DOI: 10.5880/gfz.3.1.2020.001
title: Mineralogy and Element geochemistry of volcanic rocks and sediments from the central Rift in Kenya

description: The Central Rift in Kenya (CRK) comprises the lakes Naivasha, Elementaita and Nakuru and the Longonot, Eburru and Menengai volcanos. The alkaline magmas, produced by the volcanoes within the CRK, lead to solid rocks likes trachytes, phonolites, and fewer basalts and accompanied soft rocks like ashes, tuffs, pumices and ignimbrites (e.g. Macdonald et al., 1987; Macdonald, 2014). Lacustrine sediments and beds of diatoms are remnants of former lake level variations caused by climate variability and topographic changes (e.g. Stoof-Leichsenring et al., 2011). The samples have been taken within the frame of a VW-Foundation funded project that tries to detect, map and monitor groundwater pollution from anthropogenic and natural sources. For a previous VW-Foundation funded project (grant 85465), also the groundwater fluoride enrichment in the CRK have been studied (Olaka et al., 2016). This data report presents the metadata, inclusive GPS data from 52 solid volcanic rock and sediment samples taken during a field excursion during May 2017. A geological map with all data locations is included in this report. After sample preparation, we performed X-ray diffraction (XRD) analyses to get the mineral content and X-ray fluorescence (XRF) as well as Ion-Chromatographic (IC) analyses to get the elemental concentration of those samples. The results are given together with analytical limitations and few additional information despite a graphic visualization of the XRD-data. The data are presented in tab-delimited text format and described in the dataset description.
description: Sampling method The samples were taken by hand with a geologic hammer or plastic shovel and packet separately in plastic bags. At GFZ Potsdam, samples were dried and pulverized using a jaw crusher and disc mill. Sieving was done to receive grain size fractions &lt;62 µm. Analytical procedures To achieve quantitative XRD data, approx. 4 g from all samples were flushed in 20 ml cyclohexane and milled for 5 min in a micronizing mill to receive a grain size of ≤ 10 µm. The sample container was flushed 2 times afterwards with cyclohexane and the samples dried for 24h in the fume hood. The samples were sieved again before carefully loaded into sample holders preventing any morphology on the sample surface. We used a Cu (K-alpha) X-ray tube within a PANalytical Empyrean Diffractometer to get 4.6-85°2Theta values. Main element determinations took place on glass tablets. The ground samples were mixed in a ration of 1:5 or 1:17 with a flux media, melted in platinum crucibles at approx. 1050°C using fan burners and poured out in a platinum dish to glass tablets. The loss on ignition of the samples (LOI) was determined parallel. XRF analyses were performed using a PANanlytical Axios. If a signal was received for Cl and F, IC analyses have been carried out. For this, 0.1 g of grounded rock was mixed with 0.3 g tungsten oxide and digested via pyro hydrolysis. The distilled sample is measured by IC. Hydrogen and Carbon from grounded rock samples were determined using the element analyzers EltraCS 2000 and Elementar vario ELIII. Data processing XRD data were processed qualitatively by the program EVA and quantitatively using Rietveld calculations by BGMN-Profex (Döbelin, N., & Kleeberg, R 2015). The detection limit (DL) is at about 1%. Only crystallized material can be detected via XRD, which is in turn normalized to 100%. The amount of glass in each sample is not given. XRF data were not processed. Major elements are given in wt% whereas trace elements are given in ppm. The DL is at 10 ppm. For sulfur as SO3, lead and arsenic just XRF signals are given which mean that those elements are present but have not been measured quantitatively due to missing proper calibrations. Those values are semi-quantitative at most. All samples show uranium concentrations &lt;30 ppm and La, Ce and Nd concentrations from 100-200 ppm. Some XRF results can be directly linked to XRD results. Those are color coded and explained in the table.