High-pressure And High-temperature Experimental Investigation On The Subducted Carbonate Recycling And Its Modification To The Lithospheric Mantle Beneath The North China Craton

Subduction drives the continuous exchange of materials and volatile recycling between Earth’s surface and interior.As one of the most important volatile components involved in the aforementioned processes,the recycling of carbon affects the interaction between the Earth’s spheres and plays an important role in the enrichment of ore-forming elements and global climate changes.However,much attention has been paid to the recycling of subducted carbonates and its modification to the sub-arc mantle wedge as well as its regulation on the arc volcanic CO₂ emission in the regime of shallow subduction zone.Researches on deep and ultra-deep sourced samples exposed in the intraplate environment have revealed that the recycling of subducted carbonates could extent to depths reaching mantle transition zone or even lower mantle.The release of carbonates introduced into the deeper part of subduction zone may have a great impact on the composition and physicochemical properties（e.g.,melting point,oxygen fugacity）of deep mantle,which further influences the lithospheric evolution and the related magmatism and CO₂outgassing in the intracontinental region.Due to the scarcity of deep-and ultra-deep-derived natural samples,it is highly necessary to present a systematic high-pressure and high-temperature（HPHT）experimental study on the above processes.The North China Craton（NCC）is a typical Archean craton affected by multi-slab subduction systems,and the eastern part of which experienced intense lithospheric modification and replacement in the context of subduction tectonics in the Mesozoic.The integrated studies of seismic tomography and natural samples have shown that the subducted slabs around the NCC have carried massive surficial carbonates into the deep mantle,which could also have contributed to the rapid lithospheric evolution of the NCC in the Mesozoic.Hence,the NCC is a good natural laboratory for understanding the effect of deep recycling of subducted carbonates on the evolution of continental lithosphere.The objective of this dissertation is to provide experimental constraints on the deep recycling process of subducted carbonates and its impact on the evolution of the intraplate continental lithosphere while taking the NCC as an example.The detailed contents include:an experimental investigation on the reaction of subducted carbonates with deep mantle peridotite,an experimental investigation on the carbonate melt-harzburgite reaction and a synthetic study on the contribution of recycled carbonate melt infiltration to the Mesozoic reactivation of the NCC lithosphere.The main conclusions are as follows:1)In the region of shallow subduction zone（90～180 km）,the interaction of subducted calcium carbonates with simplified mantle peridotite（clinopyroxene-free）forms reaction zone consisting of clinopyroxene+magnesite/carbonate melt and pure Mg-rich carbonate layer at the contact zone.The solidus of subducted calcium carbonates-mantle peridotite reaction system（～1000℃）is far below that of pure carbonates,and the melting behavior of carbonates in contact with mantle peridotite is also quite different with that of carbonates in other carbonate-bearing lithologies（e.g.,carbonated pelites）,suggesting that slab-mantle interaction could notably promote the decomposition of subducted carbonates via melting.By combining the experimental results with thermal models of some typical modern subduction zones where abundant carbonates are being subducted at trench,we have evaluated the stability of subducted carbonates at the shallow part of subduction zone.We suggest that the subducted carbonates will not suffer melting in general case despite the notably lowed melting temperatures for calcium carbonates in contact with mantle peridotite at the slab-mantle interface,which ensures their escape from shallow devolatilization through arc volcanism and entering into deeper subduction zone.2)In the region of deeper subduction zone（>250 km）,the interaction of subducted calcium carbonates with deep mantle peridotite produces clinopyroxene/Ca-perovskite+Ca Fe³⁺-garnets+magnesite±periclase.In combination with previously reported experimental studies on Fe-carbonate reaction systems,our results suggest that the deeply subducted calcium carbonates reaching deep reduced mantle would interact with both metallic iron and mantle silicates,which could greatly affect the composition and the redox state of the deep mantle.The consumption of metallic iron would result in an increase of oxygen fugacity of deep mantle so as to beyond the Fe-Fe O buffer（IW）,while the interactions between calcium carbonates and Fe²⁺-bearing mantle silicates further oxidize deep mantle,producing Fe³⁺-and carbon-enriched mantle domains.Such carbon-enriched mantle domains with increased bulk Fe³⁺/∑Fe ratios would suffer redox melting when upwelling to depths below the base of a thick subcratonic lithosphere（>200 km）,forming carbonate melts penetrating through and modifying the overlying subcratonic lithospheric mantle.3)The lithological consequences and reaction kinetics of interaction between recycled carbonate melt and harzburgite/dunite have been explored with long-duration experiments（1.5～3 GPa,1000～1100℃）and short-duration experiments（3 GPa,1100～1300℃）respectively,and mafic granulite-derived silicate melt-harzburgite reaction experiments were for comparison.It shows that silicate melt-peridotite reaction is restricted to melt-rock interface,and is characterized by precipitation of orthopyroxene followed by crystallization of clinopyroxene±garnet at the expense of former orthopyroxene and evolved reacting melts at the contact zone.Whereas extensive reactive infiltration of carbonate melts into hazburgite/dunite occurred,giving rise to various extent of clinopyroxene enrichment in the cases of carbonate melt-hazburgite/dunite reaction.Carbonate melt-harzburgite reaction is proceeded by diffusion-governed processes including:（1）rapid reactive infiltration of melt into the harzburgite matrix along grain boundaries forming melt-impregnated harzburgite;（2）simultaneous dissolution of orthopyroxene and olivine and precipitation of clinopyroxene at melt-rock interface forming clinopyroxenite;（3）preferential replacement of orthopyroxene by clinopyroxene forming lherzolite above the clinopyroxenite.Collectively,melt-rock reaction involving carbonate melt is much more extensive than silicate melt inside the continental lithospheric mantle,and the width of modified mantle region by carbonate melts is expected to be 3～5 orders of magnitudes higher than that by silicate melts within the same time on the basis of melt-rock reaction rates determined by our experiments.4)By comparing lithology and mineral compositions obtained from melt-peridotite reaction experiments with those from mantle-derived samples,it shows that varying extent of carbonate melt modification resulted in clinopyroxene re-enrichment to varying degrees in the NCC lithospheric mantle.The resulted lithologies cover a wide spectrum fromclinopyroxene-poorharzburgite/dunitetoclinopyroxene-rich lherzolite/wehrlite/clinopyroxenite.In contrast,silicate melt-peridotite reaction inside the NCC lithosphere produced restricted pyroxene/garnet-rich lithologies（e.g.,orthopyroxenite,websterite,garnet-pyroxenite,garnetite）.Meanwhile,it has been revealed that the ancient lithospheric mantle have ubiquitously experienced carbonate melt metasomatism which could at least date back to as early as the Ordovician,and persist to the Early Cretaceous as a result of ongoing subductions around craton edges.Since the termination of lithospheric replacement at≤110 Ma,the carbonate melt metasomatism had greatly diminished in the newly accreted lithospheric mantle.Prolonged and ubiquitous carbonate mantle modification may have greatly contributed to the Mesozoic replacement of the eastern NCC.5)A synthesis of HPHT experimental investigations,field studies on mantle-derived samples and quantitative calculations shows that the widespread carbonate melt modification inside the ancient lithospheric mantle of the NCC should be the result of multiple episodes of carbonate melt infiltration induced by multiple subductions,which lead to progressive oxidation and ultimate carbonation coupled with chemical enrichment（e.g.,Ca-enrichment）in the deep NCC lithosphere.The enriched and carbonated basal lithosphere could suffer melting and be easily eroded and removed in the context of enhanced mantle convection and upwelling during the retreat or collapse of Paleo-Pacific slab in the Early Cretaceous.Hence,the prolonged infiltration of carbonate melts induced by deep recycling of subducted carbonates may have provided a prerequisite for the Mesozoic replacement of the eastern NCC.Meanwhile,the infiltrated carbonate melts into shallow levels could be transformed into CO₂ storage in the NCC lithospheric mantle,and the estimated mantle CO₂ degassing at the time of extensional tectonics of the eastern NCC（140～110 Ma）is approximately 18～36 Mt yr^-1.These values account for about22～45%of the total CO₂ flux through global rifts activated in the Early Cretaceous,suggesting the remarkable contribution of NCC lithospheric thinning to the contemporary Cretaceous greenhouse climate.We thus strongly propose that there are intrinsic relations among deep recycling of subducted carbonates,evolution of intraplate continental lithosphere and global climate change.