In:
Solid Earth, Copernicus GmbH, Vol. 10, No. 3 ( 2019-06-24), p. 907-930
Abstract:
Abstract. Few data exist that provide insight into processes
affecting the long-term carbon cycle at shallow forearc 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 channel 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 represent portions of the subducting
lithosphere, including ocean island basalt, that were altered at lower
blueschist facies conditions and were subsequently transported to the
forearc seafloor by serpentinite mud volcanism. Euhedral aragonite and
calcite and the lack of deformation within the veins suggest carbonate
formation in a stress-free environment after peak metamorphism affected
their hosts. Intergrowth with barite and marked negative Ce anomalies in
carbonate attest the precipitation within a generally oxic environment, that
is an environment not controlled by serpentinization. Strontium and O
isotopic compositions in carbonate (87Sr∕86Sr = 0.7052 to
0.7054, δ18OVSMOW = 20 to 24 ‰)
imply precipitation from slab-derived fluids at temperatures between
∼130 and 300 ∘C. These temperature estimates 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 calcareous sediment and/or oceanic crust.
These findings provide 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.
Type of Medium:
Online Resource
ISSN:
1869-9529
DOI:
10.5194/se-10-907-2019
DOI:
10.5194/se-10-907-2019-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2019
detail.hit.zdb_id:
2543230-8