The effect of variations in cooling rates on mineral compositions in mid-ocean ridge basalts

Anders McCarthy; Cyril Chelle-Michou; Jonathan D. Blundy; Michael J. Dorais; Froukje M. van der Zwan; David W. Peate; EIMF (Edinburgh Ion Microprobe Facility)

doi:10.1016/j.chemgeo.2023.121415

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The effect of variations in cooling rates on mineral compositions in mid-ocean ridge basalts

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The effect of variations in cooling rates on mineral compositions in mid-ocean ridge basalts

Anders McCarthy, Cyril Chelle-Michou, Jonathan D. Blundy, Michael J. Dorais, Froukje M. van der Zwan, David W. Peate and EIMF (Edinburgh Ion Microprobe Facility)

Chemical geology, Vol.625, 121415

05/20/2023

DOI: 10.1016/j.chemgeo.2023.121415

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Abstract

We present textural and chemical analyses of minerals from a 150 m thick sequence of submarine mid-ocean ridge basalts from the South China Sea in order to showcase the effect of variations in magma cooling rates on mineral texture and mineral composition. Groundmass plagioclase and clinopyroxene show gradual changes in texture and composition as flows grade from slowly cooled, meter-thick massive flows to more rapidly cooled dm-thick pillow lobes with quenched glassy margins. The corresponding change in cooling-rate is estimated to vary from <1 to ≥100 °C/h. With increasing cooling rate, plagioclase forms elongated, sector-zoned swallow-tail crystals. Sector zoning is associated with increasing FeO (up to 1.5 wt%) and MgO (up to 0.6 wt%) abundances at near-constant anorthite (An), related to a two orders of magnitude increase in growth rate. Sr, Ba, Li and Ti abundances remain constant and appear unaffected by kinetic factors at such growth rates. With increasing cooling rate clinopyroxene becomes dendritic, and its composition is sensitive to changes in crystallization conditions. Increasing cooling rate (<1 to ≥100 °C/h) leads to increasing Al2O3 (average of 3.2 to 4.3 wt%), TiO2 (1.3 to 2.8 wt%) and Na2O (0.37 to 0.44 wt%) and a decrease in SiO2 (50.1 to 46.4 wt%) and Mg#Fetot, i.e., molar MgO/(MgO + FeOtot), from 71.3 to 53.1. Trace element abundances (Y, Zr, Ce, V, Sr) in clinopyroxene increase by up to an order of magnitude at cooling rates ≥100 °C/h and become more heterogeneous spatially. These results support experimental evidence that rapid crystal growth leads to significant departure of mineral compositions from equilibrium, in particular for clinopyroxene. Although plagioclase composition remains relatively insensitive to changes in growth conditions at the studied cooling rates, the sensitivity of clinopyroxene composition to growth rates imply that it should be used with caution as a tool to infer magmatic crystallization conditions.

Cooling-rate

Disequilibrium crystallization

MORB

Plagioclase

Rapid-crystal growth, clinopyroxene

Details

Title: Subtitle: The effect of variations in cooling rates on mineral compositions in mid-ocean ridge basalts
Creators: Anders McCarthy - Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
Cyril Chelle-Michou - Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
Jonathan D. Blundy - University of Oxford
Michael J. Dorais - Brigham Young University
Froukje M. van der Zwan - GEOMAR Helmholtz Centre for Ocean Research Kiel
David W. Peate - University of Iowa
EIMF (Edinburgh Ion Microprobe Facility)
Resource Type: Journal article
Publication Details: Chemical geology, Vol.625, 121415
DOI: 10.1016/j.chemgeo.2023.121415
ISSN: 0009-2541
eISSN: 1872-6836
Publisher: Elsevier B.V
Grant note: DOI: 10.13039/501100001711, name: Swiss National Science Foundation
Language: English
Date published: 05/20/2023
Academic Unit: Earth and Environmental Sciences; Honors Program
Record Identifier: 9984381024802771

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