TY - JOUR

T1 - Copper Isotope Variations During Magmatic Migration in the Mantle

T2 - Insights From Mantle Pyroxenites in Balmuccia Peridotite Massif

AU - Zou, Zongqi

AU - Wang, Zaicong

AU - Li, Ming

AU - Becker, Harry

AU - Geng, Xianlei

AU - Hu, Zhaochu

AU - Lazarov, Marina

N1 - Funding Information: We thank Hongjin Huang, Yangtao, Zhu, Tao Luo, and Haihong Chen for their assistance in the lab. We appreciate Fang Huang and Jian Huang for the measurement of Cu isotopes at USTC, Hefei. This study was supported by National Science Foundation of China (41673027, 41722302), 111Project (BP0719022), and the MOST Special Fund from GPMR-CUG (MSFGPMR10). Many thanks go to R. Mathur and one anonymous reviewer for the constructive comments, and to Stephen Parman for editorial handling, which significantly improved this manuscript. The data for this paper are listed in Tables 1, 2, and S1–S4 and available at zenodo: https://doi.org/10.5281/zenodo.3471452.

PY - 2019/12/27

Y1 - 2019/12/27

N2 - The crust shows large variations in Cu isotopic composition (δ65Cu relative to NIST 976) resulting from redox reactions and other supergene processes. The significant range of δ65Cu in mantle peridotites thus could be ascribed to recycled crustal materials and/or oxidative mantle metasomatism. However, the influence of normal magmatic fractionation processes on δ65Cu variations remains poorly understood, and it is unclear whether the magmatic processes also lead to large δ65Cu variations in mantle rocks. To address the issue, we present bulk-rock δ65Cu of fresh mantle pyroxenites from the Balmuccia peridotite massif, Northern Italy. These pyroxenites formed by melt-peridotite reaction and mineral accumulation from MORB- to OIB-like, sulfide-saturated basic magmas. Mass balance calculations show that sulfide phases host >98 wt % of the Cu budget of bulk rocks (87−484 μg/g). The pyroxenites show significant variations of δ65Cu from −0.66‰ to 0.66‰, which cover the range known for peridotites, including metasomatized ones. Three subsamples from the same pyroxenite layer display >0.3‰ decrease in δ65Cu with progressive differentiation. Nevertheless, δ65Cu does not display correlations with indicators of magmatic differentiation (e.g., Mg# and Cu/Pd) for all pyroxenites in this study. This might be attributed to variable extents of magmatic sulfide segregation for different samples and/or varying δ65Cu of the pyroxenite parent magmas, which were also affected by reactive migration through peridotites. The combined processes can change δ65Cu of evolving magmas and reacted peridotites and lead to Cu isotopic heterogeneity in the mantle, without involvement of oxidative metasomatism or recycled crustal materials.

AB - The crust shows large variations in Cu isotopic composition (δ65Cu relative to NIST 976) resulting from redox reactions and other supergene processes. The significant range of δ65Cu in mantle peridotites thus could be ascribed to recycled crustal materials and/or oxidative mantle metasomatism. However, the influence of normal magmatic fractionation processes on δ65Cu variations remains poorly understood, and it is unclear whether the magmatic processes also lead to large δ65Cu variations in mantle rocks. To address the issue, we present bulk-rock δ65Cu of fresh mantle pyroxenites from the Balmuccia peridotite massif, Northern Italy. These pyroxenites formed by melt-peridotite reaction and mineral accumulation from MORB- to OIB-like, sulfide-saturated basic magmas. Mass balance calculations show that sulfide phases host >98 wt % of the Cu budget of bulk rocks (87−484 μg/g). The pyroxenites show significant variations of δ65Cu from −0.66‰ to 0.66‰, which cover the range known for peridotites, including metasomatized ones. Three subsamples from the same pyroxenite layer display >0.3‰ decrease in δ65Cu with progressive differentiation. Nevertheless, δ65Cu does not display correlations with indicators of magmatic differentiation (e.g., Mg# and Cu/Pd) for all pyroxenites in this study. This might be attributed to variable extents of magmatic sulfide segregation for different samples and/or varying δ65Cu of the pyroxenite parent magmas, which were also affected by reactive migration through peridotites. The combined processes can change δ65Cu of evolving magmas and reacted peridotites and lead to Cu isotopic heterogeneity in the mantle, without involvement of oxidative metasomatism or recycled crustal materials.

KW - Cu isotopes

KW - magmatic differentiation

KW - mantle heterogeneity

KW - pyroxenite

KW - recycling

KW - sulfide melt

UR - http://www.scopus.com/inward/record.url?scp=85075470583&partnerID=8YFLogxK

U2 - 10.1029/2019JB017990

DO - 10.1029/2019JB017990

M3 - Article

AN - SCOPUS:85075470583

VL - 124

SP - 11130

EP - 11149

JO - Journal of Geophysical Research: Solid Earth

JF - Journal of Geophysical Research: Solid Earth

SN - 2169-9313

IS - 11

ER -