USING SCAPOLITE GROUP MINERALS TO TRACE FLUID SOURCES AND EVOLUTION IN THE MESOPROTEROZOIC WALLACE FM, NORTHWESTERN IDAHO, USA

Chlorine-rich scapolite (Na₄(Al₃Si₉)O₂₄Cl), a mineral commonly formed in salty-metamorphic rocks, can sequester halogens (Cl, Br, I), CO₂, and SO₄ that were present in its protolith host and/or from fluids that interacted with the host unit. Halogen ratios in scapolite can be used for uncovering the source of the saline fluids (evaporitic vs. residual bittern brine vs. magmatic). A detailed understanding of the behavior of the above fugitive elements coupled with the identification of saline crustal fluid sources is critical to interpret mass transfer during metamorphism and the global halogen cycle.

To learn more about how halogens behave during metamorphism, we focus on a prograde P-T sequence of the Wallace Fm within the Northwestern Idaho Mesoproterozoic Belt Supergroup. Some parts of the original salt-rich layers of the Wallace Fm. underwent multiple (regional) metamorphic events resulting in distinct halogen-rich scapolite bearing layers. Combined EPMA and LAICPMS analyses have been used to reconstruct the fluid evolution in “salty” metamorphic units. Preliminary results show decreasing Cl (~2.50 to ~0.97 wt%) and increasing CO₂ (~1.94 to ~3.18 wt%) in scapolite with increasing metamorphic grade. The SO₃ concentration of scapolite is low (<0.1 wt.%) and no correlation with the metamorphic grade was observed. High resolution quantitative chlorine maps collected via EPMA also show distinctive chemical zoning of scapolite porphyroblasts (2.75x2.75 mm) in a fine-grained pelitic matrix from the lowest grade biotite zones, indicating the presence of fluids with different salinities during scapolite crystallization (Z1: ~2.5 wt%; Z2: ~2.0 wt%). This detailed petrographic study in combination with micro-analyses will provide insights into the distribution of halogens within metamorphic rocks, the source of saline fluids in the Wallace Fm, and will help to understand whether or not halogens fractionate during prograde metamorphism.