Brine Infiltration in the Middle to Lower Crust in a Collision Zone: Mass Transfer and Microtexture Development Through Wet Grain-Boundary Diffusion

Fumiko Higashino, Tetsuo Kawakami, Noriyoshi Tsuchiya, M. Satish-Kumar, Masahiro Ishikawa, Geoffrey Grantham, Shuhei Sakata, Takafumi Hirata

Research output: Contribution to journalArticle

1 Citation (Scopus)

Abstract

Brine-induced microtexture formation in upper amphibolite to granulite facies lower crust is investigated using a garnet-hornblende (Grt-Hbl) selvage developed along a planar crack discordantly cutting the gneissic structure of an orthopyroxene-bearing gneiss (central Sør Rondane Mountains, East Antarctica). The Cl contents of hornblende and biotite, K contents of hornblende and the thickness of relatively Na-rich rims of plagioclase decrease with distance from the center of the Grt-Hbl selvage (inferred position of the crack). Biotite and hornblende arrangement defining the gneissic structure can be traced into the selvage, suggesting that the wall-rock was overprinted by the selvage formation. Addition and loss of elements to the wall-rock was examined using Zr as an immobile element. Trace elements that tend to be mobile in brines rather than in melts are added to the wall-rock, indicating that the Grt-Hbl selvage was formed by the advection of NaCl-KCl brine into a thin crack. Plagioclase in the wall-rock shows a discontinuous drop of anorthite content at the rim, indicating that coupled dissolution-reprecipitation took place and the grain boundaries were once wet. Trace element concentrations in the wall-rock minerals decrease with distance from the crack, and, in most cases show exponentially decreasing/increasing profiles depending on the elements. These profiles are best modelled by a diffusion equation, suggesting that the wet grain-boundary diffusion in the wall-rock minerals controlled the observed mass transfer and resulted in dissolution-reprecipitation of mineral rims.

Original languageEnglish
Pages (from-to)329-358
Number of pages30
JournalJournal of Petrology
Volume60
Issue number2
DOIs
Publication statusPublished - 2019 Feb 1

Keywords

  • brine
  • continental collision zone
  • diffusion
  • microtexture
  • trace element

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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