Nature and evolution of the lithospheric mantle revealed by water contents and He-Ar isotopes of peridotite xenoliths from Changbaishan and Longgang basalts in Northeast China

The nature and evolution of the lithospheric mantle underlying Northeast(NE)China were investigated by assessing the mineral chemistry,water contents,and noble gas(He-Ar)isotopes of peridotite xenoliths captured by Cenozoic basalts from the Changbaishan and Longgang regions.The xenoliths,which have863–1141℃ equilibration temperatures,primarily comprise spinel lherzolites and rare spinel harzburgites.The Mg#(Fo)values of olivine in the peridotite xenoliths vary from 86.9 to 91.3.The clinopyroxenes have high Ti/Eu and low(La/Yb)N,and their chondrite-normalized rare earth elements(REEs)exhibit light REE-depletion to-enrichment patterns,indicating that the mantle underneath the investigated region was predominantly subjected to partial melting(1%–10%)and was metasomatized by silicate melts.The measured3He/4He ratios of the Changbaishan xenoliths have a narrow range from 5.8 Ra to 8.4 Ra with an average of 7.4 Ra.The3He/4He isotopic ratios of the Longgang xenoliths varied from 4.7 Ra to8.1 Ra with an average of 5.9 Ra;slightly lower than the Changbaishan xenoliths.The whole-rock H2Ocontents of the studied peridotite ranged from 9 to 132 ppm.The high H2O contents in excess of50 ppm(up to 132 ppm)might represent newly accreted and cooled asthenospheric materials,while those with H2O contents lower than 50 ppm(as little as 9 ppm)may represent thinned,relic,ancient lithospheric mantle.These geochemical evidences,in combination with published data,indicated that the lithospheric mantle beneath the Changbaishan and Longgang in NE China is dominated by the younger and more fertile lithospheric mantle with a minor ancient and refractory keel.In addition,the lithospheric mantle of this area was metasomatized by melts related to the recent subduction event(e.g.,Pacific oceanic plate).Therefore,the westward-dipping Pacific oceanic plate subduction had an important contribution to the transformation of the lithospheric mantle beneath NE China.

The nature and evolution of the lithospheric mantle underlying Northeast（NE）China were investigated by assessing the mineral chemistry,water contents,and noble gas（He-Ar）isotopes of peridotite xenoliths captured by Cenozoic basalts from the Changbaishan and Longgang regions.The xenoliths,which have863–1141°C equilibration temperatures,primarily comprise spinel lherzolites and rare spinel harzburgites.The Mg#（Fo）values of olivine in the peridotite xenoliths vary from 86.9 to 91.3.The clinopyroxenes have high Ti/Eu and low（La/Yb）N,and their chondrite-normalized rare earth elements（REEs）exhibit light REE-depletion to-enrichment patterns,indicating that the mantle underneath the investigated region was predominantly subjected to partial melting（1%–10%）and was metasomatized by silicate melts.The measured3He/4He ratios of the Changbaishan xenoliths have a narrow range from 5.8 Ra to 8.4 Ra with an average of 7.4 Ra.The3He/4He isotopic ratios of the Longgang xenoliths varied from 4.7 Ra to8.1 Ra with an average of 5.9 Ra;slightly lower than the Changbaishan xenoliths.The whole-rock H2Ocontents of the studied peridotite ranged from 9 to 132 ppm.The high H2O contents in excess of50 ppm（up to 132 ppm）might represent newly accreted and cooled asthenospheric materials,while those with H2O contents lower than 50 ppm（as little as 9 ppm）may represent thinned,relic,ancient lithospheric mantle.These geochemical evidences,in combination with published data,indicated that the lithospheric mantle beneath the Changbaishan and Longgang in NE China is dominated by the younger and more fertile lithospheric mantle with a minor ancient and refractory keel.In addition,the lithospheric mantle of this area was metasomatized by melts related to the recent subduction event（e.g.,Pacific oceanic plate）.Therefore,the westward-dipping Pacific oceanic plate subduction had an important contribution to the transformation of the lithospheric mantle beneath NE China.