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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[sup.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 8 O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ[sup.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[sup.18]O values of the garnet rims to values ranging from 3.2 to 11.2% suggests interaction with a lower δ[sup.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[sup.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[sup.18]O garnet rims, and later as temperatures decrease, with rind formation. Keywords Franciscan Complex * Eclogite * Garnet * Oxygen isotopes * SIMS, Introduction Fluids are a primary means of mass transport in subduction zones with major consequences of arc magmatism, seismicity, and the chemical evolution of the crust and mantle. Tracing fluid [...]

Chlorine isotope ratios were determined for volcanic gas, geothermal well, ash, and lava samples along the Izu-Bonin-Mariana volcanic front, serpentinite clasts and muds from serpentine seamounts (Conical, South Chamorro, Torishima), basalts from the Guguan cross-chain, and sediments from Ocean Drilling Program (ODP) Sites 800, 801, 802, and 1149. There is no systematic variation in [[delta].sup.37]Cl values along the volcanic front in either gas or ash samples. In contrast, distinct variations occur across the arc, implying variations in the fluid source at different depths within the subduction zone. Serpentinite clasts and serpentine muds from the seamounts tap a source of ~30 km depth and have [[delta].sup.37]Cl values of structurally bound chloride of +0.4 [per thousand] [+ or -] 0.4 [per thousand] (n = 24), identical to most seafloor serpentinites, suggesting a serpentinite (chrysotile and/or lizardite to antigorite transition) fluid source. Tapping deeper levels of the subduction zone (~115-130 km depth), volcanic gases and ashes have [[delta].sup.37]Cl values averaging -1.1 [per thousand] [+ or -] 1.0 [per thousand] (n = 29), precisely overlapping the range measured in sediments from ODP cores (-1.1 [per thousand] [+ or -] +0.7 [per thousand], n = 11) and limited altered oceanic crust (AOC). Both sediments and AOC are possible Cl sources in the volcanic front. The Guguan cross-chain basalts come from the greatest depths and have an average [[delta].sup.37]Cl value of +0.2 [per thousand] [+ or -] 0.2 [per thousand] (n = 3), suggesting a second serpentine-derived source, in this case from antigorite breakdown at ~200 km depth.

The Kletno deposit in the Śnieżnik Massif (Central Sudetes, SW Poland), mined for Fe, U, Ag, Cu, fluorite, and marble through the ages, developed at the contact of marbles and orthogneiss. Here, we present a new Fe-Ti-V-ore (containing up to 14.07 wt. % Fe, 2.05 wt. % Ti, and 2055 ppm V in bulk rock) and ornamental- to gem-quality talc prospect at the southwest margin of this deposit. This newly documented Fe-Ti-V mineralization is hosted in hornblendites, dolomite veins, and chlorite schists, which, along with talc, envelopes a tectonic slice of serpentinite. Hornblendites are interpreted as metamorphosed ferrogabbros, derived from the same mafic melts as adjacent barren metagabbros. The oxygen and carbon isotope compositions of metabasites and dolomite veins (amphibole δ18O values = 8.8-9.3‰; carbonate δ18O values = 12.8-16.0‰, and δ13C values = -8.3‰ to -7.2‰), in combination with those of the country marbles (carbonate δ18O and δ13C values = 23.2‰ and +0.1‰, respectively), suggest that mineralization-bearing hornblendites formed due to interaction of the mafic magma with CO2 released during the decarbonation of the sediments. The CO2-bearing fluid interaction with gabbros likely caused carbonation of the gabbros and formation of the dolomite veins, whereas talc formed due to Si-rich fluids, possibly derived from a mafic intrusion, interaction with serpentinite, or due to the metasomatism of the serpentinite-gabbro assemblage. Moreover, fluids leaching Fe and Ti from the adjacent sediments can mix with the mafic magma causing enrichment of the magma in Fe and Ti. Consequently, the mineralization-bearing ferrogabbros became even more enriched in Fe and Ti, which can be linked with the formation of Fe-Ti-V ore bodies. [ABSTRACT FROM AUTHOR]

Garnet from skarns exposed at Empire Mountain, Sierra Nevada (California, United 18 18 States) batholith, have variable δ18O values including the lowest known δ18O values of skarn garnet (-4.0‰) in North America. Such values indicate that surface-derived meteoric water was a significant component of the fluid budget of the skarn-forming hydrothermal system, which developed in response to shallow emplacement (~3.3 km) of the 109 Ma quartz diorite of Empire Mountain. Values of δ18 O, measured in situ across single garnet crystals by secondary ion mass spectrometry, vary considerably (up to 7‰) and sometimes abruptly, indicating variable mixing of meteoric, magmatic, and metamorphic water. Brecciation in the skarns and alteration of the Empire Mountain pluton suggests that fracture-enhanced permeability was a critical control on the depth to which surface waters penetrated to form skarns and later alter the pluton. Compared to other Sierran systems, much greater volumes of skarn rock suggest an exceptionally vigorous hydrothermal system that saw unusually high levels of decarbonation reaction progress, likely a consequence of the magma intruding relatively cold wallrocks inboard of the main locus of magmatism in the Sierran arc at that time. [ABSTRACT FROM AUTHOR]