HP/UHP FLUIDS PRESERVED IN MN-RICH GARNETS FROM LAGO DI CIGNANA, ITALY

Garnet-quartz-fluorapatite ‘knots’ from the ultrahigh pressure Lago di Cignana (LDC) unit in the Western Alps preserve a complex fluid history and contain both direct and indirect evidence for the composition, oxidation state, and source of fluids generated during deep subduction of oceanic lithosphere. Mn-rich garnets contain abundant primary fluid inclusions in their cores which are overgrown by euhedral, fluid-inclusion-absent mantles. Garnet zoning is complex. Mantles have higher Mg content than cores, with truncated concentric zoning and anastomosing Fe-rich trails. Rims around fluid inclusions are enriched in Fe and indicate post-entrapment interaction between fluid and garnet. Texturally younger linear bands of smaller ‘cauliflower’ garnet occur on the mm to cm scale and have lower Mn and higher Fe content than euhedral garnets. Aqueous fluids were present in varying amounts throughout garnet growth, as evidenced by FTIR mapping of garnet cores, mantles, and ‘cauliflower’ bands. Fluid inclusions consist of liquid + vapor ± daughter crystals. Microthermometric data show that inclusions contain an aqueous fluid with a large dissolved load. Metastable solids form upon freezing and persist to >100°C during reheating, consistent with hydrates in the H₂O-Mg(-Mn-Ca)SO₄ system. Daughter crystals present in open fluid inclusions consist of jadeite, paragonite, phengite, chlorite, amphibole (?), titanite, and K-feldspar, as well as Ca-sulfates, phosphates, and oxides. Raman spectroscopic analysis of daughter crystals and fluid composition is in progress. The fluid inclusion characteristics indicate that an oxidized, highly complexed aqueous-silicate fluid was present at high- to ultrahigh-pressure conditions. The low-temperature history (≤600°C) of the LDC implies that supercritical aqueous-silicate fluids are stable at lower temperatures than previously recognized. Similar fluids have not been found (to date) in adjacent UHP lithologies, suggesting that fluids were produced in situ and did not communicate across layers. However, evidence for sulfate-rich aqueous fluids has been found in other UHP terranes (e.g. Dabie-Sulu) and rock types. Mobilization of such fluids may contribute to modification and oxidation of the mantle wedge above subducting slabs.