Details
| Original language | English |
|---|---|
| Article number | 51 |
| Journal | Contributions to Mineralogy and Petrology |
| Volume | 178 |
| Issue number | 8 |
| Publication status | Published - 28 Jul 2023 |
Abstract
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.
Keywords
- Arc magma differentiation, Arc magmatism, Ascent-driven differentiation, ASI evolution, Calc-alkaline rocks, Clinopyroxene dissolution, Crystal entrainment, Equilibrium crystallisation, Isothermal decompression, Magmatic phase equilibria
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
- Earth and Planetary Sciences(all)
- Geophysics
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In: Contributions to Mineralogy and Petrology, Vol. 178, No. 8, 51, 28.07.2023.
Research output: Contribution to journal › Article › Research › peer review
}
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 -