For granulometric, geochemical and mineralogical analyses, approximately 25 ml of each surface sample was frozen for 24 hours and subsequently lyophilized using a Christ BETA 1-8 LDplus (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany). The freeze-dried samples were homogenized and split into two aliquots. The other aliquot of the freeze-dried surface samples was ground to <63 µm with a Planetary Mill Pulverisette 5 (FRITSCH GmbH, Idar-Oberstein, Germany) and used for mineralogical and geochemical analyses.
For quantitative analyses of the inorganic element composition of the surface samples, including concentrations of selected major, minor and trace elements (Ti, K, Al, Mg, Ca, Fe, Cr and Mn), 0.5 g of dry and ground bulk sample material was digested using a near-total digestion protocol with HCl, nitric (HNO3), perchloric (HClO4) and hydrofluoric (HF) acids in heated and closed teflon vessels. Measurements were performed by means of inductively coupled plasma-mass spectroscopy (ICP-MS) at Activation Laboratories Ltd., Ancaster, ON, Canada.
Separate Si measurements were conducted by energy-dispersive X-ray fluorescence (ED-XRF) using a portable analyzer (NITON XL3t; Thermo Fisher Scientific, Waltham, MA, USA) at the University of Cologne, Germany. Triplicate measurements were performed on pellets of freeze-dried and ground sample aliquots, which were pressed into teflon rings under 12 bars, and subsequently covered with a 4 µm polypropylene film (X-ray film, TF-240-255, Premier Lab Supply, Port St. Lucie, FL, USA). Measurements were performed using a gold anode X-ray source (70 kV) and the 'mining-minerals-mode'. The secondary X-rays of element-specific photon energies were detected with a silicon drift detector and processed by a digital signal processor. Si concentrations (in ppm) were calculated from the element-specific fluorescence energies and compared with external and internal reference materials (STDS-4, BCR142R and BCR-CRM 277).
