23-7 Prograde and Retrograde Thermometry and Trace Element Mapping of Unusually Large Rutile Crystals of the Junction School Eclogite, California, USA

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism (Posters)

Presenting Author:

Authors:

Abstract:

Garnet and zircon from the Junction School eclogite (Healdsburg, California, USA) are zoned in trace elements and oxygen isotope ratios, recording a complex history of multiple burial and exhumation cycles in a changing fluid environment within the Franciscan subduction channel (Page et al., 2007, 2010). More recently, ~300µm rutile grains from the same sample were shown to also be zoned in oxygen isotopes, but did not exhibit clear zoning in trace elements.

A new sample of the same eclogite block contains the same mineralogy garnet + omphacite + phengite + rutile + zircon + quartz with minor late chlorite, glaucophance, and sphene, was found to have extremely large, elongate rutile crystals up to 2 mm wide and ~2 cm in length, in addition to sub-mm grains disseminated throughout the sample and small <100µm inclusions in garnet. A traverse by LA-ICP-MS across a 2mm wide rutile crystal shows that it is zoned in several trace elements. The central ~1mm of the grain is enriched in Cr (200 ppm) compared to the rims (~100 ppm), whereas the core zone is depleted in V (500 ppm), Nb (70 ppm) and Zr (20 ppm) relative to rims (750 ppm V, 100 ppm Nb,30 ppm Zr). These Zr concentrations are lower than small matrix Zr suggesting later growth at lower temperatures, or a decrease in the activity of Zr. Ongoing work includes trace element mapping of the large rutile grains as well as analysis of small rutile inclusions in garnet to assess prograde temperatures.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.