Serpentinite fluids and slab-melting in the Aleutian arc: Evidence from molybdenum isotopes and boron systematics

0566650 - GLÚ 2024 RIV NL eng J - Journal Article
Rojas-Kolomiets, E. - Jensen, O. - Bizimis, M. - Yogodzinski, G. - Ackerman, Lukáš
Serpentinite fluids and slab-melting in the Aleutian arc: Evidence from molybdenum isotopes and boron systematics.
Earth and Planetary Science Letters. Roč. 603, February (2023), č. článku 117970. ISSN 0012-821X. E-ISSN 1385-013X
Institutional support: RVO:67985831
Keywords : molybdenum isotopes * boron * subduction zones * arc magmas * serpentinite fluids * sediments
OECD category: Geology
Impact factor: 5.3, year: 2022
Method of publishing: Limited access
https://www.sciencedirect.com/science/article/pii/S0012821X22006069?via%3Dihub

Mo isotope compositions have been increasingly used as a source tracer in magmatic systems. Here, we investigate the relative role of subducting oceanic crust, fluid-rich sources (e.g., serpentinite, altered oceanic crust – AOC), subducted sediments and upper mantle in the chemical composition of Aleutian arc magmas with Mo and B systematics. We present elemental and Mo isotope compositions (δ⁹⁸/⁹⁵Mo) and B concentrations on Aleutian lavas (n =59), from Okmok Volcano in the east to the westernmost seamount Piip, showing absence of Mo isotope fractionation during magmatic differentiation. Additionally, we report Mo isotope systematics for serpentinized peridotites from the South-West Indian Ridge (SWIR) (n =6), AOC (n =2) and Pacific sediments (DSDP 183, ODP 886) (n=5) outboard the Aleutian arc. Molybdenum isotope composition and B enrichment (e.g., B/Ce) patterns display a step-function increase along the arc, with low, MORB-like, δ⁹⁸/⁹⁵Mo and low B/Ce values in the western section of the arc (B/Ce =0.15–1.07, δ⁹⁸/⁹⁵Mo =−0.38 to +0.01‰), that abruptly increase in the central-eastern volcanoes Korovin, Seguam and Yunaska (B/Ce =1.20–2.60, δ⁹⁸/⁹⁵Mo =+0.03 to +0.30‰) near the intersection of the Amlia Fracture Zone (AFZ) with the trench, but decrease again farther east at Okmok (B/Ce =0.76 and δ⁹⁸/⁹⁵Mo =−0.12‰ on average). These data patterns are interpreted to reflect an along-arc changing source in the Aleutian magmas. AOC and Pacific sediments have predominantly low δ⁹⁸/⁹⁵Mo (−0.47 to −0.32 and +0.17 to −1.9‰, respectively), while serpentinites have extremely high δ⁹⁸/⁹⁵Mo (up to +1.09‰) and high B/Ce (∼22000). Based on the low δ⁹⁸/⁹⁵Mo in sediments and AOC, and lack of correlation between along-arc δ98/95Mo and radiogenic sediment tracers, subducted sediments and AOC do not exert first-order controls on the observed Mo isotope compositions. Rather, low, MORB-like, δ⁹⁸/⁹⁵Mo but high Mo enrichments (e.g., Mo/Ce) in the western samples are consistent with slab melting under rutile-bearing eclogitic facies with near absent Mo isotope fractionation from the slab to the arc sources. In turn, the abrupt increase of δ⁹⁸/⁹⁵Mo and B/Ce in lavas near the AFZ are best explained by a serpentinite endmember (likely dehydration fluids) at the AFZ that is not evident elsewhere along the arc. Results from this study provide evidence for serpentinites as an additional heavy Mo isotope signature component in subduction zones and demonstrate that high δ⁹⁸/⁹⁵Mo coupled with B enrichments are a useful proxy for tracing serpentinite fluids in subduction zones.
Permanent Link: https://hdl.handle.net/11104/0337964