Journal of Conference Abstracts

Major and Trace Element Geochemistry of Basalts, Gabbros and Peridotites along the Northern Mid-Atlantic Ridge: An Assessment of the Range of Subaxial Parental and Evolved Melt Compositions

J. F. Casey Department of Geosciences, University of Houston, Houston, TX 77204 USA

jfcasey@uh.edu

S. Silantyev Vernadsky Institute of Geochemistry & Analytical Chemistry, Russian Academy of Sciences,

Kosygin Street, 19, 117975 Moscow, Russia

L. Dmitriev Vernadsky Institute of Geochemistry & Analytical Chemistry, Russian Academy of Sciences,

Kosygin Street, 19, 117975 Moscow, Russia

S. E. Smith Department of Geosciences, University of Houston, Houston, TX 77204, USA

W. Bryan Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Comparison of the bulk rock major, rare earth and trace element geochemistry with mineral chemistry in basaltic, gabbroic and ultramafic samples recovered from a variety of locations along the Northern Mid-Atlantic Ridge (including Leg 153 drill sites in the MARK region) show systematic relationships along the ridge axis. Each suite generally reflects latitudinal changes in mantle source and the extent of melting. However, bulk rock compositions and the calculated melt compositions in equilibrium with clinopyroxene within gabbroic and ultramafic samples reflect a wide range of subaxial melt compositions at any one location, (i.e., when they are compared directly to erupted mid-ocean ridge basalt (MORB) at the same location). Bulk rock LREE/HREE ratios show a much wider diversity in ultramafic and gabbroic suites than in basalt populations at the same site, ranging from those that are significantly higher to those that are significantly lower than those observed in basalts. Melts calculated to be in equilibrium with pyroxenes within mantle peridotites and gabbroic rocks also show a much wider range in highly incompatible elements than those of erupted MORBs. The residual ultramafic samples tend to be typically ultradepleted in highly incompatible trace elements with respect to MORBs, although mineral chemistry and trace element signatures of pyroxenites, wehrlites and dunites, that show intrusive relationships into residual harzburgites, exhibit evidence that primitive to highly fractionated melts, ranging from LREE/HREE depleted to highly enriched, have infiltrated the residual mantle. The geochemistry of the residual mantle and ultramafic intrusive rocks suggests that the mantle represents an open system during and after cessation of melting. When compared to erupted MORBs at the same locality, bulk rock and mineral chemistry of gabbroic samples from each locality indicate a much wider range in LREE/HREE ratios and extents of fractionation. Variations in LREE/HREE ratios in part reflect modal variation in gabbroic rocks, but also result from variations in parental melt compositions. Modelling results indicate incomplete pooling of ultradepleted to enriched polybaric melts within the ultramafic suites and more complete mixing reflected in gabbroic sections of the oceanic lithosphere in the N. Atlantic. The geochemistry of ultramafic and mafic plutonic rocks indicate larger extents of fractionation than in basalts and also supports the notion that polybaric near-fractional melting, polybaric fractionation and subsequent mixing processes may be operative at slow spreading mid-ocean ridges