Variability of magmatic and cosmogenic 3He in Ethiopian river sands of detrital pyroxenes: Impact on denudation rate determinations

Abstract

In-situ cosmogenic 3He is a robust tool for determining denudation rates or exposure ages of lavas bearing mafic phenocrysts. However, analyses are often complicated by the presence of several helium sources. In particular, in old magmatic rocks with high radiogenic 4He contents, discriminating cosmogenic 3He from magmatic 3He is not straightforward since these varieties may vary largely between aliquots.We sampled sands from the Tekeze and Mile rivers, both draining the basaltic Ethiopian highlands, an area where erosion patterns are intimately linked to the development of the Western Afar margin and to heterogeneous monsoon precipitation. From each river we analyzed ~15 aliquots of pyroxenes having variable grain sizes (0.3mm up to >1mm). The total 3He is both higher and more scattered in the bigger grains. Crushing of these largest grains and subsequent melting of the powder tends to produce more homogeneous 3He values, suggesting that magmatic 3He hosted in inclusions is responsible for most of the inter-aliquot variability. We also performed a Monte Carlo simulation based on a numerical denudation model of the two watersheds. The simulation confirms that cosmogenic 3He variability cannot be responsible for the observed scatter since the cosmogenic 3He variability is averaged away and unobservable in aliquots of ~200 grains. A compilation of previously published data also indicates that magmatic helium can be significantly variable, even between pre-crushed aliquots. Hence, magmatic helium, unlike cosmogenic 3He, is highly variable, even in the case of aliquots of hundreds of grains. We suggest this is due to a strong nugget effect, possibly due to large fluid (or melt)-inclusions contained in phenocrysts.In addition, the fact that small and big grains have comparable radiogenic 4He concentrations suggests that grain fragmentation during river transport is responsible for the lower magmatic helium content of the smallest grains.Therefore, one should preferably use small grain (0.2–0.5mm) granulometry for in-situ cosmogenic 3He analysis in mafic phenocrysts.Using the measured cosmogenic 3He, we calculate basin-averaged denudation rates of 70±20 and 57±5mmkyr−1, for the Mile and for the Tekeze river, respectively. These values are coherent with long-term denudation rates previously proposed from low-temperature thermochronology.