HR: 0800h
AN: U21B-0417    [Abstracts]
TI: Global Variations in Abyssal Peridotite Compositions II: What Determines the Major Element MORB Composition
AU: * Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. of Geology and Geophysics McLean Laboratory, MS#8, Woods Hole, MA 02543-1539, United States
AU: Warren, J M
EM: jmwarren@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. of Geology and Geophysics McLean Laboratory, MS#8, Woods Hole, MA 02543-1539, United States
AU: Shimizu, N
AF: Woods Hole Oceanographic Institution, Dept. of Geology and Geophysics McLean Laboratory, MS#8, Woods Hole, MA 02543-1539, United States
AB: Large scale global variability of abyssal basalts and peridotites was discovered over 20 years ago, with increasingly depleted peridotites correlating to increasingly enriched MORB that carries a major element signature of higher degrees of mantle melting (Dick et al., 1984; Klein & Langmuir, 1987). While time and further analysis shows that these correlations do not hold for the Pacific, they remain largely valid for the Atlantic, Arctic and Indian Oceans. Given the large variability in abyssal peridotite compositions at local and regional scales this is remarkable. Nearly the full range of mantle major, trace and isotopic composition can be found in peridotites dredged from a single fracture zone or ridge segment. Detailed analysis of large peridotite suites from the Gakkel Ridge (Dick, Hellebrand & Snow, unpub. data), and in the central Atlantic from 14° to 16°N also show large magma starved regions with highly depleted mantle compositions that must be inherited from prior melting events. Our recent investigations of SW Indian Ridge Peridotites also lead us to believe that much of the local peridotite variability at the scale of a single dredge could be inherited from an early melting event. All this leaves one wondering what, if anything the "global" (actually regional) variability of MORB and abyssal peridotites defined by Dick et al. (1984) and Klein and Langmuir (1987) means. For abyssal peridotites the correlations only show up when the mineral and modal compositions of large numbers of samples are averaged by locality. Moreover, the sample source on which the correlations are based was almost exclusively fracture zones. Thus, the samples come from the same geodynamic environment, with the same P-T-Melt Flux history, and therefore do not reflect the full variability produced beneath a ridge segment, but only the melting history at the distal end of a magmatic segment. We would contend, therefore, that the correlations found by the cited authors remain valid and show that MORB aggregates from large regions and therefore the melt aggregation process averages out the variability seen in the source at segment scales. As the previously analyzed peridotites are from fracture zone walls 0.5 to 14 m.y. old, and the ‘spatially associated basalts' are largely from the modern ridge axis, this argues for a long-term stability in magma composition and therefore mantle composition as well. There remains the question, however, to what degree the lateral variability of MORB major element compositions reflect varying degrees of mantle melting of a uniform source, and to what degree this is a function of varying mantle composition. It is certainly the case that trace and isotopic composition vary, and we suggest that it is equally likely that mantle major element composition varies significantly as well.
DE: 1025 Composition of the mantle
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
SC: Union [U]
MN: 2007 Fall Meeting