Abstract
The chemistry of primitive arc rocks provides a window into compositional variability in the mantle wedge, as well as slab-derived inputs to subduction-related magmatism. However, in the long-term cycling of elements between Earth's internal and external reservoirs, a key unknown is the importance of retaining mobile elements within the subduction system, through subduction-related metasomatism of the mantle. To address these questions, we have analysed olivine-hosted melt inclusions and corresponding bulk rocks from the Kamchatka arc. Suites of melt inclusions record evidence for entrapment along melt mixing arrays during assembly of diverse parental magma compositions. Systematic variations in parental magma B/Zr, Nb/Zr, Ce/B, and δ11B are also apparent among the different eruptive centres studied. These element ratios constrain the nature of subduction-related metasomatism and provide evidence for ambient mantle heterogeneity and variable degrees of mantle melting.High Nb/Zr and low B/Zr in back-arc rocks indicate smaller degree melts, lower slab-derived inputs, but relatively enriched mantle compositions. Similarly, small monogenetic eruptive centres located away from the main stratocones also tend to erupt magmas with relatively lower slab contribution and overall smaller melting degrees. Conversely, arc-front compositions reflect greater slab contributions and larger degree melts of a more depleted ambient mantle. Across-arc variations in δ11B (ranging from ca. −6‰ in the rear-arc and Sredinny Ridge to +7‰ in the Central Kamchatka Depression) are generally consistent with variable addition of an isotopically heavy slab-derived component to a depleted MORB mantle composition. However, individual volcanic centres (e.g. Bakening volcano) show correlations between melt inclusion δ11B and other geochemical indicators (e.g. Cl/K2O, Ce/B) that require mixing between isotopically distinct melt batches that have undergone different extents of crustal evolution and degassing processes.Our results show that while melt inclusion volatile inventories are largely overprinted during shallower melt storage and aggregation, incompatible trace element ratios and B isotope compositions more faithfully trace initial mantle compositions and subduction inputs. Furthermore, we suggest that the signals of compositional heterogeneity generated in the sub-arc mantle by protracted metasomatism during earlier phases of subduction can be preserved during later magma assembly and storage in the crust.
Highlights
Magmas erupted at subduction zones represent a surface expression of the cycling of elements between Earth’s internal and external reservoirs
These studies have demonstrated that the ambient sub-arc mantle chemistry can be more depleted than the depleted MORB mantle (DMM) or reflect mixing with enriched mantle similar to the sources of ocean-island basalts (OIB) (e.g. Pearce and Peate, 1995; Turner et al, 2017; and references therein)
We have not recalculated the melt inclusion compositions to correct for post-entrapment modification (PEC; see the Supplementary Methods for detailed discussion), but MI data interpretations are based on trace element ratios that are incompatible during olivine crystallisation (e.g., Nb, Zr, B, Ce) and 11B/10B ratios, which should be unaffected by post-entrapment crystallisation during ascent/storage
Summary
Magmas erupted at subduction zones represent a surface expression of the cycling of elements between Earth’s internal and external reservoirs Their compositions reflect ambient mantle chemistry, slab contributions, and later crustal processing (including polybaric fractionation, storage, degassing, and mixing). Constraints on subduction zone slab and mantle processes are largely derived from whole-rock chemistry These studies have demonstrated that the ambient sub-arc mantle chemistry (prior to subduction modification) can be more depleted than the depleted MORB mantle (DMM) or reflect mixing with enriched mantle similar to the sources of ocean-island basalts (OIB) de Hoog and Savov, 2018; and references therein), such that the δ11B of slab-derived contributions becomes progressively lighter with increasing depths to the slab and distances from the trench This generates arc-normal trends towards lower B and δ11B as observed in the Andean Central Volcanic Zone (Rosner et al, 2003) and Washington Cascades (Leeman et al, 2004). We show that melt inclusion arrays can preserve δ11B signatures indicating variable subduction inputs to a geochemically heterogeneous ambient mantle, despite later crustal magma assembly and mixing
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