HR: 1340h
AN: T13B-1358    [Abstracts]
TI: Fresh Abyssal Peridotites -- not an Oxymoron!
AU: von der Handt, A
EM: avdhandt@mpch-mainz.mpg.de
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: Hellebrand, E
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: * Snow, J E
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: Feig, S T
AF: Inst. fuer Mineralogie, Callinstr. 3, Hannover, 30167 Germany
AU: Gao, Y
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AU: Jovanovic, Z
AF: Univ. Salzburg, Hellbrunner Str, Salzburg, 5020 Austria
AU: Nauret, F
AF: Max-Planck-Inst. fuer Chemie, PO Box 3060, Mainz, 55020 Germany
AB: The study of the Earths mantle and the generation of its crust has been hampered so far by the lack of unaltered mantle rocks from mid-ocean ridges. Whereas ophiolites, orogenic peridotites and xenoliths sometimes provide this freshness, their often unknown tectonic setting sets limits for a one-to-one transfer of the results to processes on mid-ocean ridges. Abyssal peridotites are usually characterised by a high degree of serpentinization, usually in the range of 70% up to 100%. Even this alteration still allows the analysis of relict mineral phases and thus the investigation of major processes affecting the samples, but their study until now limited by this restriction. Completely fresh abyssal peridotites would allow bulk chemical studies and mineral separation studies with far-reaching implications for mantle evolution. So far however, unaltered abyssal peridotites have remained almost a contradiction in terms and as rare as hens teeth. A few samples exist, mostly mylonites. During the 2001 AMORE expedition to the ultra-slow spreading Gakkel Ridge, two dredge hauls recovered several kilograms of very fresh residual and plagioclase-bearing peridotites with a degree of alteration between 20% and 0%. Even including those samples, all of the truly fresh mantle peridotites dredged or drilled from the global system of mid-ocean ridges could fit into a single five-gallon sample bucket. Nevertheless, the investigation of these few fresh rocks has already been a key part in studies of radiogenic isotopes, the effects of alteration, and melt migration. More and better samples are urgently needed. In summer 2004, the Polarstern ARK XX-2 returned to the amagmatic 3$\deg$E region of Gakkel Ridge for a more detailed sampling and mapping of this region. Gakkel Ridge is an ultraslow spreading orthogonal mid-ocean ridge, where ultramafic exposure is common, particularly in a zone from 3$\deg$E to 12$\deg$E, a distance of over 100km, where exclusively ultramafic lithologies were dredged. A previous fresh dredge haul PS59-235 at 84$\deg$ 37.5'N and 4$\deg$ 13.9'E, was nearly exactly repeated, numbered as PS66-238. It recovered 275 kg of absolutely fresh peridotite. They are stunningly fresh: most are still olivine green though have sometimes a narrow rim of reddish weathered olivine. The sometimes huge size of the samples is an appealing fact. The biggest sample of dredge haul PS66-238 is a boulder 70x50x40cm weighing over 100 kg, directly followed by a 50x40x30cm boulder of 52 kg. Onboard hand sample and thin section investigations confirm the near complete lack of alteration. Most samples show 100% preservation of all primary mineral phases. After hand specimen and thin section investigations, plagioclase-bearing and plagioclase-free mantle peridotites occur in almost equal proportions (38% and 44%, respectively). Among those are most of the mantle lithologies found on mid-ocean ridges, ranging from enstatite-dunites up to lherzolites with 12% cpx. Two pyroxenite samples are found as well and some samples show pyroxene-rich bands. Another 17% consists of matrix-supported breccias with clasts of the aformentioned lithologies. The degree of deformation ranges from undeformed to one mylonitic sample. Overall, dredge haul PS66-238 will provide enough absolutely fresh sample material to allow detailed studies of mantle processes and might be therefore of great interest for the mantle petrology community.
DE: 7220 Oceanic crust
DE: 8150 Plate boundary--general (3040)
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting