Magnesium isotope geochemistry in arc volcanism

Abstract

Incorporation of subducted slab in arc volcanism plays an important role in producing the geochemical and isotopic variations in arc lavas. The mechanism and process by which the slab materials are incorporated, however, are still uncertain. Here, we report, to our knowledge, the first set of Mg isotopic data for a suite of arc lava samples from Martinique Island in the Lesser Antilles arc, which displays one of the most extreme geochemical and isotopic ranges, although the origin of this variability is still highly debated. We find the δ 26 Mg of the Martinique Island lavas varies from −0.25 to −0.10, in contrast to the narrow range that characterizes the mantle (−0.25 ± 0.04, 2 SD). These high δ 26 Mg values suggest the incorporation of isotopically heavy Mg from the subducted slab. The large contrast in MgO content between peridotite, basalt, and sediment makes direct mixing between sediment and peridotite, or assimilation by arc crust sediment, unlikely to be the main mechanism to modify Mg isotopes. Instead, the heavy Mg isotopic signature of the Martinique arc lavas requires that the overall composition of the mantle wedge is buffered and modified by the preferential addition of heavy Mg isotopes from fluids released from the altered subducted slab during fluid−mantle interaction. This, in turn, suggests transfer of a large amount of fluidmobile elements from the subducting slab to the mantle wedge and makes Mg isotopes an excellent tracer of deep fluid migration. magnesium isotopes | arc magmatism | mantle wedge | Lesser Antilles arc | Martinique Island

Magnesium Isotope Composition of Subduction Zone Fluids as Constrained by Jadeitites From Myanmar

“…Mg isotopes are not significantly fractionated during melting and crystallization (Liu et al, ; Teng et al, , ). Metamorphic dehydration only generates limited Mg isotope fractionation (<0.07‰) on a bulk‐rock scale (Li et al, ; Li, Teng, et al, ; Wang et al, ).…”

“…The blue shaded area represents the δ 26 Mg values of white jadeitites. Data sources: Seawater (Ling et al, ); oceanic island basalt (OIB), and midocean ridge basalt (MORB; Bourdon et al, ; Teng et al, ); altered MORB (Huang, ; Huang, Ke et al, ; Teng et al, ); marine sediments (Hu et al, ; Teng et al, ); abyssal peridotite (Liu et al, ); talc and antigorite (Beinlich et al, ); chlorite (Pogge von Strandmann et al, ); olivine (Handler et al, ; Yang et al, ; Young et al, ; Huang et al, ; Liu et al, 2011; Pogge von Strandmann et al, ; Xiao et al, ); carbonates (Brenot et al, ; Fantle & Higgins, ; Geske et al, ; Higgins & Schrag, ; Hippler et al, ; Huang & Xiao, ; Jacobson et al, ; Kasemann et al, ; Tipper et al, ; Wombacher et al, ).…”

“…Subducted AOC and sediments have highly variable δ 26 Mg values from −3.65‰ to +0.52‰ (Figure ), with low δ 26 Mg in carbonated rocks and with high δ 26 Mg in carbonate‐free rocks (Hu et al, ; Huang, ; Huang, Ke et al, ; Teng, ; Teng et al, ). Therefore, carbonate‐rich AOC and sediments are potential sources for the low δ 26 Mg fluids.…”

“…Unlike many incompatible trace elements and radiogenic isotopes, the Mg isotope composition of mantle wedge peridotites are insensitive to metasomatism and partial melting processes because the Mg concentrations of slab‐derived fluids are significantly lower than those of mantle peridotites. However, recent studies have revealed that some arc lavas have slightly higher δ 26 Mg values relative to the normal mantle (Li et al, ; Teng et al, ), whereas some cratonic eclogites and subduction‐related basalts have lower δ 26 Mg values (Huang, Li et al, ; Li et al, ; Wang et al, , ; Yang et al, ). These observations raise an important question on the mechanisms for transferring the heterogeneous Mg isotope signatures of the recycled slabs to the mantle.…”

Deep carbon cycle recorded by calcium‐silicate rocks (rodingites) in a subduction‐related ophiolite

“…However, the percentage of spinel in the peridotite is too low (usually less than 5%) to influence the whole‐rock Mg isotopic composition. (2) The fluid from hydrothermal circulation could modify the peridotite Mg isotopic composition toward a heavier value [ Teng et al , ].…”