Lightning Strike Glasses as an Analog for Impact Glasses:

Introduction: The process of lightning striking rock or sand is chemically very similar to impact events, especially micrometeorite impacts. Both processes cause a rapid rise in temperature followed by rapid isentropic cooling [1]. These processes result in glassy products such as fulgurites, tektites, and lunar regolith agglutinates that are often chemically reduced from their starting materials [2,3,4]. Because iron has more than one oxidation state, it is a good indicator of reduction (Fe→Fe or Fe). We chose to study fulgurites because of their abundance and the relative ease of obtaining the premelted “country rock” materials for comparison. We present the results of a Fe Mössbauer spectroscopy study of eight fulgurites and trinitite (Trinity bomb test glass) as well as their unmelted starting materials. Samples: Three samples were obtained from the Mineralogical Research Company: a basalt talus fulgurite from Cline Butte, OR; a sandstone fulgurite from the Cacapon River District, WV; and trinitite glass from White Sands Missile Range, NM. Sand samples from the Trinity bomb test site were provided by the White Sands Missile Range Public Affairs Office. Bruce and Miranda Jarnot generously provided a sand and basalt gravel fulgurite from Farmington, CT [5]. Four samples were obtained from the Smithsonian Institution: a sand and gravel fulgurite from Black Rock, UT; a rock fulgurite from Algeria; a rock fulgurite from Mt. Ararat, Turkey; and a sand fulgurite from Starke, FL. A. Sheffer collected the sand fulgurite from Monahans Sandhills, TX. Method: The fulgurite glasses were initially studied using a Cameca SX50 electron microprobe with a voltage of 15 kV and a current of 20 nA to obtain backscattered electron images and point analyses. Room temperature Mössbauer spectra were acquired to determine Fe, Fe and Fe contents. Samples were prepared by gentle crushing under acetone (to avoid oxidation), and then by handpicking out pure glassy pieces. Roughly 40-300 mg of each sample, depending on Fe content, were powdered and mounted to obtain thin absorbers. A source of 100-30 mCi Co in Rh was used on a WEB Research Co. model W100 spectrometer. Run times ranged from 1-6 days. Results were calibrated against a-Fe foil of 6 μm thickness and 99% purity. Fitting methods employed quadrupole splitting distributions (QSDs) as described in [6]. Errors on %Fe are roughly 3-5% absolute for samples with low Fe contents and 1-3% for higher Fe samples. Because there are no crystal chemical constraints imposed on Fe site occupancy or valence state in a glass, it must be noted that the glass (fulgurite) fits are especially highly non-unique and modeldependent. Results: Microprobe analyses show a range of total Fe content in the glasses (Table 1). The fulgurites that have sand as the primary starting material are predictably low in iron content (0.070.18 wt. % Fe). The other samples have higher Fe contents reflecting the contribution from their country rock (basalt in the case of the Farmington, CT, Mt. Ararat, Algeria, and Cline Butte, OR, fulgurites).