HR: 14:30h
AN: H33E-02    [Abstracts]
TI: Metasomatism of mantle-derived mafic and ultramafic rocks: Seawater? Subducted Slab? Sediment? Mantle wedge? or continental crust?
AU: * Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: Queens College and The Graduate Center of CUNY, Queens College, NYC, NY 11367, United States
AU: * Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, Rt. 94, Palisades, NY 10964, United States
AU: * Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: American Museum of Natural History, New York City, New York City, NY 10024, United States
AU: Simons, K M
EM: ksimons@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, Rt. 94, Palisades, NY 10964, United States
AU: Sorensen, S S
EM: sorensen@si.edu
AF: Smithsonian National Museum of Natural History, Washington DC, 20560, United States
AU: Harlow, G E
EM: gharlow@amnh.org
AF: American Museum of Natural History, New York City, New York City, NY 10024, United States
AU: Hemming, S R
EM: sidney@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, Rt. 94, Palisades, NY 10964, United States
AB: A variety of mantle-derived mafic and ultramafic rocks are exhumed as solid bodies as a result of tectonic processes occurring along all three plate boundary types. Mid Ocean Ridges (MOR), expose abyssal peridotite and serpentinite; subduction zones (SZ) contain serpentinite and blueschist-type or LT/HP eclogite; continental collisional zones (CC) are associated with "orogenic" serpentinite and spinel and/or garnet-bearing peridotite and eclogite and even transform faults (TF), such as the Motagua Fault Zone in Guatemala, display serpentinite, peridotite and eclogite. These rocks rarely reach the earth's surface without undergoing metasomatism. Metasomatism provides information on mantle and crustal processes while simultaneously obscuring information on the nature of the original mantle source. MOR metasomatism is the least complicated involving hydration and alteration by sea water. Subsequent SZ metasomatism occurs when fluids derived from sediments and hydrated basalts are added to the seawater signature. CC provides further complexity because orogenic peridotites and eclogites can be metasomatized in the mantle wedge by components derived from earlier MOR and SZ metasomatism and then can be metasomatized again when exposed to crustal fluids after they are transferred into the continental crust. However, the most complex metasomatism appears to have affected the serpentinites, eclogites and related rocks exposed on opposite sides of the Motagua Fault in Guatemala. Identical Cretaceous ages from eclogites on both sides require that they were exhumed recently (so that they are not offset) and high 87Sr/86Sr ratios and LIL concentrations from vein assemblages and retrograded eclogite requires that crustal metasomatism occurred during this exhumation. Cretaceous Sm-Nd ages and the enriched trace element chemistry and isotopic signature of eclogite facies minerals require prior metasomatism from sediment- and slab- derived fluids during Mesozoic subduction. Slightly enriched Sr and Nd isotope ratios from pyroxenes in serpentinized peridotite indicate metasomatism in a mantle wedge, presumably also during Mesozoic subduction. Finally, refractory trace element patterns (HFSE, HREE) and depleted Sr and Nd ratios indicate a MORB origin for eclogites and peridotites and serpentinites making it possible that some of the metasomatic signatures noted above occurred at a MOR. The Motagua Fault appears to have exploited a previous subduction zone resulting in an array of rocks with widely variable geochemical signatures.
DE: 1025 Composition of the mantle
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Hydrology [H]
MN: 2007 Joint Assembly