176Lu-176Hf and 147Sm-143Nd ages of the Martian shergottites: Evaluation of the shock-resetting hypothesis through diffusion kinetic experiments and modeling, and petrological observations
176Lu-176Hf and 147Sm-143Nd ages of the Martian shergottites: Evaluation of the shock-resetting hypothesis through diffusion kinetic experiments and modeling, and petrological observations
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DOI：
10.1016/j.epsl.2014.03.037
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发表时间：
2014-06-01
影响因子：
5.3
通讯作者：
Ganguly, Jibamitra
中科院分区：
文献类型：
--
作者：
Bloch, Elias;Ganguly, Jibamitra
我们解决的火星陨石shergottite套件的年龄争议（类似于Lu-176-Hf-176，Sm-147-Nd-143和各种其他衰变系统确定的200 Ma年龄，而不是类似于4 Ga全岩Pb-Pb年龄）通过计算重置单斜辉石中Hf和Nd同位素所需的时间尺度，在shergottites可能经历的峰值和冲击后P-T条件下，其是用于组成Lu-176-Hf-176和Sm-147-Nd-143等时线的矿物中稀土元素的主要宿主。它的结论是，影响加热是极不可能有显着重置这些陨石的Lu-176-Hf-176和/或Sm-147-Nd-143矿物等时线。这一结论得到了支持的性质的Cr和Fe的测量浓度分布的橄榄石maskelynite边界，和Ti的配置文件在shergottite RBT 04262的单斜辉石maskelynite/熔体界面。缺乏任何扩散的证据，在测得的浓度分布，再加上定性的不相容性，在这些界面上的测量晶体-熔体分馏的趋势与预期的分馏从平衡分配的性质，提供了强有力的证据表明，没有实质性的化学交换发生在峰值冲击P-T条件下的固相和熔体相之间。此外，我们还发现，共生的普通辉石和褐变岩中的主量元素的成分剖面是均匀的，反映了1143 +/- 9 ℃的岩浆温度。我们认为，辉石组成，因此（LU）-L-176-Hf-176和Sm-147-Nd-143地质年代计，是极不可能已被重置的热效应的冲击事件。RBT 04262的选定部分的X射线成像显示没有增强晶界扩散的证据。因此，与Lu-176-Hf-176和Sm-147-Nd-143记录的200 Ma年龄相似，地质年代学最可能记录了辉绿岩的结晶年龄。(C)2014爱思唯尔有限公司版权所有。
We address the age controversy of the shergottite suite of Martian meteorites (similar to 200 Ma dates determined by Lu-176-Hf-176, Sm-147-Nd-143, and various other decay systems as opposed to a similar to 4 Ga whole rock Pb-Pb date) by calculating the timescales needed to reset Hf and Nd isotopes in clinopyroxene, which is the primary host of rare earth elements amongst the minerals used to compose the Lu-176-Hf-176 and Sm-147-Nd-143 isochrons, at the peak- and post-shock P-T conditions likely to have been experienced by the shergottites. It is concluded that impact heating is highly unlikely to have significantly reset the Lu-176-Hf-176 and/or Sm-147-Nd-143 mineral isochrons of these meteorites. This conclusion is bolstered by the nature of measured concentration profiles of Cr and Fe across olivine-maskelynite boundaries, and Ti profiles across clinopyroxene-maskelynite/melt interfaces in the shergottite RBT 04262. The lack of any evidence of diffusion in the measured concentration profiles, coupled with the qualitative incompatibility of the measured crystal-melt fractionation trends at these interfaces with the nature of fractionation expected from equilibrium partitioning, provides strong evidence that no substantial chemical exchange took place between the solid and melt phases during peak-shock P-T conditions. Furthermore, we find that the compositional profiles of major elements in coexisting augite and pigeonite are homogenous and reflect a magmatic temperature of 1143 +/- 9 degrees C. We argue that the pyroxene compositions, and consequently the (LU)-L-176-Hf-176 and Sm-147-Nd-143 geochronometers, were highly unlikely to have been reset by the thermal effect of a shock event. X-ray mapping of a selected section of RBT 04262 shows no evidence of enhanced grain boundary diffusion. We thus conclude that the similar to 200 Ma dates recorded by the Lu-176-Hf-176 and Sm-147-Nd-143, geochronometers most likely record the crystallization age of the shergottites. (C) 2014 Elsevier B.V. All rights reserved.