星。在小行星内,有机质经历了不同程度的水蚀变和热变质作用。球粒陨石中的有机化合物尽管是非生命成因,但组成极为复杂,主要是类似于干酪根的大分子物质,以及少量可溶性有机物。大部分可溶有机分子也发现于地球生物圈,但前者可具有完全不同的H、C、N等同位素组成,这也是它们来源于地球之外的重要证据。星云中宇宙线和紫外线（UV）的辐射、小行星的热变质和水蚀变,是地外有机质演化的主要过程。球粒陨石中的有机质是地球生命起源的物质基础,是生命起源不可或缺的重要环节。同样重要的是,大量的火星探测表明,火星历史上有过满足生命存在的基本条件,而在火星陨石中还发现了一些生物活动相关的线索。未来很可能首先在火星上发现地外生命存在的证据。 相似文献