Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516
Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516
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DOI：
10.1016/s0016-7037(02)00835-9
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发表时间：
2002-06-01
影响因子：
5
通讯作者：
Reese, Y
中科院分区：
文献类型：
--
作者：
Borg, LE;Nyquist, LE;Reese, Y
详细的Rb-Sr和Sm-Nd同位素分析完成了lh77005和LEW88516的辉长闪长岩。ALH77005的Rb-Sr年龄为185 +/- 11 Ma， Sm-Nd年龄为173 +/- 6 Ma，而LEW88516的Rb-Sr和Sm-Nd年龄分别为183 +/- 10和166 +/- 16 Ma。ALH77005的初始Sr同位素组成为0.71026 +/- 4，初始ε (Nd)值为+ 11.1 +/- 0.2。这些值与LEW88516不同，其初始Sr同位素组成为0.71052 +/- 4，初始epsilon（Nd）值为+8.2 +/- 0.6。部分矿物渗滤液和全岩渗滤液偏离Rb-Sr和Sm-Nd等时线，表明陨石的同位素系统受到了干扰。渗滤液的Sm-Nd同位素组成似乎是原生火成岩磷酸盐和低Nd-143/Nd-144比值的蚀变组分的混合物，这可能是添加到火星陨石中的。LEW88516矿物组分的渗滤液-残渣对与整块岩石之间的连线具有几乎相同的斜率，对应的Rb-Sr年龄为90 I Ma。这个年龄可能记录了一次主要的冲击事件，从矿物晶界的晶格位置分选出Rb/Sr。另一方面，渗滤液中也可能含有次生蚀变产物，且结线平行斜率可能重合。几乎相同的矿物模式、成分和年龄表明这些陨石是非常密切相关的。然而，它们的初始Sr和Nd同位素组成在分析不确定性之外不同，需要从独特的来源推导。同化-分馏-结晶模型表明，这两种辉长岩陨石只有在Sr/Nd比值接近1和Sr同位素组成为放射性成因的情况下，才可能与一种共同的母岩浆有关。辉长岩陨石组的地球化学和同位素系统对演化组分的进一步限制表明：(a)形成于类似于4.5 Ga的年代，(b)具有较高的La/Yb比值，(c)是氧化剂，(d)成分为玄武岩或强烈富集不相容元素。岩石的组成和同位素系统。演化的成分不同于任何演化的月球或陆地火成岩。其不寻常的地球化学和同位素特征可能反映了火星地壳成分的含水变化或火星地幔内的含水变质作用。版权所有2002爱思唯尔科学有限公司
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 epsilon(Nd) 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 epsilon(Nd) 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 Nd-143/Nd-144 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 I 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 similar to4.5 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 metasamatism within the Martian mantle. Copyright (C) 2002 Elsevier Science Ltd.