Platinum-group Elements as a Tracer for Melt-Rock Interactions in Peridotites. Example:
the Nan-Uttaradit Ophiolite, NE Thailand

B. Orberger Laboratoire d'Etudes physiques et chimiques appliquées à la Terre,

Université de La Rochelle, Rue Marillac, 17000 La Rochelle, France;

and Laboratoire de Geochronologie, Universite Paris-7, 2 Place Jussieu, 75251 Paris, France

J. P. Lorand Laboratoire de Mineralogie du Musée National d'Histoire Naturelle,

Unite associée au CNRS No. 286, 61 rue de Buffon, 75005 Paris, France

R. R. Keays Laurentian University, Sudbury, Canada

S. Reeves Department of Geology, School of Earth Sciences, Melbourne University,

Parkville, Victoria, 3052, Australia

J. C. C. Mercier Laboratoire d'Etudes physiques et chimiques appliquées à la Terre,

Université de La Rochelle, Rue Marillac, 17000 La Rochelle, France

The ultramafic sequence and associated chromitites represent the upper mantle of the NU ophiolite which was formed in a transitional island arc-backarc environment. These rocks comprise irregular bodies of dunite, harzburgite, orthopyroxene-rich lherzolite and orthopyroxene-rich harzburgite, clinopyroxene-rich dunites and intrusive clinopyroxenite bodies. Interactions between percolating boninitic type melt with depleted peridotites are suggested for their origin. The chromitite bodies and the intrusive clinopyroxenites and websterites are segregated from these boninitic-type melts under varying oxygen-fugacities (Orberger et al. 1995).

The influence of melt percolation and melt-rock interaction on the platinum-group element-(PGE) and chalcophile element distribution in the peridotites and orthopyroxenites is constraint by:

1) lower Se/S-ratios than that of magmatic sulfides, but varying S and Se contents. It is suggested that the decrease of the S/Se ratios originated primarly by partial melting. Second stage melting preserves the low S/Se-ratio, because of complete melting of the residual sulfides, including Se and PGE. The evolution of the ultramafic rocks versus sulfur saturation is the result of the interaction with a second stage island arc basaltic or boninitic melt. Slight differences in the S/Se ratios might be the result of initially different degrees of partial melting.

2) low Cu/S ratios which were also observed in percolated peridotites from the Lanzo massif, the Bay of Islands and Oman ophiolite. In the NU-ophiolite, Cu-concentrations are controlled by the modal cpx rather than by the sulfur content.

3) the Pd/Ir > 1. Usually, ratios of Pd/Ir < 1 are characteristic for refractory rocks.

4) in orthopyroxenites of the NU ophiolite, PGEs are not fractionated, whereas orthopyroxenite segregates from the Bay of Islands ophiolite are strongly fractionated, resulting in Pt, Pd enrichment. Melt-rock interaction is favored to explain PGE non fractionation.

The percolating melt was sulfur poor thus PGEs are not sulfur-, but silicate melt controlled.

Chromitites are strongly depleted in Ir. Segregates from boninites and low-Ti lavas are generally strongly depleted in Os and Ir. This would explain the unusual Ir impovrishment in the first segregates due to low fO2 conditions. The unusual fractionation of Pt and Pd in the chromitites within peridotites and orthopyroxenites is suggested to occur prior to the second stage melting whereas Pt-Pd-fractionation within the ultramafic cumulates can be related to subsolidus re-equilibration during cooling.


Orberger, B., Lorand, J.P., Girardeau, G., Mercier, J.C.C. & Pitragool, S., Lithos 35, 153-182 (1995).