The transition between blueschist and eclogite plays an important role in subduction zones via dehydration and densification processes in descending oceanic slabs. There are a number of previous petrological studies describing potential mineral reactions taking place at the transition. An experimental determination of such reactions could help constrain the pressure–temperature conditions of the transition as well as the processes of dehydration. However, previous experimental contributions have focused on the stability of spontaneously formed hydrous minerals in basaltic compositions rather than on reactions among already formed blueschist facies minerals. Therefore, this study conducted three groups of experiments to explore the metamorphic reactions among blueschist facies minerals at conditions corresponding to warm subduction, where faster reaction rates are possible on the time scale of laboratory experiments. The first group of experiments was to establish experimental reversals of the reaction glaucophane+paragonite to jadeite+pyrope+quartz+H2O over the range of 2.2–3.5 GPa and 650–820°C. This reaction has long been treated as key to the blueschist–eclogite transition. However, only the growth of glaucophane+paragonite was observed at the intersectional stability field of both paragonite and jadeite+quartz, confirming thermodynamic calculations that the reaction is not stable in the system Na2O–MgO–Al2O3–SiO2–H2O. The second set of experiments involved unreversed experiments using glaucophane+zoisite ±quartz in low‐Fe and Ca‐rich systems and were run at 1.8–2.4 GPa and 600–780°C. These produced omphacite+paragonite/kyanite+H2O accompanied by compositional shifts in the sodium amphibole, glaucophane, towards sodium–calcium amphiboles such as winchite (☐(CaNa)(Mg4Al)Si8O22(OH)2) and barroisite (☐(CaNa)(Mg3Al2)(AlSi7)O22(OH)2). This suggests that a two‐step dehydration occurs, first involving the breakdown of glaucophane+zoisite towards a paragonite‐bearing assemblage, then the breakdown of paragonite to release H2O. It also indicates that sodium–calcium amphibole can coexist with eclogite phases, thereby extending the thermal stability of amphibole to greater subduction zone depths. The third set of experiments was an experimental investigation at 2.0–2.4 GPa and 630–850°C involving a high‐Fe (Fe#=Fetotal/(Fetotal+Mg)≈0.36) natural glaucophane, synthetic paragonite and their eclogite‐forming reaction products. The results indicated that garnet and omphacite grew over most of these pressure–temperature conditions, which demonstrates the importance of Fe‐rich glaucophane in forming the key eclogite assemblage of garnet+omphacite, even under warm subduction zone conditions. Based on the experiments of this study, reaction between glaucophane+zoisite is instrumental in controlling dehydration processes at the blueschist–eclogite transition during warm subduction.
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Formation of mixed-layer sulfide-hydroxide minerals from the Tochilinite-Valleriite group during experimental serpentinization of olivine
We report the formation of minerals from the tochilinite-valleriite group (TVG) during laboratory serpentinization experiments conducted at 300 and 328 °C. Minerals in the TVG are composed of a mixture of sulfide and hydroxide layers that can contain variable proportions of Fe, Mg, Cu, Ni, and other cations in both layers. Members of this group have been observed as accessory minerals in several serpentinites, and have also been observed in association with serpentine minerals in meteorites. To our knowledge, however, TVG minerals have not previously been identified as reaction products during laboratory simulation of serpentinization. The serpentinization experiments reacted olivine with artificial seawater containing 34S-labeled sulfate, with a small amount of solid FeS also added to the 300 °C experiment. In both experiments, the predominant reaction products were chrysotile serpentine, brucite, and magnetite. At 300 °C, these major products were accompanied by trace amounts of the Ni-bearing TVG member haapalaite, Ni,Fe-sulfide (likely pentlandite), and anhydrite. At 328 °C, valleriite occurs rather than haapalaite and the accompanying Ni,Fe-sulfide is proportionally more enriched in Ni. Reduction of sulfate by H2 produced during serpentinization evidently provided a source of reduced S that contributed to formation of the TVG minerals and Ni,Fe-sulfides. The results provide new constraints on the conditions that allow precipitation of tochilinite-valleriite group minerals in natural serpentinites.
more » « less- Award ID(s):
- 2153786
- NSF-PAR ID:
- 10488191
- Publisher / Repository:
- Mineralogical Society of America
- Date Published:
- Journal Name:
- American Mineralogist
- Volume:
- 109
- Issue:
- 1
- ISSN:
- 0003-004X
- Page Range / eLocation ID:
- 61 to 72
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
Quantifying the concurrent changes in rock volume and fluid composition during serpentinization remains a major challenge in assessing its physicochemical effects during continental rifting, seafloor spreading, and subduction. Here we conducted a series of 11 hydrothermal laboratory experiments where cylindrical cores of natural dunite, harzburgite, and pyroxenite were reacted with an aqueous solution at 300 °C and 35 MPa for up to 18 months. Using three-dimensional microcomputed tomography and thermogravimetry, we show that rock volume systematically increased with time and extent of reaction, leading to a volume increase of 44% (±8%) in altered rock domains after 10–18 months of serpentinization. The volume expansion was accompanied by Mg-Ca exchange between fluid and rock, while Fe and Si were largely conserved. We find that the protolith composition (olivine/orthopyroxene ratio) plays a significant role in controlling the fluid chemistry and the proportions of hydrous secondary minerals during serpentinization. Agreement between alteration mineralogy, composition of reacting fluids, and measured volume changes suggests that serpentinization under static conditions is a volume-increasing process in spite of demonstrable mass transfer. Volume expansion implies an increased water carrying capacity and buoyancy force of serpentinite per unit mass of protolith, while Mg-Ca exchange during serpentinization may affect the Mg/Ca ratio of seawater on Earth and possibly other ocean worlds.more » « less
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Quantifying the concurrent changes in rock volume and fluid composition during serpentinization remains a major challenge in assessing its physicochemical effects during continental rifting, seafloor spreading, and subduction. Here we conducted a series of 11 hydrothermal laboratory experiments where cylindrical cores of natural dunite, harzburgite, and pyroxenite were reacted with an aqueous solution at 300 °C and 35 MPa for up to 18 months. Using three-dimensional microcomputed tomography and thermogravimetry, we show that rock volume systematically increased with time and extent of reaction, leading to a volume increase of 44% (±8%) in altered rock domains after 10–18 months of serpentinization. The volume expansion was accompanied by Mg-Ca exchange between fluid and rock, while Fe and Si were largely conserved. We find that the protolith composition (olivine/orthopyroxene ratio) plays a significant role in controlling the fluid chemistry and the proportions of hydrous secondary minerals during serpentinization. Agreement between alteration mineralogy, composition of reacting fluids, and measured volume changes suggests that serpentinization under static conditions is a volume-increasing process in spite of demonstrable mass transfer. Volume expansion implies an increased water carrying capacity and buoyancy force of serpentinite per unit mass of protolith, while Mg-Ca exchange during serpentinization may affect the Mg/Ca ratio of seawater on Earth and possibly other ocean worlds.more » « less
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null (Ed.)A series of three laboratory experiments were conducted to investigate how pH affects reaction pathways and rates during serpentinization. Two experiments were conducted under strongly alkaline conditions using olivine as reactant at 200 and 230°C, and the results were compared with previous studies performed using the same reactants and methods at more neutral pH. For both experiments, higher pH resulted in more rapid serpentinization of the olivine and generation of larger amounts of H 2 for comparable reaction times. Proportionally greater amounts of Fe were partitioned into brucite and chrysotile and less into magnetite in the experiments conducted at higher pH. In a third experiment, alkaline fluids were injected into an ongoing experiment containing olivine and orthopyroxene to raise the pH from circumneutral to strongly alkaline conditions. Increasing the pH of the olivine-orthopyroxene experiment resulted in an immediate and steep increase in H 2 production, and led to far more extensive reaction of the primary minerals compared to a similar experiment conducted under more neutral conditions. The results suggest that the development of strongly alkaline conditions in actively serpentinizing systems promotes increased rates of reaction and H 2 production, enhancing the flux of H 2 available to support biological activity in these environments. This article is part of a discussion meeting issue ‘Serpentinite in the Earth System’.more » « less
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Sulfide breakdown during subduction releases oxidizing fluids that transport chalcophile and siderophile elements (CSE) such as Ni ,Co, and As. These fluids are reincorporated into high-pressure rocks such as eclogites during exhumation and rehydration along the slab-mantle interface. Evidence for these rehydration reactions takes the form of large sulfide (pyrite, pyrrhotite, chalcopyrite) grains (up to 5 mm) associated with hydrous Fe3+-bearing minerals. Here we present results of trace element determination by LA-ICP-MS coupled with mass balance calculations for sulfide-silicate reactions in rehydrated eclogites from the Mariánské Lázně Complex and Moldanubian Zone, Bohemian Massif, Czech Republic. One key texture observed in these rocks is the breakdown of garnet + omphacite in the presence of fluid to produce hornblende + diopside + plagioclase + pyrite. This rehydration reaction involves the oxidation of Fe2+ in garnet to Fe3+ in hornblende. In order to oxidize the iron from the garnet, we propose that sulfate is brought into the rock by an infiltrating fluid, where it is reduced to form pyrite, consistent with the observed textures. Trace element analyses reveal the Co distribution within rehydrated eclogite: Co is measurable in garnet (~50 μg/g), omphacite (~26 μg/g), hornblende (~80 μg/g), and pyrite (~5000 μg/g). Mass balance calculations suggest that of the total amount of Co present in the rehydration products, only ~35 % can be supplied by the breakdown of garnet and omphacite, leaving ~65 % of the Co to be supplied by another source. Average concentrations of Ni are: in garnet (1–4 μg/g), omphacite (~57 μg/g), hornblende (~90 μg/g), and pyrite (~2500 μg/g). Mass balance calculations suggest that of the total amount of Ni present in the rehydration products, ~70 % comes from the breakdown of garnet + omphacite, with the other 30 % supplied external to this reaction. Arsenic is not present in the silicate minerals, but is in the 10s of μg/g range in pyrite, and must be supplied externally to the rock, likely from a fluid. We conclude that the fluids released from subducting slabs carry sulfate and CSEs, which infiltrate the slab-mantle interface and eventually make their way into the sub-arc mantle, where they can be incorporated into the arc magmatic system.more » « less
