Melt-Peridotite Reaction: Constraints On Spinel Compositional Variation From Mantle Xenoliths

High-temperature and high-pressure experiments on the reactions between Hb-Gt pyroxenite and spinel-lherzolite as well as between Hb-Gt-Py- plagiogneiss and spinel-harzburgite were carried out to constrain the composional variation of spinels from the mantle xenoliths and reveal the lithospheric deep processes.1 Melt-peridotite reaction and results(1) Am-Gt-pyroxenite and spinel-lherzolite reactionExperiment on reaction of Hb-Gt pyroxenite with Sp-lherzolite was carried out at 1500℃and 3.5GPa for 24 hours in a Cubic model HTHP apparatus in the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry at Jilin University. The results indicate that the Hb-Gt pyroxenite was melted, that the dunite, harzburgite, and lherzolite zones were formed during the melt-Sp-lherzolite, and that the spinel in lherzolite had been changed into the chromite in dunite. Cr#'s of spinels gradually decrease and Mg#'s of spinels increase from dunite, harzburgite to lherzolite zones, whereas Mg#'s of olivines increase. In contrast, the Mg#'s of melts increase from the dunite to the melts far from the melt-lherzolite interface.(2) Hb-Gt-Py plagiogneiss and spinel-harzburgite reactionExperiment on the reaction of Hb-Gt-Py plagiogneiss with Sp-harzburgite was carried out at 1350℃and 1.5GPa for 6 hours in Brown University by Liang Yan. Differently, dunite layer is not observed. Instead, a thin Opx layer is formed at the melt-rock interface. The irregular orthopyroxenes occur around or partly around relict olivine near the interface. The bright rim of the spinel on EBS images implied a relatively high Cr₂O₃ content and low Al2 O3 content, similar to the above mentioned experimental results. Melt compositional profiles from the melt-lherzolite interface to the melt far from the interface show the increases of SiO₂ , CaO, Al₂O₃ and decrease of MgO, FeO.2 Compositional variation of spinel in melt-lherzolite reactionMost of the spinels had changed into chromites during the melt-lherzolite reaction. Relict spinel cores can be only observed in the lherzolite zone.Compared with the initial spinel, reactive spinels generally display the decreases of Al₂O₃ , MgO and the increases of Cr₂O₃ , FeO as well as minor increase of TiO₂, MnO and CaO. From the dunite to lherzolite zones, the Mg#'s of spinels increase from 46 to 76 and the Cr#'s of spinels decrease from 68 to 12, whrereas the Mg#'s of olivines increase from 75 to 90, the latter is similar to the initial spinel and olivine in the Sp-lherzolite.3 Compositional variation of spinel in peridotite xenoliths from the Taihang MountainsThe peridotite xenoliths are discovered in Fushan high-Mg diorites with the Early Cretaceous ages from southern Taihang Mountains. The host rocks consist mainly of gabbro-diorite, diorite and monzodiorite. Detailed studies on petrography and mineral chemistry indicate that the xenoliths are composed of chromite-bearing dunite, Sp-harzburgite and Sp-lherzolite. The Sp-harzburgite and Sp-lherzolite are dominated in the xenoliths. The peridotites have SiO₂=40.16-45.99%, Al₂O₃ =0.15-1.47%, MgO=43.67-47.5%, TiO₂=0.01-0.03%,Mg# =90-93. They have the REE abundances of 0.59-5.16 ppm and are characterized by enrichment in light rare earth elements (LREEs) and high field strength elements (HFSEs) such as Nb, Zr, Hf, depletion in heavy rare earth elements (HREEs) and minor positive Eu anomalies (Eu/Eu*=0.83-1.52).Spinel compositional profiles from core to rim show the decrease of Mg#'s and increases of Cr#'s and fO₂ , which is consistent with the presences of hornblende and phlogopite around the spinels. The Mg#'s increase and Cr#'s decrease for spinels can be found from the dunite-harzburgite to the lherzolite. 4 Melt-peridotite reaction constrains the evolution of continental lithospheric mantleThe experimental results have revealed that dunite and harzburgite can be formed by melt-peridotite reaction, different from the traditional view being residue of highly degree of partial melting of mantle. Two main reactions in the lithospheric mantl are as follows: Olivine + SiO₂ (silica-rich melt)=Opx (+silica-poor melt) (1) Opx + silica-poor melt=Olivine + SiO₂ (silica-rich melt) (2)The two types of reactions are the main reasons to result in the mantle heterogeneity.The results of the lherzolite dissolution depend on the composition of the reacting melt. Generally, the melt is characterized by enrichment in Si, Ca, Fe and depletion in Mg, Ni. Melts-peridotite reaction could result in enrichment in Si, Ca, Fe and depletion in Mg, Ni, i.e., the formation of low Mg peridotite.The North China Craton (NNC) is one of the oldest Archean cratons in the world. It has been generally believed that a large scale of thinning beneath the North China Craton happened in the Yanshanian and Himalayan from a number of recent studies. The discovery of late Triassic eclogite xenoliths in the early Cretaceous adakitic rocks from the Xu-Huai region indicates that the lower crustal thickening in the eastern NCC took place during Early Mesozoic, which resulted from the Triassic collision between the Yangze craton (YC) and the NCC. The thickening, foundering and partial melting of the Archean NCC mafic lower crust, as well as subsequent adakitic melt-mantle interaction have been demonstrated by chronological and geochemical data of eclogite xenoliths and host adakitic rocks in the Xu-Huai area. Dunite is believed to be the ideal pathways by which mantle melts travel through the overlying mantle to be erupted to the surface. Melt-peridotite reaction is common in the Mesozoic and Cenozoic lithospheric mantle in East China. It is an important mechanism for the compositional transformation and of lithospheric mantle, and result in the enrichment in the lithospheric mantle. Melt-peridotite reaction is one of the main reasons for the thinning of the NCC.