Abstract
Serpentinite is a major carrier of fluid-mobile elements in subduction zones, which influences the geochemical signature of arc magmatism (e.g. high abundances of Li, Ba, Sr, B, As, Mo and Pb). Based on results from Neoproterozoic serpentinites in the Arabian-Nubian Shield, we herein report the role of antigorite in the transportation of fluid-mobile elements (FME) and light rare earth elements (LREE) from the subducted slab to arc-related magma during subduction. The serpentinites contain two generations of antigorites: the older generation is coarse-grained, formed at a temperature range of 165–250 °C and is enriched in Li, Rb, Ba and Cs, whereas the younger generation is finer-grained, formed at higher temperature conditions (425–475 °C) and has high concentrations of B, As, Sb, Mo, Pb, Sr and LREE. Magnesite, on the other hand, remains stable at sub-arc depths beyond the stability field of both antigorites, and represents a potential reservoir of FME and LREE for deeper mantle melts. Magnesite has high FME and LREE absorbing capacity (over 50–60%) higher than serpentine phases. Temperature is the main controlling factor for stability of these minerals and therefore the release of these elements from subducted slabs into arc magmatism. As the liberation of these elements varies along the length of the slab, the resulting cross-arc geochemical variation trend can help to determine the subduction polarity of ancient arcs.
Highlights
Regardless of the tectonic setting in which they form, serpentinites are a major potential carrier of water and incompatible fluid-mobile elements (FME) such as Li, B, As, Sb, Pb, Ba, Cs, U and Sr1–3 into the subduction zone and the overlying mantle wedge
We provide evidence for the systematic distribution of FME and rare earth elements (REE) in subduction zones that depends on temperature conditions of the subducting slab, and we illustrate how the resultant cross-arc geochemical trends may help to determine the subduction polarity of ancient arcs in Earth’s history
In situ trace element analyses reveal a heterogeneous distribution of FME between the CA- and FA-groups
Summary
Regardless of the tectonic setting in which they form, serpentinites are a major potential carrier of water (up to 15%) and incompatible fluid-mobile elements (FME) such as Li, B, As, Sb, Pb, Ba, Cs, U and Sr1–3 into the subduction zone and the overlying mantle wedge. Numerous in situ studies were previously carried out on serpentinites with the aim of defining the behavior of fluid-mobile elements and rare earth elements (REE) during subduction These studies highlighted the role of the original minerals (i.e., olivine vs pyroxene) and temperature on trace element distributions during the subduction process[5,9,10,11,12] but did not consider the distribution of trace elements at higher-temperature conditions within the stability field of antigorite. We use the results of a detailed petrological, mineralogical and geochemical study from selected Neoproterozoic serpentinite bodies in the Arabian-Nubian Shield (a typical arc-accretion orogen)[15] to constrain the role of temperature within the stability field of antigorite on the distribution of FME and REE in subduction zones. Some ophiolites were previously thought to have formed in mid-ocean ridges[18], there is a general consensus that the majority of the ANS ophiolites formed in subduction-related tectonic settings[15,19,20,21,22,23,24,25]
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