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Parageneses of TiB2 in corundum xenoliths from Mt Carmel, Israel: Siderophile behaviour of Boron under reducing conditions
American Mineralogist ( IF 3.1 ) Pub Date : 2020-11-01 , DOI: 10.2138/am-2020-7375
William L. Griffin 1, 2 , Sarah E.M. Gain 1, 3 , Martin Saunders 3 , Luca Bindi 4, 5 , Olivier Alard 1 , Vered Toledo 6 , Suzanne Y. O'Reilly 1
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Abstract Titanium diboride (TiB2) is a minor but common phase in melt pockets trapped in the corundum aggregates that occur as xenoliths in Cretaceous basaltic volcanoes on Mt. Carmel, north Israel. These melt pockets show extensive textural evidence of immiscibility between metallic (Fe-Ti-C-Si) melts, Ca-Al-Mg-Si-O melts, and Ti-(oxy)nitride melts. The metallic melts commonly form spherules in the coexisting oxide glass. Most of the observed TiB2 crystallized from the Fe-Ti-C silicide melts and a smaller proportion from the oxide melts. The parageneses in the melt pockets of the xenoliths require f O 2 ≤ Δ I W − 6 , $f_{\mathrm{O}_{2}} \leq \Delta \mathrm{IW}-6,$probably generated through interaction between evolved silicate melts and mantle-derived CH4+H2 fluids near the crust-mantle boundary. Under these highly reducing conditions boron, like carbon and nitrogen, behaved mainly as a siderophile element during the separation of immiscible metallic and oxide melts. These parageneses have implications for the residence of boron in the peridotitic mantle and for the occurrence of TiB2 in other less well-constrained environments such as ophiolitic chromitites.

中文翻译:

以色列卡梅尔山刚玉捕虏体中 TiB2 的共生:还原条件下硼的亲铁行为

摘要 二硼化钛 (TiB2) 是一种次要但常见的相,它被困在刚玉聚集体中的熔体口袋中,该聚集体在 Mt. 的白垩纪玄武岩火山中以捕虏体形式出现。卡梅尔,以色列北部。这些熔体口袋显示了金属 (Fe-Ti-C-Si) 熔体、Ca-Al-Mg-Si-O 熔体和 Ti-(氧)氮化物熔体之间不混溶的广泛纹理证据。金属熔体通常在共存的氧化物玻璃中形成小球。大多数观察到的 TiB2 是从 Fe-Ti-C 硅化物熔体中结晶出来的,一小部分是从氧化物熔体中结晶出来的。捕虏体熔袋中的共生需要 f O 2 ≤ Δ IW − 6 , $f_{\mathrm{O}_{2}} \leq \Delta \mathrm{IW}-6, $可能通过壳幔边界附近演化出的硅酸盐熔体和地幔衍生的 CH4+H2 流体。在这些高度还原的条件下,硼,像碳和氮一样,在不混溶的金属和氧化物熔体的分离过程中主要表现为亲铁元素。这些共生对硼在橄榄岩地幔中的驻留以及 TiB2 在其他约束较少的环境(如蛇绿岩铬铁矿)中的出现具有影响。
更新日期:2020-11-01
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