TY - JOUR

T1 - Ascent-driven differentiation

T2 - a mechanism to keep arc magmas metaluminous?

AU - Marxer, Felix

AU - Ulmer, Peter

AU - Müntener, Othmar

N1 - Funding Information: We thank Lukas Martin, Eric Reusser, and Julien Allaz for assistance during SEM-EDS and EPMA analyses and Andy Huber, Andreas Jallas, Thomas Good, Uli Kroll, Andreas Reimer, and Julian Feige for technical support in the experimental labs and thorough sample preparation. Thomas van Gerve and Olivier Namur are gratefully acknowledged for assistance during Raman measurements at KU Leuven. We are grateful to Tom Sisson, Manuel Pimenta Silva, François Holtz, Olivier Bachmann, and Max Schmidt for discussions. Thoughtful and constructive reviews from Madeleine Humphreys and Dawnika L. Blatter as well as the efficient editorial handling by Dante Canil are gratefully acknowledged. This work was supported by the ETH research grant ETH-14 16-1 and the DFG project HO1337/47 (part of the Forschungsgruppe FOR 2881 "Diffusion chronometry of magmatic systems") covering F.M.

PY - 2023/7/28

Y1 - 2023/7/28

N2 - Arc magmatism is fundamental to the generation of new continental or island arc crust. However, the mechanisms that add to the chemical complexity of natural calc-alkaline magmas ranging from basaltic to rhyolitic compositions are debated. Differentiation mechanisms currently discussed include magma mixing, assimilation, crustal melting, or (fractional) crystallisation. In this contribution, the differentiation of arc magmas by decompression-driven crystallisation is investigated. We performed a set of equilibrium crystallisation experiments at variable crustal pressures (200–800 MPa) on a hydrous high-Al basalt (3.5 wt.% of H 2O in the starting material) with run temperatures varying from near-liquidus conditions (1110 °C) to 900 °C. Oxygen fugacity was buffered at moderately oxidising conditions close to the NNO equilibrium. Combining these novel experiments with previous polybaric fractional crystallisation experiments (Marxer et al., Contrib Mineral Petrol 177:3, 2022) we demonstrate the effects of pressure on the crystallisation behaviour of calc-alkaline magmas with respect to liquid and cumulate lines of descent, mineral chemistry, and phase proportions. Decompression shifts the olivine-clinopyroxene cotectic curve towards melt compositions with higher normative clinopyroxene and enlarges the stability field of plagioclase. This exerts a key control on the alumina saturation index of residual liquids. We argue that near-adiabatic (or near-isothermal) decompression accompanied by dissolution of clinopyroxene entrained during residual melt extraction in the lower crust keeps arc magmas metaluminous during crystallisation-driven differentiation thereby closely reproducing the compositional spread observed for natural arc rocks.

AB - Arc magmatism is fundamental to the generation of new continental or island arc crust. However, the mechanisms that add to the chemical complexity of natural calc-alkaline magmas ranging from basaltic to rhyolitic compositions are debated. Differentiation mechanisms currently discussed include magma mixing, assimilation, crustal melting, or (fractional) crystallisation. In this contribution, the differentiation of arc magmas by decompression-driven crystallisation is investigated. We performed a set of equilibrium crystallisation experiments at variable crustal pressures (200–800 MPa) on a hydrous high-Al basalt (3.5 wt.% of H 2O in the starting material) with run temperatures varying from near-liquidus conditions (1110 °C) to 900 °C. Oxygen fugacity was buffered at moderately oxidising conditions close to the NNO equilibrium. Combining these novel experiments with previous polybaric fractional crystallisation experiments (Marxer et al., Contrib Mineral Petrol 177:3, 2022) we demonstrate the effects of pressure on the crystallisation behaviour of calc-alkaline magmas with respect to liquid and cumulate lines of descent, mineral chemistry, and phase proportions. Decompression shifts the olivine-clinopyroxene cotectic curve towards melt compositions with higher normative clinopyroxene and enlarges the stability field of plagioclase. This exerts a key control on the alumina saturation index of residual liquids. We argue that near-adiabatic (or near-isothermal) decompression accompanied by dissolution of clinopyroxene entrained during residual melt extraction in the lower crust keeps arc magmas metaluminous during crystallisation-driven differentiation thereby closely reproducing the compositional spread observed for natural arc rocks.

KW - Arc magma differentiation

KW - Arc magmatism

KW - Ascent-driven differentiation

KW - ASI evolution

KW - Calc-alkaline rocks

KW - Clinopyroxene dissolution

KW - Crystal entrainment

KW - Equilibrium crystallisation

KW - Isothermal decompression

KW - Magmatic phase equilibria

UR - http://www.scopus.com/inward/record.url?scp=85166736813&partnerID=8YFLogxK

U2 - 10.1007/s00410-023-02035-7

DO - 10.1007/s00410-023-02035-7

M3 - Article

VL - 178

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 8

M1 - 51

ER -