HR: 0800h
AN: V41G-1541    [Abstracts]
TI: Os Isotope Heterogeneity of the Convecting Upper Mantle: The MayarĄ-Baracoa Ophiolitic Belt (Eastern Cuba)
AU: * Frei, R
EM: robertf@geol.ku.dk
AF: Geological Institute, University of Copenhagen, Oster Voldgade 10, Copenhagen, 1350 Denmark
AU: Gervilla, F
EM: gervilla@ugr.es
AF: Departemento de Mineralogia y Petrologia, Universidad de Granada, Avda. Fuentenueva, Granada, 18002 Spain
AU: Meibom, A
EM: meibom@mnhn.fr
AF: Museum National d'Histoire Naturelle, Laboratoire d'Etude de la Matiere Extraterrestre , Laboratoire d'Etude de la Matiere Extraterrestre , 57, rue Cuvier, Paris, 75005 France
AU: Proenza, J A
EM: joaquin@geo.ub.es
AF: Departament de Cristallografia, Mineralogia i Diposits Minerals, Universitat de Barcelona, Marti i Franques, Barcelona, 08028 Spain
AB: Chromite separates from a set of historically important chromite deposits from the 90 Ma old Mayarí-Baracoa Ophiolitic Belt in eastern Cuba were inspected for Re-Os isotopic systematics in an attempt to quantify the extent of Os isotopic heterogeneities within a restricted upper mantle portion represented by a single ophiolite. Compositional variations of chromites indicate their crystallization from hydrous melts varying in composition from back-arc basin basalts (Al-rich chromites; Cr# = 0.43-0.55; low Pd/Ir) to boninites (Cr-rich chromites; Cr# = 0.60-0.83; high Pd/Ir) in a supra-subduction zone setting. Initial Os isotopic compositions of the studied chromites can be grouped according to their distribution in 3 regional districts. Results indicate systematically negative calculated initial γOs values varying from -1.06 ± 0.79 (Moa-Baracoa district), -1.77 ± 0.80 (Sagua de Tanamo district) and -2.79 ± 0.31 (Mayari district). These suprachondritic values are distinctly (3.5-5.2%) less radiogenic than the estimated minimum ¹⁸⁷Os/¹⁸⁸Os composition of the primitive upper mantle of 0.1296 ± 8 and can be explained by Re depletion during ancient partial melting and melt percolation events. Old Os isotope model ages (<2100 Ma)of some of the chromites (or platinum-group minerals included in them) show and confirm previous findings that ancient Os isotopic signatures can survive in the Earth's upper mantle. Our systematically negative initial γOs values do not improve the definition of an already statistically poorly defined present-day Os isotopic composition of the convecting upper mantle, but instead indicate a complex history for the convecting upper mantle which precludes the calculation of a uniform regional Os isotopic signature for this reservoir.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3042 Ophiolites (8140)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005