HR: 0800h
AN: V31B-1436    [Abstracts]
TI: Evidence for a Depleted Mantle Component in Mildly Alkalic High-MgO Basalts From Mt. Marion Dufresne, Kerguelen Archipelago, Southern Indian Ocean
AU: * Annell, H
EM: hannell@eos.ubc.ca
AF: UBC, Dept of EOS, 6339 Stores Rd, Vancouver, BC V6T 1Z4 Canada
AU: Weis, D
EM: dweis@eos.ubc.ca
AF: UBC, Dept of EOS, 6339 Stores Rd, Vancouver, BC V6T 1Z4 Canada
AU: Scoates, J S
EM: jscoates@eos.ubc.ca
AF: UBC, Dept of EOS, 6339 Stores Rd, Vancouver, BC V6T 1Z4 Canada
AU: Giret, A
EM: Andre.Giret@univ-st-etienne.fr
AF: Universit\'{e} Jean Monnet, 23 Rue Dr Paul Michelon, St-Etienne, 42023 France
AB: Kerguelen Archipelago flood basalts (29-24 Ma) were erupted onto the Northern Kerguelen Plateau when the Southeast Indian Ridge was 200-400 km northeast of the Kerguelen hotspot. A depleted mantle component is evident in older (28-29 Ma) transitional-tholeiitic basalts on the archipelago, but is absent in younger (24-25 Ma) mildly alkalic lavas. This trend towards more alkalic volcanism primarily reflects decreasing extents of partial melting and increasing depths of melting in the mantle source. A $\sim$700 m vertical section of lavas exposed at Mt. Marion Dufresne, in the southern part of the Plateau Central, contains both transitional-tholeiitic and mildly alkalic basalts (A.I. -0.8 to +2) that are geochemically and isotopically variable. The base of the section is dominated by mildly alkalic, highly fractionated, aphyric to plagioclase-phyric basalts (3 to 5.7 wt% MgO) that trend towards decreasing alkalinity with increasing height. These basalts show a limited range of isotopic compositions ($^{87}$Sr/ $^{86}$Sr= 0.7048-0.7050; $^{143}$Nd/ $^{144}$Nd= 0.5126-0.5127; $^{207}$Pb/ $^{206}$Pb= 15.54-15.57) that are similar to enriched 24-25 Ma mildly alkalic lavas in the southeastern part of the archipelago. These lavas are overlain by lower SiO$_{2}$, high-MgO (6.7-11.4 wt%) olivine-phyric basalts (Fo$_{80-88}$) that straddle the tholeiitic-alkalic divide. These high-MgO basalts record a prominent depleted mantle signature similar to that observed in the older 28-29 Ma basalts and span a range of isotopic compositions from $^{87}$Sr/ $^{86}$Sr= 0.7037-0.7048, $^{143}$Nd/ $^{144}$Nd= 0.5127-0.5129, and $^{207}$Pb/ $^{206}$Pb= 15.48-15.52. This study demonstrates the first occurrence of a depleted mantle component in mildly alkalic basalts from the Kerguelen Archipelago. The superposition of younger lavas with a depleted mantle signature over older, mildly alkalic, enriched lavas is also a situation that has not yet been observed on the Kerguelen Archipelago. This depleted signature is not interpreted to indicate intrinsic plume heterogeneity but is due to mixing between plume-sourced magmas and a depleted asthenospheric component.
DE: 8400 VOLCANOLOGY
DE: 9340 Indian Ocean
DE: 3640 Igneous petrology
DE: 1040 Isotopic composition/chemistry
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting