T159 Frontiers in Metamorphic Petrology: Integrating Mass Transport, Heat Transport, Reaction Kinetics, and Tectonics with Mineral Equilibria

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Room 203

FLUID SOURCES AND SCALES OF ELEMENT MOBILITY IN SUBDUCTION ZONE MELANGE: OXYGEN ISOTOPE AND REE SYSTEMATICS ACROSS A JADEITITE-SERPENTINITE CONTACT

Paper number: T159-2
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

Among the outstanding questions of subduction zone metasomatism are fluid sources and the scales of mass transfer. A~4m wide, mined contact between a jadeitite block and serpentinite mélange matrix (Sierra de las Minas, Guatemala) consists of foliated albitite (cut by jadeitite veinlets) and meta-ultramafic rocks. Eleven samples were analyzed by XRF, ICP-MS, CL and SEM (Sorensen et al.,2010, International Geology Review 52: 899-940). Rare earth element (REE) data and new whole rock δ18O values redefine fluid sources and the scale of REE transfer.

The δ18O values group by rock type: those of serpentinites and meta-ultramafic rocks are +7.8 to +8.5‰; albitites, +12.8 to +13.2‰; and jadeitites, +9.3 and +9.4‰. The δ18O values of meta-ultramafic rocks are heavier than “typical mantle” of ~+5.8‰. Jadeitite δ18O values are heavier than jadeite data from Johnson & Harlow (1999, Geology) but range to ion microprobe values of jadeite (Sorensen et al., 2006, American Mineralogist). Albitite values are heavier than reported elsewhere, but resemble δ18O signatures of the low-T products of seawater-rock-sediment interaction. Guatemalan jadeitite and albitite are low-T rocks (e.g., Sorensen et al, 2006). Low-T-metamorphic seawater is a reasonable fluid source.

Albitites contain poikiloblastic clinozoisite, titanite and zircon, all likely residence sites of REE. A jadeitite veinlet in albitite, sampled near the meta-ultramafic rock contact, displays ~2x block LREE values. The block is LREE-depleted (La<Sm), the veinlet, LREE-enriched (La>Sm). The abundance and fractionation of REE in the veinlet could not be derived from the jadeitite block, albitite or ultramafic rocks, and requires m-scale REE transport along the contact. Jadeitite luminescence indicates fluids of varied compositions contributed to the block, whereas serpentinite-equilibrated (or derived?) fluid formed the veinlet (& albitite). Serpentinite (mélange matrix) shows a large negative Ce-anomaly, suggesting low-T, rock-seawater interaction. Four of 5 albitite, both jadeitite, and one of 3 meta-ultramafic rock samples also display negative Ce-anomalies, again suggesting seawater. Albitite (retrogressed jadeitite) probably formed from subduction zone fluids with seawater-derived Ce-anomalies and a capacity for REE transport at m-scales.