Tracing subducted black shales in the Lesser Antilles arc using molybdenum isotope ratios

Abstract

Lesser Antilles arc lavas have trace element and radiogenic isotope characteristics indicative of a continent-derived contribution. It is hotly debated whether this continental signature represents terrigenous sediment that has been subducted with the Atlantic plate and added to the magma sources in the mantle wedge or portions of the sub-arc crust that are assimilated during magma ascent. Here we present Mo isotope data for Lesser Antilles arc lavas and sediments off-board the Lesser Antilles trench. Sequences of black shales, present in the subducting sediment piles, are highly enriched in Mo and have unusually high 98 Mo/ 95 Mo. Despite their low mass fraction in the sediment package (<10 % in DSDP Site 144), they dominate the Mo content and isotopic composition of the bulk sediment subducting at the Lesser Antilles trench. We show that lavas from the southern part of the Lesser Antilles arc also have high 98 Mo/ 95 Mo ratios, implicating the addition Manuscript Click here to download Manuscript G38344-Freymuth-eXtyled_revised.doc

Ce/Mo and Molybdenum Isotope Systematics in Subduction‐Related Orogenic Potassic Magmas of Central‐Southern Italy

“…In our data set, however, Ca-poor shales show variable δ 98/95 Mo, including isotopically heavy values, and in general they are not lighter than Ca-rich marls ( Figure 5); this reinforces the notion that local redox conditions of the depositional basin (e.g., basin restriction, productivity upwelling, and occurrence of Fe-Mn nodule fractionation) rather than the lithology exert the main control on Mo isotopes. Similar isotopic variability within different sediment types had been extensively reported by studies where Mo isotopes are used as oceanic paleoproxy, entailing that the isotopic range defined by sediments deposited under variable suboxic to anoxic conditions covers almost the entire isotopic range comprised Freymuth et al, 2015Freymuth et al, , 2016Gaschnig et al, 2017). Mo isotope composition of MORB is from Freymuth et al (2015), while its Ce/Mo is the average (±1σ) from Gale et al (2013).…”

“…The sediment data from our study are shown in Figure 5 together with those of the sedimentary samples from the Pacific crust (ODP Leg 129; Freymuth et al, 2015, andDSDP Site 144, Freymuth et al, 2016;Gaschnig et al, 2017) to have a comparison with oceanic sediments to be recycled in subduction zones that have been analyzed for Mo isotopes. The sedimentary samples of this study overlap the whole range spanned by literature data (consisting of very low δ 98/95 Mo pelagic clays, radiolarites, and volcanoclastic sediments, Freymuth et al, 2015, and isotopically heavier calcareous-carbonaceous black shale, Freymuth et al, 2016) slightly extending to higher and lower δ 98/95 Mo values, although at a slightly less variable Ce/Mo.…”

“…Mo isotopes experience notable mass-dependent isotopic fractionation in natural systems, allowing their use as paleoproxy for oceanic redox conditions (e.g., Barling et al, 2001;Siebert et al, 2003). The occurrence of isotopically distinct reservoirs of superficial Mo in sediments that are then recycled into the mantle raised the interest in the use of Mo isotopes as tracers in subduction zones (e.g., Freymuth et al, 2016). Relatively low δ 98/95 Mo are also reported for suboxic or mildly anoxic sedimentary environments, due to nonquantitative absorption processes.…”

“…On the other hand, in strongly reducing conditions, Mo is quantitatively scavenged from seawater into anoxic sediments, without any significant isotopic fractionation, as shown by the isotopically heavy Mo recorded in black shales (e.g., Arnold et al, 2004;Barling et al, 2001;Neubert et al, 2008). The occurrence of isotopically distinct reservoirs of superficial Mo in sediments that are then recycled into the mantle raised the interest in the use of Mo isotopes as tracers in subduction zones (e.g., Freymuth et al, 2016). Some studies (e.g., Voegelin et al, 2014;Wille et al, 2018) suggested that differentiation of high-temperature hydrous magmas results in measurable isotopic fractionations, although at much smaller magnitude than the fractionations observed in low-temperature environments.…”

“…The colored fields represent the sedimentary material of the drilled cores nearby Mariana (light red field) and Lesser Antilles Arcs (gray field;Freymuth et al, 2015Freymuth et al, , 2016Gaschnig et al, 2017). The colored fields represent the sedimentary material of the drilled cores nearby Mariana (light red field) and Lesser Antilles Arcs (gray field;Freymuth et al, 2015Freymuth et al, , 2016Gaschnig et al, 2017).…”