Abstract
In Central Evia (Aegean Sea-Greece) variably serpentinized ultramafics that are crosscut by rodingite dike intrusions, crop out in the wider regions of Pagondas and Kimi. These rodingites are distinguished into four types according to their protoliths: (i) rodingites from island-arc tholeiitic dolerites (RIATD), (ii) rodingites from Mg-rich tholeiitic gabbros (RMTG), (iii) rodingites from alkaline basalts (RAB) and (iv) rodingites from calc-alkaline basalts (RCAB). Rodingites that were derived from tholeiitic dolerites and gabbros are predominantly composed of grossular-rich garnet, relict clinopyroxene, chlorite and amphibole. Rodingitized alkaline and calc-alkaline basalts include garnet (either with balanced grossular/andradite end-members or andradite-rich), clinopyroxene neoblasts (diopside), vesuvianite, chlorite and minor calcite. In these two types of rodingites vesuvianite is abundant in the highly metasomatized varieties, whereas the RCAB exhibit remarkably high amounts of di ...
In Central Evia (Aegean Sea-Greece) variably serpentinized ultramafics that are crosscut by rodingite dike intrusions, crop out in the wider regions of Pagondas and Kimi. These rodingites are distinguished into four types according to their protoliths: (i) rodingites from island-arc tholeiitic dolerites (RIATD), (ii) rodingites from Mg-rich tholeiitic gabbros (RMTG), (iii) rodingites from alkaline basalts (RAB) and (iv) rodingites from calc-alkaline basalts (RCAB). Rodingites that were derived from tholeiitic dolerites and gabbros are predominantly composed of grossular-rich garnet, relict clinopyroxene, chlorite and amphibole. Rodingitized alkaline and calc-alkaline basalts include garnet (either with balanced grossular/andradite end-members or andradite-rich), clinopyroxene neoblasts (diopside), vesuvianite, chlorite and minor calcite. In these two types of rodingites vesuvianite is abundant in the highly metasomatized varieties, whereas the RCAB exhibit remarkably high amounts of diopside. Rodingitization processes were evolved into three successive stages of increasing metasomatic degree. Stage-1 affected the protoliths of all rodingite types and was characterized by the crystallization of grossular-rich garnet, diopside, chlorite and prehnite. During this stage Ca and volatiles (indicated by LOI) were increased, whereas Si and rare earth elements (REE) were decreased, due to dissolution of primary clinopyroxene and amphibole. Stage-2 was recognized mainly in RAB and RCAB, resulting in the formation of andradite, vesuvianite and chlorite at the expense of Stage-1 minerals. During this stage the marginal zones of the rodingite dikes were enriched in Mg and REE. Stage-3 disturbed the margins of the RCAB, causing Fet enrichments accompanied by mobilization of most trace elements and REE from the margins towards the center (cores) of the rodingite dikes. The δ18Ο-δ13C isotopic composition of stage-3 calcite, suggests that carbonation processes took place at shallow depths below the Tethyan ocean floor probably within a temperature range between ~44 and ~176 oC. Precipitation of calcite was favored by mixing between seawater and serpentinization-derived fluids. Multistage rodingitization of variable magmatic protoliths within the same region, including the unusual case of alkaline basalts, renders the Central Evia rodingites, as far as we know, unique occurrences of metasomatic rocks in the worldwide literature. Thermodynamic modeling outcomes using PERPLEX software, coupled with geothermometric results provided by the chemical composition of chlorite from the first metasomatic stage, indicate that this stage was probably developed within a P-T range of 4 to ~ 5 kbar and ~350 to ~450 oC. Decrease of P-T conditions during the exhumation of mantle wedge serpentinitic/rodingitic rocks, resulted in the establishment of the second rodingitization stage (major stage of metasomatism) at intermediate P-T conditions of ~2 to ~4 kbar and ~200 to ~350 oC. The final metasomatic stage of derodingitization/carbonation followed at even shallower depths and lower P-T conditions of ≤ 2 kbar and 100 to ~200 oC. Serpentinized ultramafic rocks in Central Evia crop out in several localities between the Kandyli-Pyxaria mountains, as well as in the regions of Chalkida and Kimi. Except serpentinized peridotites and serpentinites these include ophicarbonates, which are classified mainly as ophimagnesites and ophicalcites, as well as listvenites that crop out only between the Kandyli and Pyxaria mountains. Ophicarbonates are composed of serpentinite fragments, which are cemented with carbonate material. Ophicalcites comprise serpentine (mainly lizardite) + calcite + spinel ± dolomite ± Fe-Ti-oxides ± clinopyroxene, whereas ophimagnesites are composed of serpentine (mainly lizardite) + magnesite + olivine + clinopyroxene + orthopyroxene + spinel + magnetite + dolomite ± chromite ± talc ± cuprite. Listvenites, which are fine to medium grained rocks, comprise magnesite + dolomite + quartz + serpentine + spinel + magnetite ± iddingsite ± clinopyroxene ± brucite ± talc ± cuprite ± opaque oxides ± sulfides. The δ18Ο-δ13C isotopic data of the analyzed carbonate minerals from the studied ophicarbonates and listvenites indicate that calcite and magnesite could have probably formed within the temperature windows of ~34 to ~99 oC and ~22 to ~124 oC respectively. Isotopic results suggest that carbonation in Central Evia was favored by mixing between hydrothermal fluids and meteoric water or seawater. Thermodynamic modeling results using PERPLEX software combining with isotope geothermometry and a postulated geothermal gradient, suggest that Central Evia ophicalcites were formed at pressures between 0.5 and 1 kbar and temperatures of ~34 to 99 oC. Formation of ophicalcites was associated with Ca2+, TOT/C and volatiles enrichments, which are attributed to the precipitation of calcite. In a similar context, ophimagnesites could have been formed probably at temperatures between ~28 and ~117 oC, characterized by gains in Mg2+, volatiles and TOT/C, which are assigned to the crystallization of magnesite and to lesser extent dolomite. Similarly, listvenites were formed at temperatures of ~22 to ~124 oC. Listvenitization was developed at three successive stages (namely carbonate, silica-carbonate and silica stage) of gradually increasing metasomatic degree, which were characterized by gradually increasing quartz contents and decreasing amounts of carbonate minerals. The observed Si enrichments are attributed to the precipitation of quartz, whereas further increase of listvenitization degree led to considerable Mg2+ loss due to magnesite dissolution. Formation of ophimagnesites and listvenites were two closely related processes since the latter were derived from the metasomatism of the former at higher XSiO2 and XCaO conditions (XSiO2: 0.14-0.95 and XCaO: 0.1-0.35 respectively). It is estimated that ophimagnesitization and listvenitization processes occurred at depths ≤ 4 km, which correspond to pressures ≤ 1.2 kbar. Contrary to listvenites, ophimagnesites were favored at XCaO and XSiO2 conditions ≤ 0.3 and ≤ 0.5 respectively. Regarding the ophicalcitization processes, these were developed at higher XCaO ratios compared to listvenites (XCaO: ~0.4-0.45), either from direct precipitation of calcite or the replacement of previously formed dolomite by calcite. Steatitization processes in Central Evia were highly depended on the silica activity of the metasomatic fluids. In the Pagondas region steatitization resulted from high SiO2 activities that occasionally affected the whole mass of thin rodingite dikes that were located close to the slab-mantle interface. In the region of Kimi steatites were derived from metasomatic fluids with lower SiO2 activities compared to those of Pagondas. The presence of calcite within the groundmass of Kimi steatites suggests that Si-metasomatism was probably evolved at very shallow depths that enhanced the channeling of metasomatic fluids via faults. The relative time scales of the variable metasomatic processes were found based on diffusivity of chemical elements, thermodynamic outcomes and geothermometric calculations, which were combined with stratigraphic criteria, literature data, petrographic results and evidence of major or trace element mobilization. In particular, estimated time scales based on the diffusivity of certain elements, which were evidently mobilized during the variable metasomatic phenomena, suggest that metasomatic processes were developed with rates of 1 m in few thousand years to some hundreds of thousand years. These rates, which geologically are generally considered as very fast, are in accordance with the time scales of metasomatic processes associated with serpentinization given in the literature. The distinct rodingitization stages were developed concomitantly with major serpentinization events, whereas steatitization and blackwall alteration occurred during Stage-2 and Stage-3 metasomatism. Rodingite carbonation in Kimi region, begun during the third metasomatic stage but probably continued at shallower depths after the completion of derodingitization processes. Ophicarbonates and listvenites were the final metasomatic rocks that were formed at progressively lower P-T conditions. In the region of Kimi, the initial ophicalcitization processes probably coexisted with rodingite carbonation. Listvenitization followed the formation of ophimagnesites, and it was developed via three successive metasomatic stages ranging from carbonate-rich to silica-rich at progressively lower P-T conditions. The geotectonic environment that the studied metasomatic processes took place is mostly related to intra-oceanic subduction and exhumation of the serpentinized peridotitic mantle wedge close to the slab possibly through a subduction channel. These implications are resulted from the Ti, Al, Fe, Mg# contents and Cr# composition of relict spinel from the studied serpentinized ultramafics, which point to subduction processes probably within a fore-arc setting. Moreover, the V, Ni composition of IAT-dolerites and Mg-rich tholeiitic gabbros, as well as the low Ti contents of the latter further confirmed the implications. The intrusion of alkaline basaltic melts within the mantle wedged serpentinites was plausibly favored by slab-breakoff and upwelling of asthenospheric mantle. The three successive rodingitization stages were associated with the exhumation of the mantle wedge serpentinites via a subduction channel. Stage-1 was realized in deeper setting, stage-2 in medium depths and stage-3 at shallow levels. Carbonation, listvenitization and steatitization of the serpentinized ultramafic rocks took place during the final stages of the exhumation within the shallow to very shallow depths.Serpentinized ultramafic rocks are often used for CO2-storage and capture (CCS) purposes in order to mitigate the effects of climate change. Many of the non-carbonated serpentinized ultramafic rocks of Central Evia lie close to areas with remarkable industrial activity. Their Fe-Mg rich nature, as well as their cataclastic texture suggest that they present the appropriate physicochemical properties to serve as potential sites for CO2-storage. This is further supported by their short distance from the Evoikos Gulf and the Aegean Sea, providing the appropriate water quantities that reassure the financial feasibility of CO2-storage scenarios. Preliminary estimations on the CO2 storage capacity, considering the volume of the non-carbonated serpentinized peridotites and the total CO2-emissions of the Central Evia sanitary landfill, suggest that in situ CO2 sequestration can store up to ~ 4.5 Mt of CO2 for 80 years.
show more
