CRYSTAL MATH: USING CALORIMETRY TO DETERMINE CRYSTALLINITY IN LAVAS FROM THE KILAUEA 2018 LOWER EAST RIFT ZONE ERUPTION

Rocks produced by diverse processes, from condensation in space to impacts on planetary surfaces to volcanism, can contain both crystals and amorphous material. Crystallinity provides information on the thermal history of the sample, and is especially important in characterizing volcanic rocks and pyroclasts because lava rheology is profoundly influenced by the crystal content. Crystallinity is typically quantified via microscopy, using polarized light or high voltage electrons. However, many samples present visibly ambiguous textures such as intimate intergrowth of crystal phases, and/or crystal sizes extending down to the nanometer scale. Here we apply calorimetric methods involving heat capacity and enthalpy to assess the crystallinity of a suite of volcanic samples. We tested three different approaches, using differential scanning calorimetry (DSC), on 30-40 mg aliquots of powdered lavas from the 2018 Kilauea Lower East Rift Zone. The first approach involves determining the magnitude of the increase in heat capacity (C_P) at the glass transition (T_g). The second approach is based on the enthalpy of fusion, and the third uses enthalpy differences calculated from heat capacities measured from below the glass transition to above the liquidus. Preliminary results for lavas with microscopically determined crystallinities ranging from 11 to 98% indicate that crystallinity based on calorimetric data agrees with that obtained using microscopy and petrographic analysis. Uncertainties in total crystallinity are typically ±3% for the configurational heat capacity approach, which is preferred due to its simplicity. Application to three samples with an opaque mesostasis allowed 20-35% glass to be detected and quantified, even though it could not be quantified petrographically, illustrating the utility of the approach. Uncertainties in total crystallinity are typically lower for calorimetric methods than petrographic methods, partly due to the ability to detect nanoscale ordering or lack thereof, and in part due to sample homogenization by powdering. Conversely, petrographic methods have an advantage in that they preserve textural information such as crystal size and shape distribution. We conclude that the two techniques are complementary and provide a powerful combined approach.