HR: 1330h
AN: V12C-0611    [PDF]
TI: Mantle Heterogeneity Beneath the Southwest Indian Ridge (9$\deg$ - 25$\deg$ E)
AU: * Standish, J J
EM: jstandish@whoi.edu
AF: MIT/WHOI Joint Program, 266 Woods Hole Road MS# 8, Woods Hole, MA 02543 United States
AU: Hart, S R
EM: shart@whoi.edu
AF: Woods Hole Oceanographic Insitution, 266 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Insitution, 266 Woods Hole Road, Woods Hole, MA 02543 United States
AB: Recent isotopic measurements of 20 glasses from 9$\deg$ to 25$\deg$ E on the Southwest Indian Ridge reveal striking correlations with trace element data. The isotopic variation in Sr, Nd, Hf, and Pb of these lavas confirms that along-axis heterogeneity exists in the MORB source beneath this part of the SWIR. Major and trace element systematics and isotopic compositions indicate that the along-axis variation in basalt chemistry, specifically the contrast between orthogonal supersegment and oblique supersegment lavas, can likely be explained by 1) variable extents of melting of a heterogeneous depleted peridotite lithology, or 2) variable extents of melting of a two component MORB source; a heterogeneous depleted peridotite lithology and a pyroxenite/eclogite lithology of varying proportion. This chemical variation is coupled with distinct tectonic and morphologic differences between the orthogonal supersegment and the oblique supersegment. One of the most noticeable differences between the two segments is the spreading geometry. The orthogonal supersegment spreads nearly perpendicular to the spreading axis, while the oblique supersegment spreads at up to 35$\deg$ from the axis. This abrupt change in ridge obliquity results in an effective spreading rate or mantle upwelling rate on the oblique supersegment as low as 4.5 mm/yr (half-rate), which may dramatically influence the thermal structure of the melting zone. Systematic enrichment of the mantle source from east to west along the orthogonal supersegment (16$\deg$ to 25$\deg$ E) is contrasted with a more variable, but more highly enriched isotopic and trace element composition on the oblique supersegment (9$\deg$ to 16$\deg$ E) (Figure 1A; see link below). Present day influence from the Bouvet Hotspot is unlikely, as the oblique supersegment lies nearly 800 km to the east, and the most enriched lavas are found on the eastern end of the oblique supersegment. Interestingly, the two areas of high magmatic flux on the oblique supersegment, Joseph Mayes Seamount, an axial volcano, and Narrowgate, a long-lived cross-axis volcanic high, have very different isotopic signatures, further suggesting a difference in the source composition or possibly a difference in the amount of time-integrated depletion of each residual mantle column. Figure 1B (see link below) illustrates a well-defined linear correlation between ${143/144}$ Nd and La/Sm (normalized to chondrite) for both oblique supersegment (filled) and orthogonal supersegment (open) lavas. This strong coupling of trace element and isotopic ratios supports our interpretation of a heterogeneous mantle source, but also presents the possibility that the observed glass compositions are the result of variable melting parameters (depth, extent, temperature). Isotope ratios are also generally correlated with major element indices, such as K/Ti and Mg\#, providing additional constraints on the petrogenetic evolution of these basalts and lithologic character of the depleted upper mantle. Forward modeling of peridotite versus pyroxenite/eclogite melting should provide useful constraints on the source composition, while inverse trace and major element modeling will add additional limits on the amount of melting on each segment.
UR: http://www.whoi.edu/science/GG/jstandish/Fall03agu.jpg
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting