Differentiating Basaltic Eruption Style with X-ray Diffraction

Introduction: Investigations of Mars geology have long been focused on unveiling the planet’s volcanic history and potential habitability. Of particular interest is the apparent transition from explosive to effusive volcanism over time, and the sources of volatiles in these earlier eruptions—whether within the magma, or significant quantities of external water. Identifying the eruption style of a Martian volcanic deposit may provide valuable information about the conditions of the magma and mantle sources, or the possibility of a water-rich environment at the time of volcanism. On Earth, explosive basaltic eruptions occur in two major styles: magmatic and phreatomagmatic. Magmatic eruptions (i.e. plinian) are thought to occur in part do to high magmatic volatile content (primarily water and carbon dioxide), while phreatomagmatic eruptions occur when magma interacts with surface water or groundwater. Past efforts to distinguish between these two styles have used characteristics of the pyroclasts, such as grain shape, size and vesicularity. However, these characteristics can often overlap between styles and may change over time with alteration. An alternative method, focusing on groundmass crystallinity, may be of more practical use by current and future Mars missions. We propose that evidence of eruption style may be preserved in the groundmass crystallization of volcanic ejecta. Occurring in varying degrees between basalts (and even within single pyroclasts), groundmass crystallization often shows a strong correlation to posteruption cooling rate. Pillow lavas, for example, with their glassy rinds and crystalline interior, provide evidence that initially glassy groundmass, preserved on the outside via rapid quenching, continues to crystallize in the slower-cooling interior. Prior studies of basaltic pumice clasts reveal increasing groundmass crystallinity from exterior to interior, suggesting that crystallization continues within the groundmass after individual clasts are ejected [1]. We further investigate whether variance in crystallinity may be connected to different cooling rates between eruption styles, focusing in particular on the rapid water-quenching of phreatomagmatic eruptions vs. the slower air-cooling of magmatic eruptions. Methods: Using X-ray diffraction (XRD), a technology available to the current Mars Science Laboratory, the first step of this project aims to establish a methodology for determining eruption style via crystallinity. In order to quantify crystallinity, this method follows the procedure described by Rowe et al. (2012) of using the different X-ray diffraction patterns produced by crystalline and amorphous materials [2]. A material like basalt, with mixed crystalline and amorphous content, creates a diffraction pattern with individual high intensity, sharp peaks (produced by crystalline material) superimposed on one low, broad peak (representing the amorphous component). With a linear background applied to eliminate excess background noise, the integrated area of the pattern represents the total crystalline plus amorphous material. Adding a curvilinear background which follows the broad amorphous peak shape, the integrated area above the background then represents the peaks of the crystalline material alone. Dividing the “crystalline” area by the “total” area provides a relative crystallinity estimate for the sample.