Olivine CPO in non-deformed peridotite due to topotactic replacement of antigorite
Takayoshi NagayaSimon WallisHiroaki KobayashiKatsuyoshi MichibayashiTomoyuki MizukamiYusuke SetoAkira MiyakeMegumi Matsumoto
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Observations at the Cassiar (British Columbia) cbrysotile asbestos deposit have defined a continuous series of serpentine textures, between pseudomorphic and nonpseudomorphic. These minerals and textures are distributed with respect to shear zones in the interior of the serpentinite such that the degree of recrystallization and replacement increases as the shear zones are approached. Pafierns of spatial distribution suggest that recrystallization and replacement were caused by infiltration-driven metamorphism as the serpentinite equilibrated with an externally derived fluid. The shear zones served as conduits forthe fluid. The recrystallization and replacenent of lizardite formed after olivine and of that formed after enstatite proceed independently early in ihe process, such tiat composition of tle variols serpentine minerals was controlled by the bulk composition of the precursor. fitrt io the process, Fet*/[rlg, CrlAl and Fe3+(Mg + Si) values, and the distribution of boron, between serpentine after olivine and that formed after enstatite indicate that equilibrium was closely approached. The formation of a chrysotile + antigorite assemblage marks this transition. The recrystallization of lizardite and its replacement by.chrysotile + antigorite ocqined a1250 t 25'C and a P(H2O) of less than 1 kbar, The temperatures and pressures of serpentinization cal b9 modeled in the system MgO-SiO2-H2O 6MSII because: l) clinochlore is stabilized by the breakdown of cbromite, rather than a serpentine mineral, and 2) nagnetite is both a product and a reactaat in the conversion oflizaxdite to antigorite, indicating that it is not an essential compound in the conversion of lizardite to antigorite. The mineralogy, textures and compositions of lizardite and cbrysotile indicate that they are polymorphs in the MSH system.
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A rare massive yellowish-green serpentinized dunite, covering a minimum area up to ~50 m2, has been found in Ji’an County, Jilin Province, Northeast China. It contains primary olivine and secondary serpentine (antigorite) and brucite. Other primary minerals like orthopyroxene, clinopyroxene, and aluminum-rich phase (such as garnet, spinel, and plagioclase), frequently appearing in ultramafic rocks, have not been identified. The olivine is essentially pure forsterite, with an Mg# (100 × Mg/(Mg + Fe)) of ~99.6–99.7. Due to these distinct features, we especially name the protolith of this dunite as jianite (集安岩). The forsterite grains range up to ~2 mm, show clear equilibrium textures such as nearly straight grain boundaries and ~120° dihedral angles at their triple junctions, and display no intragranular or intergranular composition variations. They are extensively ruptured and hydrated (i.e., serpentinized), with the fractures (and the grain boundaries as well) filled by fine-grained antigorite (ideally Mg6(Si4O10)(OH)8) and brucite (ideally Mg(OH)2). These secondary phases are also extremely poor in Fe, indicating a good chemical equilibrium with the forsterite. The serpentinization reaction may have proceeded as forsterite + fluid = antigorite + brucite at temperatures of ~425(25) °C and at relatively low but undetermined pressures. The fluid was likely a B-rich, but Si-poor dilute aqueous fluid, as implied by the trace element characteristics and water-related infrared features of the forsterites in equilibrium. The petrogenesis of the jianite is presently unclear.
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Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3-6.6 GPa; 853-957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101]Mt//[001]Ol, [110]Mt//[01̄1]Ol, and [01̄1]Mt//[011]Ol; and [0001]Hm//[100]Ol and [101̄0]Hm//[001]Ol. These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe2+ to Fe3+ in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6-11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.
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Compositional variation of olivine in serpentinized peridotites provides a significant constraint on modeling the redox conditions of serpentinization and the tectonothermal history of ophiolites. Here I report the variations of Fe, Mg, Mn, and Ni contents of olivine from the Oeyama ophiolite, SW Japan and show textural and chemical evidence for compositional modification of olivine related to high–temperature (T) serpentinization. The Fe–enrichment of olivine adjacent to antigorite without significant magnetite formation indicates a reducing condition for high–T serpentinization. Systematic variations of forsterite (Fo) component with distance from antigorite suggest Mg–Fe volume diffusion took place in olivine porphyroclasts under the conditions of high–T serpentinization. In addition, a similar diffusion pattern of Mn to Fe results in a retrograde trend in MnO–Fo diagram, which could be a useful indicator of high–T serpentinization. Retrograde antigorite is different from prograde antigorite in having a shape of elongated blade, lacking a significant amount of magnetite inclusion, and being more ferrous than lizardite. The existence of retrograde antigorite provides another piece of evidence for high–T serpentinization even if olivine has been decomposed by intense low–T serpentinization. Approximate estimation of time required for the observed Mg–Fe diffusion profiles of olivine porphyroclasts reveals that a cooling duration under the conditions of high–T serpentinization was much longer than that of amphibolite–facies metasomatism previously reported. This suggests a long residence time of the forearc peridotites within the serpentinized mantle wedge following rapid exhumation immediately after the amphibolite–facies metasomatism.
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Research Article| October 01, 2010 Lizardite versus antigorite serpentinite: Magnetite, hydrogen, and life(?) Bernard W. Evans Bernard W. Evans Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310, USA Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (10): 879–882. https://doi.org/10.1130/G31158.1 Article history received: 26 Feb 2010 rev-recd: 30 Apr 2010 accepted: 04 May 2010 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Bernard W. Evans; Lizardite versus antigorite serpentinite: Magnetite, hydrogen, and life(?). Geology 2010;; 38 (10): 879–882. doi: https://doi.org/10.1130/G31158.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The serpentinization of peridotite operates according to one or the other, or a combination, of two end-member mechanisms. In low-temperature environments (50–300 °C), where lizardite is the predominant serpentine mineral, olivine is consumed by reaction with H2O but its composition (Mg#) remains unchanged. Mg-rich lizardite, magnetite, and dihydrogen gas (±brucite) are products of the reaction. At higher temperatures (400–600 °C), rates of MgFe diffusion in olivine are orders of magnitude faster, with the result that the growth of Mg-rich antigorite can be accommodated by a compositional adjustment of olivine, eliminating the need to precipitate magnetite and evolve hydrogen. This latter end-member mechanism probably best reflects the situation in the forearc mantle wedge. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Metamorphic olivine formed by the reaction of antigorite + brucite is widespread in serpentinites that crop out in glacier-polished outcrops at the Unterer Theodulglacier, Zermatt. Olivine overgrows a relic magnetite mesh texture formed during ocean floor serpentinization. Serpentinization is associated with rodingitisation of mafic dykes. Metamorphic olivine coexists with magnetite, shows high Mg# of 94-97 and low trace element contents. A notable exception is 4 µg/g Boron (> 10 times primitive mantle), introduced during seafloor alteration and retained in metamorphic olivine. Olivine incorporated 100-140 µg/g H2O in Si-vacancies, providing evidence for low SiO2-activity imposed by brucite during olivine growth. No signs for hydrogen loss or major and minor element diffusional equilibration are observed. The occurrence of olivine in patches within the serpentinite mimics the former heterogeneous distribution of brucite, whereas the network of olivine-bearing veins and shear zones document the pathways of the escaping fluid produced by the olivine forming reaction. Relic Cr-spinels have a high Cr# of 0.5 and the serpentinites display little or no clinopyroxene, indicating that they derive from hydrated harzburgitic mantle that underwent significant melt depletion. The enrichment of Mg and depletion of Si results in the formation of brucite during seafloor alteration, a pre-requisite for later subduction-related olivine formation and fluid liberation. The comparison of calculated bulk rock brucite contents in the Zermatt-Saas with average IODP serpentinites suggests a large variation in fluid release during olivine formation. Between 3.4 and 7.2 wt% H2O is released depending on the magnetite content in fully serpentinized harzburgites (average oceanic serpentinites). Thermodynamic modelling indicates that the fluid release in Zermatt occurred between 480 °C and 550 °C at 2-2.5 GPa with the Mg# of olivine varying from 68 to 95. However, the majority of the fluid released from this reaction was produced within a narrow temperature field of < 30 °C, at higher pressures 2.5 GPa and temperatures 550-600 °C than commonly thought. Fluids derived from the antigorite + brucite reaction might thus trigger eclogite facies equilibration in associated metabasalts, meta-gabbros, meta-rodingites and meta-sediments in the area. This focused fluid release has the potential to trigger intermediate depths earthquakes at 60-80 km in subducted oceanic lithosphere.
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