Experimental Studies Of Melt-Peridotite Reactions At1-2GPa And1250-1400℃

A series of experiments of the melt-peridotite reactions at pressures of1and2GPa andtemperatures of1250-1400°C have been carried out using Fuxin alkali basalt (FW1-1), Feixian alkalibasalt (FX-1), and Xu–Huai hornblende–garnet pyroxenite (JG4-1) as starting materials for the reactingmelts, and lherzolite xenoliths and synthesized harzburgite as starting materials for the lithosphericmantle. Combined with the researches on mantle peridotites xenoliths in high-Mg diorites in westernShandong and Taihang Mountains, we discussed the mechanism and deep processes for transformingthe nature of Mesozoic lithospheric mantle in North China Craton as well as the petrogenesis ofhigh-Mg adakitic rocks in this region. Our work is significant to cast a light on the destruction of NorthChina Craton.1Melt-peridotite reactions and results(1) Alkali basalt and peridotite reactionFour experiments on reaction between alkali basalt and spinel-lherzolite/harzburgite have beencarried out at1or2GPa,1300-1400°C for8hours. The basalts (FW1-1and FX-1) were totally meltedwhereas lherzolite was partially melted and harzburgite was not melted. The alkali basalt-lherzolitereaction produced low-Mg dunite—lherzolite sequence. The alkali basalt-harzburgite reaction producedlow-Mg dunite—harzburgite sequence. There are interstitial melts among the olivine grains in dunitezone. The reaction zone that formed at1GPa is much wider than that formed at2GPa over the samelength of time. The processes of alkali basalt-peridotite reaction involve dissolution of orthopyroxeneand precipitation of olivine which can be described as:Ol1+Opx (+Cpx)+Melt1â†'Ol2+Melt2Compared with the starting melts, SiO2and MgO contents increase in the reacted melts whereasFeO and CaO contents decease in reactions between Fuxin alkali basalt and peridotite (FW0912andFH0629). In the experiments on the reaction between Feixian alkali basalt and peridotite (runsFXH0703and FXH0704), the compositional variations of the melts are similar to those in the experiments on Fuxin alkali basalt and peridotite. But, the reacted melts in runs FXH0703andFXH0704have lower MgO and higher FeO than their starting melts, which is the opposite of theexperimental results in runs FW0912and FH0629. This is because sample FX-1has a higher MgO andlower FeO than sample FW1-1. In the crystalline phase, there are compositional zoned olivine andorthopyroxene. The composition of olivine varies as a function of distance to the melt-peridotitecontact. The olivines in dunite zone have higher FeO and CaO contents and lower NiO and Mg#.(2) hornblende–garnet pyroxenite and peridotite reactionThree experiments on reaction between Hb-Grt pyroxenite and spinel-lherzolite/harzburgite havebeen carried out at1or2GPa,1250-1350°C for10or24hours. Hb-Grt pyroxenite was totally meltedat1GPa and1250°C for24hours (run JW1011), and its melts reacted with the lherzolite producingdunite—harzburgite sequence. Hb-Grt pyroxenite was totally melted at2GPa and1350°C for10hours(run JW0912), and its melts reacted with the lherzolite producing orthopyroxenite—lherzolite sequence.The differences between the two experiments could be ascribed to the incongruent melting of Opx atlow pressures (Opxâ†'olivine+Si-rich melt). Hb-Grt pyroxenite was partially melted at2Gpa and1300°C for24hours (run JH0627), its melts reacted with the harzburgite producingorthopyroxenite—harzburgite sequence. The residual olivine in orthopyroxenite indicates a process inthe reaction involving dissolution of olivine and precipitation of orthopyroxene which can be describedas:Ol+Opx1(+Cpx)+Melt1â†'Opx2+Melt2The reacted melts in run JW1011have lower Al2O3and FeO contents and higher MgO contentthan the starting melts. The orthopyroxenes near the reaction zone in all of the three experiments reflectsimilar compositional variation. The orthopyroxene rims have higher FeO, Al2O3and CaO contentsthan the cores.2Constraints on the product of melt-peridotite reactionThe experimental results indicate that the lithology in the peridotite sequence is controlled mainlyby the reacting melt composition. The saturation of mineral varies in different melts. Clinopyroxene isthe most undersaturated with respect to either reacting melt resulting in its absence in all the reactionzones. The basaltic melt is olivine saturation but clinopyroxene+orthopyroxene undersaturation whichresults in the formation of dunite. The pyroxenite melt and the partial melt of the pyroxenite (silica-rich)are orthopyroxene saturation but clinopyroxene+olivine undersaturation which results in the formationof orthopyroxenite.The systematic compositional variations in reacted melts, the compositionally zoned olivines andorthopyroxenes in dunite and harzburgite zone, and the composition of olivine varies as a function ofthe distance to the melt-peridotite contact can be attributed to the changes in the melts resulting fromthe mineral dissolution.3Constraints on the transformation of the nature of Mesozoic lithospheric mantle in NorthChina CratonMelt-peridotite interaction is prevalent in Mesozoic lithospheric mantle in North China Craton according to the previous researches, and could be an important mechanism for the compositionaltransformation of the lithospheric mantle. In our experiments, the reaction between alkali basalt andperidotite results in pyroxene dissolution and olivine precipitation which facilitates the depletion of themantle; reaction between peridotite and either pyroxenite melt or a partial melt of pyroxenite(silica-rich) results in the orthopyroxene or SiO2enrichment in the peridotite which transforms thedepleted mantle to fertile lithospheric mantle4Implications for the petrogenesis of low-Mg dunitesDunites are commonly observed in ultra-mafic intrusive complexes, mantle xenoliths and theophiolites around the world. The origins of dunites include:1) as a residue formed by a high degree ofpartial melting of peridotite;2) as a cumulate formed by the fractionation of olivine from a mafic melt;and3) as a replacement product formed during melt–peridotite reactions. Our experimental resultsindicate the low-Mg dunites are formed by reaction between the basaltic melt and the peridotite, whichis also supported by the mineral chemistry.5Constraints on the high-Mg signature of adakitic rocksThe experimental results show that the reacted melts have higher Mg#than the starting meltswhich provide experimental constraints on the high-Mg signature of adakitic rocks. Compined with theresults of isotopic geochemistry, we suggest the Early Cretaceous high-Mg#diorite in westernShandong and southern Taihang Mountains was the product of reaction between the partial melts ofrecycled continental crust and the mantle peridotite. This is consistent with the presence oforthopyroxenite veins and metasomatic amphibole, phlogopite and orthopyroxene in dunite andharzburgite which is representative of adakitic melt metasomatism.