Understanding the emplacement of Martian volcanic rocks using petrofabrics of the nakhlite meteorites
Understanding the emplacement of Martian volcanic rocks using petrofabrics of the nakhlite meteorites
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利用 nakhlite 陨石的石油结构了解火星火山岩的就位
DOI：
10.1016/j.epsl.2019.05.050
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发表时间：
2019
影响因子：
5.3
通讯作者：
Daly L
中科院分区：
文献类型：
--
作者：
Daly L
为了验证计算出的火星地壳年龄，我们需要精确的放射性测年从火成岩在火星表面的出处是已知的。火星陨石已经被精确和定量地确定了年代，但发射地点目前尚不清楚。推断一组相关火星陨石的形成环境可以限制它们形成的火山系统的性质和复杂性。Nakhlite陨石是这样一组富含Augite的岩石，它们对火星的玄武岩地壳进行了采样，因此可以提供对其火山过程的独特见解。利用电子背散射衍射，我们已经确定了四个nakhlites（瓦拉达雷斯总督，拉斐特，米勒范围03346和Nakhla）的形状优选和晶体学优选取向的岩组，以了解其母岩形成的条件。在所有的样品中，有一个明确的联系之间的形状择优取向（SPO）和晶体学择优取向（CPO）的辉石斑晶。这种关系揭示了使用CPO作为3D SPO的代理的普通辉石晶体的三维形状，并且还使得能够进行定量的三维岩组分析。所有四个nakhlites表现出叶理定义的CPO的辉石轴在一个平面上，虽然个别陨石显示微妙的纹理变化。Nakhla和瓦拉达雷斯总督在其轴面理内显示出弱CPO线理，解释为在纯剪切/简单剪切流相结合的情况下开发，表明其母岩作为陆上双曲熔岩流侵位。相比之下，拉法叶和米勒范围03346的叶理为主的CPO岩组表明，在纯剪切为主的制度，双曲线流的影响很小的形成。这些CPO岩组表明结晶沉淀在冷却岩浆体的停滞部分，或扩展熔岩流的平坦区域。拉法叶的CPO叶理明显弱于米勒系列03346，可能是由于其较高的斑晶密度导致阻碍组构发育的粒-粒相互作用。所确定的CPO岩组也可以用来确定火星表面的近似平面和岩浆流动线，范围在± 20°以内。我们的研究结果表明，nakhlite发射坑采样了一个复杂的火山建筑物，该建筑物由至少三个不同的岩浆系统提供，限制了这些岩石可能起源于火星的可能位置。
In order to validate calculated ages of the Martian crust we require precise radiometric dates from igneous rocks where their provenance on the Martian surface is known. Martian meteorites have been dated precisely and quantitatively, but the launch sites are currently unknown. Inferring the formation environment of a correlated suite of Martian meteorites can constrain the nature and complexity of the volcanic system they formed from. The nakhlite meteorites are such a suite of augite-rich rocks that sample the basaltic crust of Mars, and as such can provide unique insights into its volcanic processes. Using electron backscatter diffraction we have determined the shape-preferred and crystallographic-preferred orientation petrofabrics of four nakhlites (Governador Valadares, Lafayette, Miller Range 03346 and Nakhla) in order to understand the conditions under which their parent rocks formed. In all samples, there is a clear link between the shape-preferred orientation (SPO) and crystallographic-preferred orientation (CPO) of augite phenocrysts. This relationship reveals the three-dimensional shape of the augite crystals using CPO as a proxy for 3D SPO, and also enables a quantitative 3-dimensional petrofabric analysis. All four nakhlites exhibit a foliation defined by the CPO of the augite axis in a plane, although individual meteorites show subtle textural variations. Nakhla and Governador Valadares display a weak CPO lineation within their axis foliation that is interpreted to have developed in a combined pure shear/simple shear flow regime, indicative of emplacement of their parent rock as a subaerial hyperbolic lava flow. By contrast, the foliation dominated CPO petrofabrics of Lafayette and Miller Range 03346 suggest formation in a pure shear dominated regime with little influence of hyperbolic flow. These CPO petrofabrics are indicative of crystal settling in the stagnant portion of cooling magma bodies, or the flattening area of spreading lava flows. The CPO foliation of Lafayette's is substantially weaker than Miller Range 03346, probably due to its higher phenocryst density causing grain-grain interactions that hindered fabric development. The CPO petrofabrics identified can also be used to determine the approximate plane of the Martian surface and the line of magma flow to within ∼20°. Our results suggest that the nakhlite launch crater sampled a complex volcanic edifice that was supplied by at least three distinct magmatic systems limiting the possible locations these rocks could have originated from on Mars.
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