REE SYSTEMATICS OF GARNET, LAWSONITE, TITANITE AND APATITE IN GUATEMALAN LAWSONITE ECLOGITE: FLUID HISTORIES FROM A COLD SUBDUCTION ZONE

Lawsonite eclogite represents unusually high-P, low-T conditions and contains evidence for fluid-mineral partitioning histories. Lawsonite eclogite from Jalapa Department, Guatemala contains several generations of lawsonite, two generations of garnet, as well as titanite and apatite in microstructures that suggest domainal fluid-rock interaction. Two samples (01GSn2-14 and MVE02-SS-07) were analyzed by bulk ICP-MS and LA-ICP-MS.

Sample 2-14 shows a sloped, LREE-rich whole-rock pattern, with La 50× and Sm 30× Chondrite. It contains lawsonite as: 1) 50-100μm, subidiomorphic inclusions in inclusion-rich cores of 3-5mm Garnet₁; 2) 200-400 μm, irregular inclusions in inclusion-poor rims of Garnet₁; 3) 100-300 μm idiomorphic matrix grains, partly consumed by retrograde amoeboid titanite; 4) 100-300 μm, idiomorphic grains with 100-200 μm idiomorphic matrix grains of Garnet₂; and 5) 300-600 μm idiomorphic grains in late veins. Each lawsonite type manifests distinct LREE abundances and fractionations. Sample SS-07 displays a flat, ~20× Chondrite LREE pattern. It contains two types of lawsonite: 25-50 μm inclusions in mm-sized garnet, and 10-20 μm inclusions in titanite. A lawsonite core near a garnet core contains 400× Chondrite La, in a steep LREE-rich pattern, whereas a lawsonite core in the garnet rim contains 20× Chondrite La in a flatter pattern. Lawsonite inclusions in titanite display flat LREE at 60-70× Chondrite. Titanite and apatite are LREE-poor and range from ~4-20× Chondrite La.

In these rocks, lawsonite-producing and consuming reactions record a sequence of domainal equilibration of LREE with the same (or similar) major mineral assemblages. Because lawsonite contains most of the LREE in the rock, the variations of composition are likely due to differences in fluid compositions attending near-peak and retrograde crystallization, all under extremely low-T conditions.