Dating Clinopyroxene Phenocrysts in Submarine Basalts Using 40Ar/39Ar Geochronology

Dating submarine basalts using 40Ar/39Ar geochronology is often hindered by a lack of potassium-bearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to low-temperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for 40Ar/39Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002-0.4, suggesting that other K-bearing phases hosted within the clinopyroxene are likely degassing to yield the 40Ar/39Ar age information. There are three possible origins for the K and corresponding 40Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (>60% 39Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values >0.05. We conclude that clinopyroxene dating by the 40Ar/39Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents.