Interactions of magmas and highly reduced fluids during intraplate volcanism, Mt Carmel, Israel: Implications for mantle redox states and global carbon cycles

Oxygen fugacity (fO2) controls the speciation of COH fluids in Earth’s mantle; a major question is whether the sublithospheric mantle is metal-saturated, maintaining fO2 near the Iron-Wüstite (IW) buffer reaction. If so, then COH fluids from this source will be dominated by CH4 + H2, rather than the more oxidized CO2-H2O fluids commonly considered in petrological studies. A key to this question is found in rare but widespread examples of natural mineral assemblages that require unusually low fO2. We summarize an investigation of super-reduced mineral assemblages in corundum xenocrysts from Late Cretaceous alkalibasalt volcanoes on Mt Carmel, northern Israel and related Plio-Pleistocene alluvial deposits. P-T estimates indicate that the corundum xenocrysts crystallized in the uppermost mantle. The welldocumented geological controls on the origin of these deposits, and radiometric dating of the superreduced phases, ensure the ‘‘naturalness” of the controversial assemblages and make these mineral parageneses a benchmark for evaluation of related occurrences worldwide. The tuffs contain a ‘‘basalt-megacryst” mineral suite (zircon, sapphire, ilmenite, spinel). The megacryst chemistry and the geochronology of the zircons indicate that the megacrysts crystallized from broadly syenitic melts that differentiated at subcrustal levels (P ca 1 GPa) within a thick gabbroic underplate built up from Permian through Pliocene time and perhaps into the Pleistocene. Reaction of mantle-derived CH4-H2 fluids with these syenitic melts led to the separation of immiscible Fe0 and Fe-Ti oxide melts near fO2 = IW. Trace-element distributions suggest the syenitic melts then separated into immiscible Si-Al-Na- K-rich and FeO-rich oxide melts; the latter were enriched in HFSE, REE, P and Zr as in other natural and synthetic examples of melt-melt immiscibility. In a model magma chamber the FeO-rich melts would sink, leaving the Si-Al-Na-K melts in an upper zone, both still fluxed by CH4-H2 fluids. At fO2 of DIW-6 to -7 the removal of immiscible Fe-Ti-Si-C silicide melts from the FeO-rich melt would leave a desilicated Ca-Al-Si oxide melt that crystallized high-Ti corundum hibonite cumulates with inclusions requiring fO2 from DIW + 2 to DIW-9, while the lessreduced conjugate silicate melts in the upper levels crystallized low-Ti corundum. Aggregates of skeletal, strongly Ti-zoned corundum crystals. reflect rapid crystallization from very reduced melt-fluid mixtures, probably in fluid-escape channels. Explosive eruptions sampled individual magma chambers at different depths and with different initial compositions, fluid mixtures and fluid dynamics to produce Mt Carmel’s mineralogical diversity. A review of similar occurrences worldwide suggests that the Mt Carmel assemblages reflect a fundamental process – the rise of CH4-H2 fluids into the upper mantle -- that accompanies mantle-derived magmatism in many tectonic settings. The interaction of these fluids with lithospheric mantle rocksand melts can lead to extreme fractionation via the separation of immiscible Fe-Ti-Si-C melts and residual desilicated melts. The oxidation of CH4-H2 fluids in the lithospheric mantle may be the ultimate source of metasomatic fluids dominated by CO2 + H2O, and of many diamonds. More attention should be paid to the role of methane and other reduced fluids in mantle petrology, and their relevance to metasomatic processes and global carbon cycles.