Understanding deep cumulates: insights into crust-building processes across time and tectonic setting
Abstract
The Moho is a complex zone of mass, heat and chemical transfer between mantle to crust. How heterogeneous is this transition, to what extent does mantle and crust material mix, and what are the consequences for magmatic differentiation, crustal growth, and crustal destruction? To answer these questions, we examine deep lithospheric mafic and ultramafic cumulates. These rocks represent the earliest crystal fractionation products of magmas, and provide the best opportunity to investigate the crust-mantle interface without potential contamination from pre-existing crust or complications from shallow-level differentiation. We survey global cumulate compositions from the literature across diverse tectonic settings (mid-ocean ridges, continental rifts, island arcs, continental arcs), and show that distinctive major element trends emerge based on tectonic setting. At mid-ocean ridges, cumulate compositions closely match MELTS models of instantaneous cumulates derived from low-pressure fractional crystallization of a MORB-like melt. In contrast, cumulates from continental rifts show earlier enrichment in FeO and TiO2 when plotted against MgO. These distinct continental rift trends more closely resemble modeled trends of accumulated (not instantaneous) cumulates. We find that, compared to cumulates from all rifts, cumulates from arcs show considerably more scatter relative to modeled fractionation trends, suggesting that other processes, such as mixing between cumulates and melts, may operate in addition to crystal fractionation alone. We propose that the prolonged magmatism beneath continental settings (both at rifts and arcs) may enhance secondary differentiation processes such as mixing and assimilation, whereas plate spreading at mid-ocean ridge settings may reduce the chances of these processes taking place. We also explore the role of other parameters such as crustal thickness and tectonic stress regime against petrologic indices of differentiation (e.g., Mg#). Our results provide a new global, deep lithospheric perspective on crustal growth processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.V31D..08C
- Keywords:
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- 1020 Composition of the continental crust;
- GEOCHEMISTRYDE: 1021 Composition of the oceanic crust;
- GEOCHEMISTRYDE: 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY