Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock–buffered fluid interaction in the mantle wedge

• Published in: Contributions to mineralogy and petrology Vol. 166; no. 4; pp. 1161 - 1176
• Main Authors: Errico, Jessica C., Barnes, Jaime D., Strickland, Ariel, Valley, John W.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.10.2013; Springer; Springer Nature B.V
• Subjects: Continental dynamics; Earth and Environmental Science; Earth Sciences; Geology; Isotopes; Low temperature; Metamorphism; Mineral Resources; Mineralogy; Oceanic crust; Original Paper; Oxygen isotopes; Petrology; Precious stones; Rocks; Seismology; Germany, Berlin; USA, California, San Rafael Mts., Franciscan Complex; Garnet; SIMS; Franciscan Complex; Oxygen isotopes; Eclogite
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-013-0915-0

The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ 18 O value by ~1–3 ‰ from core to rim at a distance of ~120 ± 50 μm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ 18 O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ 18 O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ 18 O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ 18 O fluid during the final stages of growth during eclogite facies metamorphism (450–600 °C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185–240 °C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ 18 O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ 18 O garnet rims, and later as temperatures decrease, with rind formation.