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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.

Chlorine, lithium, and boron are trace elements in rhyolite but are enriched in groundwater flowing through rhyolite because they tend to partition into the fluid phase during high‐temperature fluid‐rock reactions. We present a large data set of major element and δ37Cl, δ7Li, and δ11B compositions of thermal water and rhyolite from Yellowstone Plateau Volcanic Field (YPVF). The Cl/B, Cl/Li, δ37Cl (−0.2‰ to +0.7‰), and δ11B (−6.2‰ to −5.9‰) values of alkaline‐chloride thermal waters reflect high‐temperature leaching of chlorine, lithium, and boron from rhyolite that has δ37Cl and δ11B values of +0.1‰ to +0.9‰ and −6.3‰ to −6.2‰, respectively. Chlorine and boron are not reactive, but lithium incorporation into hydrothermal alteration minerals result​s in a large range of Cl/Li, B/Li, and δ7Li (−1.2‰ to +3.8‰) values in thermal waters. The relatively large range in δ7Li values of thermal waters reflects a large range of values in rhyolite. Large volumes of rhyolite must be leached to account for the chloride, lithium and boron fluxes, implying deep groundwater flow through rhyolite flows and tuffs representing Yellowstone's three eruptive cycles (∼2.1 Ma). Lower Cl/B values in acid‐sulfate waters result from preferential partitioning of boron into the vapor phase and enrichment in the near‐surface water condensate. The Cl/B, Cl/Li, δ7Li (−0.3‰ to +2.1‰), and δ11B (−8.0‰ to −8.1‰) values of travertine depositing calcium‐carbonate thermal waters which discharge in the northern and southern YPVF suggest that chlorine, lithium, and boron are derived from Mesozoic siliciclastic sediments which contain detrital material from the underlying metamorphic basement. Plain Language Summary: High concentrations of chlorine, lithium, and boron are typically measured in groundwater that chemically react at elevated temperatures with rocks such as rhyolite that have high silica (SiO2) concentrations. The elevated concentrations result from the partitioning of these elements into groundwater during chemical reactions. Understanding how the stable isotopes of chlorine, lithium, and boron partition between groundwater and rocks in continental hydrothermal systems can provide guidance for geothermal energy and mineral deposit exploration and for characterizing degassing in active magmatic systems. We present a large data set of major element and stable isotope compositions of thermal waters and rhyolite from the Yellowstone hydrothermal system. Results suggest that the ratios of chlorine to boron and chlorine to lithium and the stable isotope compositions of these three elements mostly reflect the compositions of the rocks with which the hot water reacts. Some lithium is subsequently incorporated into hydrothermal alteration minerals and boron is enriched in vapor when the groundwater boils. Key Points: Cl, Li, and B in alkaline‐chloride thermal water in the Yellowstone Caldera are derived from leaching large volumes of rhyoliteCl and B are not incorporated into hydrothermal alteration minerals, but small amounts of Li are transferred from thermal water to rockIn calcium‐carbonate thermal waters which deposit travertine, Cl, Li, and B are probably derived from Mesozoic siliciclastic sediments [ABSTRACT FROM AUTHOR]