DOI: 10.5281/zenodo.14196687
title: Supplementary data for the paper "Erratic behavior of platinum-group elements along an olivine - Cr-spinel cotectic in sulfide-undersaturated basaltic magma"

description: This dataset comprises supplementary data for the paper "Erratic behavior of platinum-group elements along an olivine - Cr-spinel cotectic in sulfide-undersaturated basaltic magma". Whole-rock geochemical data is also submitted to PetDB and currently pending approval.

Supplementary Information – a detailed description of analytical techniques employed in the study.

Figure S1. High-resolution back-scattered electron SEM panoramic photographs of polished thin sections of picrites 21AK9c, 21AK17 and basalt 21AK61a used in this study. A colored inset at (A) is the EDS element map for the particular site of the thin section: note that Cr-spinel is only included within or intergrown with olivine, while in groundmass oxides are represented by magnetite. Abbreviations: Ol – olivine, Cpx – clinopyroxene, Srp – serpentine, Chr – Cr-spinel, Grnd – groundmass, Chl – chlorite, Mag- magnetite.

Figure S2. Chemical variation plots with concentrations of oxides and main chemical ratios of Cr-spinel from picrites 21AK9c and 21AK17. Mg# = Mg/(Mg+Fe) mole %, Cr# = Cr/(Cr+Al) mole %.

Figure S3. Chemical variation plots with PGE concentrations in the studied bulk rocks plotted versus Cr concentrations.

Figure S4. Chemical variation plots with PGE concentrations in Cr-spinel from 21AK9c picrite plotted against major component parameters of Cr-spinel. Spearman correlation coefficients and p-values for the Student’s significance level test are given for each pair.

Figure S5. Chemical variation plots with PGE concentrations in Cr-spinel from 21AK9c picrite plotted against each other. Spearman correlation coefficients and p-values for the Student’s significance level test are given for each pair.

Figure S6. Field-emission SEM photomicrographs of representative PGM inclusions in Cr-spinel from 21AK9c picrite (A-S) and 21AK17 picrite (T, U). Bright phases are the Pt-Fe alloys unless specifically mentioned, grey and black matrix – Cr-spinel. (B, D-H, J-S) – secondary electron photos, (A, C, I, T, U) – back scattered electron photos. Scale bars are 1 µm. Sil – silicate attachments.

Figure S7. Representative time-resolved laser ablation ICP MS spectra revealing presence of PGM inclusions in Cr-spinel. Data from CUG, Wuhan, China. From 0 to 30 s – blank gas signal, from 30 to 90 s – sample signal.

Figure S8. Representative time-resolved laser ablation ICP MS spectra revealing presence of PGM inclusions in Cr-spinel. Data from Institute of the Earth’s Crust, Irkutsk, Russia. Background (before 30 s) did not contain signal from PGEs and was excluded.

Figure S9. A BSE photo of a mount with Cr-spinel concentrate utilized for a TIMA search of Pt-bearing phases (A) and its recolored image with Cr-spinel grains red, magnetite - white.

Figure S10. Distribution density plots of relative errors (uncertainty) for the estimation of integral Pt-Fe alloy to Cr-spinel volume ratio based on flat sections of Cr-spinel-hosted Pt-Fe inclusions. Results of the Monte-Carlo statistical modelling (see Methods for details). (A) – assuming that the inclusions are equal in size; (B) – introducing a real size distribution. Zero error corresponds to the value expected from the calculation based on the Delesse principle.

Figure S11. A time-resolved LA ICP MS spectrum of Cr-spinel with a PGM inclusion and a schematic representation of a pit with the main parameters used in LA ICP MS -based calculation of the volume and mass proportions of the PGM inclusions (see Methods and Table S6 for details).

 

Table S1. Results of FA ICP MS analyses of bulk rock PGE contents in the Tumrok range picrites and basalts. Major and trace element concentrations, including Cr, are from Chayka et al. (2023).

Table S2. Electron microprobe (sheet 1) and LA ICP MS (sheets 2 and 3) analyses of Cr-spinel from 21AK9c and 21AK17 picrites. Concentrations obtained by microprobe are in weight %, by LA ICP MS – in ppb. PGE concentrations in Cr-spinel from 21AK9c were obtained using a modified procedure with an increased sweep time for PGE. PGE concentrations in Cr-spinel from 21AK17 were obtained using a routine procedure (see Supplementary Information for the detail). Mg# = Mg/(Mg+Fe) mole %, Cr# = Cr/(Cr+Al) mole %. FeOtotal was recalculated to FeO and Fe2O3 using method of Droop (1979).

Table S3. LA ICP MS analyses of silicates from the 21AK9c picrite. The concentrations are in ppm.

Table S4. A summary of the areas and characteristic dimensions of the PGM inclusions in Cr-spinel, according to SEM data. A calculation is given for the Pt content stored as PGM inclusions in Cr-spinel based on Delesse principle. For the calculation details see Methods in the main text.

Table S5. SEM EDS analyses of the PGM inclusions in Cr-spinel. Raw data is followed by the composition of the host Cr-spinel and recalculation of the PGM composition on the Cr-spinel-free basis. For positions 1-25 Cr-spinel compositions were determined by EPMA, for 26-43 – by EDS. Concentrations are in weight %; empty cells – not analyzed.

Table S6. A spreadsheet with LA ICP MS data on PGE spikes interpreted as PGM inclusions in Cr-spinel and their quantification to obtain parameters for the mass balance calculations, volumes and characteristic dimensions. EPMA data on host Cr-spinel are provided as well. Empty cells – no data. See Methods in main text for the details on the calculation procedure. Element concentrations are given in ppm, oxides in EMPA analyses – in weight %.

Table S7. A summary on occurrence and composition of the PGM inclusions found by LA ICP MS. Data obtained during sessions in CUG with “routine PGE analyses” were not included as they might underestimate occurrence of the inclusions due to low proportion of spectrometer time spent on PGE measurement.

Table S8. A spreadsheet with the mass balance calculations of PGE-in-Cr-spinel contents, their role in the bulk rock PGE budget and Cr-PGE fractionation trends. The details are provided on the 3rd sheet in the file and Methods section in the main text.

Table S9. Secondary reference material data for FA ICP MS in LabMaTer Universite du Quebec Chicoutimi (Chicoutimi, Canada)

Table S10. Secondary reference material data for LA ICP MS in CUG (Wuhan, China)

Table S11. Secondary reference material data for LA ICP MS in UTAS (Hobart, China)