The transfer of material from subducting slabs to the overlying mantle is one of the most important processes regulating Earth’s geochemical cycles. A major part of this material cycling involves slab devolatilization and the release of sediment- and slab-derived fluids to the mantle wedge, triggering melting and subsequent arc volcanism. Previous geodynamic, geophysical, and geochemical studies have revealed many important controls on fluid fluxing to the mantle and its manifestations in arc magmas. However, it remains difficult to identify the specific mineral breakdown reactions that control element fluxes from the subducting slab into the overriding mantle. To address this challenge, we combine global arc whole-rock compositional data with geophysical information (e.g., depths to slab) and thermodynamic data. We observe three peaks in Ba/Nb in global arc magma whole-rock compositions corresponding to depths to slab of 60, 120, and >290 km. Using published thermodynamic and geodynamic models of slab evolution, we show that these peaks can be linked to the progressive breakdown of hydrous minerals (e.g., epidote, actinolite, lawsonite) in subducting slabs.
Copper, sourced from porphyry deposits formed in arc settings, is an increasingly scarce yet critical resource. The processes that shape the copper contents of magmas remain poorly understood. One theory is that magmas must be copper-rich in order to form porphyry deposits. Mature arcs have up to now played an outsized role in shaping existing models of copper systematics in magmas. Here we take a Big Data approach, compiling multiple data sets of volcanic whole rock compositions using open-source software. We show the global ubiquity of the "copper paradox," where rocks with high Sr/Y (and high ore potential) have the lowest copper concentrations. These calc-alkaline, ore-forming magmas undergo iron depletion caused by extensive amphibole and/or garnet fractionation, promoting sulphide fractionation and copper depletion. Despite their paucity in copper, these magmas are associated with porphyry deposits, implying that magma fertility depends on factors other than a magma's bulk copper content.
The majority of geochemical and cosmochemical research is based upon observations and, in particular, upon the acquisition, processing and interpretation of analytical data from physical samples. The exponential increase in volumes and rates of data acquisition over the last century, combined with advances in instruments, analytical methods and an increasing variety of data types analysed, has necessitated the development of new ways of data curation, access and sharing. Together with novel data processing methods, these changes have enabled new scientific insights and are driving innovation in Earth and Planetary Science research. Yet, as approaches to data-intensive research develop and evolve, new challenges emerge. As large and often global data compilations increasingly form the basis for new research studies, institutional and methodological differences in data reporting are proving to be significant hurdles in synthesising data from multiple sources. Consistent data formats and descriptions as well as appropriate information on data quality are becoming crucial to enabling reproducibility and integration of results and fostering confidence for data reuse. Here, we explore the key challenges faced by the geo- and cosmochemistry community and, by drawing comparisons from other communities, recommend possible approaches to overcome them. The first challenge is bringing together the numerous sub-disciplines within our community. One key factor for this convergence will be gaining endorsement from the international geochemical, cosmochemical and analytical societies and associations, journals and institutions. Increased education and outreach, spearheaded by ambassadors recruited from leading scientists across disciplines, will further contribute to raising awareness, and to uniting and mobilising the community. Appropriate incentives, recognition and credit for good data management as well as an improved, user-oriented technical infrastructure will be essential for achieving a cultural change towards an environment in which the effective use and real-time interchange of large datasets is common-place. Finally, the development of best practices for standardised data reporting and exchange, driven by expert working groups, will be a crucial step towards making geo- and cosmochemical data more Findable, Accessible, Interoperable and Reusable by both humans and machines (FAIR).
Wall-rock assimilation can cause effective sulfide saturation in magmas and lead to the formation of base and precious metal sulfide deposits. Detailed assessments of how assimilation affects the sulfur content at sulfide saturation (SCSS) in magmas have been scarce because of the lack of suitable thermodynamic modeling tools. The Magma Chamber Simulator (MCS) is the first geochemical modeling software that accounts for thermodynamic wall-rock phase equilibrium in open magmatic systems experiencing recharge-assimilation-fractional crystallization. We used the MCS to model SCSS in a magmatic system corresponding to the parental melt of the Partridge River intrusion of the Duluth Complex, Minnesota. This intrusion hosts several Cu-Ni deposits in troctolitic and noritic rocks, which both show evidence of assimilation of the adjacent Virginia Formation black shale. Our simulations show that the dominantly troctolitic rocks can form via fractional crystallization if the parental melt is hydrous (≥ 1 wt % H2O), while gabbroic rocks dominate when the parental melt is H2O poor (≤ 0.14 wt % H2O). Formation of norite from the hydrous parental melt requires ~20–30% of selective assimilation of black shale partial melts or bulk assimilation of stoped blocks. In the fractional crystallization simulations, increasing the H2O content of the parental melt lowers SCSS. In the hydrous fractional crystallization scenarios, SCSS is lowered further by the depletion of FeO from the residual melt, owing to enhanced olivine stability. In the assimilation simulations, the residual melt in the magma subsystem becomes enriched in SiO2, Al2O3, K2O, and H2O with simultaneous depletion in FeO, MgO, CaO, and Na2O. These compositional changes promote sulfide saturation—an effect that is more pronounced in selective rather than in bulk assimilation scenarios. Trace element models, used as a proxy for the efficiency of sulfur assimilation, show that sulfur should behave as an incompatible element (DWR (S) ≤ 1) to wall rock in the selective assimilation simulations, i.e., enriched in early-assimilated wall-rock fluids and/or partial melts, in order to fulfill the natural sulfur isotope criteria of the Duluth Complex. Bulk assimilation may also be efficient enough to modify the sulfur isotope composition, but it requires a large amount of crystallization in the magma and is, hence, considered less likely to be the main process for sulfur assimilation. If wall-rock sulfur is effectively transported to the magma, in situ precipitation of sulfides without notable subsequent upgrading by dynamic processes could produce the sulfide grade of an average Cu-Ni deposit in the Partridge River intrusion.
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