NEW DETRITAL METHODOLOGY FOR MAFIC TERRAINS: SINGLE BASALT GRAIN INCREMENTAL 40AR/39AR AGE DETERMINATIONS

Single-grain detrital geochronology is a powerful tool for assessing the first-order temporal evolution of a region. However, current methodologies tend to focus on mineral phases such as zircon, sanidine, hornblende, and rutile, that in ocean island environments like Iceland are rare or absent, and fail to capture the regional temporal magmatic history. Here we present preliminary results using ⁴⁰Ar/³⁹Ar incremental heating geochronology on single basaltic grains recovered from river catchments in Iceland. To test the viability of the method we selected 19 1-2mm sized grain samples from 5 catchments in Iceland. These river samples erode tholeiitic to transitional/alkaline volcanic rocks with a predicted age range of 100 ka to 15 Ma.

Samples were broken into three distinct preparation groups, one set cleaned through sonication in acetone and two groups with variable acid leaching. Samples were heated using a vacuum furnace and gas was analyzed using a NGX noble gas mass spectrometer with ATONA amplifiers housed within the Nevada Isotope Geochronology Laboratory. Experiments consisted of 19 to 23 individual heating steps. In total 15 of the 19 incremental heating experiments produced concordant age plateaus with ³⁹Ar percent varying from 55 – 100% and probability of fit factors >5%. Ten of the heating experiments had tightly concordant subatmospheric inverse isochron intercepts (typically ⁴⁰Ar/³⁶Ar_int of 293-295; relative to a ⁴⁰Ar/³⁶Ar_Atmo of 298.56) consistent with their glassy composition. However, irradiation recoil effects appear to be minimal or non-existent. Uncertainties (2σ) ranged from 1% to 7% for lava flows older than 1 Ma (n=7) and 9% to 86% for flows younger than 1 Ma (n=8). The furnace blank to sample ratio was the primary control on the uncertainties and using a low blank laser system would improve the error considerably. We found that less altered grains with abundant glass yielded more precise age data due to the higher ³⁹Ar_K and ⁴⁰Ar* signals. The initial results here are very promising and show ⁴⁰Ar/³⁹Ar incremental heating experiments on single grains is a viable detrital geochronology method in mafic terrains. Future work will focus on testing and refining the method by experimenting with heating through a diode laser to further increase efficiency.