Magmatic evolution biases basaltic records of mantle chemistry towards melts from recycled sources
The chemistry of erupted magmas provides a crucial window into the composition and structure of Earth’s convecting mantle. However, magmatic evolution in the crust makes it challenging to reconstruct mantle properties from volcanic rocks in important but incompletely understood ways. Here we investi...
| Published in: | Earth and Planetary Science Letters |
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| Main Authors: | , , , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
2019
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| Subjects: | |
| Online Access: | https://research.manchester.ac.uk/en/publications/29c8b14b-0541-4ef6-96ff-3d1376a3caeb https://doi.org/10.1016/j.epsl.2019.06.003 https://pure.manchester.ac.uk/ws/files/103064628/basalt_evolution_r_02.pdf |
| Summary: | The chemistry of erupted magmas provides a crucial window into the composition and structure of Earth’s convecting mantle. However, magmatic evolution in the crust makes it challenging to reconstruct mantle properties from volcanic rocks in important but incompletely understood ways. Here we investigate how mantle-derived compositional variability in primary oceanic basalts determines their phase equilibria relations and the nature of the geochemical signals they record. By performing experiments on synthetic analogues of compositionally extreme primitive lavas from the Reykjanes Peninsula of Iceland at realistic magma storage conditions (300 MPa, 1140–1260 °C), we show that melts from enriched mantle domains retain higher melt fractions as they cool than those generated by melting of typical fertile lherzolite (i.e. they crystallise less mass over any interval of decreasing temperature). These melt fraction differences arise because plagioclase crystallisation is suppressed in Na- and H2O-rich but Ca- and Al-poor liquids derived from enriched source lithologies. Thus, compositional characteristics inherited from the mantle have a first-order control on the efficiency with which cooling basalts crystallise. This means that enriched melts will be more likely to survive crustal processing than depleted melts. Basalt chemistry will therefore be disproportionately influenced by melts from volumetrically minor enriched lithologies compared with melts from the upper mantle’s most common lithology, lherzolite,systematically biasing basaltic records towards melts from recycled mantle sources. We combine our experimental observations from Iceland with thermodynamic simulations on mid-ocean ridge basalt compositions and show that mantle derived variability in crystallisation efficiency can explain two enigmatic features of the global oceanic basalt record: firstly, the anomalous over-enrichment of incompatible elements during the differentiation of mid-ocean ridge basalts, which may reflect a progressive bias towards ... |
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