Northwest Africa 5790: Revisiting nakhlite petrogenesis
Northwest Africa 5790: Revisiting nakhlite petrogenesis
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DOI：
10.1016/j.gca.2016.06.032
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发表时间：
2016-10-01
影响因子：
5
通讯作者：
Devouard, B.
中科院分区：
文献类型：
--
作者：
Jambon, A.;Sautter, V.;Devouard, B.
西北非洲5790是最新发现的Nakhlite，由58vol. %普通辉石、6%橄榄石和36%玻璃斑状积云物质。它的岩石学与以前发现的钠闪石相当，但有关键的区别：（1）金红石核显示出在Mg# 54和60之间的不寻常的环带;（2）橄榄石粗晶具有富Fe的原生核成分（Mg# = 35）：（3）中静态模态比例是裸石中最高的;（4）与MIL 03346一样，是最富磁铁矿的赤霞岩，组构各向异性最小。复杂的原生分带表明，由于复杂的过程，如前堆在一个新的岩浆批次的再活化的再吸收。纹理的关系表明，明确的橄榄石周围生长的再吸收辉石，橄榄石的增长是连续的，而辉石的增长恢复在最后阶段。橄榄石核心组成（Mg# = 35）是不平衡的普通辉石核心组成（Mg# 60-63），并与先前推断的nakhlite母岩浆（Mg# = 29）。橄榄石和普通辉石的振荡环带的存在排除了亚固相线扩散，可能有修改橄榄石组合物。NWA 5790证明至少有两个岩浆批次喷发前，这意味着熔体在平衡与普通辉石核心从来没有接触橄榄石。因此，以前关于赤霞岩成因的设想必须修正。第一次原生母岩浆的Nakhlites产生不同的辉石累积深度（镁# 66-60），因为他们分化到不同程度。随后更进化的岩浆批次夹带积累的普通辉石晶体的表面，在那里他们被部分再吸收，而橄榄石结晶。微量元素的变化明确地表明，它们代表了同源但不同的岩浆批次。不同的Nakhlites之间的成分差异表明了一些连续的熔岩流。为了解释所有的观测结果，我们提出了一个基于几个（至少三个）厚流的Nakhlites的成岩模型。虽然NWA 5790属于一个流的最顶部，但基于缺乏iddingsite，它应该来自采样的最低流。(C)2016爱思唯尔有限公司版权所有
Northwest Africa 5790, the latest nakhlite find, is composed of 58 vol.% augite, 6% olivine and 36% vitrophyric intercumulus material. Its petrology is comparable to previously discovered nakhlites but with key differences: (1) Augite cores display an unusual zoning between Mg# 54 and 60; (2) Olivine macrocrysts have a primary Fe-rich core composition (Mg# = 35); (3) The modal proportion of mesostasis is the highest ever described in a nakhlite; (4) It is the most magnetite-rich nakhlite, together with MIL 03346, and exhibits the least anisotropic fabric. Complex primary zoning in cumulus augite indicates resorption due to complex processes such as remobilization of former cumulates in a new magma batch. Textural relationships indicate unambiguously that olivine was growing around resorbed augite, and that olivine growth was continuous while pyroxene growth resumed at a final stage. Olivine core compositions (Mg# = 35) are out of equilibrium with the augite core compositions (Mg# 60-63) and with the previously inferred nakhlite parental magma (Mg# = 29). The presence of oscillatory zoning in olivine and augite precludes subsolidus diffusion that could have modified olivine compositions. NWA 5790 evidences at least two magma batches before eruption, with the implication that melt in equilibrium with augite cores was never in contact with olivine. Iddingsite is absent.Accordingly, the previous scenarios for nakhlite petrogenesis must be revised. The first primary parent magmas of nakhlites generated varied augite cumulates at depth (Mg# 66-60) as they differentiated to different extents. A subsequent more evolved magma batch entrained accumulated augite crystals to the surface where they were partly resorbed while olivine crystallized. Trace element variations indicate unambiguously that they represent consanguineous but different magma batches. The compositional differences among the various nakhlites suggest a number of successive lava flows. To account for all observations we propose a petrogenetic model for nakhlites based on several (at least three) thick flows. Although NWA 5790 belongs to the very top of one flow, it should come from the lowest flow sampled, based on the lack of iddingsite. (C) 2016 Elsevier Ltd. All rights reserved.