HR: 1330h
AN: V12A-0564    [PDF]
TI: Insights into the dynamics and timing of crustal contamination of Kerguelen plume magmas using "Crystal Stratigraphy".
AU: * Kinman, W S
EM: Kinman.1@nd.edu
AF: Dept. of Civil Eng. and Geological Sciences, University of Notre Dame, Notre Dame, IN 46530 United States
AU: Neal, C R
EM: Neal.1@nd.edu
AF: Dept. of Civil Eng. and Geological Sciences, University of Notre Dame, Notre Dame, IN 46530 United States
AU: Jon, D P
AF: Dept. of Geological Sciences University of Durham, Science Laboratories, South Road, Durham, DH1 3LE United Kingdom
AB: Basalts represented by the 123-130 Ma Bunbury basalt (BB) Casuarina flow (BB), SW Australia, and drill core from the 107-108 Ma ODP Leg 183 Site 1137 (Elan Bank or EB) are purported to be products of the Kerguelen plume. Whole-rock geochemical data indicate these basalts have assimilated continental crust (Frey et al., 1996, EPSL 144:163; Ingle et al., 2002, EPSL 197:35) and combined, these 2 sites represent $\sim$22 m.y. of plume-crust interactions. In this study we use the crystal stratigraphy of (up to 1 cm) plagioclase phenocrysts in basalts from BB and EB to determine magmatic evolution (cf. Davidson et al., 1998, EOS 77:185). Core-to-rim trace element abundances were measured by LA-ICP-MS, major elements by EPMA, and 87Sr/86Sr compositions were determined by multicollector ICP-MS after computer-controlled microdrilling of discrete zones within each crystal. The BB plagioclase has overall lower Sr, Ba, REE, Ti and Ga abundances than the EB sample. Isotopically, the BB plagioclase contains a less radiogenic I(Sr) core (0.70414) and a more radiogenic rim composition (0.70567). The BB plagioclase exhibits core to rim Ce, Ti, Nd, and Ba enrichments that exceed those expected by fractional crystallization for the observed core-to-rim decrease in An content (66 to 60). It contains several resorption features defined by inclusion-rich zones mirroring crystal shape and a marked decrease in Sr and Ba abundances. Progressive core-to-rim enrichments in major and trace element (An, Sr, Ba, Ce, and Ti) abundances, as well as I(Sr), cut by relatively depleted Sr and Ba zones suggest initial growth in a relatively uncontaminated magma, progressive crustal assimilation, with periodic magma recharge. While a core I(Sr) value has not yet been determined, the EB plagioclase contains a less radiogenic rim (0.70618) relative to an intermediate zone (0.70645), which is adjacent to a resorption surface and also contains relatively enriched abundances of Sr, REE, and Ba. EB plagioclase trace element abundances suggest it initially grew in a highly crustal contaminated magma. The three distinct resorption surfaces observed in the EB plagioclase correspond to increased trace element abundances. We suggest the EB plagioclase initially grew in a highly contaminated magma where mixing with a highly REE enriched and radiogenic Sr end member occurred. Frey et al. (1996, EPSL 144:163) reported I(Sr) isotope ratios of Bunbury Casuarina basalts ranging from 0.70411-0.70534, less radiogenic than our rim I(Sr) value of 0.70567. Ingle et al. (2002, EPSL 197:35) reported I(Sr) isotope ratios for Elan Bank basalts ranging 0.70563-0.70573, again less radiogenic than our two I(Sr) measurements of 0.70618 and 0.70645. Disequilibrium exists between the plagioclase phenocrysts and the whole rock. We attribute the less radiogenic Sr signatures of whole rock samples relative to rims of their respective plagioclase phenocrysts to a large influx of parental plume melt just prior to eruption.
DE: 1065 Trace elements (3670)
DE: 1094 Instruments and techniques
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting