Geochemical and mineralogical dataset of tephra from the 2021 Tajogaite eruption, La Palma, Canary Islands. Description of methods used for collection/generation of data: The analytical package 4LITHORES from Actlabs laboratories was used to determine the concentrations of major and trace elements in bulk tephra samples. It combines packages 4B (lithium metaborate/tetraborate fusion and ICP-OES analysis) and 4B2 (trace elements by ICP-MS). X-ray diffraction files were acquired at the analytical facilities of GEO3BCN of CSIC by using a diffractometer Bruker D8-A25 (acquisition with Cu Kα radiation, tube voltage, and current of 40 kV and 40 mA, respectively; scans from 4° to 60° in 2θ, with a step size of 0.035°, and an equivalent counting time of 192 s per step. Mineral identification and semi-quantification were carried out with the DIFFRAC.EVA software from Bruker. The leaching experiments of tephra were conducted using batch tests. In these tests, 1 gram of each ash sample was mixed with 10 millilitres of Milli-Q Plus ultrapure water (18.2 MΩ/cm) in 14x100 mm polypropylene test tubes and shaken for 4 hours at 20 rpm. Initial (pH₀, SC₀) and final (pHf, SCf) pH and specific conductivity (SC) were measured using a HANNA HI8424 pH Meter. Leachates were then filtered through 0.45 μm PVDF syringe filters and analyzed for anions (chloride, sulfate, fluoride, nitrate, nitrite) using ion chromatography (IC) on a Metrohm 850 Professional IC AnCat – MCS system. For major and trace element analysis, a portion of the leachate was acidified with 1% HNO₃, diluted to 100 ml, and analyzed by inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent 7700X instrument. All analyses were conducted at the Technical Research Services of the University of Alicante. The spreadsheets are in CSV format UT-F8 and delimited by commas.

Financial support was provided by Grant SD RD 1078/2021 LA PALMA (Monitorización, evaluación y seguimiento multidisciplinar de la erupción volcánica de La Palma - MESVOL) funded by Ministerio de Ciencia e Innovación of Spain to the University of Las Palmas de Gran Canaria, Grant PGC2018-101027-B-I00, funded by MCIN/AEI/10.13039/501100011033 and by ERDF 'A way of making Europe', Project PID2022-139990NB-I00 from Ministry of Science, Innovation and Universities (MCIU), and a pre-doctoral research fellowship to NGH (Grant FPU20/05157).

Filename 0: Tajogaite-chemistry_xrd-metadata.xml Short description of filename 0: Dataset metadata. Filename 1: 1-Tajogaite-tephra_sampling_data-IGSN.csv Short description of filename 1: Identification of samples. Filename 2: 2-Tajogaite-whole-rock_geochemistry.csv Short description of filename 2: Whole-rock geochemistry. Filename 3: 3-Tajogaite-XRD.csv Short description of filename 3: X-ray diffraction data. Filename 4: 4-Tajogaite-leaching.csv Short description of filename 4: Leachate chemistry of tephra.

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