Multiple origins of zircons in jadeitite
Multiple origins of zircons in jadeitite
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DOI：
10.1007/s00410-009-0453-y
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发表时间：
2010-06
影响因子：
3.5
通讯作者：
B. Fu;J. Valley;N. Kita;M. Spicuzza;C. Paton;T. Tsujimori;M. Bröcker;G. Harlow
中科院分区：
文献类型：
--
作者：
B. Fu;J. Valley;N. Kita;M. Spicuzza;C. Paton;T. Tsujimori;M. Bröcker;G. Harlow
硬玉岩形成于俯冲环境中高压变质作用的热液流体中，但硬玉岩中锆石的成因尚不确定。本文报道了日本Osayama蛇纹混杂岩、希腊Syros混杂岩、危地马拉Motagua断层带和加州Franciscan杂岩中硬玉岩全岩锆石δ 18 O和Ti的离子探针分析结果以及大块δ 18 O数据。在Osayama硬玉岩中，通过阴极发光鉴定出两组结构截然不同的锆石，它们的δ 18 O值不同：无特征或弱环带的锆石，δ 18 O = 3.8 ± 0.6‰（2 SD，VSMOW）;危地马拉多硅白云母硬玉岩和锡罗斯硬玉岩块体中的锆石δ 18 O值与大坂硬玉岩块体中的相似：危地马拉锆石为4.8 ± 0.7‰，硬玉岩中的锡罗斯锆石为5.2 ± 0.5‰，伴生绿辉石岩、蓝闪石和绿泥-阳起石皮中的锡罗斯锆石为5.2 ± 0.4‰。上述绝大多数锆石的δ 18 O值落在现代洋壳氧化物辉长岩和斜长花岗岩中火成锆石的离子探针测得的δ 18 O值（5.3 ± 0.8‰）和与原始岩浆成分或地幔成分平衡的锆石激光显微镜测得的δ 18 O值（5.3 ± 0.6‰）范围内。这些锆石中的钛浓度在1至19 ppm之间，在全世界火成岩锆石的范围内。4个地区的硬玉岩δ 18 O（全岩）值均大于硬玉岩δ 18 O（硬玉岩）值，变化范围为6.3 ‰ ~ 10.1‰，其δ 18 O和Ti值均高于热液锆石的预测值，且在合理的变质温度下，多数锆石的δ 18 O值与共存硬玉岩的δ 18 O值不平衡。我们的结论是，虽然有些锆石可能是热液的起源，大多数的锆石研究最好的解释为遗留的火成岩晶体继承的前体岩石，他们没有直接从热的含水流体沉淀如先前所假设的。因此，这些锆石的U-Pb年龄可以确定岩浆结晶的时间，而不能确定高压变质或热液活动的时间。
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 δ18O and Ti in zircons, and bulk δ18O data for the jadeitite whole-rock from four terranes: Osayama serpentinite mélange, Japan; Syros mélange, 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 δ18O: featureless or weakly zoned zircons with δ18O = 3.8 ± 0.6‰ (2 SD, VSMOW), and zircons with oscillatory or patchy zoning with higher δ18O = 5.0 ± 0.4‰. Zircons in phengite jadeitite from Guatemala and a jadeitite block from Syros have similar δ18O 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 chlorite-actinolite rinds. The δ18O 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 δ18O (whole-rock) ≅ δ18O (jadeite) and vary from 6.3 to 10.1‰ in jadeitites in all four areas.These values of δ18O and Ti are higher than predicted for hydrothermal zircons, and the δ18O 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.