TY - JOUR

T1 - Magmatic evolution biases basaltic records of mantle chemistry towards melts from recycled sources

AU - Neave, David A.

AU - Namur, Olivier

AU - Shorttle, Oliver

AU - Holtz, François

N1 - Funding Information: We thank Harald Behrens, Renat Almeev, Stefan Linsler, Robert Balzer, Julian Fiege and Ulrich Kroll for their assistance with experiments and analyses. We thank Eric Brown and Deborah Eason for their supportive and constructive comments, Marc Hirschmann and an anonymous reviewer for their helpful comments on an earlier version of this manuscript and Rajdeep Dasgupta for his efficient editorial handling. This work and D.A.N. were supported by the Alexander von Humboldt Foundation , the German Research Foundation ( NE2097/1-1 ) and a Presidential Fellowship from the University of Manchester . F.H. acknowledges support from the German Research Foundation ( HO1337/35-1 ). D.A.N. devised the project and carried out the experiments with the guidance of O.N. and F.H. All authors contributed towards the interpretation of experimental data and the intellectual development of the written manuscript.

PY - 2019/8/15

Y1 - 2019/8/15

N2 - 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 enriched compositions as differentiation proceeds; and secondly, the frequently documented cargoes of highly anorthitic plagioclase crystals carried by evolved and enriched liquids from which they cannot have crystallised. These crystals can now be understood as the solidified remnants of depleted, lherzolite-derived melts that have been entrained into melt mixtures from more enriched sources. Increases in the degree of enrichment of cumulate rocks sampled from progressively shallower horizons of the oceanic crust can also be interpreted in terms of enriched melts surviving crustal processing in preference to depleted melts.

AB - 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 enriched compositions as differentiation proceeds; and secondly, the frequently documented cargoes of highly anorthitic plagioclase crystals carried by evolved and enriched liquids from which they cannot have crystallised. These crystals can now be understood as the solidified remnants of depleted, lherzolite-derived melts that have been entrained into melt mixtures from more enriched sources. Increases in the degree of enrichment of cumulate rocks sampled from progressively shallower horizons of the oceanic crust can also be interpreted in terms of enriched melts surviving crustal processing in preference to depleted melts.

KW - basalt phase equilibria

KW - geochemical variability

KW - Iceland

KW - magmatic evolution

KW - mantle heterogeneity

KW - MORB

UR - http://www.scopus.com/inward/record.url?scp=85066975633&partnerID=8YFLogxK

U2 - 10.1016/j.epsl.2019.06.003

DO - 10.1016/j.epsl.2019.06.003

M3 - Article

AN - SCOPUS:85066975633

VL - 520

SP - 199

EP - 211

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

ER -