Institute of Mineralogy, Leibniz Universtät Hannover, 30167, Hannover, Germany.
State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, 100083, Beijing, China.
Nat Commun. 2018 Apr 12;9(1):1415. doi: 10.1038/s41467-018-03761-4.
The origin of iron oxide-apatite deposits is controversial. Silicate liquid immiscibility and separation of an iron-rich melt has been invoked, but Fe-Ca-P-rich and Si-poor melts similar in composition to the ore have never been observed in natural or synthetic magmatic systems. Here we report experiments on intermediate magmas that develop liquid immiscibility at 100 MPa, 1000-1040 °C, and oxygen fugacity conditions (fO) of ∆FMQ = 0.5-3.3 (FMQ = fayalite-magnetite-quartz equilibrium). Some of the immiscible melts are highly enriched in iron and phosphorous ± calcium, and strongly depleted in silicon (<5 wt.% SiO). These Si-poor melts are in equilibrium with a rhyolitic conjugate and are produced under oxidized conditions (~FMQ + 3.3), high water activity (aHO ≥ 0.7), and in fluorine-bearing systems (1 wt.%). Our results show that increasing aHO and fO enlarges the two-liquid field thus allowing the Fe-Ca-P melt to separate easily from host silicic magma and produce iron oxide-apatite ores.
铁氧化物-磷灰石矿床的成因颇具争议。有人提出，铁镁质硅酸盐熔体不混溶以及富铁熔体的分离是形成这些矿床的原因，但在天然或合成的岩浆体系中，从未观察到过组成上类似于矿石的富 Fe-Ca-P 和贫 Si 的熔体。在这里，我们报告了在 100 MPa、1000-1040°C 和氧逸度条件（∆FMQ=0.5-3.3（FMQ=铁橄榄石-磁铁矿-石英平衡）下，中间岩浆中发展出不混溶性的实验。一些不混溶的熔体富含铁和磷±钙，而硅严重亏损（<5wt.%SiO）。这些贫硅熔体与流纹质岩浆处于平衡状态，并且在氧化条件（~FMQ+3.3）、高水活度（aHO≥0.7）和含氟体系（1wt.%）下产生。我们的结果表明，增加 aHO 和 fO 会扩大两液相区，从而使富 Fe-Ca-P 的熔体容易从寄主硅质岩浆中分离出来，形成铁氧化物-磷灰石矿石。
