HR: 1340h
AN: V13B-0543    [Abstracts]
TI: A Textural and Microanalytical Evaluation of Magma Mixing in Detroit Seamount Lavas.
AU: * Kinman, W S
EM: wkinman@nd.edu
AF: Dept. Civil Eng. and Geological Sciences University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556 United States
AU: Neal, C R
EM: neal.1@nd.edu
AF: Dept. Civil Eng. and Geological Sciences University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556 United States
AB: Basalts from DSM are incompatible trace element and isotopically depleted relative to other tholeiitic basalts from the Emperor Seamount Chain (ESC) and the Hawaiian Islands. Sources of this depletion have been debated, and may include variable partial melting of depleted components in the Hawaiian plume source and/or magma mixing that involved an exchange of liquid and crystals between plume and MORB magmas. To evaluate what roles, if any, magma mixing played in the genesis of DSM basalts we conducted a quantitative textural and microanalytical study focused on plagioclase phenocrysts. We measured plagioclase crystal size distributions (CSDs) in three pillow basalts from DSM, ODP Site 1203 Units 3, 14, and 31 at the top middle and bottom of the cored section, respectively. All three basalts have concave up plagioclase CSDs, although upward deflections of the Unit 14 and 31 CSDs are more pronounced. Based upon CSD slopes we suggest each of the three basalts contains mixed populations of plagioclase crystals indicative of magma mixing (upward CSD deflection may be due to textural coarsening, overgrowth, or magma mixing). From the textural data a minimum of two crystal populations are present in each basalt: Population A = smaller crystals; Population B = larger crystals. To test the magma mixing hypothesis, we quantified major and trace elements in crystals from each population. Unit 3 crystals from population A are An_62-88 and population B are An_71-81. Population A crystals from Units 14 and 31 are An_66-75 and population B are An_80-87. Unlike the distinct An contents of population A and B crystals of Units 14 and 31, the overlapping An content of Unit 3 crystals is inconsistent with a mixed crystal population. Parent magma compositions for populations A and B were calculated using partition coefficients that account for An content and crystallization temperature. Parent magmas of population A and B from Unit 3 have overlapping Ti, Sr, Y, Ba, and LREE abundances. Parent magmas of population A and B crystals from Units 14 and 31 are distinct from one another. In both Units 14 and 31 population A parent magmas have lower Sr and Y. Unit 14 population B parent magmas tend to be less LREE enriched, whereas Unit 31 population B parent magmas are more LREE enriched than those of respective population A crystals. Parent magmas of all Unit 3 crystals are more LREE depleted than those of Unit 14 and 31 crystals. This trace element evidence is used to support our initial interpretation that only Units 14 and 31 basalts contain both textural and compositional evidence of magma mixing. While the CSD of the Unit 3 basalt is slightly concave up, major and trace element signatures recorded in plagioclase indicate that the upward CSD deflection is attributed to greater error associated with shape measurements of larger crystals. Magma mixing appears to have played a role in the petrogenesis of Detroit Seamount lavas.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005