HR: 13:45h
AN: V42E-01    [PDF]
TI: Eclogitic Minerals in Diamond: MORB Reincarnated
AU: * Schulze, D J
EM: dschulze@utm.utoronto.ca
AF: Department of Geology, University of Toronto, Erindale College, 3359 Mississauga Road, Mississauga, On L5L 1C6 Canada
AU: Harte, B
EM: BHARTE@glg.ed.ac.uk
AF: Department of Geology and Geophysics, University of Edinburgh, King's Buildings, Edinburgh, EH9 3JW United Kingdom
AU: Valley, J W
EM: valley@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton St., Madison, WI 53706 United States
AU: Channer, D
EM: dchanner@genesisbci.net
AF: El Toco Mining Company, Centro Mohedano, Caracas, 1060 Venezuela
AB: Following participation in creation of oceanic lithosphere, MORB and related rocks can be altered, contaminated and eventually subducted. Evidence for these processes survives in Alpine eclogites, subcratonic eclogite xenoliths and especially in some diamonds. One of the key pieces of evidence exists as anomalous oxygen isotope values of silicates in these high-pressure assemblages. Sea floor alteration changes the d18-O VSMOW values of MORB from +5.7 +/- 0.2 per mil in fresh samples, to values as high as +27 per mil in examples that have undergone low-T ocean floor weathering. High d18-O values are also found in ophiolite basalts (to +20 per mil) and Alpine eclogites (to +16 per mil), but d18-O values of mantle eclogite xenoliths are only somewhat anomalous (to +9.3 per mil). Using SIMS, we have studied the stable isotope composition of diamonds and their eclogitic mineral inclusions from Guaniamo, Venezuela, in situ in polished section. The d18-O values of six coesite inclusions in four diamonds are in the range +10.2 to +16.9 per mil, and eight garnets in five diamonds are in the range +8.8 to +12.9 per mil, with a ninth example at +5.5 per mil. As oxygen isotope fractionations are known to be small under mantle conditions, these data provide compelling evidence for an altered ocean floor basalt protolith for this eclogitic diamond suite. Carbon isotope values (d13-C PDB) of host diamonds are very low (approximately in the range -12 to -22 per mil), values considered by many workers to indicate a significant component of biogenic carbon in the diamonds. Two stones have higher values of d13-C (-7 to -8 per mil), more typical of mantle carbon, but also consistent with a crustal source. Although isotopically light carbon can be created by mantle fractionation processes, the correlation we have shown here between anomalous oxygen and carbon isotope signatures within single diamonds strongly supports a crustal biogenic source for the carbon in these diamonds.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting