Immiscible hydrous Fe-Ca-P melt and the origin of iron oxide-apatite ore deposits.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 29650951.
- Also identified by DOI 10.1038/s41467-018-03761-4 and PMC identifier 5897329.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
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<sub>2</sub>) 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<sub>2</sub>). These Si-poor melts are in equilibrium with a rhyolitic conjugate and are produced under oxidized conditions (~FMQ + 3.3), high water activity (aH<sub>2</sub>O ≥ 0.7), and in fluorine-bearing systems (1 wt.%). Our results show that increasing aH<sub>2</sub>O and fO<sub>2</sub> 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.