The Mantle Source Of Hannuoba Basalts And Its Release Of Chalcophile Elements

The subduction and volcanism modulate the exchange of materials and energy between the Earth's surface and deep mantle,which is critical for the habitability of the earth,evolution of the mantle,and migration of metallogenic elements.The deep mantle-derived intraplate basalts are widely occurred in intraplate settings and have the chemical spectrum from tholeiitic to alkaline.Therefore,intraplate basalts could provide a uniquely rich picture of the deep mantle.The subducted altered oceanic crust(AOC)is increasingly regarded to play a key role in the compositional diversity of intraplate basalts.However,the widely proposed experiment-based model for the origin of intraplate basalts by melting of subducting carbonated AOC remains enigmatic because the recognition of low-Mg O primitive alkaline basalts as predicted by experiments is scarce.The mechanisms operating during intraplate basalts generation and evolution are also controversial.In addition,subduction introduces oxidized materials and volatile such as carbonate into the mantle,which can significantly affect the redox state and melting behaviour of the mantle.Sulfide is the main budget of chalcophile elements in the mantle.The stability of sulfide is sensitive to the redox reactions and melting behaviour of the mantle,which controlled the behaviours of chalcophile elements during partial melting and magma differentiation.Thus,the chalcophile elements and Cu isotopes of intraplate basalts could provide insights into the effects of altered oceanic crust on the liberation of chalcophile elements from a mantle source.However,a variable fraction of chalcophile elements are dissolved in silicate phases,especially for basalts,which cannot be efficiently extracted from silicates by the traditional digestion method.Besides,the affinities to sulfides for chalcophile elements and the heterogeneous distribution of sulfide have significant effects(nugget effect)on the uncertainties of measurement results.We thus established an analytical method that can obtain the mass fractions of elements with variable chalcophile affinities from the same test portions of basalts.In addition,the influence of normal magmatic fractionation processes on?⁶⁵Cu variations remains poorly understood,and it is unclear whether the magmatic processes also lead to large?⁶⁵Cu variations in mantle rocks.The Cu isotope of pyroxenites that formed by melt-peridotite reaction and mineral accumulation from sulfide-saturated basic magmas were studied to evaluate the?⁶⁵Cu as a tracer of recycling crustal materials and redox conditions.Based on the above research,we further focus on major,trace element and Sr-Nd isotopic compositions of the Cenozoic intercalated intraplate continental tholeiitic and alkaline basalts from Hannuoba,eastern China to illustrate the genetic relationship between intraplate basalts and carbonated AOC.We also conducted Cu isotope and systematic chalcophile element studies on Hannuoba basalts,using the aforementioned established analytical method,to explore the liberation of chalcophile elements from a mantle source associated with altered oceanic crust.The recycled Pacific oceanic crust has been demonstrated to contribute to the mantle sources of Hannuoba basalts,and alkaline basalts have been demonstrated to be genetically linked to carbonates from the recycled oceanic crust.Here we show that Cenozoic continental intraplate alkaline basalts at Hannuoba,eastern China display geochemical features consistent with their origin as low-degree partial melts of carbonated eclogites.Their Mg O contents correlate positively with Ca O,Ba/Th and Ti/Eu,but negatively with Dy/Yb,Al₂O₃ and?_Nd.Remarkably,the most primitive alkaline basalts are characterized by low Mg O(<5.25 wt.%),low heavy rare earth elements and Sc contents,low Ca O/Al₂O₃(<0.41),low Ti/Eu(<3380),but Dy/Yb(>7.1)higher than OIBs.These features cannot be ascribed to differentiation from high-Mg O alkaline basalts because significant amounts of crystallization of clinopyroxene and garnet during ascent did not occur.Differentiation also cannot account for the correlations of time-integrated Sr-Nd isotopes with Mg O,Dy/Yb and Ba/Th.Instead,the linear correlations mainly reflect the strong interaction between the primitive alkaline melts and the lithospheric mantle.The compositions of primitive alkaline basalts reflect the key control of garnet and clinopyroxene in the mantle residue(eclogites),and the Ti,Zr and Hf anomalies further indicate the critical effect of carbonates in the eclogite source.Partial melting of the carbonated eclogites likely occurred in the uppermost asthenosphere.The production of silica-undersaturated alkaline basalts with low-Mg O contents by partial melting of carbonated eclogite below the peridotite solidus in an intraplate setting has been overlooked and the magmas instead considered to be highly evolved.The carbonated AOC may thus not only cause metasomatism of the deep mantle but may also serve as a direct source of mafic melts.These results on natural rocks support the experiment-based model for subducted carbonated ocean crust and also indicate its diverse fate in the mantle.Based on the petrogenesis of Hannuoba basalt.The chalcophile element content and Cu isotopes of Hannuoba basalt were analyzed by the aforementioned establishing analytical method to constraint the liberation of chalcophile elements from a mantle source associated with altered oceanic crust.The primitive Hannuoba alkaline basalts show significant fractionation of Cu-Ag(Cu/Ag:961–2720)and lower Cu content(Cu=33.5-58.7?g/g)than those in mid-ocean ridge basalts(MORBs;Cu/Ag:3600±400).Similar low Cu/Ag(1600)and Cu(?40?g/g)content also apply for primitive tholeiitic basalts.Clinopyroxene thermo-barometry results indicate that both the alkaline and tholeiitic basalts are above the liquids of sulfide.Given similar partition coefficients of Cu and Ag between sulfide melt and silicate melts,the low Cu/Ag ratio and Cu content cannot result from the migration of Hannuoba basalts during the lithosphere.The low Cu content and the BSE-like?⁶⁵Cu for alkaline basalts indicate the sulfide is stable in a carbonated mantle source.The low Cu/Ag ratio is probably due to the partial melting of carbonated eclogites under the conditions of high pressure,low temperature and residual monosulfide solid solution(MSS)in the source.The low Cu/Ag ratio and Cu content of tholeiitic basalts reflect the MSS or the recycled crustal materials in the mantle source.Although some tholeiites may be affected by crustal contamination,they exhibit much more homogeneous?⁶⁵Cu than that of the spinel peridotite xenoliths from Hannuoba.This may be ascribed to efficient homogenization.These results reveal that the low-Mg O alkaline basalts in the Hannuoba region are primitive melts of a carbonated eclogite source,which provides a strong link between the crust and the mantle origin of alkaline basalts,highlighting the importance of the subduction-driven deep volatile cycle.The results also provided some new critical constraints on the petrogenetic relations between alkaline basalt and tholeiitic basalt.The chalcophile elements cannot be efficiently extracted from a carbonated alerted oceanic crust.