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Multiple origins of zircons in jadeitite

机译：翡翠中锆石的多种起源

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Jadeitites form from hydrothermal fluids during high pressure metamorphism in subduction environments; however, the origin of zircons in jadeitite is uncertain. We report ion microprobe analyses of δ~(18)O and Ti in zircons, and bulk δ~(18)O data for the jadeitite whole-rock from four terranes: Osayama serpentinite melange, Japan; Syros melange, Greece; the Motagua Fault zone, Guatemala; and the Franciscan Complex, California. In the Osayama jadeitite, two texturally contrasting groups of zircons are identified by cathodoluminescence and are distinct in δ~(18)O: featureless or weakly zoned zircons with δ~(18)O = 3.8 ± 0.6‰ (2 SD, VSMOW), and zircons with oscillatory or patchy zoning with higher δ~(18)O = 5.0 ± 0.4‰. Zircons in phengite jadeitite from Guatemala and a jadeitite block from Syros have similar δ~(18)O values to the latter from Osayama: Guatemala zircons are 4.8 ± 0.7‰, and the Syros zircons are 5.2 ± 0.5‰ in jadeitite and 5.2 ± 0.4‰ in associated omphacitite, glaucophanite and chloriteactinolite rinds. The δ~(18)O values for most zircons above fall within the range measured by ion microprobe in igneous zircons from oxide gabbros and plagiogranites in modern ocean crust (5.3 ± 0.8‰) and measured in bulk by laser fluorination of zircons in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6‰). Titanium concentrations in these zircons vary between 1 and 19 ppm, within the range for igneous zircons worldwide. Values of δ~(18)O (whole-rock) δ~(18)O (jadeite) and vary from 6.3 to 10.1‰ in jadeitites in all four areas. These values of δ~(18)O and Ti are higher than predicted for hydrothermal zircons, and the δ~(18)O values of most zircons are not equilibrated with the coexisting jadeite at reasonable metamorphic temperatures. We conclude that while some zircons may be hydrothermal in origin, a majority of the zircons studied are best explained as relic igneous crystals inherited from precursor rocks; they were not precipitated directly from hot aqueous fluids as previously assumed. Therefore, U–Pb ages from these zircons may date magmatic crystallization and do not establish the timing of high pressure metamorphism or hydrothermal activity.
机译：在俯冲环境中高压变质过程中，热液中形成翡翠。但是，锆石在硬玉中的来源尚不确定。我们报告了锆石中δ〜（18）O和Ti的离子微探针分析，以及来自四个地层的翡翠全岩的整体δ〜（18）O数据：日本Osayama蛇纹石混合物;希腊Syros melange;危地马拉的莫塔瓜断层带;和加利福尼亚的方济各会建筑群。在Osayama翡翠中，通过阴极发光鉴定出两个质地不同的锆石组，并且在δ〜（18）O中不同：δ〜（18）O = 3.8±0.6‰（2 SD，VSMOW）的无特征或弱带状锆石，锆石具有较高的δ〜（18）O = 5.0±0.4‰的振荡或斑片带。危地马拉的变质辉石翡翠中的锆石和锡罗斯（Syros）的翡翠块中的锆石具有与Osayama后者相似的δ〜（18）O值：危地马拉的锆石为4.8±0.7‰，锡罗斯的锆石在翡翠中为5.2±0.5‰和5.2±0.4 ‰在伴有的辉石，青石棉和绿泥石阳晶石皮中。大部分以上锆石的δ〜（18）O值都在离子微探针测量的氧化铁辉石和斜长花岗岩的火成锆石中的现代锆石中（5.3±0.8‰）的范围内，并且通过与平衡达到平衡的锆石的激光氟化进行散装测量原始岩浆成分或地幔（5.3±0.6‰）。这些锆石中的钛浓度在1至19 ppm之间变化，在全球火成锆石的范围内。四个地区的翡翠中δ〜（18）O（整体岩石）δ〜（18）O（硬玉）的值在6.3〜10.1‰之间。 δ〜（18）O和Ti的这些值高于热液锆石的预测值，并且大多数锆石的δ〜（18）O值在合理的变质温度下都无法与共存的翡翠平衡。我们得出的结论是，尽管某些锆石可能起源于热液，但大多数研究的锆石最好地解释为从前体岩石继承的遗留火成岩晶体。它们不是像先前所假定的那样直接从热水性流体中沉淀出来的。因此，这些锆石中的U–Pb年龄可能是岩浆结晶的日期，并且没有确定高压变质或热液活动的时间。

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