title: Trace Element Mobility in Aqueous Fluids and the Cause of Melting in Subduction Zones
Creators: name: Rustioni, Greta
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: subduction zone fluids
subject: salinity
subject: trace elements
subject: arc magmas
subject: diamond trap
subject: 550

description: The formation of magmas in subduction zones is the main mechanism for the growth of the continental crust today. Nevertheless, there is still no consensus concerning the mechanism that causes melting in the mantle wedge above the subducted plate. Most recent studies suggest that water-bearing sediment melts infiltrate the mantle wedge and reduce the solidus of the mantle peridotite. However, this idea is difficult to reconcile with the striking similarity of primitive arc basalt compositions in all subduction zones worldwide, irrespective of the presence, amount, and nature of sediment subducted. In this dissertation, I therefore investigated the hypothesis that aqueous fluids released from the basaltic portion of the subducted slab are the main trigger for melting in volcanic arcs. In order to test this hypothesis, I studied the composition of subduction zone fluids in high-pressure experiments and compared them with the trace element enrichment pattern seen in natural subduction zone magmas. In contrast to previous studies, I also investigated in detail the effect of fluid salinity (NaCl content) on the trace element enrichment pattern, as this may be strongly altered by selective complexing with chloride. For measuring fluid compositions at high pressures and temperatures, the “diamond trap” method was used, where a layer of diamond powder inside an Au or Pt capsule containing the charge is used to trap the fluid inside its pore space. The composition of the fluid is then determined by laser-ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) after quenching the run and freezing the charge. Since this method was essential for this study, a number of tests were carried out in simple and well-studied model systems to check the reliability of the results. In general, the method yields data within a factor of two of the expected value. However, systems that are highly susceptible to dissolution and re-precipitation in temperature gradients may lead to erroneous results. Freezing the sample before analysis is essential for obtaining high-quality data. Experiments to determine the fluid/eclogite partition coefficients of a large suite of trace elements were carried out at 700 – 800 ˚C and 4 – 5 GPa in a piston cylinder apparatus. Additional experiments at 6 GPa were done in a multi anvil press. Starting materials were synthetic glasses of MORB compositions and aqueous solutions with variable NaCl contents. By using periodic oscillations of temperature during the experiments, it was possible to enhance the grain growth of minerals due to Ostwald ripening, such that from all solid phases crystals large enough for laser-ablation ICP-MS analysis could be obtained. Under run conditions, the starting glasses converted to a well-crystallized eclogitic assemblage of omphacite and garnet, with minor rutile and kyanite. The coexisting aqueous fluid contained typically 30 - 40 wt. % of solutes, mostly silica. With increasing salinity (up to about 7 wt. % Cl), the fluid/eclogite partition coefficients of the large-ion lithophile elements (e.g. Cs, Rb, Ba, Sr), of the light rare earth elements (e.g. La, Ce), of U, Th, and Pb increased up to three orders of magnitude. On the other hand, the partitioning of the typical high-field strength trace elements (e.g. Ti, Nb, Ta) was unaffected by salinity. Attainment of equilibrium was demonstrated by truly reversed experiments; runs starting with the trace elements doped either in the solution or in the solid glass yielded consistent results. Increasing pressure and temperature generally enhances the partitioning into the fluid, but does not fundamentally change the enrichment pattern. The experimental data were used to quantitatively model the composition of partial melts of the mantle wedge after metasomatic enrichment by an aqueous fluid released from the basaltic part of the subducted slab after amphibole dehydration. The models show that trace element enrichment pattern of primitive arc basalts can be fully reproduced by adding just 2.5 wt. % of a saline fluid with 7 – 10 wt. % Cl to the source. Lower salinities would require higher fractions of fluid additions, while a purely aqueous fluid (without any Cl) is unable to produce the observed trace element enrichment pattern. In contrast to this finding, sediment melts generated under plausible slab-surface conditions have difficulties to yield the trace element pattern in primitive arc basalts. The present study therefore demonstrates that primitive arc basalts are produced by the partial melting of a mantle peridotite enriched by saline fluids released from the basaltic part of the subducted slab. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible and the importance of sediment melts for magma formation in subduction zones has likely been grossly overestimated.