The Chromite Crisis in the Evolution of Continental Magmas and the Initial High δ²⁶Mg Reservoir

Xiao, Yan1; Yuan, Meng1,2; Su, Ben-Xun2,3; Chen, Chen4; Bai, Yang5; Ke, Shan6; Sun, Yang7; Robinson, Paul T.1

2023-04-01

10.1093/petrology/egad019

JOURNAL OF PETROLOGY   Impact Factor & Quartile

0022-3530

1460-2415

Fractional crystallization of Fe-Ti oxides can induce detectable Mg isotopic changes during late-stage basalt differentiation. Because chromite and olivine are early crystallizing phases during basaltic melt differentiation, the effect of chromite crystallization on the fractionation of Mg isotopes during early-stage basalt differentiation is still poorly understood. Here, we examine the possibility of chromite induced Mg isotope fractionation with a Mg isotopic study of chromite-olivine pairs in dunites and chromitites collected from major types of basaltic intrusive rock suites formed by fractional crystallization in different tectonic settings. The chromite delta Mg-26 (= [(Mg-26/Mg-24)(sample) / (Mg-26/Mg-24)(DSM3)-1] x 1000) values range from -0.19 parts per thousand to 0.30 parts per thousand in the Luobusa ophiolite, -0.09 parts per thousand to 0.78 parts per thousand in the Kizildag ophiolite, -0.04 to 0.42 parts per thousand in the Gaositai Alaskan-type complex, similar to those previously reported from the Stillwater layered intrusion (-0.05 to 0.84 parts per thousand; ). They are significantly higher than those of coexisting olivine (delta Mg-26 = -0.48 to -0.10 parts per thousand). The Delta Mg-26(Chr-Ol) (= delta Mg-26(Chr) - delta Mg-26(Ol)) values in the rock suites investigated here fall largely between equilibrium fractionation lines of spinel-olivine pairs and magnesioferrite-olivine pairs, indicating equilibrium Mg isotopic fractionation. Furthermore, the Delta Mg-26(Chr-Ol) values increase with decreasing Cr content of chromite in the dunites and chromitites, showing that high Mg-26 has a greater affinity to Al-rich chromite than Cr-rich chromite. Fractional crystallization of such isotopically high chromite is expected to progressively lower the Mg isotope values of the in the remaining magma. Furthermore, continental basaltic magmas typically experience early crystallization of olivine and Al-rich chromite. Their delta Mg-26 values correlate positively with MgO (FeO, Cr,) and CaO/Al2O3 ratios and negatively with total alkali contents (Na2O + K2O). This indicates that detectable Mg isotopic fractionation occurred in intra-continental basalt magmas, probably by fractional crystallization of olivine and chromite. The observed low-delta Mg-26 intra-continental basalts can be accurately modeled by olivine + chromite fractionation with fractionation factors (Delta Mg-26(Chr-Melt)) of 0.20 parts per thousand, 0.60 parts per thousand, and 1.18 parts per thousand as observed in the chromitites investigated during this study. Therefore, the early-stage basaltic melt differentiation involving separation of olivine and chromite may induce resolvable Mg isotopic fractionation, and the delta Mg-26 values of continental basalts should be used with caution in petrogenetic studies.

intra-continental magmatism isotope fractionation chromite segregation fractional crystallization magnesium isotopes

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English

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Second Tibetan Plateau Scientific Expedition and Research Program (STEP)[2019QZKK0801] ; National Natural Science Foundation of China["41973012","91755205","41772055"] ; Ministry of Science and Technology National Key R&D Program of China[2022YFC2903501]

Geochemistry & Geophysics

Geochemistry & Geophysics

WOS:001008020400001

OXFORD UNIV PRESS

GEOSCIENCES

Web of Science

1.Chinese Acad Sci, Inst Geol & Geophys, State Key Lab Lithospher Evolut, Beijing 100029, Peoples R China
2.Univ Chinese Acad Sci, Beijing 100049, Peoples R China
3.Chinese Acad Sci, Inst Geol & Geophys, Key Lab Mineral Resources, Beijing 100029, Peoples R China
4.Chinese Acad Sci, Guangzhou Inst Geochem, Key Lab Mineral & Metallogeny, Guangzhou 510640, Peoples R China
5.Taiyuan Univ Technol, Coll Min Engn, Taiyuan 030024, Peoples R China
6.China Univ Geosci, State Key Lab Geol Proc & Mineral Resources, Beijing 100083, Peoples R China
7.Southern Univ Sci & Technol, Dept Earth & Space Sci, Shenzhen 518055, Guangdong, Peoples R China

Xiao, Yan,Yuan, Meng,Su, Ben-Xun,et al. The Chromite Crisis in the Evolution of Continental Magmas and the Initial High δ²⁶Mg Reservoir[J]. JOURNAL OF PETROLOGY,2023,64(4).

Xiao, Yan.,Yuan, Meng.,Su, Ben-Xun.,Chen, Chen.,Bai, Yang.,...&Robinson, Paul T..(2023).The Chromite Crisis in the Evolution of Continental Magmas and the Initial High δ²⁶Mg Reservoir.JOURNAL OF PETROLOGY,64(4).

Xiao, Yan,et al."The Chromite Crisis in the Evolution of Continental Magmas and the Initial High δ²⁶Mg Reservoir".JOURNAL OF PETROLOGY 64.4(2023).