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In situ Os dating of peridotite xenoliths,Tariat, northern Mongolia
KUO-LUNG WANG1,2, SUZANNE Y. O’REILLY
2,WILLIAM L. GRIFFIN
2, NORMAN J.PEARSON2
1Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
([email protected])2GEMOC Key Centre, Department of Earth and Planetary
Sciences, Macquarie University, Sydney, Australia (soreilly@
els.mq.edu.au; [email protected]; [email protected])
Cenozoic basalts are associated with regional intensive riftingactivity across much of Mongolia. The Tariat volcanic field, northof the Hangai Mountains, central Mongolia contains abundantmantle xenoliths. TRD model ages of sulfides in such xenoliths,irrespective of their Re/Os ratios, can provide minimum estimatesfor the age of the subcontinental lithospheric mantle (SCLM) andrecord later metasomatic events. Sulfides from the Tariat regionhave TRD model ages ranging from 1.8 to 0.5 Ga, with peaksaround 1.6, 1.3, 1.0 and 0.6 Ga. These events recognized in theSCLM are consistent with those known in the overlying crust asrecorded by Nd model ages of granitoids (1.7–0.7 Ga, Jahnet al., 2004; 1.4–1.1 Ga, Kovalenko et al., 2004) and Hf modelages of detrital zircons (Griffin, Unpublished data). The sulfideage data indicate that parts of the SCLM beneath the regionare as old as the oldest known crust in the Precambrian microcon-tinental blocks (1.8–2.0 Ga) of the Altaid Orogenic belt. Youngersulfide ages may actually date metasomatic events in the SCLM,related to mantle thermal events that affected the overlying crust,including crustal formation in the Altaid Orogen since Paleozoictime.
Spinel lherzolites from the Haer and Zala volcanic centres ofthe Tariat field represent relatively fertile SCLM. While a smallpercentage of sulfides show superchondritic 187Os/188Os(>0.127), none has 187Os/188Os lower than 0.116 (correspondingto TDM = 1.8 Ga, Shirey and Walker, 1998). The large range in187Re/188Os (0.036–1.049) may suggest recent addition of Re relat-ed to Cenozoic magmatic activity. The least-disturbed sulfide,with 187Re/188Os = 0.036, yield similar TMA and TRD model ageof 0.7 ± 0.1 Ga (2r), marking significant SCLM formation duringthe Paleozoic Altaid Orogeny. Sulfides with low Re/Os (0.042 and0.044) and more radiogenic 187Os/188Os (0.1293 and 0.1248) sug-gest the formation of new SCLM from the primitive mantle; thisprocess may be related to Cenozoic rifting and the upwelling ofasthenospheric mantle, which cooled to form new, weakly deplet-ed, SCLM (Zheng et al., 2006).
ReferencesGriffin W.L. unpublished data.Jahn, B.-M. et al., 2004. J. Asian Earth Sci. 23, 629–653.Kovalenko, V.I. et al., 2004. J. Asian Earth Sci. 23, 605–627.Shirey, S.B., Walker, R.J., 1998. Adv. Refract. Effects Predict. Syst. 26,
423–500.Zheng, J.P. et al., 2006. CMP 151, 457–479.
doi:10.1016/j.gca.2006.06.1495
High-latitude forcing of interhemisphericanti-phased precipitation duringthe last glacial-interglacial cycle
X. WANG1, A.S. AULER
2, R.L. EDWARDS1, H. CHENG
1
1Department of Geology and Geophysics, University of Minne-
sota, Minneapolis, MN 55455, USA ([email protected])2CPMTC, Instituto de Geociencias, Universidade Federal de
Minas Gerais, Belo Horizonte, MG 31270-901, Brazil
A high resolution oxygen isotopic record of cave calcite, span-ning the last 90,000 years, was obtained from Caverna Botuvera,southern Brazil. The record broadly follows local insolationchanges and shows significant millennial-scale variation withamplitudes as large as 3&. Tests for equilibrium conditions showthat oxygen isotopic variations are primarily caused by climatechange. We interpreted the oxygen isotopic variations in termsof meteoric precipitation changes at this low-latitude location.Established with independent absolute-dated uranium series chro-nologies, our record anti-correlates remarkably with the contem-poraneous oxygen isotopic records of cave calcite from easternChina (e.g. Wang et al., 2001, 2005; Yuan et al., 2004) (anti-phaseprecipitation changes), but positively correlates with the record ofspeleothem growth periods from northeastern Brazil (Wang et al.,2004) (in-phase precipitation changes) on millennial time scales.
These observations suggest that a rainfall seesaw may haveexisted between the southern and northern low latitudes on mil-lennial time scales in the past. This interhemispheric anti-phasingof rainfall is probably caused by feedbacks between North Atlan-tic thermohaline circulation and the intertropical convergencezone (ITCZ), with affecting not only North Atlantic climate butalso the displacement of the mean position of the ITCZ and asso-ciated asymmetry of the Hadley cell. The Brazilian speleothemrecords further indicate that millennial-scale abrupt climatechange is largely tied to meridional shifts rather than zonal shifts,and are therefore unlikely to have been triggered by tropicalWalker circulation changes. The modern appearance of a strongEl Nino-Southern Oscillation (ENSO) signal in tropical Atlanticclimate was perhaps induced by the current intermediate stateof the mean ITCZ position and relatively weak asymmetry ofthe Hadley circulation, which can be monitored by the differencesin speleothem calcite oxygen isotopic compositions.
ReferencesWang, Y. et al., 2001. Science 294, 2345–2348.Wang, Y. et al., 2005. Science 308, 854–857.Wang, X. et al., 2004. Nature 432, 740–743.Yuan, D. et al., 2004. Science 304, 575–578.
doi:10.1016/j.gca.2006.06.1496
Goldschmidt Conference Abstracts 2006 A687
