PRACTICAL GUIDE TO IMPROVED MAGNETITE (U-TH)/HE GEOCHRONOMETRY - A CASE STUDY FROM THE COLUMBIA RIVER BASALT GROUP

Blackburn and others (2007) demonstrated the feasibility of magnetite (U-Th)/He (MHe) dating as an alternative method for obtaining precise and reliable absolute age constraints from mafic volcanic rocks. This study provides a practical guide to dating basaltic magnetite and potential pitfalls. Absolute eruption ages determined by groundmass and plagioclase K-Ar and ⁴⁰Ar/³⁹Ar geochronology have inherent difficulties related to excess ⁴⁰Ar and/or recoil, lack of suitable mineral phases such as sanidine and zircon, and susceptibility to alteration. However, not all basalt samples are suitable for MHe dating due to a variety of reasons ranging from magnetite texture, grain size, and other complications. In an effort to refine MHe dating and further develop the practical methodology, the mid-Miocene Columbia River Basalt Group (CRBG) was selected as a case study given the vast amount of existing petrologic and geologic data. In light of the ubiquitous occurrence of magnetite in CRBG, the procedures described by Blackburn and others (2007) were followed to determine eruption age for the aphanitic and/or altered CRBG by MHe dating. However, detailed petrographic examination of CRBG samples showed that magnetite occurs in more or less conducive textures, grain sizes, and shapes for MHe dating, reinforcing the clear need for detailed petrographic analyses (optical and BSE) to ensure suitability for magnetite He dating. After petrographic screening, isotopic analyses were conducted only on solid basaltic magnetite >100 microns and possessing a uniform internal structure free of skeletal growth and no silicate matrix intergrowth. In addition, we evaluated the impact of different methodologies with respect to He implantation from the matrix, as matrix uranium concentrations (~2 ppm) on average are one order of magnitude greater than that of magnetite (~100 ppb). Methodological experiments were conducted to compare ages from air-abraded grains (Blackburn et al., 2007) to unabraded, internal fragments from larger magnetite crystals. In conclusion, this study stresses the critical importance of careful petrographic selection of suitable magnetite prior to isotopic analyses in order for the magnetite (U-Th)/He technique to serve as a reliable alternative method for dating basalts.