ALTERATION OF THE COLUMBIA RIVER BASALTS: IMPLICATIONS FOR 40AR/39AR GEOCHRONOLOGY

A recent paper by Kasbohm and Schoene (2018) shows that 95% of the Columbia River Basalts (CRBs) erupted between 16.7 and 15.9 Ma (via U-Pb TIMS). This directly contradicts decades of ⁴⁰Ar/³⁹Ar studies, which collectively show that the CRBs did not reach 95% of its final erupted volume until 15.0 Ma (Barry et al. 2013). The discrepancy is well outside the nominal uncertainties of the U-Pb TIMS and ⁴⁰Ar/³⁹Ar methods for rocks of the age and composition in question.

Here we use a combined rock-magnetic and petrological approach to show that unrecognized alteration of the CRBs is common, and that this may explain erroneous ⁴⁰Ar/³⁹Ar ages. This study uses thermomagnetic susceptibility curves to detect alteration of basalt. This novel approach is not affected by magmatic history, and provides an independent and quantitative constraint on the degree of alteration.

We present results from 100+ samples taken from throughout the CRBs. All samples were taken from the massive interior of flows, and visible alteration was avoided at all times.

Our results suggest two main alteration mechanisms are at work. In some samples, presence of maghemite suggests high-temperature oxidation of basalts. In other samples, the presence of goethite and lepidocrocite suggest long-term alteration via groundwater. In either case, alteration products can be present even in ‘fresh’ samples from the massive interiors of flows.

Overall, our results show that aqueous alteration of the CRBs is common. Such alteration may explain erroneous ⁴⁰Ar/³⁹Ar ages, as K and Ar are more easily mobilized than Fe. This alteration is not easily identified in the field, and is heterogeneous at the outcrop scale. Finally, we present a simple test for such alteration of basalt that can be completed quickly, routinely, and for little cost.