Brucite as an important phase of the shallow mantle wedge: Evidence from the Shiraga unit of the Sanbagawa subduction zone, SW Japan

Kawahara, Hiroyuki; Endo, S.; Wallis, Simon; Nagaya, Teruo; Mori, Hiroshi; Asahara, Yoshihiro

doi:10.1016/j.lithos.2016.02.022

Cited by 29 publications

(45 citation statements)

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“…While the influence of the terminal breakdown of antigorite on element recycling (Garrido et al 2005 ; Harvey et al 2014 ; Hermann et al 2005 ; Marchesi et al 2013 ; Ryan and Chauvel 2014 ; Savov et al 2005 ; Scambelluri et al 2014 , 2019 ; Spandler and Pirard 2013 ), subduction zone earthquakes (Dobson et al 2002 ; Ferrand et al 2017 ; Gasc et al 2017 ; Hacker et al 2003 ; Jung and Green 2004 ; Padrón-Navarta et al 2011 ; Peacock 2001 ) and tectonics (Gerya et al 2002 ; Guillot et al 2015 ) has been widely discussed in the literature, less attention has been paid to the antigorite + brucite reaction (Bloch et al 2018 ; Kawahara et al 2016 ; Kempf and Hermann 2018 ; Peretti et al 1992 ; Peters et al 2020 ; Scambelluri et al 1991 , 1995 ; Sippl et al 2019 ; Spandler et al 2011 ), which is frequently exposed in high-pressure ophiolites. Brucite formed during ocean floor serpentinization (Klein et al 2014 ) is thought to be produced at low oxygen fugacity conditions (Frost 1985 ; Peretti et al 1992 ).…”

Section: Introductionmentioning

confidence: 99%

The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

et al. 2020

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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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Section: Introductionmentioning

confidence: 99%

The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

et al. 2020

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“…The olivine has high Fo values (reaching 98), suggesting that it was formed by the following terminal reaction; antigorite = forsterite + talc + fluid (H 2 O) (reaction line is shown in Figure ). The peak metamorphic conditions of the Shiragayama serpentinites are estimated to be 1 GPa and 500 °C (Kawahara et al, ); therefore, from the terminal reactions shown in åwe estimate the temperature to be 580 °C at 1 GPa. That is, the shear crack underwent metamorphism at a temperature 80 °C higher than that of the surrounding Shiraga serpentinite.…”

Section: Discussionmentioning

confidence: 99%

“…Kunugiza, ). Previous geological mapping has interpreted that the belt comprises a single serpentinite sheet repeated by thrusts (Shiota, ), and recent petrologic data indicate that the Shiragayama serpentinite body originated from a forearc mantle wedge (Kawahara et al, ). To reconstruct the Shiragayama serpentinite body's geological structure, we carried out a detailed geological mapping of the body in lithological order (Figure ).…”

Section: Geological Outline and Sample Descriptionmentioning

confidence: 99%

Metamorphic olivine after dehydration embrittlement in Serpentinite: Case study from the Shiraga Serpentinite mass in the Sanbagawa high P/T metamorphic belt, central Shikoku, Japan

2019

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Metamorphic olivine veins in serpentinites are considered to be a metamorphism dehydration product. However, the previous descriptions of these veins have been from paleo‐collision belts such as those in the Alps, Norwegian Caledonides, and Spain. Here the unique metamorphic olivine veins in a mass of serpentinite in the high‐pressure/low‐temperature (high P/T) Sanbagawa belt was reported. The Sanbagawa belt is a product of the subduction of oceanic plates during the Cretaceous. Elongated olivine‐talc veins were observed along shear bands in serpentine mylonite. The olivine's chemical composition is nearly pure forsterite; magnetite is distributed along the boundary of the olivine vein. This clearly suggests that the olivine was formed by the antigorite terminal reaction at a pressure less than 1.5 GPa. The reaction was caused by local heating in the mylonite as there is little evidence of the continuous antigorite breakdown reaction in the serpentinite. Metamorphic olivines are possibly formed in the intraslab earthquake zone of a relatively warm subduction zone such as southwest Japan after the antigorite terminal reaction at P less than 1.5 GPa.

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“…; 3 Takasu, 1989Takasu, , 1997Mouri and Enami, 2008 30 km Aoya et al, 2013a Mizukami et al, 2014;Kawahara et al, 2016 1 50,000 , 2013b; (Higashino, 1990a, b;Aoya et al, 2013b;Kawahara et al, 2016). (b) Cross-section along the line A-B shown in (a) (Aoya et al, 2013b).…”

Section: 1938mentioning

confidence: 99%

Geologic traverse of the Sambagawa metamorphic belt in central Shikoku, SW Japan:

Aoya¹,

Mizukami²,

Endo³

2017

Jour. Geol. Soc. Japan

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Brucite as an important phase of the shallow mantle wedge: Evidence from the Shiraga unit of the Sanbagawa subduction zone, SW Japan

Cited by 29 publications

References 59 publications

The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

Metamorphic olivine after dehydration embrittlement in Serpentinite: Case study from the Shiraga Serpentinite mass in the Sanbagawa high P/T metamorphic belt, central Shikoku, Japan

Geologic traverse of the Sambagawa metamorphic belt in central Shikoku, SW Japan:

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