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Geochemical evidence for exhumation of eclogite via serpentinite channels in ocean-continent subduction zones

Ulyana Horodyskyj^1,*, Cin-Ty A. Lee^1, and Peter Luffi¹

¹ Department of Earth Science, MS-126, Rice University, 6100 Main Street, Houston, Texas 77005, USA

Correspondence: Corresponding author: ctlee{at}rice.edu.

Retrograde eclogites (ranging from unaltered eclogite to retrograde blueschist and greenschist mantling the eclogite boulders) from Ring Mountain on the Tiburon Peninsula, near San Francisco, California, were examined for whole-rock major and trace elements to assess protolith compositions and the geochemical signature of fluids associated with retrogression. High field strength elements are highly correlated, indicating relatively immobile and conservative behavior during retrogression. These immobile elements were used to assess the relative losses or gains of other elements during retrogression. Rare earth elements and FeO content show only minimal open-system behavior. The rare earth abundance patterns, FeO contents, and Nb/Ta and Nb/La ratios show that the protoliths of these rocks were most likely normal to enriched mid-oceanic ridge basalts. Mass-balance considerations reveal two independent styles of metasomatic enrichment during retrogression. One style involves coupled enrichment in large ion lithophile elements (Cs, Rb, Ba, K, and Tl), likely caused by fluids from sediments or reaction with sediments. Another style involves coupled enrichment in Cr, Mg, Ni, and Pb, which may reflect overprinting by reaction of eclogite boulders with serpentinite, the latter of which are highly enriched in these elements. Pb is shown here and elsewhere to be nearly universally enriched in serpentinites and is likely to be selectively mobilized into eclogite-serpentinite reaction zones. Because all retrograde lithologies show reaction with serpentinites and sediments, exhumation of the eclogite must have been accompanied by chemical interaction with serpentinites along the entire retrograde path. The simplest interpretation is that the eclogites were transported within a deeply rooted serpentinite channel, presumably formed along the slab-mantle interface by infiltration of slab-derived fluids into the overlying mantle wedge. Physical models of channel flow show that the rapid exhumation rates required to preserve eclogites in a hydrous carrier matrix, such as serpentinite, are possible due to the buoyant and low-viscosity nature of serpentinite. However, the most rapid ascent rates occur during oblique subduction, suggesting that eclogite exhumation could be favored by, but not confined to, oblique subduction zones.