Multi-anvil experiments were performed both in static and deformation geometries on olivine polymorphs and MORB (Mid-Ocean Ridge Basalt) -composition garnetite. Faults were induced by uniaxial compression in both garnetite and wadsleyite but only the garnetite showed evidence of thermal run away to melting. These results suggest that strain localisation by thermal feedback might be an important mechanism for triggering deep earthquakes in the crustal portion of subducted slabs in the deeper transition zone (~500-660 km depth).

We collected major element, trace element and Nd isotopes of cumulate plagioclase and clinopyroxene in lower crustal gabbros from Hess Deep oceanic crust (~2°15'N, 101°30'W) to investigate the Nd isotopic heterogeneity of melts delivered to a complete section of Hess Deep oceanic crust, accreted at the fast-spreading (133 mm/yr) East Pacific Rise (EPR). These data are presented in Cooper et al. (2025) (https://doi.org/10.1130/G52872.1). Elemental maps of 58 samples were initially obtained prior to selecting a subset of 25 samples for in-situ microanalysis. We targeted the Nd isotope record of cumulate plagioclase and clinopyroxene from lower crustal gabbro samples, representing early crystallisation products of melts delivered to the crust. These samples were collected in several expeditions: Ocean Drilling Program (ODP) Leg 147; RSS James Cook cruise JC21; Integrated Ocean Drilling Program (IODP) Expedition 345 (Site U). Combined, these studies provide the most complete composite section of fast-spreading EPR crust to date (stratigraphic depth of 4350 m to 25 m). In our study, we selected 25 samples for in situ Nd isotope microanalysis, covering the range of mineralogy and textural diversity, and over the full stratigraphic depth. For a comparison to local MORB compositions, we selected a set of 13 upper-crustal sheeted dikes collected on the RSS James Cook cruise JC21. Our data reveal that the mantle is heterogeneous at the scale of melt extraction, and the crystal record from the lower crust shows greater 143Nd/144Nd heterogeneity than the overlying MORB. Hence, Pacific MORBs do not reflect the full heterogeneity of their mantle source, and some aggregation of melts occurs within the crust. Data was collected between 2020 and 2023 by George Cooper, Johan Lissenberg and Max Jansen at Cardiff University, UK, as part of NERC Grant NE/T000317/1:HiDe: A Highly Heterogeneous Depleted Upper Mantle? Mineral isotopic analyses were performed on a Thermo Scientific TRITON Plus at the Vrije Universiteit in Amsterdam. The long-term average and reproducibility (2019–2022) for the JNdi-1 standard is 0.512094 ± 0.000011 2 SD (standard deviation; n = 28) with 1011Ω resistors (used for clinopyroxene) and 0.512105 ± 0.000044 2 SD (n = 45) with four 1013Ω resistors (used for plagioclase). Full methodology can be found within the supplemental Material of Cooper et al. (2025) at https://doi.org/10.1130/GEOL.S.28485770 The DOI is a supplement to https://doi.org/10.1130/G52872.1 Methodology: https://gsapubs.figshare.com/articles/journal_contribution/Supplemental_Material_Crustal_versus_mantle-level_aggregation_of_heterogeneous_melts_at_mid-ocean_ridges/28485770?file=52665137

Whole rock Hafnium (Hf) isotope data for mid-oceanic ridge basalt (MORB) samples from the Reykjanes ridge, Arctic Ocean, Equatorial MAR, South MAR, SEIR. The Hf isotope results are not yet published but will contribute to the mantle characterisation efforts detailed in Béguelin et al. (2025) (https://doi.org/10.1029/2025GC012357). Mid-Ocean Ridge Basalt (MORB) glasses were crushed and sieved to ~600 µm. Chips were then hand-piked under a binocular microscope to remove alteration (only optically clear chips selected). Hf fractions were purified using the column chemistry protocol of Béguelin et al. (2017) and references therein (https://doi.org/10.1016/j.gca.2017.09.015). Hf isotope ratios were measured on a Nu Instruments Plasma II mass spectrometer equipped with an Aridus desolvating nebuliser, on 50 ppb solutions. Accuracy and precision were monitored with repeated measurements of the JMC-475 standard (reported in dataset). Scanning Electron Microscopy-Energy Dispersive Spectrometry (SEM EDS) and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) major and trace elements maps of eclogites from the Raspas complex (Ecuador). Samples were prepared as 1-inch epoxy mounts. Samples are described in John et al. (2010) (https://doi.org/10.1007/s00410-009-0427-0). The objective of this (present) work is to trace U, Pb and Th fluxes during subduction, as documented by obducted eclogites. Dataset includes SEM mapping of major elements with a Zeiss Sigma HD Field Emission Gun Analytical Scanning Electron Microscope (SEM) using energy dispersive X-ray spectroscopy (EDS). Elements measured are Fe, Ca, Cu, Ni, Mn, K, Na, Al, Mg, Ti, O, Cr, S, Si, P, Zr. Resolution is 25µm. Dataset also includes LA-ICP-MS mapping of trace elements using an Elemental Scientific ESL213 laser coupled to a Thermo Scientific iCap Q mass spectrometer. Elements measured are Rb, Sr, Mo, Cs, La, Yb, Pb, Th, U. Parameters: Raster spacing (resolution): 40µm, scan speed: 80 µm/s, rep rate: 20 Hz, fluence: 4.5 J/cm2, dwell time per element: 50 ms, injector: ESI dual concentric injector (DCI). SEM data are in weight %, LA-ICP-MS data are in µg/g (ppm). Data are unpublished, but further details can be found in the poster included with the dataset. Dataset was produced at the School of Earth and Environmental Sciences, Cardiff University, in the context of NERC grant NE/T012633/1 Mantle Circulation Constrained (MC2): A multidisciplinary 4D Earth framework for understanding mantle upwellings.