| The destruction and lithosphere thinning of the eastern North China Craton havebeen widely recognized by Geoscientists. However, the time, mechanism and thedynamic background of the destruction and lithosphere thinning remains controversial.So far, at least three destruction mechanisms have been proposed: delamination,lithosphere hydration and thermal-mechanical-chemical erosion. All of thesedestruction mechanisms are inevitably related to the physicochemical property of themelts of the upper mantle. As the most prevalent fluid, water in the mantle has animportant influence on the nature of geophysical evolution. Hydrous minerals such asmica and Hornblende inclusions carried by intrusive vulcanicity are powerfulevidence of the existence of water in the mantle. High temperature and high pressureexperiments have demonstrated that water can effectively reduce the mechanicalstrength of Pyroxene and Olivine, improve their ion diffusion rate and conductivity.Therefore, it is very important to discover the water content of mantle and its presencemodes, especially for the rheology, phase equilibrium and frit reaction.In this thesis, we systematically studied the density, velocity and elastic property ofhydrous basalt melts under high temperature and high pressure, and found some newphenomena. Study results are listed as follow:1. Hydrous basalt glasses with water content of0%~6.82%were synthesized byYJ-3000ton press at P=1.0-2.0GPa and T=1200-1400°C starting from naturalMesozoic basalts of the eastern North China Craton (NCC). We conductedinfrared absorption spectroscopy and electron microprobe to these hydrous glasses.The test rests show that hydroxyl is predominant when the total water content is less than2wt%, therefore when the total water content is over2wt%, themolecular water is predominant. We also proposed a new method to calculate theNBO/T of the silicate melts, especially the hydrous melts. The revised formulainclude the influence of water to the degree of polymerization of silicate melts,which is more accurately reflect the interaction of the silicate group with thenetwork-former and modifier cations.2.(1) We conducted thermal expansion experiments to these―condensed‖and―relaxed‖basalt glasses. The partial molar volumes of water (VH2O) under ambientconditions which is independent of composition with a value of11.6±0.8,10.9±0.6and11.5±0.5cm3/mol for FX, FW and SHT basalt glasses, respectively.However, The partial molar volumes of water (VH2O) at high pressure and hightemperature suggest that increase of temperature or decrease of pressure wouldlead to the increase of the partial molar volume of water in basalt melts.(2) Thesound velocities and elastic properties of hydrous basalt glasses were measured byBrillouin scattering spectroscopy as a function of water content. The increase ofwater content reduces both of the longitudinal (VP) and shear (VS) wave velocities.Higher synthesis pressure results in a denser glass and finally leads to the increaseof VP. Water has two effects to the silicate glass: to reduce its total density andincrease its compressibility. As the degree of depolymerization (NBO/T) increases,the VP, VSand bulk moduli (G and KS) of hydrous basalt glasses decrease, but theadiabatic compressibility (S) increases. The O-T coordination number of thehydrous melt is substantially less than that of the anhydrous melt, demonstratingthat added water disrupts interpolyhedral linkages. The breaking of the relativelystable tetrahedral bridging oxygen structure caused the silicate system even morecompressible. The comparison of the adiabatic compressibility (βS) the glassessynthesized at1GPa and those synthesized at2GPa indicates that, the basaltglasses synthesized under higher pressure would yield a lesser compressibility. Itmeans that the denser the glass the lesser the compressibility. If regardless of thechange of compression, the addition of water would reduce the density of thesilicate glasses. However, water can decrease the polymerization and increase the compressibility of the silicate system, slowing down the decrease of the density.Hence addition of water has two effects to the silicate glass: to reduce density ofthe glasses but increase its compressibility at certain synthesis pressure. Althoughthere is an overall drop of the compressibility under higher pressure, theincreasing rate is larger than the counterpart under lower pressure. So water notonly reduces the melt density of the sample, the degree to which the addition ofwater reduces density is slightly lowered at higher pressure.(3) To understand theeffect of hydration on the elastic properties of silicate melts, we conducted in situhigh-pressure Brillouin scattering measurements on two hydrous basalt glasseswith different water contents in diamond anvil cells. Second-order phasetransitions were observed in the hydrous basalt glasses and are due to thetopological rearrangement of the silicate network to a high [Si, Al]–Ocoordination. Up to a pressure of10GPa at300K, the extra2.23wt%H2O lowersthe elastic moduli of FX-2basalt glass (2.69wt%H2O) by10%–18%, but doesnot affect the pressure derivatives of the elastic moduli, compared with FX-1(0.46wt%H2O) basalt glass. The phase transition takes place at a higher pressure inFX-2compared with FX-1, possibly because of the depolymerization of water tosilicate glass. Water interacts with network-forming cations and creates Si–OHand Al–OH groups, and prohibits nonbridging oxygen ions from being connectedto other nearby framework cations (i.e.,[5,6](Si, Al)), resulting in the hysteresis ofthe second-order phase transition. The density contrasts of our hydrous basaltmelts with previous mid-ocean ridge basalt and preliminary reference Earth modeldata indicate that basalt melts may need very low water content (<0.46wt%H2O)to maintain gravitational stability at the base of the upper mantle. Our results showthat the elastic properties of hydrous silicate melts may have importantimplications for the dynamic evolution and chemical differentiation of the mantle. |
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