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

Content Provider	Springer Nature Link
Author	Errico, Jessica C. Barnes, Jaime D. Strickland, Ariel Valley, John W.
Copyright Year	2013
Abstract	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 δ18O 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 δ18O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ18O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ18O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ18O 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 δ18O 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 δ18O garnet rims, and later as temperatures decrease, with rind formation.
Starting Page	1161
Ending Page	1176
Page Count	16
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	166
Issue Number	4
e-ISSN	14320967
Language	English
Publisher	Springer Berlin Heidelberg
Publisher Date	2013-07-06
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Franciscan Complex Eclogite Garnet Oxygen isotopes SIMS Geology Mineral Resources Mineralogy
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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