The Molybdenum Isotope System as a Tracer of Slab Input in Subduction Zones: An Example From Martinique, Lesser Antilles Arc

Abstract

Molybdenum isotopes are fractionated by Earth‐surface processes and may provide a tracer for the recycling of crustal material into the mantle. Here, we examined the Mo isotope composition of arc lavas from Martinique in the Lesser Antilles arc, along with Cretaceous and Cenozoic Deep Sea Drilling Project sediments representing potential sedimentary inputs into the subduction zone. Mo stable isotope composition (defined as δ98Mo in ‰ deviation from the NIST 3134 standard) in lavas older than ∼7 million years (Ma) exhibits a narrow range similar to and slightly higher than MORB, whereas those younger than ∼7 Ma show a much greater range and extend to unusually low δ98Mo values. Sediments from DSDP Leg 78A, Site 543 have uniformly low δ98Mo values whereas Leg 14, Site 144 contains both sediments with isotopically light Mo and Mo‐enriched black shales with isotopically heavy Mo. When coupled with published radiogenic isotope data, Mo isotope systematics of the lavas can be explained through binary mixing between a MORB‐like end‐member and different sedimentary compositions identified in the DSDP cores. The lavas older than ∼7 Ma were influenced by incorporation of isotopically heavy black shales into the mantle wedge. The younger lavas are the product of mixing isotopically light sedimentary material into the mantle wedge. The change in Mo isotope composition of the lavas at ∼7 Ma is interpreted to reflect the removal of the Cretaceous black shale component due to the arrival of younger ocean crust where the age‐equivalent Cretaceous sediments were deposited in shallower oxic waters. Isotopic fractionation of Mo during its removal from the slab is not required to explain the observed systematics in this system.

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

“…%), which is not coupled with a correlated variation of δ 98/95 Mo. This is different from the evidence reported for Kos (Voegelin et al, 2014) and Banda Arc (Wille et al, 2018) but consistent to other volcanic suites (e.g., Casalini, 2018;Freymuth et al, 2015;Gaschnig et al, 2017;König et al, 2016;Yang et al, 2015). Considering the whole data set, the relatively small variation of δ 98/95 Mo with respect to the large MgO range (9.3-3.0 wt.…”

“…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, work on Iceland (Yang et al, 2015) and other studies on subduction-related settings (e.g., Casalini, 2018;Freymuth et al, 2015;Gaschnig et al, 2017) found no evidence indicating δ 98/95 Mo fractionation during magma differentiation, but the latter did identify Mo isotopic fractionation during the processes of their genesis. 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).…”

“…Therefore, other residual phases retaining Mo must have been present during sediment melting. (Freymuth et al, , 2015Gaschnig et al, 2017;König et al, 2016;Voegelin et al, 2014;Wille et al, 2018) and MORB average (Gale et al, 2013). Sulfides represent another preferential host phases for Mo (e.g., Li & Audétat, 2012), and the net partitioning effects in these Ca-poor sediment melts (Skora et al, 2017) can produce Ce/Mo values consistent with those required for the TMP magmas (Figure 6c).…”

“…Vesuvius magmas along with other subduction-related magmas from literature(Freymuth et al, 2015Gaschnig et al, 2017;König et al, 2016;Voegelin et al, 2014;Wille et al, 2018) and MORB average(Gale et al, 2013). Vesuvius magmas along with other subduction-related magmas from literature(Freymuth et al, 2015Gaschnig et al, 2017;König et al, 2016;Voegelin et al, 2014;Wille et al, 2018) and MORB average(Gale et al, 2013).…”

“…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).…”