Sulfur determination by laser ablation high resolution magnetic sector ICP-MS applied to glasses, aphyric lavas,and micro-laminated sediments

Sulfur(S) is an important element for understanding redox processes, ore formation, environmental chemistry, volcanism and climate. Here, we present a method for in situ S concentration measurement by LA-ICP-MS at the50–100 lm length scale using a 213 nm laser ablation set up coupled to a single-collector magnetic sector ICP-MS with high resolution capabilities. Analyses were performed in medium mass resolution(m/Dm = 3000) mode, which allows for the separation of32 S and34S from molecular interferences. S is simultaneously analyzed along with all major and minor elements so that a priori knowledge of an internal standard concentration is not necessary; this allows for in situ bulk analysis of aphyric groundmass in volcanic rocks or other fine-grained samples. The primary limitation in analyzing S by laser ablation was found to be long-term instrumental drift in fractionation of elemental S relative to other elements,presumably due to drift in laser dynamics. A method for correcting for such fractionation over time was implemented.After correcting for such drift, measurements of homogeneous basaltic glasses are reproducible to within 10 % for high concentration samples([500 ppm) and to within 20 % for low concentration samples(\200 ppm). The applicability of the method was demonstrated using natural and synthetic glasses, aphyric lavas, and micro-laminated sediments.

Continental crust formation at arcs, the arclogite ‘‘delamination''cycle, and one origin for fertile melting anomalies in the mantle

The total magmatic output in modern arcs,where continental crust is now being formed, is believed to derive from melting of the mantle wedge and is largely basaltic. Globally averaged continental crust, however, has an andesitic bulk composition and is hence too silicic to have been derived directly from the mantle. It is well known that one way this imbalance can be reconciled is if the parental basalt differentiates into a mafic garnet pyroxenitic residue/cumulate(‘‘arclogite'') and a complementary silicic melt, the former foundering or delaminating into the mantle due to its high densities and the latter remaining as the crust.Using the Sierra Nevada batholith in California as a case study, the composition of mature continental arc crust is shown in part to be the product of a cyclic process beginning with the growth of an arclogite layer followed by delamination of this layer and post-delamination basaltic underplating/recharge into what remains of the continental crust.A model is presented, wherein continuous arc magmatism and production of arclogites in continental arcs are periodically punctuated by a delamination event and an associated magmatic pulse every *10–30 My. The recycling flux of arclogites is estimated to be *5 %–20 % that of oceanic crust recycling by subduction. Delaminated arclogites have the necessary trace-element compositions to yield time-integrated isotopic compositions similar to those inferred toexist as reservoirs in the mantle. Because of their low melting temperatures, such pyroxenites may be preferentially melted, possibly forming a component of some hotspot magmas.