Water chondritic origin

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

D/H ratios in carbonaceons chondrites may hint on the origin of water on Earth. Robert (2001) suggested that since the contribntion of cometary water to terrestrial water should be less than 10%, most of the water on Earth should derive from a meteoritic source. 

EUer and Kitchen (2004) have re-evaluated the hydrogen isotope composition of water-rich carbonaceous chondrites by stepped-heating analysis of very small amounts of separated water-rich materials. Their special aim has been to deduce the origin of the water with which the meteorites have reacted. They observed a decrease in 5D with increasing extent of aqueous alteration from 0%c (least altered, most volatile rich) to —200%c (most altered, least volatile rich). 

Amino acids - and more generally organic substances - are mainly detected in carbonaceous chondrites, a minor, carbon-rich class of meteorites. These are believed to have originated from parent bodies having underwent alteration by liquid water at some stages of their existence, as attested by geochemical studies [63]. 

Murchison (meteorite) A carbonaceous chondrite, type II (CM2), suspected to be of cometary origin due to its high water content (12 percent)... 

The Cl chondrites have long been cited as the classic example of asteroidal aqueous alteration, because of the presence of ubiquitous sulfate veins (DuFresne and Anders, 1962 Richardson, 1978 Fredriksson and Kerridge, 1988). These veins crosscut the dark, fine-grained matrix and can extend across the entire meteorite sample or stone. These veins have commonly been attributed to the widespread movement of water within the Cl parent body. However, Gounelle and Zolensky (2001) have reappraised the origin of these veins and concluded that they are terrestrial, not asteroidal, in origin. Their preferred interpretation is that the veins formed as a result of the dissolution, local transport, and precipitation of extraterrestrial sulfates by absorbed terrestrial water. Thus, one of the widely accepted lines of evidence to support parent-body alteration should now be treated with caution. 

We can estimate how much water was dissociated by considering the oxidation of the mantle. If the Earth started out with the oxidation state of chondrites (although not necessarily of total chondritic composition), it would take the oxygen from about one present-day ocean volume to reach the oxidation state of the mantle inferred from mantle-derived rocks. Alternatively, if we start with a more reduced ensemble, suggested by condensation calculations from a solar nebula, we will have primarily enstatite (MgSiOs) and metallic iron as the original source to be oxidized. The oxidation reaction would then be as shown in eqn [1]. 

A meteorite that landed in Monahans, Texas, in 1998 was cut open and water was found in it it was the first time that scientists have detected water in a meteorite, an essential ingredient for life of primordial origin (Zolensky et al. 1999). The fluids are dominantly sodium chloride-potassium chloride brines, but they also eontain divalent cations such as iron, magnesium, or calcium. The Monahans belongs to a class of meteorites known as ordinary chondrites, which astronomers have believed are fragments of asteroids that contain little or no water. One explanation for the water in this meteorite is that its parent asteroid acquired it after the rock formed. A water-rich, icy projectile, such as a comet, could have plowed into the newborn asteroid and spilled some of its water. Alternatively, the water might have been incorporated into the asteroid as it coalesced (Kargel 1992, Schmitt et al. 2007)). 

Ashworth, and Hutchison, 1975 [11] made electron microscopic observations of the hydrous alteration products of olivine in an achondrite and in an ordinary chondrite. Their conclusion was that the Nakhla achondrite, and possibly the Weston chondrite, contain water of extraterrestrial origin which was mobilized by mild shock deformation. Carbonaceous chondrites are believed to be unaltered material left over from the formation of the solar system. They contain substantial amounts of reduced carbon and of water in the form of hydroxyl ions. The oxidation state of iron in some carbonaceous chondrites has been determined by means of Moess-bauer spectroscopy, and it is demonstrated that there is a correlation between the oxidation state of iron and the content of water and reduced carbon in the meteorites (Roy-Poulsen et al., 1981 [284]). 

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