Hydrous Mg-Silicates

Hydrous Magnesium Silicates Lin-gun Liu Lab for Study of the Earth's Interior and Geofluids Institute of Geochemistry Chinese Academy of Sciences Guiyang, Guizhou 550002, China Is there significant amount of water (H2O) in the Earth’s deep interior? 1. Water Content 2. Water Reservoirs (Ferro-)Magnesium Silicates Forsterite Mg2 SiO4 (Olivine) Enstatite MgSiO3 (Pyroxene) Pyrope 3MgSiO3. xAl2 O3 (Garnet) x ≤ 1 Water reservoirs in the Earth’s deep interior: 1. Water solubility in magnesium silicates 2.Water solubility in silicate melts (magma) 3.Hydrous magnesium silicates Natural Hydrous Magnesium Silicates Norbergite Mg2 SiO4.Mg(OH, F)2 Chondrodite 2 Mg2 SiO4.Mg(OH, F)2 Humite 3 Mg2 SiO4.Mg(OH, F)2 Clinohumite 4 Mg2 SiO4.Mg(OH, F)2 Hydrous Magnesium Silicates At HP-HT phase D The densest hydrous magnesium silicate known (3.50 g/cm3) The only hydrous silicate possesses a SiO6 octahedral structure The only possible hydrous material may exist in lower mantle Natural Hydrous Magnesium Silicates Weight percentages for the various phases in the reaction forsterite + enstatite + water (solid lines)  forsterite + enstatite + phase A (dashed lines) as a function of H2 O content. One of the Conclusions For a “pyrolite” upper mantle in which the MgO/SiO2 molecule ratio ~1.5, there should be no Mg2 SiO4 -olivine in the “pyrolite” upper mantle, if H2 O content is greater than 4.3 wt%. DEHYDRATION DURING ACCREDEHYDRATION DURING ACCRETIONTION Lange and Ahrens (1984) and their earlier works on shock-wave experimental studies of serpentine and other hydrous minerals Dehydration starts at ~200 kbar completes at ~600 kbar HH22 O BURIED INSIDE THE GROWING O BURIED INSIDE THE GROWING TERRESTRIAL PLANETSTERRESTRIAL PLANETS H2 O content in the infalling planetesimals Ringwood (1979) ~3 wt%. Fukai and Suzuki (1986) ~2 wt% Liu (1988) 0.33 wt% The following amount of H2 O was buried inside Venus, Earth and Mars during accretion 1.4 x 1024 g This is equivalent to the mass of today’s oceans on the Earth estimated by Holland (1984). MM11 = = HH22 OO buried inside the growing Earthburied inside the growing Earth = 1.4 x 10= 1.4 x 102424 g (~ one ocean water) (Liu, 1988)g (~ one ocean water) (Liu, 1988) MM22 == HH22 OO escaped & lost to outerescaped & lost to outer--spacespace MM11 + + MM22 > ~10> ~102626 g (Donahue, 1986); critical massg (Donahue, 1986); critical mass beyond which the lose of beyond which the lose of HH22 O O to outerto outer-- space is negligiblespace is negligible MM33 = The amount of = The amount of HH22 O O derived from Mderived from M33 isis ~ ~ 6 x 106 x 102424 g (~4.5 ocean water)g (~4.5 ocean water) There are about 4.5 times of There are about 4.5 times of todaytoday’’s ocean water that were s ocean water that were released to the protoreleased to the proto-- atmosphere (or dissolved in the atmosphere (or dissolved in the magma ocean) when the magma ocean) when the EarthEarth’’s radius grew over Rs radius grew over R22 , if, if the primordial planetesimals contained 0.33 wt% contained 0.33 wt% HH22 O.O. It can be concluded that the It can be concluded that the Earth contained at least 5 Earth contained at least 5 ocean waters during and ocean waters during and right after accretion, ifright after accretion, if the primordial planetesimals contained 0.33 wt% contained 0.33 wt% HH22 O.O. Are there other water Are there other water reservoirs in the Earth’s interior? High pressure experiments showed that High pressure experiments showed that some some FeHFeH?? compound(?) and compound(?) and FeOFeO?? alloy are stable at HPalloy are stable at HP--HT. HT. There is no convincing scenario to put There is no convincing scenario to put HH22 into Earthinto Earth’’s Fe core. s Fe core. FeHFeH?? is not able to reduce much of the is not able to reduce much of the density of Earthdensity of Earth’’s core.s core. Basic Research Opportunities in Earth Science U.S. National Research Council (2001) Newly discovered mineral phases that are formed at high pressures lock water, carbon dioxide, and other “volatile” molecules into their crystal structures. The Earth’s interior is thus likely to contain far more water and other volatile species than the hydrosphere, fundamentally altering current views of how the oceans and atmosphere have evolved over geologic time. (p. 58) Origin and Evolution of Earth: Research Questions for a Changing Planet U. S. National Research Council (2008) 10 Grand Research Questions in Solid-Earth Science for the 21st Century Question 4: How does Earth’s interior work, and how does it affect the surface? Question 6: How are Earth processes controlled by material properties? Question 10: How do fluid flow and transport affect the human environment? 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