Squeezing water from a stone: H2O in nominally anhydrous minerals from granulite xenoliths and deep, hydrous fractional crystallization
Squeezing water from a stone: H2O in nominally anhydrous minerals from granulite xenoliths and deep, hydrous fractional crystallization
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从石头中挤出水：麻粒岩捕虏体中名义上无水矿物中的 H2O 和深层含水分步结晶
DOI：
10.1002/essoar.10503635.1
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发表时间：
2020
影响因子：
--
通讯作者：
Farmer, Lang
中科院分区：
文献类型：
--
作者：
Chin, Emily J.;Curran, Sean T.;Farmer, Lang
水是地球板块构造的关键，而板块构造又对大陆地壳的形成至关重要。尽管弧熔岩以富含挥发物的状态喷发，而钙碱性弧岩体的特征是存在角闪石和黑云母等含水矿物，但弧岩浆在其演化早期（在地壳深处）的含水量仍然受到很少的限制。在这里，我们报告了通过二次离子质谱法对来自美国科罗拉多州的元古代深部地壳捕虏体在岩相薄片上原位测量的名义无水矿物中的 H2O 含量。单斜辉石、斜方辉石和石榴石的平均 H2O 含量分别为 75–760、233–410 和 42–139 ppm。重建的散装岩石 H2O 含量范围为 ~60 至 ~650 ppm。矿物间 H2O 比率与实验矿物/熔体 D 值重叠，用于计算最后与捕虏体达到平衡的熔体的 H2O。我们认为这些捕虏体代表了在高（~1 GPa）压力下从原始含水（≥1 wt.% H2O）熔体中分馏出来的堆积物，类似于现代俯冲带的条件，并可能与形成前寒武纪北美核心的广泛弧增生有关。
Water is key to plate tectonics on Earth, which, in turn, is vital to the production of continental crust. Although arc lavas erupt in a volatile‐rich state and calc‐alkaline arc plutons are distinguished by the presence of hydrous minerals such as hornblende and biotite, the water content of arc magmas earlier in their evolution—in the deep crust—remains poorly constrained. Here, we report H2O contents in nominally anhydrous minerals measured in situ on petrographic thin sections by secondary ion mass spectrometry of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average H2O contents ranging from 75–760, 233–410, and 42–139 ppm, respectively. Reconstructed bulk rock H2O contents range from ~60 to ~650 ppm. Intermineral H2O ratios overlap experimental mineral/meltDvalues and are used to calculate H2O of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% H2O) melt at high (~1 GPa) pressures, similar to conditions in modern subduction zones and potentially associated with widespread arc accretion that formed the core of North America in the Precambrian.