Deciphering variable mantle sources and hydrous inputs to arc magmas in Kamchatka
•Ambient mantle metasomatism and melting degree traced through Nb/Zr, Ce/B, and δ11B.•δ11B consistent with isotopically heavy slab component(s) and DMM-OIB mantle source.•Greater δ11B complexity in melt inclusion suites reveals melt mixing processes.•Combined melt inclusions and whole-rocks record a...
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Veröffentlicht in: | Earth and planetary science letters 2021-05, Vol.562, p.116848, Article 116848 |
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Sprache: | eng |
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Zusammenfassung: | •Ambient mantle metasomatism and melting degree traced through Nb/Zr, Ce/B, and δ11B.•δ11B consistent with isotopically heavy slab component(s) and DMM-OIB mantle source.•Greater δ11B complexity in melt inclusion suites reveals melt mixing processes.•Combined melt inclusions and whole-rocks record assembly of diverse melt batches.•Contributions from shallow crustal repositories of B remain poorly constrained.
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 l |
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ISSN: | 0012-821X 1385-013X |
DOI: | 10.1016/j.epsl.2021.116848 |