HR: 08:30h
AN: V51E-03    [Abstracts]
TI: Refractory Garnet Peridotite Origin by Extreme High Temperature (>1800 °C) and Pressure (>8 GPa) Melting
AU: * Spengler, D
EM: spengler@geo.uu.nl
AF: Universiteit Utrecht Faculteit Geowetenschappen, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AU: Roermund, H L
EM: hermanvr@geo.uu.nl
AF: Universiteit Utrecht Faculteit Geowetenschappen, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AU: Drury, M R
EM: martynd@geo.uu.nl
AF: Universiteit Utrecht Faculteit Geowetenschappen, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AU: Mason, P R
EM: mason@geo.uu.nl
AF: Universiteit Utrecht Faculteit Geowetenschappen, Budapestlaan 4, Utrecht, 3584 CD Netherlands
AU: Davies, G R
EM: gareth.davies@falw.vu.nl
AF: Vrije Universiteit Amsterdam Faculteit der Aard- en Levenswetenschappen, De Boelelaan 1085, Amsterdam, 1081 HV Netherlands
AU: Ottolini, L
EM: ottolini@crystal.unipv.it
AF: Consiglio Nazionale delle Ricerche Istituto di Geoscienze e Georisorse - Sezione di Pavia, Via Ferrata 1, Pavia, 27100 Italy
AU: Pearson, D G
EM: d.g.pearson@durham.ac.uk
AF: Durham University Department of Geological Sciences, South Road, Durham, DH1 3LE United Kingdom
AB: We present mineral chemical evidence that garnet peridotites from Otrøy, western Norway, are residues after high degrees of melt extraction of about 30--40%. Strongly fractionated HREE in garnet with (Dy/Yb)_N ranging between 0.4--0.05 indicate that the melting occurred within the garnet stability field. For polybaric decompression melting, upwelling at very high temperature (>1800 °C) and high initial pressures (>8 GPa) are required to explain strongly depleted garnet-bearing residues. This temperature regime is higher than expected in the early Proterozoic but in agreement with the temperature regime proposed for Archean plumes. Os whole rock model ages of garnet-bearing and garnet-absent peridotites range with 3.3-2.8 Ga and confirm an Archean age for the melting event. The refractory origin of garnet as an aluminium and HREE rich phase suggests that the Otrøy peridotites may be the complementary residue to Archean aluminium-poor komatiites. Despite variable degrees of depletion recorded in garnet and garnet aggregates (garnetites), both contain intra- and intercrystalline pyroxene decompression microstructures after the break-down of a majoritic garnet precursor and imply their ultimate exhumation from extraordinary depths ≥350 km. Sm-Nd isotope systematics on the exsolution microstructure yield a garnet-clinopyroxene isochron age of 1.4 Ga and model ages of 2.9-2.5 Ga. The Proterozoic age suggests two possible scenarios for the history of the depleted mantle peridotites: a) the decompression microstructure formed during and survived the Archaean decompression melting event, implying isotope systems remained open in the lower lithosphere, until the end of the Gothian orogeny at 1.5 Ga; b) decompression postdates the melting, implying Proterozoic lithosphere was formed by accumulation of diapirs of Archean lithosphere, that was recycled in the convective mantle suggesting that the formation of crust and sub-continental lithosphere have not always been linked in time.
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 3654 Ultra-high pressure metamorphism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005