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. Olivine-Ti-clinohumite veins and their relation to partial dehydration of high pressure serpentinites János Kodolányi*, Carl Spandler*, Timm John**, Marco Scambelluri*** and Thomas Pettke* * Institute of Geological Sciences, University of Berne, 1-3 Baltzerstrasse, Berne, CH-3012, Switzerland (janos@geo.unibe.ch) ** Physics of Geological Processes, University of Oslo *** Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova A major question concerning the dehydration of subducted ultramafic (UM) rocks is whether they have the potential to leave a chemical fingerprint on the mantle wedge above them. Fluids released by UM rocks at high P and T are not directly accessible. However, they can be traced by studying the chemistry of serpentinites that were subducted to depths of serpentinite dehydration. In the present study we explore peak-metamorphic veining in high pressure (HP) serpentinites of the Erro-Tobbio Unit (ET), Ligurian Alps, Italy, in order to learn about the chemical composition of the fluid released by these rocks during partial dehydration. The ET Unit represents ultramafic mantle (UM) that was hydrated on the Tethys ocean floor. The UM rocks were then subducted to depths of 65-80 km at around 550-600°C and developed olivine (Ol) + Ti-clinohumite (Ti-Cl) + antigorite (Atg) + clinopyroxene (Cpx) + magnetite (Mag) peak mineral assemblage. The same minerals dominate adjacent HP veins, but their proportions are different. The ET HP serpentinites went through partial dehydration to form Ol + Atg from brucite (Brc) + low-T serpentine polymorph (Srp) (Scambelluri et al., 2001). Olivine has abundant polyphase (serpentine ± magnetite ± methane) inclusions in wall rocks as well as in the veins, and is often accompanied by inclusion-rich Ti-Cl. The abundance and appearance of these inclusions and the presence of methane indicate that these polyphase inclusions represent fluid inclusions modified by post-entrapment modification(s) during exhumation. Trace element analysis of these inclusions reveals subtle but significant differences in dehydration fluid chemistry, which can be linked to different mineral proportions and water/rock ratios during fluid release and migration. Fluid in the ET rocks was partly channelized and migrated as attested by the veins. Vein bulk rock trace element concentrations show enrichment in Ti, Ba, Nb, Li, HREE and Cu relative to the wall rocks, accompanied by depletion in Cr. Based on trace element distribution in Srp of recent ocean-floor serpentinites decomposition of Brc + Srp produces relatively dilute fluids. This requires high net fluid flux to produce HFSE-rich veins. However, disequilibrium in and around fluid conduits may enhance reactions and trace element uptake from the fluid. High concentration of certain trace elements in vein Cpx (e.g. Sr 191-202 µg/g) and Ol (e.g. Li 9.8-35.5 µg/g) are therefore not necessarily indicative of significant addition of these elements from an external source. Our study emphasizes the importance of mineral stability and trace element chemistry in influencing bulk rock and fluid geochemical evolution. REFERENCES Scambelluri M., Rampone, E. and Piccardo, G.B. 2001 Fluid and Element Cycling in Subducted Serpentinite: a Trace Element Study of the Erro-Tobbio High Pressure Ultramafites (Western Alps, NW Italy) Journal of Petrology 42, 55-67. .10 Trace element uptake into quartz cement – a function of temperature or fluid characteristics? Lehmann Katja*, Driehorst Frauke*, Ramseyer Karl*, Pettke Thomas*, Wiedenbeck Michael** *Institut für Geologie, Baltzerstr. 1+3, CH-3012 Bern (katja.lehmann@geo.unibe.ch) **GeoForschungszentrum Potsdam, Telegrafenberg 326, D-14473 Potsdam In the Interior Oman Sedimentary Basin (Fig. 1) siliciclastic sediments of the Permo-Carbonifereous Haushi Group were deposited under both glacial (Al Khlata Formation) and temperate to semi-arid (Gharif Formation) conditions. Due to a varied subsidence, these sediments today cover depths ranging from surface outcrop in the SE to almost 5000 m in the NW. Burial is maximal in the NW part of the basin, whereas in the SE uplift of up to 2000 m is indicated from basin modeling results. Basin-wide authigenic quartz cementation began concurrent with the Late Cretaceous obduction of the Oman Mountains, 221 Symposium 7: Geofluids and related mineralization: from shallow to deep
222 Symposium 7: Geofluids and related mineralization: from shallow to deep and thus, independent of burial depth or temperature (Juhasz-Bodnar, 1999). Crystallization temperatures vary between 85 and 150°C. Pressure solution is the major source of silica since no other cogenetic reactions occurred which could release silica. Fig. 1: Map of the sampling area. Well locations are marked by black dots. LA-ICPMS and SIMS analyses of authigenic quartz reveal Al and charge balancing Li as the predominant impurities ranging from 60 to 1700 µg/g and 0.5 to 60 µg/g, respectively. Li and Al are correlated with an atomic ratio of 0.11 (R 2 =0.94) in the Al Khlata Formation and 0.14 (R 2 =0.80) in the Gharif Formation. SIMS analyses indicate a positive correlation between H and Al. Na is below 10 µg/g and not correlated with any other trace element. Be, B, K, Ti and Fe are below the element-specific limits of detection of ca. 0.5 µg/g (using a 32 µm spot for analysis). The mean Al concentration is independent of the stratigraphy and crystallization temperature but the Al concentration varies by a factor of 3 on a regional scale. Additionally, the measured Li/Al ratio is higher than published values for quartz cement formed between 80 and 120°C (Demars et al., 1996), but is lower than ratios in hydrothermal vein quartz formed at temperatures between 200 and 300°C (Perny et al., 1992). The low Al/Li atomic ratio and the absence of significant other cations required for charge balancing Al in quartz implies the presence of additional positive charged ions such as H + , which was only semi-quantitatively determined by SIMS. In addition, the highly variable concentrations of Al and Li and hence H, at constant atomic ratios indicate a regionally and temporally variable pH-value. This variability suggests that pH-value is not buffered internally (Landtwing and Pettke, 2005) but externally by processes such as organic matter maturation and/or hydrocarbon migration.
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