The Fate Of Subducted Continental Crust: An Experimental Study

Previous works have demonstrated that deep subduction of continental crust is required toexplain the formation of coesite,micro-diamond and other index minerals in the ultra-highpressure metamorphic(UHPM)rocks from continental affinities.The possibility of continentalcrust being subducted into greater depth;perhaps even to the mantle transition zone has also beenproposed with the findings of oriented exsolutions of ilmenite and chromite/magnetite andtitanite and omphacite containing exsolved coesite rods and plates.More recently,subduction ofcontinental crust into depth of at least 350 km and return to the surface has been suggested withthe finding of former stishovite pseudomorphy in UHPM rocks from the Altyn Tagh,westemChina and relevant experiments.The idea of deep subduction of continental crust,along withUHP eclogite has also been supported by geochemical isotope signatures,and more recentlycoesite-bearing zircons from paragneisses within UHPM terrains.Subduction and recycling ofthese components into the mantle play a significant role in the evolution of mantle heterogeneityin terms of trace element abundances,volatile contents and radiogenic isotope systems.The mineralogy of subducted continental sediments at high pressure has been inferred fromhigh P-T experiments performed on compositionally analogies of average continental crust.Inorder to avoid complication related to reaction kinetics and to easily establish chemicalequilibrium,most of the previous experimental studies carried out on gels,glasses and sinteredoxide mix starting materials with bulk compositions close to that of pelagic and terrigenoussediments and hypothetical continental crust.Only few experiments were performed usingnatural rock powders as starting material.Experimental results suggest that at shallow depth thesubducted continental material,mainly consisting of coesite,clinopyroxene,orthoclase,garnet,and minor hydrous phases,is buoyant compared to the surrounding mantle in terms ofzero-pressure density.The intrinsic buoyancy of continental crust would oppose the entrainmentof subducted slab and would provide buoyancy-based exhumation force for UHMP rocks.However,it is not well established experimentally that to what depth this buoyancy contrastwould be reversed,thus the fate of the subducted continental crust in deep mantle is questionable.In addition to the continental crust,oceanic trench where a few hundred meters to severalkilometer thick sediments eroded from continental crust and deposited on the seafloor is anotherenvironment in which continental crust material may enter deep part of the upper mantle.It hasbeen shown that the subducted oceanic crust becomes much denser than the surrounding mantlethroughout the upper mantle,the sedimentary layer could be delivered,along with the basaltic crust,into the deep mantle and produce remarkable geochemical heterogeneous.Another poorly constrained issue is the maximum stability of some hydrous minerals andthe role of continuous dehydration reactions in the subducted continental crust.The emergingquestion of water recycling at continental subduction zones involve not whether water is recycledbut how it is recycled.What phases are involved in which reactions under what conditions?Phase transformations in the subducted continental slabs are mostly described on the basis ofexperiments in the simplified model systems.In these systems,solid solutions are inhibited bythe absence of proper chemical components.This would further favor discontinuous dehydrationreaction which implies a focused release of fluid at specific uninvariant P-T conditions.Incontrast,solid solutions are more common in natural minerals,thus continuous reaction plays anessential role in volatile releasing in complex natural systems,even though some hydrousminerals do not show significant compositional variability.The breakdown reaction of somehydrous minerals may involve a number of solid solutions and is therefore continuousdehydration.Consequently,in most natural systems dehydration reactions in the subducted slabsare continuous,and fluid release tends to be smeared out over a wide pressure and temperaturerange rather than at a specific P-T conditions.Thus,it requires experimental investigations onnatural rocks to better understand these complex issues.In order to determine the mineralogy and density of subducted upper continental crust,wecarried out experiments on natural biotie bearing para-gneiss and granitic gneiss powders fromShuanghe UHP terrain,Dabie Mt.,Eastern China.Those rocks are compositionally similar to thatof the classic rocks from UHPM terrains over a wide pressure and temperature ranges of 3.5GPa-24 GPa and 750℃-1800℃,corresponding to the P-T path through the upper mantle.Theexperiments provide data on the subsolidus phase relations and buoyancy relationship of thesubducted upper continental crust and its surrounding mantle as a function of pressures andtemperatures.The fate of the subducted upper continental crust and water releasing andtransportation during continental subduction will be discussed in light of the new experimentaldata.We also explore the micro-structural and textural characteristics of the phase transformationas a result of breakdown reactions and replacement by high-pressure phases,especially ofhydrous minerals.The observations provide a close link between the high-pressure experimentsand the occurrences of natural UHPM minerals.The experimental charges show well-crystallized textures of the coexisted phasescharacterized with straight or flat grain boundaries and homogeneous chemical compositions,implying that close chemical equilibrium was achieved.The starting material was firsttransformed into an assemblage consisting of coesite+jadeite+garnet+phengite+epidote/lawsonite in the pressure range of 3.5-9 GPa at temperatures up to 900℃.Withincreasing pressure,the assemblage transforms to stishovite+jadeite+K-hollandite+garnet+phengite or K-mica depending on temperature in the pressure range of 9-14 GPa.Epidote/lawsonite was smeared out above 9 GPa,while phengite was eliminated at a pressure of14 GPa.The assemblage consisting of stishovite+jadeite+K-hollandite+garnet remains to bestable at pressures up to 18 GPa,where Ca-perovskite was identified.The mineral assemblages in the pressure range of 18-24 GPa showed minor variation.However,significant variations of thechemical compositions and modal proportion of minerals were observed.Jadeite and coesiteformed fine-grained aggregates as breakdown products of plagioclase in experiments at pressurerange 3.5-6 GPa.Both jadeite and coesite from these aggregates were identified by Ramanspectroscopy with the diagnostic peaks at 699 cm^-1for jadeite and 523 cm^-1for coesite.Therewere also porphyroblastic or coarse grained coesite,characterized by the most intensive Ramanpeak at 523 cm^-1,formed from the direct transformation of the original quartz in the startingmaterial.Symplectitic intergrowth of phengite with Si content of 3.5-3.8 pfu rimmed along theinterfaces of K-feldspar and biotite and of plagioclase and biotite,was observed in thelow-temperature experiments.At higher pressures and temperatures,fine garnet grains together with K-rich mica andK-hollandite formed myrmekitic microstructure indicate dehydration reactions of biotite.Coesitewas replaced by stishovite and K-hollandite was present in runs at pressures＞9 GPa.Similar tocoesite occurrence,stishovite appeared in two textures(fine and coarse grains)and wasconfirmed by its characteristic Raman shift at 759 cm^-1and 589 cm^-1.K-hollandite wasconfirmed by diagnostic Raman shift at 765 cm^-1.The fine grain stishovite is the product of thephengite/K-mica dehydration;whereas coarse grains were formed from the direct transformationof coarse grain quartz.Both phengite and K-mica were eliminated at 14 GPa through thereactions 5 and 6 above.Epidote/lawsonite was observed only in experiments below 9 GPa and attemperature＜1000℃.It was eliminated and replaced by garnet at higher temperature throughmelting.It is worth noticing that no major hydrous minerals were found in experimentsexceeding 14 GPa.The transformed assemblage consists of stishovite,jadeite,garnet andK-hollandite.There is no significant change of phase assemblage within this pressure range.The chemical compositions of minerals showed systematic changes with both pressure andtemperature.For example,clinopyroxene gradually dissolved into garnet which becameincreasingly majoritic and Na rich with increasing pressure between 14-24 GPa.Likewise,atconstant temperature,the jadeite content in pyroxene tends to increase with increasing pressure,while it decreases with increasing temperature at constant pressure.At pressures greater than 14GPa,the composition of clinopyroxene from the experiments approaches that of pure jadeite.Garnet contains a significant amount of majorite component(Si=3.08-3.30 pfu)at pressuresexceed 11 GPa as compared with stoichiometric garnet(Si=3 pfu).The Ca content in garnetdecreases rapidly with increasing pressure as a result of the occurrence of CaSiO3 perovskite atpressures＞18GPa.The composition of coesite from experiments at 3.5 and 6 GPa were close tothat of pure SiO₂.Less than 0.1 wt% Al₂O₃ and Fe₂O₃ were soluble in coesite.The solubility ofAl₂O₃ and Fe₂O₃ in stishovite increases with pressure and it slightly increases with increasingtemperature at pressures exceeding 9 GPa where coesite transforms to stishovite.At 24 GPa and1800℃,stishovite contains 4.23 wt% Al₂O₃.The solubilities of Lingunite(NaAlSi₃O₈)andCaAl₂Si₂O₈ in K-hollandite showed strong positive correlations with increasing temperature.Themaximum solubility of Lingunite and CaAl₂Si₂O₈ in K-hollandite occurred at the highestpressure and temperature of this study(24 GPa,1800℃).At this P-T condition,42 mol% NaAlSi₃O₈ and 11 mol% CaAl₂Si₂O₈ were dissolved into K-hollandite.We used third-order high-temperature Birch-Murnaghan(HTBM)equation of state,combined with mineral proportions present in subducted continental crust at elevated pressuresand temperatures obtained in this study,to calculate the density profiles of the subductedcontinental crust rocks along geotherms of cold subduction,hot subduction and normal mantle,respectively.The comparison of the calculated density profiles of the subducted continental crustwith that of surrounding mantle provides some insight into the fate of the subducted continentalcrust.The subducted continental crust is relatively buoyant compared to the surroundingmantle at depths showller than c.250 km.The intrinsic buoyancy of the subducted continentalcrust would oppose entrainment to greater depth,and the buoyancy force would be an importantexhumation mechanism.Our experimental results demonstrate that the density of continentalcrust would be equal to or denser than that of mantle rock when those rocks have beentransported to depth＞250 km(8-9 GPa).Thus,the'point of no return'derived from ourexperimental results would be located at 250 km-300 km.Accordingly,once these rocksbeing subducted together with eclogite and/or dragged by oceanic subduetion exceeding thecritical depth,the high-pressure phases in the subducted continental crust would creategravitational instability which would favor continental subduction into the lower part of thetransition zone.In that case,entrainment by the sinking slab could become the predominantinfluence until the base of the mantle transition zone,which may play an important role in thegeochemical evolution of the Earth.