Primordial water

Primordial water was formed in the primordial (nebular) gas and dust that later collapsed to form the Sun and planets.^[1] This dust cloud was rich in hydrogen and oxygen, which are the first and third most abundant elements in the Universe. These elements bound to small dust particles in the cloud. The small size of the dust particles created a large surface area for the attachment of these elements allowing for a large accumulation of hydrogen and oxygen, the building blocks of water. Data suggests a strongly negative deuterium to hydrogen ratio (δD) was added to the Earth during initial formation, during the Hadean eon, via dust particles with adsorbed H₂O inherited directly from the protosolar nebula (–870‰).^[2] During the early Earth phases the temperature was high, 1000 to 500 K would still allow adsorption of 25% to 300% of Earth's water onto fractal grains during Earth's accretion.^[3] Under the high heat and pressure of the Earth's formation, these dust particles and elements were smashed together and formed minerals and in some instances these minerals had H₂O inclusion bodies. Chemical models produced by Genda and Ikoma (2008) suggest an increase in the atmospheric D/H (heavy H2)/(light H1) value by a factor of 2 to 9 since Earth's formation.^[4]

Primary water or magmatic water

Understanding primordial water is important to understand the origins of primary or magmatic water. Primordial water and its off-spring primary water is the Earth's deep-water reservoir. Originally it was thought that low hydroxyl contents in upper mantle minerals collected near the Earth surface was proof of low solute H₂O contents in the source region.^[5][6][7][8] However, recent papers have shown that minerals from deep Earth regions with fair to high concentrations of solute hydroxyls can have these reduced through a redox conversion which consumes solute hydroxyls in the matrix of minerals by converting them into peroxy plus molecular H₂.^[5] This redox conversion of solute H₂O to peroxy plus H₂ is now thought to be a universal redox reaction applied to all rocks residing in the Earth’s rock column at temperatures below ~500˚ C.^[5] This also means that materials transported up to the surface from the deep mantle have a solute H₂O content that is no longer expressed in form of hydroxyls and they may not be "dry" as originally hypothesized. Freund and Freund (2015) state that better analytical techniques are needed to study the "true" solute H₂O contents of minerals.^[5] They also stated, "Earth’s upper mantle has most likely contained in the past—and continues to contain today—plenty of 'water' in the form of solute hydroxyls, probably enough to fill the world's oceans."^[5]

Primary water is water that has been solubilized from deep earth primary minerals bound with primordial (nebular) water elements or deep earth materials like diamonds with primordial water inclusions. The solubilization of primordial water from anhydrous or hydrous deep Earth minerals and rocks occurs under high heat and pressure conditions. A table of these can be found in Deep Earth: Physics and Chemistry of the Lower Mantle and Core, by Hidenori Terasaki and Rebecca A. Fischer, 2016, on page 267.^[9] The transition zone and lower mantle contain a vast amount of stored water due to high water solubility of its major olivine phase polymorph mineral constituents, wadsleyite and ringwoodite. A compiled table of the elasticity was put together in Zhu et al. (2016), which includes water (wt. %), of different compositions of wadsleyite and ringwoodite.^[10] These mantle regions subsequently store a significant amount of primary water solubilized from these deep earth minerals locally.^[9]

Total ocean-water mass is approximately ∼1.4 × 1021 kg, a tiny fraction (∼0.02%) of the bulk Earth mass.^[4] Based on recent literature, Nestola and Smyth (2016) updated water-content estimates based on calculations by Bodnar et al. (2013).^[11] They calculated that the transition zone would contain 3.46 x 1024 g (8640 ppm) of H₂O, which corresponds to about 2.5 times of all the Earth's oceans.^[12] Based on the hydrous ringwoodite and hydrous wadsleyite; hydrous ringwoodite models the % of the earth’s water in the transition zone is between 28.48% and 17.8% respectively and for the lower mantle, 50.23% and 57.74% respectively.^[12] Our biosphere, while originally thought to be the majority of this percentage from previous work by Brodner et al. (2013), is now thought to contain only about 11.61% to 13.34%, respectively, based on hydrous ringwoodite and hydrous wadsleyite and only on hydrous ringwoodite models. What is clear is that our understanding of water reservoirs is expanding rapidly and continuing to evolve as the field of investigation and analytical equipment do.

Only deep Earth primary rock, primary minerals, and primary water that have not participated in mixing processes related to convection action are likely to preserve Earth's initial D/H ratio.^[13] When minerals, rocks, and water are exposed to the Earth's atmosphere, a preferential loss of the lighter hydrogen isotopes occur, driven by thermal atmospheric escape or plasma interactions with gases.^[13] D/H Ratios when compared to Vienna Standard Mean Ocean Water (VSMOW, D/H = 1. 5576 Å~ 10–4) using δD = [[(D/H)unknown/(D/H)VSMOW] – 1] Å~1000, in units of parts per thousand (permil (‰)) gives us a tool to delineate Primordial Water; and the lowest measured D/H value (δD=–218‰) potentially provides an upper limit on the D/H of early Earth.^[13] One possibility is that this is strongly negative; primordial water has a high deuterium (H2)/hydrogen(H1) ratio.^[14] It is predicted that water solubilized from primordial water resources would have the same signature. Deuterium is a heavier isotope of hydrogen, with a neutron in its nucleus, and its prevalence compared with that of normal hydrogen serves as a useful fingerprint for tracing an object's history.^[14]

References