Digital Appendix to "Role of chamber replenishment in the formation of the Merensky Reef and its footwall anorthosite"

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Supplementary material to "Role of chamber replenishment in the formation of the Merensky Reef and its footwall anorthosite" submitted to Journal of Petrology in 2024. The archive contains: (1) element maps from electron probe microanalysis; (2) raw data files and processed data from electron backscatter diffraction analysis; (3) raw CSDCorrections files for crystal size distribution analysis; (4) input and output thermodynamic modelling files from Magma Chamber Simulator.
Lineage: The EPMA map of chromite crystals in the lower chromitite was produced using a Cameca SX-Five EPMA at the Centre de Microcaractérisation Raimond Castaing (University Paul Sabatier, Toulouse). The accelerating voltage was 15 kV, the beam current 40 nA, the step size 5 microns, and the electron beam diameter was 1 µm. We used a counting time of 0.5 s and only Al2O3, Cr2O3, FeO, MgO and TiO2 were measured. Analyses with a Cr2O3 content of at least 35 wt.% and a total oxide content above 95 wt.% were considered to isolate chromite crystals and to remove the silicate fraction and alteration products (e.g., magnetite). Pixels missing data were interpolated using a moving median filter and the resulting data were smoothed using a mean filter over a rectangle of 7 x 7 pixels.

The electron backscatter diffraction (EBSD) maps were produced at Cardiff University using a Zeiss Sigma HD Analytical Field Emission Gun Scanning Electron Microscope equipped with two Oxford Instruments 150 mm2 energy dispersive spectrometers and a Nordlys EBSD detector inserted to 191 mm. For EBSD, colloidal silica-polished samples were mounted at 70° to the incident electron beam at a working distance of 20 mm. Multifield large area maps were collected with a step size of 20 µm, using 2 x2 binning with an exposure time of ~ 60 ms with a Hough resolution of 60 and a minimum number of bands of 8. Removing of wild spikes and interpolation of EBSD data using 8, 7, 6 and 5 identical neighbours was performed using the Channel5 software. Data with a mean angular deviation above 1.3 were discarded together with crystals with less than 10 pixels to limit indexing errors. The crystallographic preferred orientation (CPO) of pyroxene is visualized using an orientation density function (ODF) calculated with the mean orientation of each pyroxene crystal and a 10° halfwidth. Due to the prevalence of twinning in plagioclase, their CPO is visualized using an ODF calculated from all available measurements. The fabric strength for each crystallographic axis of interest is quantified by the J-indices.

The physical properties of chromite crystals were acquired from the EBSD data using MTEX software. Crystal size distribution (CSD) profiles of chromite were determined using the program CSDCorrections v1.6 (Higgins 2000). Profiles were computed using a box length measurement, a 1:1:1 isometric shape, a roundness of 0, and a massive fabric. The profiles were corrected using the volumetric phase abundance determined from the EBSD data.

Thermodynamically constrained forward models simulating the reaction between cumulate rocks and replenishing magma(s) were produced using the Magma Chamber Simulator (Bohrson et al. 2014). These models were assimilation-fractional crystallization models that were completed at 2 kilobars using 2-5°C temperature decrements. Prior to modelling, FeO/Fe2O3 was calculated for each footwall composition at 800°C and ΔFMQ, and hydrous equivalents (denoted as h-) with ~ 2 wt.% H2O were also produced. Furthermore, each floor rock was set slightly above solidus temperature so that 8-10 wt.% of interstitial melt was present.

Bohrson WA, Spera FJ, Ghiorso MS, Brown GA, Creamer JB, Mayfield A (2014) Thermodynamic model for energy-constrained open-system evolution of crustal magma bodies undergoing simultaneous recharge, assimilation and crystallization: The magma chamber simulator. J Petrol 55:1685–1717
Higgins MD (2000) Measurement of crystal size distributions. Am Mineral 85:1105–1116