Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs,as well as fluid and element transfer between the Earth’s mantle and crust.However,...Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs,as well as fluid and element transfer between the Earth’s mantle and crust.However,fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained.Here we present two constant temperature(230℃)time-series of natural peridotite(77.5%olivine;13.7%enstatite;6.8%diopside;2%spinel)serpentinization experiments:at 13.4 MPa;and 20.7 MPa.Al-enriched lizardite was the main secondary mineral in all runs after olivine(olv)and orthopyroxene(opx)serpentinization(without any detectable brucite,talc or magnetite),while primary spinel and diopside partially dissolved during the experiments.Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite,as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine,while the opposite is true for Mg and Fe,with homogenization of serpentines compositions after 40 days.The Ni/Cr ratios of serpentines,however,remain diagnostic of the respective primary mineral.Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day^(-1)(0.26 mmol·day^(-1))and 0.26 wt.%·day^(-1)(0.13 mmol·day^(-1))at 13.4 and 20.7 MPa,respectively.Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al,which accelerates olivine serpentinization while delays orthopyroxene dissolution.Additionally,time-dependent increase in solid products masses suggests rock volume expands linearly 0.37%±0.01%per serpentine wt.%independently of pressure.Mass balance constrains suggests olv:opx react at~5:2 and~3:2 M ratios,resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series(13.4 MPa)and high-pressure series(20.7 MPa),respectively.Elevated starting peridotite olv:opx ratio展开更多
Peculiar and rare occurrences of serpentinite-hosted magnetite deposits with mineable sizes are found in the Mesozoic ophiolites of Greece(Skyros), Iran(Nain and Sabzevar) and Oman(Aniba). These deposits have diverse ...Peculiar and rare occurrences of serpentinite-hosted magnetite deposits with mineable sizes are found in the Mesozoic ophiolites of Greece(Skyros), Iran(Nain and Sabzevar) and Oman(Aniba). These deposits have diverse thickness(from a few centimeters up to 50 m) and length(2 to >500 m). Magnetite ores show variable textures, including massive, nodular and banded ores, veins, net and fine-grained disseminations in serpentinites. Intriguingly, the investigated magnetite deposits can be mistaken for chromitite pods. Serpentinite-hosted magnetite deposits show three modes of occurrences including:(i) boulders strewn across the serpentinites(i.e. Skyros Island);(ii) ore bodies along the nonconformity contacts between serpentinites and limestones(i.e. Aniba);(iii) irregular and discontinuous trails of massive and semi-massive ore bodies within highly sheared serpentinite masses(i.e. Nain;Sabzevar). In all of these magnetite ore bodies, relicts of chromian spinel grains are occasionally enclosed in magnetite crystals. The chemistry of Cr-spinel relics found in these magnetite bodies are comparable to those of accessory Crspinels in the surrounding serpentinized peridotites. BSE images and elemental mapping revealed that magnetite occurs as a nucleation on chromian spinels but not being involved in reaction either with chromite or ferritchromite. Low-grade metamorphic transformation of chromite into Fe-chromite is documented along the cracks and fractures of a few chromite grains. Generally, magnetite has typical hydrothermal compositions, characterized by low Cr, V and Ti and high Mg and Mn. It is crucial to note that a few magnetite grains with metamorphic origin are characterized by high Cr and low Ti and Ni. The potential source of iron is essentially the Fe-rich olivine. We believe that multi-episodic serpentinization of peridotite systems at high fluid-rock ratios is the main process responsible for precipitation of magnetite at ore levels whereas low-grade metamorphic transformation of chromite to magnetite has min展开更多
基金We thank Petrobras for sponsoring the grad studies of the main author and its financial support through project 8310-0 FAURGS-UFRGS-PETROBRAS.
文摘Serpentinization reactions are paramount to understand hydro-geothermal activity near plate boundaries and mafic–ultramafic massifs,as well as fluid and element transfer between the Earth’s mantle and crust.However,fluid-rock element exchange and serpentinization kinetics under shallow hydrothermal conditions is still largely unconstrained.Here we present two constant temperature(230℃)time-series of natural peridotite(77.5%olivine;13.7%enstatite;6.8%diopside;2%spinel)serpentinization experiments:at 13.4 MPa;and 20.7 MPa.Al-enriched lizardite was the main secondary mineral in all runs after olivine(olv)and orthopyroxene(opx)serpentinization(without any detectable brucite,talc or magnetite),while primary spinel and diopside partially dissolved during the experiments.Initial serpentinization stages comprises intrinsically coupled reactions between olivine and enstatite,as Al and Si are progressively transferred from orthopyroxene-derived to olivine-derived serpentine,while the opposite is true for Mg and Fe,with homogenization of serpentines compositions after 40 days.The Ni/Cr ratios of serpentines,however,remain diagnostic of the respective primary mineral.Estimated average serpentine content indicates fast serpentinization rates of 0.55 wt.%·day^(-1)(0.26 mmol·day^(-1))and 0.26 wt.%·day^(-1)(0.13 mmol·day^(-1))at 13.4 and 20.7 MPa,respectively.Approximately 2x faster serpentinization kinetics at lower pressure is likely linked to enhanced spinel dissolution leading to one order of magnitude higher available Al,which accelerates olivine serpentinization while delays orthopyroxene dissolution.Additionally,time-dependent increase in solid products masses suggests rock volume expands linearly 0.37%±0.01%per serpentine wt.%independently of pressure.Mass balance constrains suggests olv:opx react at~5:2 and~3:2 M ratios,resulting in Si-deficient and Si-saturated serpentines at the end of the low-pressure series(13.4 MPa)and high-pressure series(20.7 MPa),respectively.Elevated starting peridotite olv:opx ratio
基金granted by the Iran National Science Foundation(Grant No.98000178)the Iranian Ministry of ScienceResearch and Technology
文摘Peculiar and rare occurrences of serpentinite-hosted magnetite deposits with mineable sizes are found in the Mesozoic ophiolites of Greece(Skyros), Iran(Nain and Sabzevar) and Oman(Aniba). These deposits have diverse thickness(from a few centimeters up to 50 m) and length(2 to >500 m). Magnetite ores show variable textures, including massive, nodular and banded ores, veins, net and fine-grained disseminations in serpentinites. Intriguingly, the investigated magnetite deposits can be mistaken for chromitite pods. Serpentinite-hosted magnetite deposits show three modes of occurrences including:(i) boulders strewn across the serpentinites(i.e. Skyros Island);(ii) ore bodies along the nonconformity contacts between serpentinites and limestones(i.e. Aniba);(iii) irregular and discontinuous trails of massive and semi-massive ore bodies within highly sheared serpentinite masses(i.e. Nain;Sabzevar). In all of these magnetite ore bodies, relicts of chromian spinel grains are occasionally enclosed in magnetite crystals. The chemistry of Cr-spinel relics found in these magnetite bodies are comparable to those of accessory Crspinels in the surrounding serpentinized peridotites. BSE images and elemental mapping revealed that magnetite occurs as a nucleation on chromian spinels but not being involved in reaction either with chromite or ferritchromite. Low-grade metamorphic transformation of chromite into Fe-chromite is documented along the cracks and fractures of a few chromite grains. Generally, magnetite has typical hydrothermal compositions, characterized by low Cr, V and Ti and high Mg and Mn. It is crucial to note that a few magnetite grains with metamorphic origin are characterized by high Cr and low Ti and Ni. The potential source of iron is essentially the Fe-rich olivine. We believe that multi-episodic serpentinization of peridotite systems at high fluid-rock ratios is the main process responsible for precipitation of magnetite at ore levels whereas low-grade metamorphic transformation of chromite to magnetite has min
