Compositional Variations with Depth in Icelandic Cores: Applications to Integrated Mars Remote Sensing Data

Introduction: The Martian crust is globally dominated by primary igneous minerals found in basalt (plagioclase, pyroxene and olivine) with limited degrees of surface alteration at high-latitudes [e.g. 1-4]. On local scales, a diversity of secondary hydrated mineral-phases (phyllosilicate, sulfate, opal, and carbonate) has been identified [e.g. 5-6]. Ice and transient water appear to have a regional influence on the production of secondary phases, whereas the effects hydrothermal alteration and prolonged high water/rock ratios are discovered in unique and locally diverse geological settings. The depth of alteration in the Martian crust is, however, poorly constrained. This is important for understanding both the timing and duration of aqueousrelated processes on Mars. For example, GRS measured chemistry in Acidalia Planitia reveals a basaltic crust (based on measured Si abundances) with little evidence of aqueous fractionation of elements [7-8]. The relatively constant K/Th ratio across Mars is not consistent with subaqueous or deep (centimeters to meters) subaerial aqueous weathering as K would fractionate from Th in both scenarios. However, TES and OMEGA observations of Acidalia Planitia are most consistent with basalt and alteration coatings of amorphous SiAl-Fe rich mineral-phases [9]. To better constrain the regional effects and depth of alteration at high-latitudes on Mars, we are examining compositional variations with depth in Icelandic basaltic cores. Our goal is to develop a consistent geologic model with integrated GRS chemical and TES and OMEGA mineralogical data sets. In this study, we report initial near-infrared spectral properties and XRF measured chemistry of Icelandic basaltic cores and investigate the extent of aqueous alteration at high-latitudes on Mars. Iceland: A young volcanic island in the North Atlantic, Iceland has an abundance of basaltic rocks which have been subjected to a variety of physical and chemical weathering conditions. The interior, specifically, in the rain shadow of the Vatnajökull icecap, includes basaltic deserts and terrain covered in extensive lava flows and tephra falls from the Krafla volcanic system and Askja caldera. Areas north and south of the icecap have been subjected to occasional jokulhaup activity, resulting in flood plain deposits, later subject to aeolian redistribution of materials. Basaltic materials in Iceland's interior appeal to a possible early model of the Martian environment and thus serve as a good terrestrial analog for our sampling depth questions. Field Approach: A total number of X core samples were collected across Iceland’s interior region using a specially modified lever and coring device. Core samples are 60 cm deep, which is analogous to GRS sampling depths. Surface rocks and soils were also collected for each sampling location. In the field, cores were initially classified according to common visual characteristics, based primarily on color and texture (ie – light, “rockier” tephra cores vs. dark, “fine” basaltic sandy cores). For this study, two largely basaltic cores were down-selected for chemical and spectral analyses. The samples include representatives of river plain/sand deposits from Jokulsa, north of the icecap, and Skaftafell, south of the icecap (post-glacial). Chemical Analyses: The Jokulsa and Skaftafell cores were initially scanned with a hand-held XRF gun to obtain Fe, Ca, and K measurements every 2 cm, with two 60-sec count intervals. Ground-truth points (with measurements made at UMass-Amherst via standard XRF methods) were then selected for particularly homogeneous sections within the cores (typically on the 2 cm scale) to assess the accuracy of the hand-held XRF scanner data. Figure 1 shows an image of the Jokulsa basaltic core with hand-held XRF scans of Fe, Ca, and K. Apart from the obvious tuff layer at ~ 45 cm, resulting in a decrease in Ca, there are no significant variations across Fe, Ca, and K for the Jokulsa core. Ground-truth XRF data from 10-12 and 32-34 cm (red boxes on image) are consistent with the handheld XRF data providing confidence in the relative homogeneity of the basaltic core. The average bulk SiO2 content for the Jokulsa core is 50.15 wt. %.