Study of the closure temperature of(U-Th)/He in detrital zircon obtained from natural evolution samples

Zircon(U-Th)/He thermochronometry was used as an effective indicator to study the thermal history of deep sedimentary basins with high temperatures.Zircon He ages and closure temperatures are very important parameters.In this paper,detrital zircon He closure temperature was studied by establishing the evolutionary pattern between zircon He ages and burial depth of borehole samples obtained from the Cenozoic strata in the Bohai Bay and Tarim basins,which have different thermal settings.The results show that the zircon He closure temperature of natural evolution samples is 200℃,which is higher than the temperature obtained from thermal simulation experiments(183℃).The temperature range of 140-200℃corresponds to the zircon helium partial retention zone.By properly understanding zircon He closure temperatures,zircon(U-Th)/He thermochronometry can provide a correct explanation of the He ages,and significant guidance in the study of the evolution of source rocks and the process of hydrocarbon accumulation in deep sedimentary basins.

Diffusion of Sm-Nd in Scheelite and its Significance to Isotopic Dating and Tracing

As the principal ore mineral in various tungsten(-gold)deposits,scheelite(CaWO_(4))plays an important role in directly dating the timing of ore formation,and in tracing associated material sources through the study of its Sm-Nd geochronology and Nd isotopic characteristics.Since the retention of Sm-Nd systematics within scheelite is presently unconstrained,equivocal interpretations for isotopic data resulting from this method have occurred quite often in previous studies that apply these isotopic data.In order to better elucidate the closure of Sm-Nd in scheelite,the kinetics of Sm and Nd within this mineral lattice were investigated through calculation of diffusion constants presented herein.The following Arrhenius relations were obtained:D_(Nd)=4.00exp(-438 kJ·mol^(–1)/RT)cm^(2)/s D_(Sm)=1.85exp(-427 kJ·mol^(–1)/RT)cm^(2)/s showing diffusion rate of Nd is near identical to Sm in scheelite when at the same temperature.However,compared to other rare earth elements(REEs),which have markedly different atomic radii to either Nd or Sm,these are shown to exhibit a great variation in diffusivities.The observed trends in our data are in excellent agreement with the diffusion characteristics of REEs in other tetragonal ABO4 minerals,indicating that ionic radius is a key constraint to the diffusivity of REEs in the various crystal lattices.With this in mind,the same substitution mechanism and a very slight discrepancy in radii will allow us to infer that significant Sm/Nd diffusional fractionation in scheelite is unlikely to occur during most geological processes.Based upon the diffusion data determined herein,Sm and Nd closure temperatures and retention times in scheelite are discussed in terms of diffusion dynamics.Those results suggest that closure temperatures for Sm-Nd within this mineral are relatively high in contrast to the temperature ranges of ore-formation responsible for scheelite-related deposits,and any later thermal environments.It is likely,therefore,that relevant isotopic information could be easily retained under most geological conditions,since initial crystallization of the scheelite.In addition,comparison of this mineral-element pair over a range of temperatures with some other common minerals used as geochronometers(e.g.,zircon and apatite)indicates that Sm-Nd system has a slower diffusive rate in scheelite than for Sr in apatite or Ar in quartz,and only a little faster than for Pb in zircon.It should be noted,within most hydrothermal deposits where zircon has crystallized,its size is typically no more than 100μm,whereas scheelite commonly occurs as macroscopic grains.For this reason,the larger dimensions of scheelite would provide a robust Sm-Nd system more able to resist perturbations,relating to any later thermal process.As such Sm-Nd investigations of scheelite are akin to U-Pb within zircon samples used in isotopic dating.These observations indicate that Sm-Nd age and isotopic information can provide reliable data in all but the most extreme case,especially when data are extracted from macroscopic grains of scheelite that are chosen to be“pristine”(i.e.,free of surface alteration and/or fractures).

Thermochronologic Constraints on Uplifting Events since the Early Cretaceous in the North Margin of the Luxi Rise, Eastern China

Two ^40Ar/^39Ar ages and six fission track ages from monzonite and the Jurassic- Cretaceous rocks provide new geochronologic constraints on the timing of uplifting events in the north margin of the Luxi （鲁西） rise, eastern China. ^40Ar/^39Ar age 111.1±2.4 and 111.2±2.5 Ma of biotite and K-feldspar sampled from the monzonite may record the cooling age at 300 and 150-300 ℃, respectively. Fission track ages of zircon and apatite from the monzonite changing from 75±7 to 40±3 Ma record the cooling age at 250 and 120 ℃, respectively. The apatite from the Jurassic-Cretaceous sandstone and volcanic rocks yielded a different T-t path. The results indicate that there are two phases 111-46.9 and 13.4 （6.5）-0 Ma of rapid uplifting happened to the north margin of the Luxi rise; the first one is a tilted uplift from north to south in Zibo （淄博） during 111-46.9 Ma and in the south in Mengshan （蒙山） during 70-40 Ma; the second one is a tilted uplift from south to north in Mengshan during 32-20 Ma, and in turn in Taishan （泰山） and Zibo during 23-20 and 13.4-0 or 6.5-0 Ma, respectively. The aging coincidence between magmatism and tectonic uplifting implies there are two phases of uplifting induced by large scale extension and lithospheric thinning.