The role of volatile exsolution and sub-solidus fluid/rock interactions in producing high 56Fe/54Fe ratios in siliceous igneous rocks

Highly differentiated igneous rocks can, in some cases, have 56Fe/54Fe ratios that are significantly higher than those of mafic- to intermediate-composition igneous rocks. Iron isotope compositions were obtained for bulk rock, magnetite, and Fe silicates from well-characterized suites of granitic and volcanic rocks that span a wide range in major- and trace-element contents. Sample suites studied include granitoids from Questa, N.M. (Latir volcanic field) and the Tuolumne Intrusive Series (Sierra Nevada batholith), and volcanic rocks from Coso, Katmai, Bishop Tuff, Grizzly Peak Tuff, Seguam Island, and Puyehue volcano. The rocks range from granodiorite to high-silica granite and basalt to high-silica rhyolite. The highest δ56Fe values (up to +0.31‰) are generally restricted to rocks that have high Rb (>100 ppm), Th (>∼15 ppm) and SiO2 (>70 wt.%) but low Fe (<2 wt.% total Fe as Fe2O3) contents. Magnetite separated from these rocks has high δ56Fe values, whereas Fe silicates have δ56Fe values close to zero. Although in principle crystal fractionation might explain the high δ56Fe values, trace-element ratios in high-δ56Fe igneous rocks indicate that crystal fractionation is an unlikely explanation. The highest δ56Fe values occur in volcanic and plutonic rocks that contain independent evidence for fluid exsolution, including sub-chondritic Zr/Hf ratios, suggesting that loss of a low-δ56Fe ferrous chloride fluid is the most likely explanation for the high δ56Fe values in the bulk rocks. Based on magnetite solubility in chloride solutions and predicted Fe isotope fractionations among Fe silicates, magnetite, and ferrous chloride fluids, the increase in δ56Fe values of bulk rocks may be explained by isotopic exchange between magnetite and FeCl20, which predicts an increase in the δ56Fe values of magnetite upon fluid exsolution. This model is consistent with the δ56Fe values measured in this study for bulk rocks, as well as magnetite and Fe silicates. Our results suggest that fluid exsolution from siliceous hydrous magmas, which sometimes produce porphyry-style Cu, Mo, or Cu-Au mineralization, may be traced using Fe isotopes.