Oxide minerals, as small parts of the hybrid granitoids, are useful for studying the crystallization conditions of hybrid granitoid systems. They may also provide evidence for magma interactions in a magma chamber because their compositions change much faster than those of silicates during temperature fluctuation that may be caused by magma mixing. Electron microprobe analyses of Fe–Ti oxides in a time series (1995–2002) of andesites were used in two-oxide geothermometry (Andersen & Lindsley, 1988). Rim-to-rim transects of titanomagnetite grains were also obtained to detect the potential development of near-rim Ti diffusion gradients.
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 2) 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 (. The origin of orebodies composed of low-Ti iron oxide minerals (magnetite and/or hematite) and apatite in (sub)volcanic rocks is controversial,. These rocks, essentially free of silicates and sufficiently enriched in Fe to be recoverable, have been classified as Kiruna-type or iron oxide-apatite (IOA) deposits.
Their enrichment in Fe and P has been variously attributed to magmatic and hydrothermal ore-forming processes. Metasomatic replacement of the host igneous rocks by convecting fluids, proposed as a likely mechanism due to the pervasive hydrothermal alteration of the ore, is supported by the low Ti content and trace element characteristics of magnetite crystals. Alternatively, IOA deposits may represent volcanic flows or shallow magma intrusions as suggested by several field relationships including discordant veins and dykes of magnetite-apatite ores intruding their host rocks, magma flow structures, vesicular textures, and volcanic bombs. In this case, the formation of Fe-rich and P-rich rocks might be explained by liquid immiscibility and segregation of a Fe–P-rich immiscible magma from its rhyolitic counterpart. The development of immiscibility is supported by the coexistence of two types of melts in glassy matrices and inclusions hosted by phenocrysts in the ore and in andesitic wall rocks.
However, none of these immiscible melts have compositions representative of IOA ores. Experimental evidence for the formation of such silica-poor iron oxide melts at magmatic conditions is also lacking.Evolved basaltic magmas can split into immiscible rhyolitic (dacitic) and ferrobasaltic melts along their crystallization path at temperatures below 1040–1020 °C. P 2O 5 in the bulk composition promotes the development of silicate liquid immiscibility and this oxide strongly concentrates in the Fe-rich melt. Experimental and natural Fe-rich immiscible melts generally contain 35–45 wt.% SiO 2 and only a few wt.% P 2O 5,.
Silicate phases predominantly crystallize from such melts, producing oxide-apatite gabbros of moderate economic interest. Extreme enrichment of apatite and iron oxide over silicate minerals, as observed in IOA deposits, cannot simply result from differential crystal settling in an iron-rich silicate melt.
This is because, with the exception of plagioclase, common silicate minerals (actinolite and diopside) are denser than the melt and would sink along with the oxides. A more efficient mechanism for the production of IOA deposits would be direct crystallization of a Fe–P-rich and Si-depleted magma.Here, we provide an original solution to this challenging issue based on results obtained from experiments performed in realistic conditions of pressure and temperature in an internally heated pressure vessel (IHPV). We used experimental starting material which was prepared from a series of mixtures between two mafic end-members and a rhyolitic composition (Supplementary Fig. And Supplementary Tables, ). We show that liquid immiscibility develops in the intermediate magmas at conditions relevant to the magmatic reservoirs of most subvolcanic IOA deposits ( P = 100 MPa, T = 1000–1040 °C).
With elevation of oxygen fugacity and water activity, nearly pure Fe–Ca–P melts that are compositionally identical to typical IOA ores are produced by liquid immiscibility. This finding allows us to conclude that liquid immiscibility is the key process in the formation of IOA deposits. This is extremely important for the establishment and refinement of a petrogenetic model for IOA ores. Phase equilibria and immiscibility texturesExperimental conditions and phase assemblages are summarized in Supplementary Table. All run products contain crystal phases and either a single homogenous melt or two distinct immiscible melts quenched to glass. Solid phases are magnetite, apatite, fayalite (or fayalitic olivine), a silica phase (tridymite), and occasionally titano-hematite and clinopyroxene. A single homogeneous melt is found in some experiments with high bulk P 2O 5 contents (1.1–2.3 wt.% P 2O 5; Supplementary Table ), indicating that, despite the critical role of phosphorus on the development of liquid immiscibility, other compositional parameters must contribute significantly to the onset of unmixing.
We note that a single melt is also observed in experiments performed at the highest temperature (i.e., 1040 °C) suggesting that in our multicomponent system the apex of the binodal lies beneath 1040 °C, as already identified in dry ferrobasalts. We also note that all experiments performed below 1040 °C under oxidizing conditions (fayalite-magnetite-quartz equilibrium) (FMQ + 3) developed immiscibility while some experiments performed at identical temperature under more reduced conditions do not show immiscibility.Experimental products with distinct immiscibility typically show sharp two-liquid interfaces (Fig. ). Immiscible melts form globules or domains of various sizes (including nano-scale droplets). We observe no compositional difference between small and large melt pools in individual experiments, suggesting complete equilibration of the two melts. In runs with sufficiently large globules, the Fe-rich melt droplets display very small wetting angles with magnetite, apatite, and fayalite, and these phases form euhedral crystals preferentially concentrated in the Fe-rich melt (Fig. ). Experiments in which we added an FeS component (HP22–27) also contain large spherical or ovoid droplets of sulfide melt dispersed in the silicate glasses: our experimental products therefore contain three immiscible liquids (Fig. ). Back-scattered electron images of selected experiments showing liquid immiscibility between Fe-rich and Si-rich glass.
A, b Typical irregularly shaped (coalesced) patches of Fe-rich silicate glass (liq Fe) within Si-rich glass (liq Si). Magnetite and/or apatite are preferentially enclosed in the immiscible Fe-rich silicate glasses. C Fe–Ca–P glass (liq Fe–Ca–P) separated from the Si-rich glass (liq Si). Magnetite and apatite are crystalline phases in both liquids.
D Irregularly shaped (coalesced) patches of Fe–P glass (liq Fe–P) within Si-rich glass. Oxide minerals (Ti-rich hematite and magnetite) are predominantly hosted by the Fe–P glass. Abbreviations: Mt, magnetite; Ti-Hem, solid solution of ilmenite and hematite; Ap, apatite; Sul, sulfide; liq Fe, Fe-rich silicate glass; liq Fe–Ca–P, Fe–Ca–P glass; liq Fe–P, Fe–P glass; liq Si, Si-rich glass.