Os Isotopic Systematics Accompanying the Interaction Between Pelitic Xenoliths and Mafic Magma of the Duluth Complex, Midcontinent Rift System

Os isotope ratios of massive sulfides from the Babbitt Ni-Cu deposit of the Midcontinent Rift-related Duluth Complex (DC) are strongly radiogenic, with γ_Os values at 1.1 Ga ranging from 1691 to 3544. The isotope ratios are attributed to the assimilation of sulfidic rocks from the ~1.85 Ga pelitic Virginia Formation (VF). However, sedimentary sulfides in VF xenoliths show evidence of extensive uptake of Os with a chondritic signature at 1.1 Ga. To further investigate this relationship we have analyzed a xenolith and surrounding troctolite from the Partridge River Intrusion.

Pyrrhotite from the xenolith shows well-defined zoning in ¹⁸⁷Os/¹⁸⁸Os. The ~30m thick xenolith is characterized by the high-grade assemblage cordierite, biotite and orthopyroxene. Pyrrhotite from the core of the xenolith plots along a 1.85 Ga reference isochron, whereas samples from the margins show progressively more anomalous Os isotopic ratios, with samples in contact with the surrounding troctolite falling only slightly above a chondritic 1.1 Ga reference isochron. Surprisingly, troctolite in contact with the xenolith also fall along the 1.85 Ga reference isochron, suggesting that the magmas were strongly contaminated by the VF.

Our new data indicate xenolith cores that have reached temperatures as high as 800°C may resist Os isotope “resetting”. Interaction with mafic magma characterized by a chondritic Os isotope ratio occurred only near the margins of the xenolith, with the addition of Os resulting in anomalous isotopic ratios displaced to lower values. The highly anomalous character of the troctolite that now encloses the xenolith suggests that the magma from which the troctolite crystallized was not the magma from which the chondritic Os was attained. The variability in Os isotope compositions of VF pyrrhotite and DC troctolite attests to the dynamic nature of magma-country rock interaction in the rift setting, and the emplacement of variably contaminated pulses of mafic magma.