HR: 15:25h
AN: V22F-08    [PDF]
TI: Strontium and Oxygen Isotopic Profiles Through a Complete Sequence of Oceanic Crust Exposed on Macquarie Island
AU: * Teagle, D A
EM: dat@soc.soton.ac.uk
AF: Southampton Oceanography Centre, University of Southampton, Southampton, SO14-3ZH United Kingdom
AU: Coggon, R M
EM: rmc01@soc.soton.ac.uk
AF: Southampton Oceanography Centre, University of Southampton, Southampton, SO14-3ZH United Kingdom
AU: Alt, J C
EM: jalt@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 United States
AU: Davidson, G J
EM: Garry.Davidson@utas.edu.au
AF: School of Earth Sciences, University of Tasmania, Hobart, Tas 7001 Australia
AB: Macquarie Island, approximately 1000 km south of New Zealand, is a thin sliver of ocean crust exposed by recent transpression along the Pacific - Australian plate boundary. The crust formed $\sim$11 Ma-ago most probably on a short spreading segment of a slow spreading mid-ocean ridge. Uplift and erosion on Macquarie Island has exposed unique sections through the ocean crust and an opportunity to describe and sample regions of the ocean basement that have yet to be sampled in situ despite 30 years of scientific ocean drilling. Here we present the first strontium and oxygen isotope profiles for a complete ocean basement stratigraphy from a single location. Although there are local complications, the isotope profiles yield general trends of decreasing seawater Sr and increasing temperatures of alteration with depth. $^{87}Sr$/$^{86}Sr$ ratios of the uppermost lavas are highly elevated ($\sim$0.703-0.704) compared to the primary ratios determined from fresh glass ($^{87}Sr$/$^{86}Sr$ $\sim$0.70255-0.70275) due to reaction with seawater. These lavas yield heavy oxygen isotopic compositions ($\delta$$^{18}$O $\sim$8-10 $\permil$) indicative of low temperature exchange with seawater. Relatively radiogenic $^{87}Sr$/$^{86}Sr$ of prehnites and chlorite bearing samples with elevated $\delta$$^{18}$O indicate that there may be a lower temperature overprint of some samples. Dikes from deeper in the crust yield a more restricted range of $^{87}Sr$/$^{86}Sr$ ($\sim$0.703-0.7035). Oxygen isotopic compositions of these samples bracket the primary mantle value and show a trend of decreasing $\delta$$^{18}$O with depth representing greater extents of exchange at higher temperatures. Sr isotope ratios of gabbros, dikes and layered gabbros from the lower Macquarie crust are slightly elevated from primary igneous values ($^{87}Sr$/$^{86}Sr$ $\sim$0.7025-0.703) indicating that there is minor penetration of seawater-derived strontium to the lowermost regions of the crust. These samples consistently yield relatively light oxygen isotopic compositions ($\delta$$^{18}$O $\sim$3-6 $\permil$) requiring significant exchange with hydrothermal fluids at high temperatures. Two peridotite samples from near the crust-mantle boundary have near seawater $^{87}Sr$/$^{86}Sr$ ratios ($\sim$0.7085-0.7088) but visible late stage carbonate may be responsible for these highly elevated values. Heavy oxygen isotope ratios for some peridotites may indicate serpentinization at low temperatures although this occurred with little exchange of seawater strontium. The Macquarie Island section reveals similar trends in strontium and oxygen isotopic compositions with depth to those from the uppermost crust drilled into in situ ocean crust (ODP Holes 504B and 896A) although the rocks are less altered than samples from the major supra-subduction zone ophiolites (Troodos and Oman).
DE: 0330 Geochemical cycles
DE: 1040 Isotopic composition/chemistry
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3035 Midocean ridge processes
DE: 8135 Hydrothermal systems (8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting