Determining the origin of inclusions in garnet: Challenges and new diagnostic criteria

IF 1.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
T. Griffiths, G. Habler, R. Abart
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引用次数: 3

Abstract

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.
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(奥地利东阿尔卑斯山)的二叠纪变晶岩石榴石含有丰富的亚微米至微米大小的金红石、刚玉、铁锰磷酸盐、钛铁矿、捕虏体、锆石和磷灰石包裹体。包裹体丰度、相组合、习性和大小的变化定义了石榴石中的扇形和同心带,追踪低指数石榴石刻面。分区是由石榴石熔体界面发生的一个过程产生的,沿每个面均匀,但对其晶面敏感。此外,包裹体和宿主晶格相互作用,产生宿主-包裹体晶体取向关系(CORs)。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形成的夹杂物。岩浆石榴石边缘含有长达100μm的金红石针,显示出界面依赖的形状优选取向(SPO),这无法用出溶模型来解释。此外,针的COR分布是独特的,并且意味着大的3D晶格失配。这些现象表明,针状物是通过石榴石界面的定向非均匀成核和随后两相同时生长而产生的。核心区域中均匀包裹体的起源尚不清楚。在一些假设下,集成组合物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)保持相容。尽管如此,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CORs的频率不仅在核和边缘之间变化很大,而且在石榴石核域之间也变化很大。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定向成核和随后同时生长来识别夹杂物形成的标准。
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来源期刊
American Journal of Science
American Journal of Science 地学-地球科学综合
CiteScore
5.80
自引率
3.40%
发文量
17
审稿时长
>12 weeks
期刊介绍: The American Journal of Science (AJS), founded in 1818 by Benjamin Silliman, is the oldest scientific journal in the United States that has been published continuously. The Journal is devoted to geology and related sciences and publishes articles from around the world presenting results of major research from all earth sciences. Readers are primarily earth scientists in academia and government institutions.
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