HR: 14:25h
AN: V23D-04    [Abstracts]
TI: Ancient Melt Extraction From the Oceanic Upper Mantle Revealed by 187Re-187Os Isotopes in Abyssal Peridotites
AU: * Harvey, J
EM: j.harvey@open.ac.uk
AF: The Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Department of Earth Sciences, The Open University, Walton Hall., Milton Keynes, MK7 6AA United Kingdom
AU: Gannoun, A
EM: a.gannoun@open.ac.uk
AF: The Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Department of Earth Sciences, The Open University, Walton Hall., Milton Keynes, MK7 6AA United Kingdom
AU: Burton, K W
EM: k.w.burton@open.ac.uk
AF: The Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Department of Earth Sciences, The Open University, Walton Hall., Milton Keynes, MK7 6AA United Kingdom
AU: Rogers, N W
EM: n.w.rogers@open.ac.uk
AF: The Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Department of Earth Sciences, The Open University, Walton Hall., Milton Keynes, MK7 6AA United Kingdom
AU: Alard, O
EM: O.Alard@open.ac.uk
AF: The Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Department of Earth Sciences, The Open University, Walton Hall., Milton Keynes, MK7 6AA United Kingdom
AU: Parkinson, I J
EM: i.j.parkinson@open.ac.uk
AF: The Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Department of Earth Sciences, The Open University, Walton Hall., Milton Keynes, MK7 6AA United Kingdom
AB: Abyssal peridotites represent residual mantle that has lost variable but generally large amounts of melt, which subsequently forms mid-oceanic ridge basalts (MORB). Evidence suggests that much of this melting is very recent, and isotope variations that do exist are usually explained by compositional heterogeneity in the source rather than by age differences. However, studies have indicated that many abyssal peridotites may have experienced multiple episodes of melt extraction. The 187Re-187Os isotope system offers some unique insights into the timing and nature of melt extraction from abyssal peridotites. This is because Os behaves as a compatible element during melting, whereas Re is moderately incompatible [1], hence mantle rocks may develop low 187Re/188Os ratios, and evolve to unradiogenic Os isotope compositions with time. Whole-rock model ages do reveal ancient melting events not preserved by other isotope systems, but perturbation of the Re-Os system means that these can only be minimum ages. However, if individual minerals such as sulphide remain closed to diffusion or reaction then, in principle, age information on ancient melt depletion may be accessible. This study presents Re-Os isotope data for abyssal peridotites, on both a whole-rock and individual sulphide grain scale, from ODP Leg 209 Hole 1274A in the 15§20'N fracture zone on the mid-Atlantic ridge. All of the samples possess 187Os/188Os isotope compositions that are less radiogenic than estimates for the primitive upper mantle [2]. Many are less radiogenic than previous estimates for the depleted MORB mantle and are lower than any yet reported for mid ocean ridge abyssal peridotites, and whole rock 187Os/188Os ratios are consistent with melt loss over, at least, the past 1.5 Ga. Many sulphide grains show evidence for recent recrystallisation or diffusional modification either due to partial melting or seawater alteration. However, some grains are extremely unradiogenic (187Os/188Os = 0.114) providing unequivocal evidence for at least some degree of melt depletion at ca. 2 Ga. Taken with recently published data [3] these results suggest that ancient melt depletion may be a widespread feature of the oceanic upper lithosphere, even though evidence for this depleted reservoir has not yet been observed in the Re-Os chemistry mid-ocean ridge basalts. References: [1] Burton et al., (2002). EPSL 198, 63-76. [2] Meisel et al., (1996). Nature 383, 517-520. [3] Alard et al., (2005). Nature 436, 1005-1008.
DE: 1025 Composition of the mantle
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005