Tracing fluid infiltration into oceanic crust up to ultra-high-pressure conditions.

Autor: Rubatto D; Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland.; Institut des Sciences de la Terre, University of Lausanne, 1015 Lausanne, Switzerland.; The Australian National University, Research School of Earth Sciences, Canberra, ACT 2601 Australia., Williams M; The Australian National University, Research School of Earth Sciences, Canberra, ACT 2601 Australia.; CSIRO, Mineral Resources, Kensington, WA 6151 Australia., Markmann TA; Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland., Hermann J; Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland., Lanari P; Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland.
Jazyk: angličtina
Zdroj: Contributions to mineralogy and petrology. Beitrage zur Mineralogie und Petrologie [Contrib Mineral Petrol] 2023; Vol. 178 (11), pp. 79. Date of Electronic Publication: 2023 Oct 19.
DOI: 10.1007/s00410-023-02060-6
Abstrakt: Fluid-rock interaction within the altered oceanic crust and across the slab-mantle boundary during subduction facilitates element transfer, but the dynamics of fluid transport and fluid-rock exchange during upward fluid migration are still unclear. A study of metamorphic fluid-rock interaction within a section of subducted oceanic crust was carried out on eclogites and metasediments of the ultra-high-pressure Lago di Cignana Unit (NW Italian Alps). The P - T modeling of a quartzschist shows that garnet grew during the prograde and sporadically during the retrograde path and that phengite mainly records the peak to retrograde conditions. Microscale geochemical analysis of garnets has revealed a systematic evolution of oxygen isotopic composition with garnet major element zonation, with extreme within-sample core-rim variations in δ 18 O between 18 and 4‰ providing evidence for external fluid influx. Garnet in eclogites and calcschists, as well as garnet cores in quartz-rich lithologies, shows normal compositional zoning, as expected for prograde garnet growth, and a relatively constant oxygen isotopic composition. The outer garnet growth zones within a few metasediments show reverse or discontinuous zoning and progressively lower δ 18 O. Despite major element zoning, the isotopic composition of mica is homogeneous across chemical zoning in one eclogite and one quartzschist, but shows 6‰ variability in another quartzschist. In the underlying Zermatt-Saas serpentinites, antigorite from nine serpentinite samples shows some variation in δ 18 O, with average δ 18 O values for individual samples ranging from 1 to 6‰. These results provide evidence for two main stages of external fluid infiltration: (i) fluids from the dehydration of mafic lithologies entered the sequence at peak conditions around 3 GPa, as indicated by the oxygen composition of intermediate zones of mica and garnet, and (ii) low δ 18 O fluids from serpentinites infiltrated parts of the sedimentary package during exhumation prior to 1.5 GPa, as recorded by the 4‰ garnet outer rims. Samples recording external fluid infiltration are concentrated in the lower part of the sequence, indicating channelized fluid flow, suggesting focused fluid infiltration due to permeability contrasts between metasedimentary and eclogitic lithologies. Channelized fluid flow in the ultra-high-pressure metasediments of Lago di Cignana has not resulted in systematic decarbonation of the metasediments.
Supplementary Information: The online version contains supplementary material available at 10.1007/s00410-023-02060-6.
Competing Interests: Conflict of interestDaniela Rubatto is Associate Editor of this journal. The authors have no other competing interests to declare that are relevant to the content of this article.
(© The Author(s) 2023.)
Databáze: MEDLINE
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