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Magma Chambers: Products and Processes |
Grant Institute of Geology, University of Edinburgh, Edinburgh EH9 3JW, Scotland
Electron-microprobe analyses of submarine basalt glass and coexisting olivine from the Galapagos Rise and the Mid-Atlantic Ridge (60–63°N) reveal that olivine phenocrysts are virtually unzoned and olivine compositions are identical to those predicted by assuming that the olivine crystallized at the liquidus temperature of the host glass. The absence of chemical zoning of olivine indicates that their host magmas must have cooled by less than 15°C and the MgO/FeO of the magma must have been nearly constant (less than 3 wt% olivine removed) during the time that olivine phenocrysts existed in the magma. Olivine phenocrysts re-equilibrate with their host magma relatively quickly and any changes in temperature or composition that occurred in the 24 h previous to eruption would be recorded in the olivine composition. The settling rates of 1-mm olivine phenocrysts are too slow compared to magma ascent rates to allow early-formed olivine to settle out. There, many magmas that erupt at mid-ocean-ridge crests must ascend from their final reservoir nearly isothermally and isochemically, or alternatively they ascend at superliquidus temperatures and cool below the liquidus just prior to eruption. In either case the magmas identify the temperature (±15°C) and the composition of the magma reservoir immediately prior to eruption. The depth of these magma reservoirs may be estimated through experimental and empirical determination of the pressure-temperature phase diagrams of oceanic basalts. For Galapagos Rise and Mid-Atlantic Ridge basalts these reservoirs are at 3–25 km beneath the ridge crust and in some cases individual reservoirs may be located within ±3 km.
