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

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.