Ordering And Lattice-preferred Orientation Of Dolomite From Ultrahigh-pressure Metamorphic Marble In The Dabie Mountains

Dolomite[CaMg(C03)2]widely occurs in dolostone,limestone and marble.The wide P-T stability conditions of dolomite make it an important mineral for understanding deformation in subduction channels,mantle metasomatism and global carbon cycle.Previous studies have found the influence of temperature,pressure and composition on the ordering of dolomite,as well as deformation mechanisms and dominant slip systems of dolomite under pressure lower than 1 GPa.At 1 to 3 GPa,the transition temperature from complete disorder/order of dolomite is 1150-1200?,which will be significantly decreased by addition of Fe,Mn or Cr.Naturally deformed dolomite often develop the basal c slip.However,the influence of pressure,composition and ordering on rheological properties of dolomite is still unknown.Here we first presented a review on ordering and rheological properties of dolomite,then we selected dolomite marble from Shuanghe in the Dabie ultrahigh-pressure metamorphic belt to analyze composition,microstructure and ordering of dolomite.Using the electron backscatter diffraction technique,we obtained the crystallographic preferred orientation(CPO)of dolomite.The results indicate that the c slip characterized by(0001)[2110]slip system controlled deformation of our samples,meanwhile the CPO of some dolomite grains suggests activation of the slip system(1014)[12T0],i.e.,the r slip,which may be related with ultrahigh-pressure metamorphism.In addition,dolomite grains in our samples show compositional variations.The strength of the r slip is stronger in Fe-poor dolomite grains than in Fe-rich dolomite grains.The ordering degree of dolomite,s(h),can be defined by the strength ratio between the X-ray diffraction peaks(010)and(110).We found that with increasing Fe content,the measured s(h)of dolomite decreases.Theoretical calculations indicate that the value of s(h)depends on not only crystallographic positions of Mg2+?Ca2+?Fe2+ irons,but also Fe content.The crystallographic ordering of dolomite may reduce the critical shear stress of the c slip,resulting in the dominant activation of the c slip in dolomite.However,addition of Fe2+ content may affect the critical shear stress of the c slip and r slip,making the r slip dominant in Fe-poor dolomite.The different LPOs between Fe-poor and Fe-rich dolomite in the Shuanghe marble imply that distribution of Fe2+ may affect dislocation creep of dolomite under ultrahigh pressure.As a stable carbonate mineral in subduction zones,further experiments on the fabric development of dolomite under ultrahigh pressure will clarify the formation mechanism of the r slip,as well as the relationship between ordering and rheological properties of dolomite.Such knowledge will improve our understanding of subduction and exhumation mechanisms of carbonates,and provide new constraints to trace deformation history of dolomite-rich rocks.