Toronto Geological Discussion Group
Jim Mungall, Associate Professor, Department of Geology, University of Toronto will give a presentation on
Formation of iron oxide and phosphate tephra of the El Laco volcano, Chile from an Fe-P-O-S magma.
Download the TGDG presentation here
The 2 Ma El Laco volcano of Chile is noted for its controversial iron ore deposits, which many researchers regard as examples of vesicular magnetite lava flows with flow-top breccias and textural evidence of slag-like quench growth of magnetite from a rapidly degassing melt. Unconsolidated Fe oxide block, lapilli, and ash deposits show fine air-fall stratification and bomb sags. The tephra comprises polycrystalline aggregates and single crystals of hematite, magnetite, and Fe phosphate minerals (identified by XRD and EMP as being predominantly Fe5(PO4)4(OH)3-2H2O). The bulk composition of an ash sample is approximately 86 mol% Fe2O3, 12 mol% FePO4, 2 mol% SiO2 and traces of other components including S. Ovoid cavities and intercrystalline reentrants in the oxide lapilli are wholly or partially occupied by a vesicular mass of Fe-phosphate, silica, and traces of monazite interpreted to be finely crystallized melt with a composition nearly identical to that of the eutectic in the FePO4-Fe2O3 system at 1 atm (84 mol% FePO4, 1070 C) according to phase equilibria recently published by Zhang et al. (2011, J. Amer. Ceram. Soc.). An oxide-phosphate bomb contains minor but ubiquitous occurrences of perlitic shoshonite glass that form menisci partially lining cavities near the bomb margin. Heating of the tephra to 1081 C for 6 hours in an evacuated silica tube yielded hematite and magnetite crystals partially wetted by a near-eutectic iron phosphate glass. At low pressures P2O5 is known to degas vigorously from Fe-P-O liquid, leaving a residue strongly enriched in Fe2O3. We suggest here that the El Laco magnetite lavas and magnetite-hematite tephra crystallized from an Fe-P-O-S magma that had separated at depth from a shoshonitic parent magma by liquid immiscibility. We interpret the small amounts of shoshonite glass observed as the products of opening of the solvus separating immiscible silicate and Fe-P-O liquid during cooling, which led to the separation of small droplets of shoshonitic melt from the Fe-P-O liquid. At 1200 C, the estimated liquidus temperature of the shoshonite, the liquidus on the Fe-rich side of the FePO4-Fe2O3 phase diagram occurs at about 80 mol% FePO4:20 mol% Fe2O3. The Fe-P-O-S magma is therefore inferred to have lost most of its P2O5 and all of its SO3 during degassing. Violent degassing promoted the ascent of these very dense liquids and led to explosive eruption of the tephra and the emplacement of highly vesicular flows. The flows are completely degassed but degassing was arrested in the airfall units, where some vestiges of the iron phosphate melt remain among the hematite and magnetite crystals. We are currently testing this hypothesis by conducting melting experiments to try to equilibrate shoshonite melt with Fe-P-O-S melt and define the solvus between the two liquids. If our hypothesis is correct then we will have identified a new magma type that could have been responsible for the genesis of several important iron ore deposits including those at Kiruna, Sweden, the Chilean Iron Belt, and numerous others.
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