OH incorporation and retention in eclogite-facies garnets from the Zermatt–Saas area (Switzerland) and their contribution to the deep water cycle
<p>The incorporation mechanisms of OH groups in garnet were investigated in a suite of high-pressure rocks from the Zermatt–Saas area (Switzerland) using a combination of Fourier transform infrared spectroscopy (FTIR) and electron probe micro-analysis (EPMA). Investigated garnet specimens incl...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-08-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/35/679/2023/ejm-35-679-2023.pdf |
| Summary: | <p>The incorporation mechanisms of OH groups in garnet were
investigated in a suite of high-pressure rocks from the Zermatt–Saas area
(Switzerland) using a combination of Fourier transform infrared spectroscopy
(FTIR) and electron probe micro-analysis (EPMA). Investigated garnet
specimens include grossular–andradite–uvarovite solid solutions in
serpentinite and rodingite and almandine–grossular–pyrope–spessartine
solid solutions in eclogite, mafic fels and meta-sediment. All rocks
experienced the same peak metamorphic conditions corresponding to a burial
depth of <span class="inline-formula">∼</span> 80 km (<span class="inline-formula">∼</span> 540 <span class="inline-formula"><sup>∘</sup></span>C, 2.3 GPa),
allowing determination of the OH content in garnet as a function of rock
type. The capacity for OH incorporation into garnet strongly depends on its
composition. Andradite-rich (400–5000 <span class="inline-formula">µ</span>g g<span class="inline-formula"><sup>−1</sup></span> H<span class="inline-formula"><sub>2</sub></span>O) and
grossular-rich garnet (200–1800 <span class="inline-formula">µ</span>g g<span class="inline-formula"><sup>−1</sup></span> H<span class="inline-formula"><sub>2</sub></span>O) contain at
least 1 order of magnitude more H<span class="inline-formula"><sub>2</sub></span>O than almandine-rich garnet
(<span class="inline-formula"><i><</i></span> 120 <span class="inline-formula">µ</span>g g<span class="inline-formula"><sup>−1</sup></span> H<span class="inline-formula"><sub>2</sub></span>O). Microscale analyses using FTIR
and EPMA profiles and maps reveal the preservation of OH zoning throughout
the metamorphic history of the samples. The OH content correlates strongly
with Mn, Ca and Ti zoning and produces distinct absorption bands that are
characteristic of multiple nano-scale OH environments. The use of 2D
diffusion modelling suggests that H diffusion rates in these rocks is as low
as log(<span class="inline-formula"><i>D</i></span>[m<span class="inline-formula"><sup>2</sup></span> s<span class="inline-formula"><sup>−1</sup></span>]) <span class="inline-formula">=</span> <span class="inline-formula">−</span>24.5 at 540 <span class="inline-formula"><sup>∘</sup></span>C. Data were
collected for the main garnet-bearing rock types of the Zermatt–Saas area
allowing a mass balance model of H<span class="inline-formula"><sub>2</sub></span>O to be calculated. The result shows
that <span class="inline-formula">∼</span> 3360 kg H<span class="inline-formula"><sub>2</sub></span>O km<span class="inline-formula"><sup>−1</sup></span> (section of oceanic crust) yr<span class="inline-formula"><sup>−1</sup></span>
could be transported by garnet in the subducting slab beyond 80 km depth and
contributed to the deep-Earth water cycle during the Eocene subduction of
the Piemonte–Liguria Ocean.</p> |
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| ISSN: | 0935-1221 1617-4011 |