Experimental simulation of water formation on Earth from dry pebble rain
Data files
Aug 28, 2026 version files 31.24 KB
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micro_data_compiled.xlsx
26.73 KB
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README.md
4.51 KB
Sep 17, 2026 version files 31.24 KB
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micro_data_compiled.xlsx
26.73 KB
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README.md
4.51 KB
Abstract
Water is vital to planet habitability. However the sources of water for the terrestrial planets and the timing of delivery remain controversial. Here we show that water is produced within Earth's primitive atmosphere by hydrogen reduction of iron oxide during pebble accretion. Pebble accretion supports a massive H2-rich atmosphere where basal temperatures exceed the silicate liquidus. Pebbles settling through such an atmosphere melt and react with the gas, causing volatile loss, iron reduction, and H2O production. We simulate this process experimentally by melting anhydrous mid-oceanic ridge basalt (MORB), picrobasalt, and synthetic MORB in H2 streams. A significant portion of the FeO in these samples reduces to iron-rich metal and water vapor in a matter of hours. Moderately volatile elements Na and S evaporate from the melt. The metal produced strongly sequesters P, stripping P from the silicate. The metal is transferred to the core, explaining the low P content of Earth’s mantle relative to Mars as well as the low Fe/Mg ratio of the primitive mantle. Atmospheric reduction of iron oxide is a massive endogenous source of water for Earth. Earth-sized exoplanets may inherit oceans worth of water during their formation by pebble accretion, representing a critical step toward habitability.
Dataset DOI: 10.5061/dryad.fttdz0980
Description of the data and file structure
Electron microprobe analysis (EMPA): Compositions of initial and starting materials and reaction products for hydrogen reduction experiments conducted using the SiC resistance heater. Anhydrous MORB (or picrobasalt) glass: Initial MORB (or 1:1 mixture of MORB+Olivine) melted in He streams; silicate part of the melt: silicate part of the run products produced by hydrogen reduction of anhydrous MORB glass; metals: metals produced in the interior of the run products, by hydrogen reduction of anhydrous MORB glass. Initial composition, starting composition, and silicate reaction products are assumed to have all cations as oxides. For metals, all elements are considered to be elemental without any O. Low totals in starting compositions are assumed to be related to unmeasured volatile species (e.g., water, carbonate). Data points shown for a given melt are individual measurements from separate locations within the melt. All data are given in wt%.
File: micro_data_compiled.xlsx
Sheet: Initial MORB
Description: Composition of initial MORB in wt%. Data points shown for a given melt are individual measurements from separate locations within the melt.
Sheet: Anhydrous picrobasalt
Description: Composition of anhydrous picrobasalt in wt%. Data points shown for a given melt are individual measurements from separate locations within the melt.
Sheet: Synthetic MORB
Description: Composition of the P-free synthetic MORB in wt%. The composition is adapted from the OIT-1 to OIT-5 olivine tholeiite compositions reported by Bell et al. (2021).
Sheet: 1200-1
Description: Compositions of initial and starting materials and reaction products for hydrogen reduction experiments conducted using the SiC resistance heater. The samples were heated at 1200 °C for 1 hour in every phase.
Sheet: 1200-3
Description: Compositions of initial and starting materials and reaction products for hydrogen reduction experiments conducted using the SiC resistance heater. The samples were heated at 1200 °C for 3 hours in every phase.
Sheet: 1300-1
Description: Compositions of initial and starting materials and reaction products for hydrogen reduction experiments conducted using the SiC resistance heater. The samples were heated at 1300 °C for 1 hour in every phase.
Sheet: 1300-3
Description: Compositions of initial and starting materials and reaction products for hydrogen reduction experiments conducted using the SiC resistance heater. The samples were heated at 1300 °C for 3 hours in every phase.
Variables for sheets 1200-1, 1200-3, 1300-1, 1300-3
- Anhydrous MORB glass - Initial MORB melted in He stream
- silicate part of the melt- silicate part of the run products produced by hydrogen reduction of anhydrous MORB glass
- metals - metals produced in the interior of the run products by hydrogen reduction of anhydrous MORB glass.
Code/software
Electron Microprobe Analysis (EPMA) of starting products and polished cross-sections were conducted on a JEOL JXA-8200 electron microprobe at the University of New Mexico Department of Earth & Planetary Sciences/Institute of Meteoritics. Operating parameters were 15kV and 20 nA beam current with <1 um to 5 um spot size. Data were reduced using Probe for EPMA v. 13 (ProbeSoftware, Eugene, OR) utilizing the ZAF matrix correction algorithm. Overlap or interference corrections are determined by measuring the intensity for element-x on the overlapping element-y standard that contains no element-x to determine the ZAF-corrected percentage of intensity of element-y subtracted from the intensity of element-x3. All the elements were calibrated on the Taylor standard block (C.M. Taylor Corporation, Stanford, CA). The specific standards used are as follows: Na=Amelia Albite; Mg=Diopside (with a correction for Ca 3rd order SKa interference); Al=Orthoclase (with a correction for Ti 3rd order Ka interference using a Rutile Std); Si=Diopside; P=Wilberforce apatite-(F); S=Pyrite; K=Orthoclase; Ca=Sphene (with a correction for Ni 2nd order Ka interference); Ti=Sphene; Cr=Chromite; Mn=Spessartine; Fe=Pyrite; Ni=Nickel metal; Zn=Sphalerite (with a correction for W La interference); Mo=CaMo4 (with a correction for S Ka interference); W=Tungsten metal.
We simulate the effect of the interaction between silicate pebbles and H2 gas as the pebbles settle through an open nebular atmosphere. We accomplished this by melting mafic silicates in an H2-rich stream. Initial materials for our experiments include: i) a mid-ocean ridge basalt (MORB) from the East Pacific Rise; ii) a synthetic P-free mid-ocean ridge basalt (MORB, an olivine tholeiite) provided by Aaron Bell (Bell et al., 2021), and iii) a picrobasalt, prepared by mixing natural MORB and commercially purchased olivine in a 1:1 ratio.
1. Starting materials
For the silica samples, homogeneous, anhydrous glasses were prepared as starting materials by melting the initial compositions in He streams. MORB samples, each with an initial mass of ~100 mg, were heated in tungsten baskets using a silicon carbide vertical tube heater. The sample baskets were hung in a ¾” alumina tube connected to a He tank below and vented at the top. One pair of samples was heated at 1200°C (which is below the MORB liquidus) for 1 hour and 3 hours, respectively. The other pair was heated at 1300°C (above the MORB liquidus) for the same time periods. The flow rate of the He stream for (i) and (ii) was 200 10 mL/min, controlled with an electronic flow meter. Experimental temperatures were calibrated using a K-type thermocouple (to Au melting point = 1064°C) placed at the point in the heater where the melting/reaction experiments were made.
2. Reduction in H2streams
~100 mg samples of MORB glasses were subsequently heated as above in H2 streams at 200 10 mL/min. After the reaction, compositional data for the products were compiled using electron microprobe analysis (EMPA).