HR: 17:05h
AN: V24A-05    [Abstracts]
TI: Compositional variations across a dunite - harzburgite - lherzolite - plagioclase lherzolite sequence at the Trinity ophiolite: Evidence for multiple episodes of melt flow and melt-rock reaction in the mantle.
AU: * Morgan, Z T
EM: Zachary_T_Morgan@brown.edu
AF: Brown University, Dept. of Geo. Sci., Providence, RI 02912
AU: Liang, Y
EM: Yan_Liang@brown.edu
AF: Brown University, Dept. of Geo. Sci., Providence, RI 02912
AU: Kelemen, P
EM: peterk@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades, NY 10964
AB: In the preceding report we showed experimentally that the dunite-harzburgite-lherzolite (DHL) sequence found in the mantle sections of ophiolite could be formed by reactive dissolution of lherzolite in a basaltic liquid. The most striking results of our lherzolite dissolution experiments are the sharp mineralogical boundaries between adjacent lithologies and simple monotonic composition variations in minerals across the DHL sequence. Here we present a detailed compositional traverse across a dunite (3.64 m wide) - harzburgite-lherzolite (5.64 m) - plagioclase lherzolite ($>$ 10 m) sequence (referred to as DHL-PL) at the Trinity ophiolite that shows complicated composition trends and melt flow history. With the exception of a small (1 m wide) anomalous region within the dunite, less than 1 m away from the dunite-harzburgite contact, the Mg\#s of olivine (90), cpx (92.8), opx (90.4 in harzburgite) and spinel (40), as well as Al$_{2}$O$_{3}$ and TiO$_{2}$ abundance in cpx, opx and spinel are essentially constant from dunite to lherzolite. The CaO content in olivine (0.02%), opx (1%) and cpx (23.5%) are also uniform throughout the harzburgite-plagioclase lherzolite sequence. However, the Mg\# of olivine and opx, Al$_{2}$O$_{3}$, TiO$_{2}$ and Cr$_{2}$O$_{3}$ in cpx and opx, as well as Na$_{2}$O in cpx increase 2 meters into the plagioclase lherzolite. In addition, asymmetric concentration gradients are observed for CaO in olivine and Cr$_{2}$O$_{3}$, Al$_{2}$O$_{3}$, MgO, and FeO in spinel. These asymmetric concentration gradients are mostly in the dunite-side of the dunite-harzburgite contact. And finally, the 1 m wide anomalous region within the dunite is characterized by elevated Mg\# and NiO in olivine, Al$_{2}$O$_{3}$, TiO$_{2}$, Cr$_{2}$O$_{3}$, and REE in cpx, and very distinct elemental abundance in spinel. The composition variations reported here are substantially different from those of Quick (1981) who measured a smaller (1 m) DHL-PL sequence at the Trinity ophiolite. Together these two Trinity data sets show a large variation in DHL-PL chemistry within the same ophiolite. Concentration gradients across DHL-PL sequences have been observed in the mantle sections of ophiolites around the world. Based on our lherzolite dissolution experiments, preliminary numerical calculations, and previous studies we suggest that the DHL-PL sequence at Trinity was formed by pervasive melt flow and reactive dissolution of a plagioclase lherzolite in basaltic liquids. Although the details are still unknown, the complicated concentration profiles reported in this study can be explained by a model that involves multiple episodes of melt flow and melt-rock reaction in an evolving dunite channel system. Multiple episodes of melt flow in the mantle, each with distinct elemental and isotopic characteristics, have already been documented in olivine-hosted melt inclusions. The spatial distributions of the compositional variations reported here can be used to further constrain the time interval between different episodes of melt flow in the dunite channel. For example, the two (or more) episodes of melt flow that created the anomalous region in the dunite and the DHL-PL sequence could at most be separated by less than a few hundred years.
DE: 8434 Magma migration
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting