New Developments in Incremental Heating Detrital ⁴⁰Ar/³⁹Ar Lithic (DARL) Geochronology using Icelandic River Sand

Abstract. Iceland records over fifteen million years of complex volcanism resulting from the intersection of mid-ocean ridge and mantle plume upwelling. The Iceland mantle plume has been active for at least 70 Ma, with surface expressions in Greenland, the North Atlantic, and Iceland. The Iceland hotspot may exhibit periods of increased volcanic output linked to pulses of upwelling within the plume. Understanding Iceland’s magmatic history and potential pulsation could provide key insights into dynamic topography driving changes in deep-water oceanic circulation, late Cenozoic climate change and mantle plume – mid-ocean ridge interaction. Detrital geochronology is a powerful tool for capturing the magmatic history of a region. However, Iceland's fine-grained extrusive volcanic lithologies lack the typical detrital mineral phases such as zircon, sanidine, hornblende, and rutile that current geochronology methods utilize. Here we present a new methodology for capturing the magmatic history of fine grained extrusive volcanic rocks using single grain detrital ⁴⁰Ar/³⁹Ar incremental heating geochronology. The DARL (or Detrital Argon Lithics) method thus far has consisted of ⁴⁰Ar/³⁹Ar total fusion analyses, which pose a problem in the case of Iceland, due to the nature of its young glassy lava flows commonly displaying subatmospheric ⁴⁰Ar/³⁶Ar isochron intercepts and low ⁴⁰Ar*. This work represents a ⁴⁰Ar/³⁹Ar incremental heating pilot study on 19 single grains of Icelandic river sand (1–3 mm), collected from five different catchments. Fifteen of the 19 basaltic grains produced concordant age experiments that ranged from 0.2 to 13.5 Ma and uncertainties (2σ) from 1 % to 86 % with the grains under 1 Ma having the largest uncertainties. Preliminary results show that basaltic grains with less alteration (and corresponding lower atmospheric argon concentration) yield more accurate age determinations, though altered basaltic grains can still produce statistically valid age determinations. Results presented here show the validity of the incremental heating DARL methodology for capturing the magmatic history of mafic terrains. The long analysis time required for incremental heating experiments makes it infeasible to produce the large number of ages required for a detrital study. For this reason, we propose combining the aspects of the total fusion and incremental heating DARL methodologies to acquire age data for the large N values needed for detrital studies while improving the accuracy of total fusion DARL analysis.