HR: 0800h
AN: V11B-0587    [Abstracts]
TI: Are Variations in TiO2 Contents in Anorthite-Hosted MORB Melt Inclusions Controlled by Magma Compositions or Boundary Layer Phenomena?
AU: * Weinsteiger, A B
EM: weinstea@geo.oregonstate.edu
AF: Oregon State University Department of Geosciences, 104 Wilkinson Hall, Corvallis, 0R 97331, United States
AU: Kent, A J
EM: kentad@geo.oregonstate.edu
AF: Oregon State University Department of Geosciences, 104 Wilkinson Hall, Corvallis, 0R 97331, United States
AU: Nielsen, R L
EM: nielsenr@geo.oregonstate.edu
AF: Oregon State University Department of Geosciences, 104 Wilkinson Hall, Corvallis, 0R 97331, United States
AU: Tepley, F J
EM: ftepley@coas.oregonstate.edu
AF: Oregon State University Department of Geosciences, 104 Wilkinson Hall, Corvallis, 0R 97331, United States
AB: Anorthitic, AN greater than 90, plagioclase-phyric pillow basalts have been recovered from almost every Mid Ocean Ridge (MOR). These phenocrysts typically contain bands of primitive (high MgO) melt inclusions (MI's). Previous workers have described large variations in the abundance of Ti in melt inclusions within a single anorthite crystal. The origin of this anomalous Ti is uncertain. It has been ascribed to both variations in Ti (and by inference other elements) in melts from which plagioclase crystallize or to processes relating specifically to the formation of melt inclusions (e.g. diffusion controlled boundary layer effects). The goal of this study was to discriminate between these possibilities by comparing the Ti contents of melt inclusions and co-existing plagioclase. If it can be shown that the Ti content of plagioclase can be related to the Ti content of MI's through simple partitioning relationships, then this implies that inclusions represent melts with variable Ti contents. In this case, MI's potentially preserve a range of "unmixed" magma compositions that occur in the MOR magma environment. Data collected from MI's and high anorthite phenocrysts illustrate the same results for both Gorda Ridge and Southeast Indian Ridge. The range of Ti concentrations in MI's within individual phenocrysts varies from near zero (e.g. homogeneous) up to a factor of two at a given MI Mg number and constant anorthite value of plagioclase. Host feldspar analyzed next to the inclusion is characterized by a greater range of Ti concentration than that measured in the MI band. Calculated Ti distribution coefficients, based on the MI/host feldspar pairs are constant as a function of MI TiO2 and melt inclusion size. We interpret this to show that MI's represent real trapped melts, from which the plagioclase crystallized. Furthermore, MI compositions were not affected by diffusion or crystal nucleation and growth processes.
DE: 8400 VOLCANOLOGY
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting