Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab

Our plain-English summary of Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab, by T. Kawamoto, M. Yoshikawa, Y. Kumagai, et al (Proceedings of the National Academy of Sciences of the United States of America, 2013).

The study

• Authors: T. Kawamoto, M. Yoshikawa, Y. Kumagai, M.H. Mirabueno, M. Okuno, T. Kobayashi
• Journal: Proceedings of the National Academy of Sciences of the United States of America
• Published: 2013;110(24):9663-8
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure-temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.

In plain English

This is a geology study, not a medical one. It examined rocks, not people, and reports nothing about asbestos exposure or disease. The authors made thin sections of eight harzburgite xenoliths (fragments of mantle rock) from deposits of the 1991 eruption of Pinatubo volcano in the Philippines. In the least altered specimen, called P3, they studied tiny fluid pockets inside olivine crystals using Raman microscopy and freezing and heating measurements. The pockets held mainly liquid water, magnesite crystals and a vapor bubble, some with detectable carbon dioxide. Raman spectra indicated a wall layer that may be talc and/or chrysotile. Measurements on 33 inclusions gave a salt content of 5.1 plus or minus 1.0% by weight NaCl-equivalent, which the authors said can be the maximum for the original fluids. The rocks also contained the amphiboles magnesiohornblende and tremolite. The authors concluded that the inclusions represent hydration of the mantle wedge by carbon-dioxide-bearing salty fluids. They noted that the temperature at which the inclusions formed cannot be estimated.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Geological (petrological) study of rock samples using thin sections, Raman microscopy and microthermometry of fluid inclusions; not a study of people or health
• Who was studied: Eight amphibole-bearing spinel-harzburgite xenoliths (mantle rock fragments) collected from 1991 Pinatubo eruption deposits, Luzon arc, Philippines; fluid inclusions were studied in the least metasomatized specimen, P3
• Exposure: No human exposure was studied. The minerals described include talc and/or chrysotile on the walls of fluid inclusions in olivine, and the calcic amphiboles magnesiohornblende and tremolite in the rock
• Main results:
  • Fluid inclusions in olivine of specimen P3 were composed mainly of liquid water, magnesite, and a vapor bubble; some vapor bubbles contained detectable CO2
  • In specimen P3, Raman spectra indicated hydrous minerals on the walls of the host olivine that may be talc and/or chrysotile
  • Salinity of the fluid inclusions in P3, from ice melting temperatures, was 5.1 plus or minus 1.0% (wt) NaCl-equivalent, based on 33 inclusions
  • The amphibole in the xenoliths consisted of magnesiohornblende and tremolite
• The authors' conclusion: The authors supposed that the fluid inclusions represent hydration in the mantle wedge by CO2-bearing saline solutions, with salinities that can be similar to seawater
• Limitations the authors state:
  • The temperature at which the fluid inclusions formed beneath Pinatubo cannot be estimated
  • The measured salinity represents residual fluids after magnesite and talc and/or chrysotile crystallized, so it can be the maximum of the original fluids
  • The relation of graphite inclusions found in an olivine crystal to the H2O-CO2 inclusions remains unclear

Topics

The National Library of Medicine indexes this study under subjects our site covers: Tremolite.

Medical terms in this study, explained in our Lexicon: Decarbonation, Talc, Chrysotile, Tremolite, Amphibole Group.

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