40Ar/39Ar age variations among cogenetic feldspars from the Benson Mines, New York

Abstract

The alkali feldspar group constitutes one the most common mineral groups of Earth’s crust. The 40Ar/39Ar method for dating potassium feldspars has proven to be a powerful analytical tool for evaluating low-temperature thermochronologic histories. Conventionally, for a vast number of studies, 40Ar/39Ar ages are determined for bulk concentrates of alkali feldspar of a particular grain size, separated and prepared following crushing of the host rock. In early studies of orthoclase from the Benson Mines in the northwestern Adirondack Highlands, Foland (1974) concluded that argon diffusion occurred over the physical scale of orthoclase crystals, a result reaffirmed by more recent studies of Foland & Xu (1990) and Cassata & Renne (2013). If the diffusion length scale is defined by physical grain size, the crushing and grain size reduction requisite for the analysis of bulk feldspar samples can sensibly be expected to alter and misrepresent the natural distribution of 40Ar ⃰. The primary objective of the present study is to obtain 40Ar/39Ar ages of cogenetic orthoclase crystals for different lithologies of the Benson Mines. Argon analyses were accomplished via the incremental heating of mineral grains with either a CO2 laser, or a diode laser in conjunction with a thermocouple to control temperature. To further assess the impact of grain size and other physical characteristics on resultant 40Ar/39Ar ages, this study utilizes the analysis of single crystals of potassium-bearing mineral phases from lithologies recollected at the Benson Mines. Lithologies sampled from the Benson Mines include magnetite-rich orthoclase-sillimanite gneiss, magnetite-rich orthoclase-garnet gneiss, biotite-hornblende syenite, and pegmatite. In addition to orthoclase from various samples, single crystals of hornblende and biotite were also analyzed, and yield plateau ages of ca. 970 Ma and ca. 900 Ma, respectively. These results are consistent with a cooling history (≤ 1oC/m.y.) through the retention temperatures of various phases following late stages of the Mesoproterozoic Grenville orogeny. Argon release spectra produced from step-heating single crystals of orthoclase in the present study reveal significant discordance between cogenetic crystals of individual samples and also systematic differences in age between feldspar populations of different samples. Notable variation is also observed at the scale of individual grains between initial and final heating steps. Release spectra for single crystals of orthoclase from the sillimanite gneisses yield the oldest 40Ar/39Ar ages observed and generally are less discordant between crystals, considering all of the lithologies analyzed, with total gas ages ranging from 875 to 850 Ma. A highly perthitic feldspar from a hornblende syenite yields much younger ages ranging from ca. 750 to 670 Ma. In contrast, K-feldspars from post-metamorphic pegmatites yield the most discordant spectra for individual crystals, with ages ranging from ca. 800 to 500 Ma and total gas ages ranging from ca. 765 to 677 Ma. Differences in the age distributions observed for feldspars of a given rock (more or less discordance, or differences in absolute age) are consistent with analysis of fragments differing in size and shape that present variable representations of their natural grain-scale diffusion geometries. The younger and more discordant ages for the feldspars from the syenite and pegmatite are interpreted to result from a higher local water activity during cooling than was the case in the gneisses. Results of the present study expand the earlier work of Foland and others for the Benson Mines, and emphasize the importance of analytical strategies to recover natural diffusion gradients in radiogenic 40Ar* concentrations at the scale of individual feldspar grains.