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Determining the origin of inclusions in garnet: Challenges and new diagnostic criteria
American Journal of Science ( IF 2.9 ) Pub Date : 2020-11-01 , DOI: 10.2475/11.2020.01
Thomas A. Griffiths , Gerlinde Habler , Rainer Abart

ABSTRACT Permian metapegmatite garnets from the Koralpe region (Eastern Alps, Austria) contain abundant submicrometer- to micrometer-sized inclusions of rutile, corundum, Fe-Mn phosphate, ilmenite, xenotime, zircon, and apatite. Variations in inclusion abundance, phase assemblage, habit, and size define sector and concentric zones in the garnets, tracing low-indexed garnet facets. Zoning resulted from a process occurring at the garnet-melt interface, homogeneous along each facet, but sensitive to its crystallographic plane. Furthermore, inclusion and host lattices interacted, generating host-inclusion crystallographic orientation relationships (CORs). These phenomena exclude inclusion formation via overgrowth of pre-existing phases, infiltration of fluids/melts, or dissolution-reprecipitation. Magmatic garnet rims contain rutile needles up to 100 μm long, showing an interface-dependent shape-preferred orientation (SPO) that cannot be explained by exsolution models. Furthermore, the COR distribution for needles is unique, and implies large 3D lattice mismatches. These phenomena suggest that needles originated via oriented heterogeneous nucleation at the garnet interface and subsequent simultaneous growth of both phases. The origin of equant inclusions in core domains is less clear. With some assumptions, integrated compositions remain compatible with closed system exsolution or open system precipitation (OSP) involving divalent cation loss. Still, the oriented interface nucleation hypothesis seems to better explain the fact that the frequency of rutile-garnet CORs varies strongly not only between cores and rims but also between garnet core domains. Inclusion formation by oriented interface nucleation and simultaneous growth can explain many observations commonly attributed to exsolution, making distinguishing between these two mechanisms a challenge. We suggest interface-dependence of SPOs and COR frequencies as criteria for identifying inclusion formation via oriented nucleation at an interface and subsequent simultaneous growth.

中文翻译:

确定石榴石中夹杂物的来源:挑战和新的诊断标准

摘要 来自 Koralpe 地区(奥地利东部阿尔卑斯山)的二叠纪变伟晶岩石榴石含有丰富的亚微米至微米级的金红石、刚玉、磷酸铁锰、钛铁矿、磷钇矿、锆石和磷灰石包裹体。内含物丰度、相组合、习性和尺寸的变化定义了石榴石中的扇形和同心带,追踪低指数石榴石的刻面。分区是由石榴石-熔体界面处发生的过程引起的,沿每个面均质,但对其晶面敏感。此外,夹杂物和宿主晶格相互作用,产生宿主-夹杂物晶体取向关系 (COR)。这些现象排除了通过预先存在的相的过度生长、流体/熔体的渗透或溶解-再沉淀而形成的夹杂物。岩浆石榴石边缘包含长达 100 μm 的金红石针状结构,显示出无法通过出溶模型解释的界面相关形状偏好取向 (SPO)。此外,针的 COR 分布是独一无二的,并且意味着大的 3D 晶格不匹配。这些现象表明,针状晶体起源于石榴石界面处的定向异相成核和随后两相的同时生长。核心域中等值包含的起源不太清楚。在某些假设下,综合成分与涉及二价阳离子损失的封闭系统出溶或开放系统沉淀 (OSP) 保持兼容。仍然,定向界面成核假说似乎更好地解释了这样一个事实,即金红石-石榴石 COR 的频率不仅在核心和边缘之间而且在石榴石核心域之间变化很大。通过定向界面成核和同时生长形成的夹杂物可以解释许多通常归因于出溶的观察结果,这使得区分这两种机制成为一个挑战。我们建议将 SPO 和 COR 频率的界面依赖性作为通过界面处的定向成核和随后的同时生长来识别夹杂物形成的标准。
更新日期:2020-11-01
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