Modeling of Water-Rock Reactions
Because the redox characteristics of the element phosphorus are typically limited to phosphate and its acid-base chemistry under “typical” aqueous conditions (Figure 1), the effect of water-rock interactions on P are viewed mostly in the context of dissolution of phosphate [52, 53]. However, it stands to reason that highly exergonic water-rock interactions could potentially promote the more intractable redox transition to phosphite. Alternatively, the speciation of phosphorus within such rocks may begin at a lower redox state than is typically considered (as phosphite exchanging with SiO4 tetrahedra), as the speciation of P within olivine is unclear (and generally not considered as anything beyond +5 [10, 54]). As an example, the serpentinization of olivine couples the transformation of forsterite (Mg2SiO4) to serpentine minerals (Mg3Si2O5(OH)4), and the oxidation of fayalite (Fe2SiO4) to magnetite (Fe3O4), with the resulting mineralogy dependent on the initial stoichiometry of the olivine.
(Mg,Fe)2SiO4 + H2O = Mg3Si2O5(OH)4 + Fe3O4 + H2 + SiO2 + Mg(OH)2
Brucite is formed when the Mg/Fe is greater than 3, and SiO2 (typically amorphous) is formed when this ratio is less than 3. The oxidation of iron provides the electrons necessary to reduce neighboring material, whereas the serpentine mineral formation is exothermic, and provides the energy for the batch reaction [55-56]. We investigate the effect of serpentinization on P speciation by two methods: a redox calculation and by batch equilibrium models. First, we constrain the redox-pH conditions of the serpentinization reaction to show that these conditions are conducive to phosphate reduction. Thermodynamic construction of the Eh-pH diagram  are at a temperature of 298 K (25°C). The reaction of olivine (a solid solution mixture of Mg2SiO4 and Fe2SiO4) with water to give serpentine, magnetite, and hydrogen (H2 to H+ + e- as the half-cell), with SiO2 as quartz or Mg(OH)2 as brucite filling the stoichiometric balance from this reaction.
Notably, this result demonstrates that olivine serpentinization is conducive to phosphite production if the olivine is 50% forsterite or greater; however, reduction of a few percent of phosphate to phosphite still occurs at higher fayalite content (Figure 1). In general, most olivine is Mg-rich, favoring lower redox conditions.
As noted by Klein et al.  the serpentinization of mafic and ultramafic rocks generates H2 only with the oxidation of Fe(II)-bearing minerals, and some of these minerals form solid solutions with serpentinization products, as ferrobrucite, Fe(OH)2, does with brucite, and greenalite, Fe3Si2O5(OH)4, does with Mg-serpentine minerals. To this end, olivine serpentinization was modeled using HSC Chemistry for batch equilibria at higher temperature and with more consideration for solid solutions, along with changing water/rock ratios, coupled to an investigation of P speciation. These models employed the equilibrium chemistry calculator as part of HSC Chemistry (version 7.1, Outokompu Research Oy). In these models, either the water to rock ratio was set to 1:1 (by mass) and temperature slowly increased, or the temperature was set to 250°C and the water to rock ratio increased (from ~0 to 0.25). The rock composition was set to be initially equivalent to 70% forsterite and 30% fayalite. We specifically modeled a dunite rock, where olivine is the sole silicate present, though similar test models with pyroxene present did not substantially change the results with respect to phosphorus. The water reacting with the rock was set to a pH of 7.5, with 0.5 M NaCl, and low redox state (in equilibrium with an N2 atmosphere). The system was held at a constant 500 bar pressure (50 MPa). We added data for ferrobrucite, greenalite, and minnesotaite , and the remainder of the data came from the existing HSC database. Solid solutions were assumed between olivine, serpentine minerals, talc minerals, and brucite. Due to a lack of thermodynamic data for reduced P compounds, and for P dissolved in olivine/glass, phosphorus was considered to be present in the rock as P2O5 at 1000 ppm of the total rock weight. Aqueous speciation was constrained using the Debye-Huckel approximation of activity coefficients. The pH was “fixed” with a buffer consisting of Na2S/H2S in a 1:4 ratio (0.1 M total Na added) that kept pH near 7.5. The species investigated in this model are provided in the methods below.
These batch equilibria models of an olivine dunite undergoing serpentinization reactions reveal that reduction of phosphate occurs readily at incipient serpentinization (i.e., at low water-rock ratios) (Figure 2). This is because water is potentially an oxidant for phosphite, and based on thermodynamic equilibria will ultimately oxidize phosphite to phosphate (though in practice it does not readily oxidize in water on timescales of greater than 5 years . Reduced oxidation state P persists at about 0.3% of the total P even after the incipient serpentinization has completed (Figure 2b). These models, based off prior serpentinization batch equilibria models , demonstrate that reducing conditions pervade ultramafic rocks when the first interactions with water occur .
Phosphorus Reactions and Speciation
The above models demonstrate that the reduction of phosphate to phosphite is plausible within serpentinizing rock. This reduction occurs with the concomitant oxidation of iron, and is similar to prior work demonstrating iron oxidation coupled to phosphate reduction . However, in contrast to the low production (1-4%) of phosphite reported by Fe2+ Fe3+, these thermodynamic models predict the highly exergonic nature of serpentinization may be able to better power this reduction reaction than the amount produced by this diagenetic process, especially at low water to rock ratios.
We contrast these model results to P speciation within serpentinites. Serpentinites were collected from outcrops in southwestern Oregon at the Nolan Claim (N 42°10.003’ W 123°42.709’ and N 42°09.925’ W 123°42.719’) in Josephine County, OR, USA. These rocks are part of the Josephine Ophiolite in the Klamath Mountains [60, 61], and were a sequence of ultramafic rocks (dunite and harzburgite) with a formation age of 157 million years. Fresh samples were taken along the Josephine creek, then powdered and analyzed by Raman, XRD, and XRF. Both Raman and XRD show that the main mineralogy of these samples is the serpentine mineral antigorite (see SI). The composition of these rocks determined by XRF (see SI) shows they are composed primarily of magnetite and serpentine minerals, and that they are depleted in P and enriched in Cr and Ni (consistent with their ultramafic origin).
Phosphorus compounds were extracted from these serpentinites (see methods) and analyzed by 31P NMR spectroscopy (Figure 3). This spectrum shows a peak occurring within the region of phosphite that splits (doublet at 4.9 and 1.4 ppm) when the coupling to hydrogen is permitted with a JP-H coupling constant of 565 Hz. This coupling constant is diagnostic of phosphite , indicating phosphite is present within the serpentinite and is the major P species, formed during the highly reducing alteration of olivine. The other associated peaks correspond to phosphate (5.6 ppm) and pyrophosphate (-4.6 ppm). The presence of both phosphate and pyrophosphate may be due to a few causes. For one, the presence of phosphate may suggest incomplete reduction of phosphate to phosphite. Then, when the rock is serpentinizing, the exergonic/exothermic reaction results in the dimerization of phosphate. Alternatively, and perhaps more likely, the presence of pyrophosphate and phosphate may suggest that phosphite has been oxidized by free radicals such as OH , possibly formed by reaction of O2 with native metals present in the serpentinite , which may produce H2O2 that could then react to produce OH .
Modeling results and analysis of natural samples both demonstrate that P in ultramafic rocks that serpentinize is present in reduced form as phosphite. These results highlight a new role for serpentinization in planetary habitability. In addition to heat generation and low redox conditions , serpentinization also affects P speciation. Due to the higher solubility of phosphite relative to phosphate [66, 67], the serpentinization process may liberate P into water as rocks serpentinize. Notably, the serpentinized rock is significantly lower in total P content than associated unaltered rocks ([68, 69], Table S1). This may imply that as water reacts with the serpentinite that further extraction occurs due to the higher solubility of phosphite. As an illustration of this process, the addition of divalent cations (in this case, Ca2+) to a solution of both phosphate and phosphite results in the precipitation of phosphate but leaves phosphite relatively unaffected (Figure 4). This implies that the phosphite is more soluble, and more easily extracted from the serpentinizing rock than is phosphate.