Evidence for High Pressure and Temperature Crystallization in Boninite Samples Recovered During IODP Expedition

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Mineralogy and mineral chemistry data collected on unusual boninite samples (U1439C 15R-1, U1439C 25R-1) recovered during IODP Expedition 352 demonstrate highly variable mineral chemistries that suggest complex crystallization histories. The samples include early-crystallized olivines with embayed and reacted margins, some of which also show zoning (Fo87 cores; Fo77 rims), and large, euhedral pyroxene crystals, with clinoenstatite to pigeonite cores (En86 Fs12 Wo2, to En69 Fs12 Wo19), and augite rims (En43 Fs17 Wo40). Cr-rich spinels were found to be enclosed in both olivines and pyroxenes. These spinels show elevated Al2O3 and MgO contents (≈8-12%), suggesting early high pressure crystallization. Further EPMA studies of these and other Expedition 352 boninites, focused on spinel-olivine and spinel-pyroxene relationships, were conducted using the JEOL 8900R electron microprobe system at the Florida Center for Analytical Electron Microscopy (FCAEM) at Florida International University.

Olivine-spinel barometry (based on the calculations of Ballhaus and Berry, 1990) indicate pressures of crystallization between 2.0-2.3 GPa. Calculated maximum temperatures after Putirka et al (2007) range between 1364°C and 1403°C (assuming a minimum 1.5% wt. H2O, from preliminary FTIR results on U1439C glasses). Coexisting clinoenstatites and pigeonites provide pyroxene-based thermobarometric constraints (e.g., Nimis and Taylor, 2000), pointing to pressures between 1.0-2.0 GPa at temperatures between 1200-1260oC. Together these results demonstrate that boninite magmas are products of high pressure and temperature melting, as has been suggested in past studies. The late crystallizing augites in these samples have highly variable Al2O3 contents (1.9 – 13.7% wt.) that reflect disequilibrium conditions which, though thermobarometric measurements are not possible, appear to be consistent with lower pressure conditions. These augites appear have formed in response to magma mingling or mixing that introduced additional Ca, Fe, and Al into the melt.

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Geological Society of America Abstracts with Programs, v. 50, issue 5, no. 15-30