Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions
Albers, Elmar
Klein, Frieder
Menzies, Catriona D.
Lucassen, Friedrich
Teagle, Damon A. H.
DOI: https://doi.org/10.5194/se-10-907-2019
Persistent URL: http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/9609
Persistent URL: http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/9609
Albers, Elmar; Bach, Wolfgang; Klein, Frieder; Menzies, Catriona D.; Lucassen, Friedrich; Teagle, Damon A. H., 2019: Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions. In: Albers, Elmar; Bach, Wolfgang; Klein, Frieder; Menzies, Catriona D.; Lucassen, Friedrich; Teagle, Damon A. H. (2019): Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions - Solid Earth, Vol. 10, Nr. 3, p. 907-930, DOI: 10.5194/se-10-907-2019.
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Few data exist that provide insight into pro- cesses affecting the long-term carbon cycle at shallow fore- arc depths. To better understand the mobilization of C in sediments and crust of the subducting slab, we investigated carbonate materials that originate from the subduction chan- nel at the Mariana forearc (< 20 km) and were recovered during International Ocean Discovery Program Expedition 366. Calcium carbonates occur as vein precipitates within metavolcanic and metasedimentary clasts. The clasts repre- sent portions of the subducting lithosphere, including ocean island basalt, that were altered at lower blueschist facies conditions and were subsequently transported to the fore- arc seafloor by serpentinite mud volcanism. Euhedral arag- onite and calcite and the lack of deformation within the veins suggest carbonate formation in a stress-free environ- ment after peak metamorphism affected their hosts. Inter- growth with barite and marked negative Ce anomalies in car- bonate attest the precipitation within a generally oxic envi- ronment, that is an environment not controlled by serpen- tinization. Strontium and O isotopic compositions in car- bonate (87Sr/86Sr = 0.7052 to 0.7054, δ18OVSMOW = 20 to 24 ‰) imply precipitation from slab-derived fluids at tem- peratures between ∼ 130 and 300 ◦C. These temperature es- timates are consistent with the presence of blueschist facies phases such as lawsonite coexisting with the carbonates in some veins. Incorporated C is inorganic (δ13CVPDB = −1 ‰
to +4 ‰) and likely derived from the decarbonation of cal- careous sediment and/or oceanic crust. These findings pro- vide evidence for the mobilization of C in the downgoing slab at depths of < 20 km. Our study shows for the first time in detail that a portion of this C forms carbonate precipitates in the subduction channel of an active convergent margin. This process may be an important asset in understanding the deep carbon cycle since it highlights that some C is lost from the subducting lithosphere before reaching greater depths.