Sulfur isotopic composition of modern seafloor hydrothermal sediment and its geological significance

A total of 1 264 sulfur isotopic values for modern seafloor hydrothermel sediments from different hydrothermal fields have been collected.On this basis,combining our sulfur isotpic data for surface hydrothermal sediments from the Jade hydrohtermal field in the Okinawa Trough and the TAG hydrothermal field in the Mid-Atlantic Ridge,respectively,and comparing the sulfur isotopic compositions and analyzing their sources of sulfur in seafloor hydrothermal sediments from different geologic-tectonic setting,the results show that:(1) sulfur isotopic values of sulfides and sulfates in modern seafloor hydrothermal sediments are concentrated in a narrow range,δ 34S values of sulfides vary from 1×10 -3 to 9×10 -3,with a mean of 4.5×10 -3 (n=1042),δ 34S values of sulfates vary from 19×10 -3 to 24×10 -3,with a mean of 21.3×10 -3(n=217);(2) comparing the sulfur isotopic compositions of hydrothermal sediments from the sediment-hosted hydrothermal fields,the range of sulfur isotopic values for hydrothermal sediments from the sediment-free hydrothermal fields is narrow relatively;(3) the differences of sulfur isotopic compositions in sulfides from different hydrothermal fields show the differences in the sources of sulfur.The sulfur of hydrothermal sulfides in the sediment-free mid-ocean ridges is mainly from mid- ocean ridge basalt,and partially from the reduced seawater sulfate,and it is the result of partially reduced seawater sulfate mixed with basaltic sulfur.In the sediment-hosted mid-ocean ridges and the back-arc basins,the volcanics,the sediments and the organic matters also can offer their sulfur for forming hydrothermal sulfides;(4)the variations of sulfur isotopic compositions and the different sources of sulfur for hydrothermal sediments may be attributed to the various physical-chemical characteristics of hydrothermal fluids,the magmatic evolution and the different geologic-tectonic settings of seafloor hydrothermal systems.

Cretaceous source to sink system reconstruction of northeastern Asian continental margin:Insight from integrated detrital geochronology in NE China

The Cretaceous sedimentary successions in NE China are largely incomplete as the basins are inverted and younger strata are eroded.Except for the Songliao Basin,whose depositional record has remained relatively intact,the burial record to the east was interrupted in the late Early or Late Cretaceous.There is still controversy on the possible connection between the Songliao Basin and the eastern basin group,as well as the extent of the previously suggested“pan-Sanjiang”proto-basin that covered most current eastern satellite basins.To address these questions,we studied modern river catchments of variable sizes from sediment-supplying basement highs that cover most of eastern NE China to track the intense denudation events and provenance changes through the Cretaceous.We found a great inconsistency between detrital age patterns of the modern river sands and the spatial proportions of the source units.The detrital age distributions allow to re-evaluate the basement units that are mostly composed of the Triassic–Jurassic igneous suite and some metasedimentary units with Paleozoic ages,but the contribution from Proterozoic formations is negligible.Combining the newly dated modern catchment detrital U-Pb ages,the region-wide compilation of the basement emplacement ages,and the U-Pb ages in the Cretaceous formations in NE China,we systematically refine the Cretaceous provenance history.In the Early Cretaceous,the eastern satellite basins accumulated sediments from the Lesser Xing’an range(LXR)in the west,Zhangguangcai range(ZGCR)in the south,and the Nadanhada terrane(NT)in the east.The sediment of the southern Songliao Basin derived from the North China Craton(NCC)and from ZGCR but barely from the Great Xing’an Range(GXR).The Jiamusi uplift(JU)did not provide sediments to the eastern satellite basins and the Songliao Basin.In the Late Cretaceous,the eastern satellite basins received sediments from the exhumed JU but barely from ZGCR and LXR.The Songliao Basin still accumulated the sediments from the NCC,some south part of ZGCR,and GXR but minor from the central and northern ZGCR and LXR.Between the latest Early Cretaceous and early Late Cretaceous,a much wider proto-basin was suggested in NE China than previously assumed.It covered during its maximum extension period the current Songliao Basin,the eastern satellite basins,the LXR,ZGCR,JU,and partial GXR basement areas.The provenance change is induced by the Paleo-Pacific plate motion change,i.e.,with the subduction direction change,the roll-back,and twostage limited flat slab subduction trigged the formation of the basin-and-range system in NE China since the Early Cretaceous.Moreover,we suggest conducting modern sand provenance analysis to detect,verify or re-classify the ages of the zircon-bearing units,especially at a reconnaissance prospecting on areas covered by imprecise large-scale geological maps only to reveal the catchment better.

Grain-size and compositional variability of Yarlung Tsangpo sand(Xigaze transect,south Tibet):Implications for sediment mixing by fluvial and aeolian processes

Studying the grain-size dependent compositional variability in modern river sediments provides a key to decipher the information stored in the sedimentary archive and reconstruct the evolution of the Earth’s surface in the past. Bedload sand along the Xigaze cross section of the Yarlung Tsangpo(upper Brahmaputra River) ranges in mean grain size from 0.72 Φ to 3.21 Φ, is moderately to poorly sorted and slightly platykurtic to moderately leptokurtic with sub-angular to sub-spherical grains. Litho-feldspatho-quartzose to feldspatholitho-quartzose sand(Q 43%-65%;F 13%-44%;L 11%-28%) contains 3.4%-14.4% heavy minerals including amphibole(64%-89%), epidote(4%-11%), chloritoid(0-10%), and clinopyroxene(2%-6%). The marked textural and compositional variability observed across the Xigaze transect of the Yarlung Tsangpo mainstem is controlled by both fluvial and aeolian processes, including repeated reworking by westerly and glacial winds,as well as by local contributions from northern and southern tributaries draining the Lhasa Block and the Himalayan Belt, respectively. The modern sedimentary case here will shed new light on interpreting paleogeography and provenance.