Osmium and oxygen isotope constraints on magma-crust interactions and the transport of copper at the roots of arcs

• Published in: Chemical geology 2024-09, Vol.664, p.122301, Article 122301
• Main Authors: Tassara, Santiago, Ague, Jay J., Keller, Duncan S., Rooney, Alan D., Wostbrock, Jordan A.G., Axler, Jennifer A., Tardani, Daniele
• Format: Article
• Language: English
• Subjects: Copper-rich cumulates; Osmium isotopes; Oxygen isotopes; Sub-arc crust; Ultramafic rocks
• ISSN: 0009-2541
• DOI: 10.1016/j.chemgeo.2024.122301

The formation of copper-rich cumulates at the base of arc crusts has been proposed as a key process modulating the geochemical evolution of the continental crust and the genesis of giant ore deposits of copper. Despite the importance of these phenomena, the degree to which the lower crustal evolution of magmatic systems is influenced by open-system interaction and the assimilation of pre-existing crustal materials remains unclear. To tackle this issue, we provide direct isotopic constraints on the evolution of deep magmatic systems in arcs by measuring the osmium and oxygen isotope composition of hydrous copper-rich (∼730 μg·g−1 Cu) ultramafic cumulates formed at the base of the Acadian orogen (∼40 km deep) in the New England Appalachians (northeastern USA). The radiogenic 187Os/188Os initial ratios (ranging from 0.31 to 0.67) and the elevated δ18O values (8.97 ± 0.42 ‰ for orthopyroxene; 9.25 ± 0.26 ‰ for phlogopite) suggest a significant role of open-system magmatic differentiation, involving crustal assimilation, in the formation of these cumulates. Modeling of the 187Os/188Os and δ18O composition of the cumulates suggests that the observed isotopic compositions result from the initial evolution of parental magmas under sulfide-undersaturated conditions, followed by saturation after approximately 15% to 20% of assimilation and fractional crystallization progression. These results suggest that the assimilation of crustal material led to a drop in the magmatic system's fO2 (∼ΔFMQ < −1), triggering sulfide segregation and the formation of copper-rich cumulates. Our findings align with the hypothesis that magma-crust interactions can lead to the formation of lower crustal domains enriched with Cu, which may constitute a pre-stage in the formation of some porphyry copper deposits, particularly in collisional orogens. •Osmium and oxygen isotope compositions suggest significant crustal assimilation in forming copper-rich sub-arc cumulates.•Isotopic modeling indicates sulfide saturation during assimilation–fractional crystallization.•Crustal assimilation may be a necessary step for generating lower crustal domains enriched in copper.•Despite extensive assimilation, the cumulate line of descent is predominantly controlled by fractional crystallization.