HR: 17:45h
AN: U14A-07    [Abstracts]
TI: Producing Ni-rich olivine phenocrysts by mixing partial melts of eclogite and peridotite: an alternative to an olivine-free source for Hawaiian shield basalts
AU: * Wang, Z
EM: zhengrong.wang@yale.edu
AF: Woods Hole Oceanographic Institution, Geology and Geophysics Department, Woods Hole, MA 02543, United States
AU: * Wang, Z
EM: zhengrong.wang@yale.edu
AF: Yale University, Geology and Geophysics Department P.O. Box 208109, New Haven, CT 06520, United States
AU: Gaetani, G
EM: ggaetani@whoi.edu
AF: Woods Hole Oceanographic Institution, Geology and Geophysics Department, Woods Hole, MA 02543, United States
AB: It has been posited that presence of unusually Ni-rich (2500-4000 ppm) magnesian olivine phenocrysts in SiO₂-enriched Hawaiian shield-building basalts, most notably the Koolau lavas, is inconsistent with a deep, olivine-bearing source rock. Instead, Sobolev et al. (2005) proposed that these lavas are generated by a multi- stage process in which partial melts of eclogite react with peridotite within the plume to form an olivine-free source rock with high Ni concentration. As the plume continues to ascend, partial melts of this "hybrid" pyroxenite mix with peridotite melts to produce SiO₂- enriched Hawaiian shield-building lavas that crystallize high-Ni olivine. This model has also been used to argue for significant amounts of "hybrid" pyroxenite in the source regions of lavas from other ocean islands, continental basalts, and even MORB, implying that the upper mantle is highly heterogeneous (Sobolev et al., 2007). New experimental results demonstrate that Ni-rich magnesian olivine crystallizes from mixtures of peridotite partial melt and Ni-poor eclogite partial melt that have equilibrated with mantle olivine. This occurs because the concentration of Ni decreases linearly as eclogite partial melt is added to peridotite partial melt, whereas changing major element composition of the mixed melts causes D_Ni to increase hyperbolically.
Experiments were conducted in which either (1) siliceous partial melt of eclogite or (2) primitive basalt was equilibrated with San Carlos olivines at 1 bar and 1201-1350°C. Experimental results demonstrate that eclogite partial melts in equilibrium with mantle olivine retain their high SiO₂, low FeO and MgO characteristics. Theoretical modeling calibrated from these experimental results suggest that reaction of siliceous eclogite melt with mantle olivine at low pressure produces a melt containing ~300 ppm Ni. Despite its low Ni content, mixing of this melt with peridotite partial melt produces a high SiO₂ melt that crystallizes Ni-rich, magnesian olivine. The dependence of olivine-melt partition coefficients on melt composition also explains the enrichment or depletion of other minor element in Koolau olivines (e.g., Ca and Mn). Our results obviate the need for a multi-step melt generation process in which reaction with large amounts of siliceous eclogite partial melt exhausts olivine from portions of mantle peridotite within the Hawaiian plume. As a result, the amount of eclogite required to explain the composition of the Koolau lavas is greatly reduced.

Reference
Sobolev, A.V. et al., (2005), Nature, 434, 590-597
Sobolev, A.V. et al., (2007), Science, 316, 412-417
DE: 1025 Composition of the mantle
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1065 Major and trace element geochemistry
SC: Union [U]
MN: 2007 Fall Meeting