Paper No. 118-2
Presentation Time: 9:15 AM
TRACE ELEMENT AND TEXTURAL CHARACTERISTICS OF OLIVINE IN KILAUEA IKI PICRITES
BRADSHAW, Richard W., College of Earth, Ocean & Atmospheric Sciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331 and KENT, Adam J.R., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331
Mafic volcanic rocks are important to our understanding of the origin and evolution of the mantle and the processes that cause the mantle to melt. However, these rocks are commonly modified during transport and storage within the crust, which obscures their primary source information. Picrites and other olivine-rich volcanic rocks often contain olivine grains with diverse textures, including optically deformed (with kink-bands, subgrains, etc.) and undeformed crystals. A common interpretation is that deformed olivine is plastically deformed during storage in dunite cumulate piles, implying that picrites are formed as basaltic liquids that accumulate olivine from diverse sources within the crust, and thus are not primitive melts from the mantle. However, recent studies have also suggested instead that deformed olivines form by collisions or by variations in crystal growth rates. We present olivine trace element and textural data from Kilauea Iki picrites as a way to distinguish different olivine populations, giving insights into the process of picrite formation.
Trace element analyses (EMP and LA-ICP-MS) of olivine from the 1959 eruption of Kilauea Iki show chemical differences between deformed and undeformed grains, suggesting they derive from different chemical reservoirs and are not all related to the carrier magma. Chemical differences are most apparent in slowly diffusing trace elements (Al, P, Ti, V, Cr). Timescales of olivine residence are also of interest as the different models for the formation of deformed olivine predict much different residence times. X-ray element maps of both undeformed and deformed olivine show delicate P zoning, with some grains also showing similar, but more diffuse, zoning in Al and Cr. This more diffuse nature, or in most cases, lack of zoning in Al and Cr, suggests that if Al and Cr were originally zoned along with P, that they diffuse more rapidly than P. This relationship can be used to estimate residence times of olivine. Preliminary results suggest varied residence times up to tens of years to relax Cr compositional profiles or to erase them altogether. Thus trace elements concentrations and zoning patterns show that picrites sample olivine crystals that have resided in the magmatic system over a range of timescales and come from diverse chemical reservoirs.
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