Kinetics Of Reaction Between Different Melts And Peridotite

This thesis studies the effect of reacting melt composition and water on thekinetics of melt-peridotite reaction using an experimental approach. The experimentsare conducted in piston-cylinder apparatus using the reaction couple method. Theexperimental results, combined with petrologic, geochemical, and thermometricinvestigations for mantle xenoliths from the North China Craton (NCC), help toconstrain the nature of the NCC lithospheric mantle in geologic time (Mesozoic andCenozoic) and mechanism of the destruction of the NCC. Main content of this thesisare as follows:1. Effect of melt composition on the kinetics of melt-peridotite interactionEffect of melt composition on the kinetics of melt-peridotite reaction wasexamined at2GPa and1425°C. The reacting melts include a basaltic andesite, aferro-basalt, and an alkali basalt. Reactions between basaltic andesite or ferro-basaltand lherzolite produce harzburgite-lherzolite sequences, whereas reaction betweenalkali basalt and lherzolite produces a dunite-harzburgite-lherzolite sequence.Variation in reaction zone lithology depends on liquidus phase relationship of thereacting melt. Systematic variations in mineral compositions across the lithologicalunits are observed. These variations are attributed to grain-scale processes involvingdissolution, precipitation, and reprecipitation and depend strongly on reacting meltcomposition.2. Effect of water on the kinetics of melt-peridotite interactionEffect of water on the kinetics of melt and peridotite interaction was examinedexperimentally in Pt and Au-Pd capsules. The reacting melts include a hydrousbasaltic andesite (4wt%H2O) and a ferro-basalt (amphibole-bearing garnetpyroxenite). Reaction between hydrous basaltic andesite and lherzolite at1GPa and1200°C produces an orthopyroxenite-dunite sequence. Reactions between hydrousferro-basalt and lherzolite at0.8-2GPa and1250-1385°C produce anorthopyroxenite-harzburgite sequence. The reaction products have similar petrologic and mineralogical features as those of the NCC mantle xenoliths. Processes leading tothe formation of orthopyroxenite are deduced based on the lithological features,liquidus phase relations of interface melt, and reacted melt and mineral compositions.Water infiltration triggers incongruent melting of lherzolite. Efficient mixing betweenthe partial melt and the reacting basalt leads to orthopyroxene oversaturation aroundthe melt-rock interface region. Reaction between the hybrid melt and depletedperidotite forms orthopyroxenite at the melt-rock interface.3. Peridotite dissolution rate and compositional variations of the reacting meltsRate of lherzolite dissolution in basaltic melts and compositions of the reactedmelts were acquired in melt-peridotite reaction experiments conducted at2GPa and1385-1425°C. The reacting melts include a basaltic andesite, a ferro-basalt, and analkali basalt. Combined with the results of experiments reported in literature, it isconcluded that peridotite dissolution rate, extent of melt-peridotite equilibrium, andtrends of melt compositional variation are controlled by temperature, reaction kinetics,and starting melt composition (major element and water concentrations).4. Thermal history recorded by mantle xenoliths from the NCCIn order to reveal the thermal history of the NCC lithospheric mantle, we appliedthe REE-in-two-pyroxene thermometer and major element-based thermometers on theNCC mantle xenoliths. The mantle xenoliths include (1) ancient refractory peridotitesentrained by Early Cretaceous high-Mg diorites from Fushan,(2) peridotites withvarying chemical compositions and rhenium-depletion model ages entrained by theLate Cretaceous and Cenozoic alkali basalts, and (3) pyroxenites entrained by EarlyCretaceous alkali basalts from Feixian and Fangcheng. Group1xenoliths have lowmajor element-derived temperatures and slow cooling rates, owing to the shallowintrusion of their host diorites. Group2xenoliths have a more complicated thermalhistory, suggestive of the complexity of lithospheric mantle structure since the LateCretaceous. Group3xenoliths have relatively high temperatures and moderate to fastcooling rates, a sign of transformation of nature of the NCC lithospheric mantle.5. Deep processes and mechanism of destruction of the North China CratonBased on the experimental results and comparisons with field observations withthe help of thermometric data of mantle samples and seismic observations from theNCC, processes leading to the destruction of the NCC and transformation of nature ofthe NCC lithospheric mantle were deduced. Destruction of the NCC was initiated bydelamination in the Mesozoic. The delamination mainly took place in the ancient orogens with thickened lower crust. Interaction between the residual lithosphericmantle and melt derived from the recycling lower crust in the Early Cretaceous isresponsible for the petrological and chemical characteristics of mantle xenoliths fromthese regions. Subsequent accretion of the upwelling asthenospheric mantle (since theLate Cretaceous) accomplished replacement of the lithospheric mantle. Theseprocesses (delamination and accretion) not only resulted in the varying chemicalcompositions of the mantle samples but also their complex thermometriccharacteristics such as temperature and cooling history.