HR: 0800h
AN: V41D-1495    [Abstracts]
TI: Lena Trough MORB: Trace Element and Isotopic Composition of a Very Slow Spreading Ridge
AU: * NAURET, F
EM: fnauret@eos.ubc.ca
AF: PCIGR-Earth & Ocean Sciences-UBC, 6339 Stores Road, Vancouver, BC V6T1Z4 Canada
AU: WEIS, D
EM: dweis@eos.ubc.ca
AF: PCIGR-Earth & Ocean Sciences-UBC, 6339 Stores Road, Vancouver, BC V6T1Z4 Canada
AU: SNOW, J E
EM: jesnow@mpch-mainz.mpg.de
AF: Max Planck Institut fur Chemie, Postfach 3060, Mainz, 55020 Germany
AB: Extreme non-hotspot MORB compositions are less straightforward to explain than comparable OIB's, where all manner of enriched components may be derived from plume material. Here we report a new isotope and trace element study of dredged samples from the ultra-slow (7.5 mm/yr effective full-rate) Lena Trough rift in the Arctic Ocean and the western end of Gakkel Ridge (13mm/yr full rate). The few basalts found in Lena Trough are alkali-MORB with an extreme enrichment of incompatible trace elements, particularly alkalies and Ba (Ba/Th=350±50, Ba/Nb=17±1, Nb/U=69±, (La/Sm)_PM=1.39±0.25) and by a garnet-source signature with (Dy/Yb)_PM=1.22±0.06. The range of radiogenic isotopic ratios is ⁸⁷Sr/⁸⁶Sr=0.7036-0.7044, ¹⁴³Nd/¹⁴⁴Nd=0.51277-0.51230, ²⁰⁶Pb/²⁰⁴Pb=17.75-17.99, ²⁰⁷Pb/²⁰⁴Pb=15.41-15.44 and ²⁰⁸Pb/²⁰⁴Pb=37.54-37.79. In comparison, Western Gakkel Ridge MORB compositions are similar to N-MORB (Ba/Th=143±60, Ba/Nb=11±3, Nb/U=45±6, ⁸⁷Sr/⁸⁶Sr=0.7029-0.7030, ¹⁴³Nd/¹⁴⁴Nd=0.51307-0.51310, ²⁰⁶Pb/²⁰⁴Pb=17.97-18.08, ²⁰⁷Pb/²⁰⁴Pb=15.43-15.44, and ²⁰⁸Pb/²⁰⁴Pb=37.73-37.81). The Lena Trough high-precision MC-ICP-MS Pb data define linear Pb-Pb arrays extending to quite unradiogenic values. These arrays extend towards the more radiogenic values of some Gakkel MORBs, which show very limited variations on their own. Together with negative linear correlations between trace element ratios and Nd-Pb isotope systematics, we interpret these Pb linear arrays as reflecting mixing between a DMM component (similar to Gakkel Ridge MORB source) and a highly enriched component, with radiogenic Sr isotopes but low Nd and Pb isotopic ratios. The Lena Trough source contains residual garnet and is preferentially sampled by low degree partial melting. The source of the enriched component of the Lena Trough alkali MORB is interpreted to be olivine-free pyroxenite, taking into account the shallow pressure of partial melting, indicated by major elements. This is a clear indication of a "mantle vein" source for these basalts, however, given their distinctive geochemistry, such melts cannot be a large component of MORB in general, or be a mixing endmember for typical E-MORB.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005