Crystallographic and textural evidence for precipitation of rutile, ilmenite, corundum, and apatite lamellae from garnet
Crystallographic and textural evidence for precipitation of rutile, ilmenite, corundum, and apatite lamellae from garnet
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DOI：
10.2138/am-2019-6849
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发表时间：
2019-06
影响因子：
3.1
通讯作者：
D. S. Keller;J. Ague
中科院分区：
文献类型：
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作者：
D. S. Keller;J. Ague
石榴子石是一种常见的变质和火成岩矿物，具有广泛的固溶体，其稳定性可达地幔深度≥400 km。高T和/或高P石榴石可能含有其他矿物的定向层状矿物，最常见的是简单氧化物（例如，金红石、钛铁矿）、磷灰石，以及在超高磷的情况下，硅酸盐，包括辉石和角闪石。层状矿物通常被认为是沉淀特征，保留了富含层状矿物营养物的前石榴石化学的记录（例如，Ti 4+）。降水系统中的这种微观结构起源（例如，合金）的晶体取向关系（COR）和层状结构的形状优选取向（SPO）长期以来一直是通过在主体和分离相之间形成的晶体取向关系（COR）来研究的。然而，最近，沉淀的替代假说已经提出，需要在石榴石的流体，或共同生长，过度生长，或继承机制的薄板就位。这些假设表明，石榴石不能用于研究以前的石榴石化学。此外，他们预测，层状相，SPO，和COR应该有很大的不同的地方，因为层状形成将控制各种当地的岩石特定的因素，如流体的存在，流体化学，或矿物生长序列。另一方面，如果laminate特性在很大程度上是一致的地方之间，它可能反映了降水能量学的控制，而不是外部因素。在地质系统中，很少有比较COR的研究，但COR，SPO和薄片组合的综合评价应该指纹薄片生长过程。为了检验沉淀和替代假设，我们收集了康涅狄格州Brimfield片岩（≥1000 °C变质;美国中部缅因州地体）石榴石中金红石、钛铁矿和磷灰石薄片的大型电子背散射衍射（EBSD）数据集。我们分析了这些数据以及奥地利阿尔卑斯山榴辉岩相中变质的变砾岩中金红石、钛铁矿和金红石的EBSD数据（Griffiths等人，2016）。磷灰石的数据集是第一个同类的，并揭示了磷灰石优先排列其紧密堆积的方向平行于石榴石（c-axisapatite//garnet）。我们还认识到一个金红石石榴石COR与陨石中的Widmanstätten模式，是明确的产品出溶。这是我们所知道的硅酸盐-氧化物和金属-金属COR之间的直接相似性的第一个鉴定。值得注意的是，这种金红石石榴石COR在不同的地质环境中被发现，包括康涅狄格州和爱达荷州（美国），奥地利，德国，希腊和中国在一个广泛的散装岩石组成。所有层状矿物的结果表明，COR在很大程度上是一致的地方之间，而且，磷灰石，钛铁矿，和磷灰石之间共享。此外，70%和95%之间的laminae具有COR，并且每个laminae阶段都有一个占主导地位的COR。计算结果表明，d-间距比的主机laminals对可以成功地预测最常见的特定COR（那些COR与两个或两个以上的轴向对齐的主机）。这些结果，特别是相似的COR从显着不同的地质环境和低多样性的层状矿物，是完全一致的层状形成沉淀（可能通过出溶）。相反，替代假设仍然不支持COR结果以及矿物学和岩石学证据。因此，与本研究中的那些具有类似特征的石榴石应被认为是在升高或极端压力和温度下原本稳定的石榴石组合物分解过程中形成的沉淀物。
Abstract Garnet is a common metamorphic and igneous mineral with extensive solid solution that can be stable to mantle depths ≥400 km. High-T and/or high-P garnet may contain oriented lamellae of other minerals, most commonly simple oxides (e.g., rutile, ilmenite), apatite, and, in ultrahigh-P cases, silicates including pyroxene and amphibole. Lamellae have classically been considered to be precipitation features preserving a record of former garnet chemistry richer in the lamellae nutrients (e.g., Ti4+). Such microtextural origins in precipitation systems (e.g., alloys) have long been studied via the crystallographic orientation relationships (COR) that form between a host and a separating phase, and by the shape-preferred orientation (SPO) of the lamellae. Recently, however, alternative hypotheses to precipitation have been suggested that require emplacement of lamellae in garnet by fluids, or co-growth, overgrowth, or inheritance mechanisms. These hypotheses posit that lamellae cannot be used to study former garnet chemistry. Moreover, they predict that lamellae phases, SPO, and COR should differ widely between localities, as lamellae formation will be controlled by various local rock-specific factors such as fluid presence, fluid chemistry, or mineral growth sequence. On the other hand, if lamellae characteristics are largely consistent between localities, it likely reflects control by precipitation energetics, rather than external factors. There have been few comparative COR studies in geologic systems, but the integrative assessment of COR, SPO, and lamellae assemblages should fingerprint lamellae growth process. To test the precipitation and alternative hypotheses, we collected large electron backscatter diffraction (EBSD) data sets for rutile, ilmenite, and apatite lamellae in garnet from the Brimfield Schist, Connecticut (≥1000 °C metamorphism; Central Maine Terrane, U.S.A.). We analyzed these data alongside published EBSD data for rutile, ilmenite, and corundum from metapegmatites metamorphosed in the eclogite facies from the Austrian Alps (Griffiths et al. 2016). The apatite data set is the first of its kind, and reveals that apatite preferentially aligns its close-packed direction parallel to that of garnet (c-axisapatite//garnet). We also recognize a rutile-garnet COR related to those in meteorites with Widmanstätten patterns that are unequivocal products of exsolution. This is the first identification of direct similarities between silicate-oxide and metal-metal COR of which we are aware. Significantly, this rutile-garnet COR is found in diverse geologic settings including Connecticut and Idaho (U.S.A.), Austria, Germany, Greece, and China over a broad range of bulk-rock compositions. Results for all lamellae minerals show that COR are largely consistent between localities and, furthermore, are shared between apatite, ilmenite, and corundum. Moreover, between 70% and 95% of lamellae have COR and there is a dominant COR for each lamellae phase. Calculations show that d-spacing ratios of host-lamellae pairs can successfully predict the most commonly observed specific COR (those COR with two or more axial alignments with the host). These results, especially similarity of COR from markedly different geologic settings and a low diversity of lamellae minerals, are fully consistent with lamellae formation by precipitation (likely via exsolution). In contrast, the alternative hypotheses remain unsupported by COR results as well as by mineralogical and petrological evidence. Lamellae with similar traits as those in this work should thus be considered precipitates formed during unmixing of garnet compositions originally stable at elevated or extreme pressures and temperatures.