Experimental apparatus
All experiments were conducted in a 6–8 multi-anvil apparatus with 32 mm edge length WC cubes featuring 11 mm truncations and 18 mm edge octahedra, calibrated against the transitions of quartz–coesite, garnet–perovskite in CaGeO 3 , and coesite–stishovite. The assembly consists of a Cr-doped MgO-octahedron, a zirconia sleeve, a stepped graphite furnace, inner MgO parts and a PtRh B-type thermocouple. Full details, including pressure calibration, are provided in ref. 24. Based on previous experience with similar temperatures and melt compositions, equilibrium is typically achieved within 1–2 h. Consequently, our run times ranged from 2 h to 8 h, increasing with decreasing temperature. The starting mixtures were loaded into Re capsules folded from foil, placing a thin graphite layer at the bottom and top. These were then inserted into a Pt capsule, which was welded shut to contain the volatiles. The graphite–CO 2 melt pair yields the appropriate oxygen fugacity for redox melting, in which C0 oxidizes to CO 2 .
Starting compositions
The experiments (Extended Data Table 1) equilibrate erupted surface melts with a lherzolite mantle at 7 GPa and temperatures of 1,420–1,630 °C. These conditions correspond to mantle potential temperatures (T P ) of 1,350–1,560 °C, encompassing a typical mid-ocean ridge adiabat15,16 and an excess temperature of 210 °C, similar to the hottest mantle plumes32,33. Initial starting materials (Supplementary Table 1) include an average primitive ocean-island basanite (APB, see below) and an average of close-to-primitive MORBs, corrected for olivine fractionation (MBB). The OIB composition contains 5.4 wt% CO 2 and 1.9 wt% H 2 O, based on melt inclusion data20. For the MORB, we selected the maximum observed CO 2 content of 1.0 wt% as in the popping rocks19 and 0.5 wt% H 2 O, which is in the upper range of the MORB average (0.28 ± 0.24 wt% H 2 O; ref. 30). As erupted melts are largely degassed, reconstituting their volatile concentrations was necessary. Moreover, we extended our previous study on kimberlites31 to 1,580 °C, using a 1,400 °C experimental melt31 derived from a starting bulk composition with 7.5 wt% CO 2 and 3.5 wt% H 2 O (JER1555).
All starting compositions were mixed by weight from previously dried chemicals, that is, SiO 2 , TiO 2 , Cr 2 O 3 , NiO, MgO, Al 2 O 3 , MnO and Na 2 SiO 3 , and from synthetic leucite, wollastonite, apatite and fayalite. All iron was present as FeII in the starting material. H 2 O and CO 2 were introduced as Mg(OH) 2 , synthetic CaCO 3 and natural magnesite. Powders were mixed in an agate mortar and homogenized in alcohol in a planetary mill. The starting materials were then kept in a desiccator and again dried at 110 °C before use.
Iterative forced multiple saturation experiments
The starting melt compositions were iteratively equilibrated with a four-phase lherzolite (olivine, orthopyroxene, clinopyroxene and garnet) modelled after peridotite KLB-1. Capsules were loaded either in a sandwich configuration with the melt placed in between two layers of synthetic powders representing mantle peridotite or as homogeneous mixtures of both components. Multiple iterations were necessary to (1) saturate the melt in all four lherzolite mantle phases and (2) to obtain large melt pools rather than interstitial melts (see Supplementary Fig. 1 for back-scattered electron images of the experiments). Melts rich in CO 2 + H 2 O rarely quench into glass but rather form intergrown crystallites, requiring measurement areas larger than the crystallites to ensure accurate melt compositions. To address these challenges, subequal melt/peridotite starting proportions are required, and a suite of 13 and 11 experiments were conducted on the MORB and OIB compositions, respectively, to obtain equilibrium melt compositions at 1,420 °C, 1,480 °C and 1,630 °C (7 GPa). During these iterations, the peridotite component was adjusted by modifying the relative proportions of olivine, orthopyroxene, clinopyroxene and garnet, counteracting complete dissolution of any single mineral. Simultaneously, the melt composition was progressively refined (MBB → MORB2 → MORB10 and APB → OIB5 → OIB11; Extended Data Table 1 and Supplementary Table 1) until saturation in all four mantle minerals was reached and mineral compositions closely resembled those found in the mantle. A further experiment was run on a kimberlitic composition at 1,580 °C, which is complemented by the 1,400 °C melt composition previously obtained for kimberlites using a similar method31.
Preparation of experimental run products
The CO 2 + H 2 O-bearing melts quench into a mixture of clinopyroxene, calcite and Na-rich interstitial carbonate-rich material and some interstitial voids, well observable (see detailed images in Supplementary Fig. 1). When left in the open, this quench material grows whiskers of several mm length within hours or days, leading to the destruction of the experimental charge. Thus, all preparation of the experimental run products was done minimizing the presence of humidity and avoiding water. Samples were exclusively handled in a purpose-built glove box that maintains a <2% relative humidity environment. Grinding and polishing were achieved using first dry SiC abrasive paper and then dry diamond powder. The very final polish was done using kerosene, after which samples were immediately coated and loaded either into the scanning electron microscope (for first imaging) or electron microprobe (for element quantification). Immediately after measurement, samples were impregnated with an epoxy droplet, protecting them from humidity. For any re-measurement, the polishing procedure was then repeated.
Experimental textures
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