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The source of earth water has always been a hot spot in earth science and planetary science. The hydrogen isotopic composition is the most important basis for tracing the source of the earth's water. Existing research results show that there are huge differences in the hydrogen isotope composition of celestial bodies in the solar system: the sun, Jupiter, and Saturn have similar hydrogen isotope compositions (δD approximately -865‰), and are the same as the hydrogen isotope composition of interstellar gas. It is considered to be the initial value of the solar nebula; compared with the sun, terrestrial planets, chondrites and comets have significantly higher hydrogen isotopic compositions, and the difference is obvious, such as the earth’s ocean water (δD=0‰), carbon and Ordinary chondrites (δD=-220-1600‰, except R type), comets (δD=~300‰, except 103P/Hartley 2).
Based on the hydrogen isotope composition, many studies have suggested that carbonaceous chondrites and comets are the main sources of earth’s water (Morbidelli et al., 2000; Hartogh et al., 2011; Marty, 2012), but they cannot explain the earth. There is a clear difference in hydrogen isotopes between water and carbonaceous chondrite and comet water. In recent years, more and more high-precision isotope analyses have shown that enstatite chondrites (EC) are almost identical to the earth in terms of O, Cr, Ti, and Ca isotopic compositions. May be the main material for building the earth. Enstatite chondrites (Figure 1) are formed in a very reducing environment, in which Na and K elements can be produced in the form of sulfides, so enstatite chondrites are generally believed to be formed near the sun. (figure 2). From the perspective of nebula evolution, it is impossible for hydrogen near the sun to combine with minerals in the form of hydroxyl groups or water molecules, but there are still reports that water-containing minerals can be found in enstatite chondrites, such as Djerfisherite (Fuchs, 1966) . Recently, Dr. Laurette Piani of the University of Lorraine, France, performed water content and hydrogen isotope analysis on 13 enstatite chondrites (type 3-6) with different degrees of thermal metamorphism. At the same time, they also analyzed a piece of enstatite The heated product of pyroxene chondrite-enstatite achondrite Aubrite. The analysis results show that the whole rock water content of the enstatite chondrite is 0.08-0.54 wt%, while the water content of the enstatite achondrite is 0.3 ± 0.2 wt%, which is significantly lower than the water-rich carbonaceous chondrite Meteorite (7.2-9.1wt%). The average hydrogen isotope values of EH3 and EH4 (δD = -103±3‰) are lower than the current Earth's ocean water, and the hydrogen isotope compositions of EH5, EH6 and Aubrite are lower (δD = -127±15 ‰) (Figure 3). At the same time, the in-situ water content and H isotope analysis of the glass components in Sahara 97096 pellets were carried out using ion probes. The analysis results show that the water content in the glass composition is 2700-12300 ppm, and the hydrogen isotope ratio is uniform (δD = -147±16 ‰). Since there is no evidence of water alteration in Sahara 97096, it can be considered that the matrix of the pellet has not been disturbed by events such as later water alteration. Statistical results show that the matrix water content of the pellets accounts for about 13% of the total rock water, and the organic water only accounts for 7.7%. Where does the remaining 80% of the water come from? Is it from the main constituent mineral-enstatite (belonging to low calcium pyroxene)? Previous studies have shown that the water content of pyroxene on the S-type small celestial body Itokawa can reach 700-1000 ppm, and the water content of pyroxene in ordinary chondrite (hermes outlet) Larkman Nunatak 12036 can reach 600-1300 ppm (Jin and Bose, 2019) ). The water content of enstatite in Aubrite can reach 5300 ppm, combined with the mode content of enstatite in EC (50 vol%), it is estimated that the water content of enstatite accounts for 15% of the whole rock water (based on OC) Or 58% (based on Aubrite). The study shows that the earth's water can be provided entirely by enstatite chondrites. Because the hydrogen isotopic composition of some water-rich CM-type carbonaceous chondrites also falls within the range of the mantle, in order to further confirm that enstatite chondrites are the source of earth's water, additional isotopic indexes are needed to distinguish them. The hydrogen-nitrogen isotopic composition is a very good indicator. The analysis results show that only the hydrogen-nitrogen isotopic composition of enstatite chondrites is within the range of the mantle rock (Figure 4), so it can be considered as enstatite chondrites. Meteorites not only provide water, but are also the main material that constructs the earth, which is consistent with the results of high-precision isotope analysis. Piani et al. applied the analytical data to the theoretical model of the formation of the earth, and found that substances similar to enstatite chondrites can contribute 3.4-23.1 times the earth’s ocean water, glass components and organic matter can contribute 3-4 Times the ocean water, which is consistent with the estimate of mantle water content.
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