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Chlorite REE geochemistry to unveil hydrothermal processes at the incipient stage of oceanization of Mid-Triassic Dinaridic Tethys

The hydrothermal metamorphism that takes place in the oceanic lithosphere facilitates our understanding of crustal evolution and offers deep insights into the processes characteristic for hydrothermal systems. Mafic extrusive and intrusive rocks of the oceanic crust react with circulating and percolating hydrothermal fluids resulting with transformation of primary magmatic minerals into new deuteric assemblages. This process commonly results with partial recrystallization and prevalent formation of clay phyllosilicates which are essentially out of thermodynamic equilibrium and hence adumbrate broad compositional ranges. These hydrothermal processes are metasomatic and allochemical, prone to a selective addition and removal of chemical components. This research pays attention to REE mobility during ocean metasomatism which had long been a matter of concern because of its bearing on a life cycle of the oceanic crust. Chlorite is omnipresent in the studied crustal rocks being either a clay-size replacement of a pyroclastic/effusive matrix and/or a coarse vein or vesicules infill. As such chlorite serves as an excellent polygon to investigate the mobility of REE in hydrothermal media controlled by temperature, redox potential, acidity and ionic complexation of pervasive fluids. The purpose of this research is therefore to report on the REE distribution in hydrothermal, texturally diverse, chlorite and to test a possible relation between its REE geochemistry and prevailing hydrothermal conditions. Our preliminary results have shown that clay- matrix and columnar topotactic chlorite is Fe-rich with REE content similar to those of black smokers albeit enriched for several orders of magnitude. Conversely, the coarse amygdaloidal Mg-rich chlorite outlines REE levels on par with those of seawater. An inference can be made that in Fe- chlorite virtually all of the REE stem from the host rock while for Mg-rich chlorite amygdules this is not necessary the case. This means that chlorite from basaltic amygdules originated from the cold seawater that penetrated down in the recharge zone of the hydrothermal system but did not reach the hot reaction zone, or alternatively, the hydrothermal system was temporarily inactive or cooled down. In such solutions REE are mostly complexed ; LREE are however less complexed than HREE with the former displaying a considerable complexing by Cl-, SO42- and CO32-, while the latter is commonly complexed by CO32-, which explains our experimental normalization curves. On the other hand, matrix and replacive Fe-rich chlorite is the product of fluid-facilitated fast chemical degradation of primary minerals, notably plagioclase, which takes place far from equilibrium. Plagioclase REE budget thus effectively controlled the REE content of deep- seated, high-temperature hydrothermal fluids parental to the second geochemical type of investigated chlorite.

Chlorite ; REE geochemistry ; hydrothermal processes ; Mid-Triassic ; Dinaridic Tethys

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