Geochemical evidence for a chondritic impactor in altered impact glass from the Stac Fada Member impactite, NW Scotland

Abstract

The Stac Fada Member (Stoer Group) is a ∼1.2 Ga melt-rich impact breccia whose source crater and, therefore, proximity remains debated. We present a detailed in situ geochemical dataset for altered impact melt clasts within Stac Fada samples from Bay of Stoer and Second Coast. These altered impact melt clasts are now predominantly composed of clinochlore. Geothermometry of this clinochlore indicates formation temperatures within the ejecta blanket at 188-231°C, in agreement with previous estimates. A positive correlation between platinum group elements Ir, Pt, and Rh — apparently independent of Ni — indicates two geochemical sources for platinum group elements enrichment. We propose that these represent an Mg-Ni rich chondritic impactor and a mafic/ultramafic layered body within the target lithology. Spatial variation in Ni content suggests that geochemical data, in combination with field observations, may be instrumental in assessing the likely location of the impact crater.