Partial melt or aqueous fluid in the mid-crust of Southern Tibet? Constraints from INDEPTH magnetotelluric data

Abstract In 166 isotope dilution analyses of Ba, Rb, Cs on fresh basalts from mid-ocean ridges and oceanic islands, Ba/Rb and Cs/Rb ratios are nearly constant. From this, we conclude that Ba/Rb and Cs/Rb ratios are essentially constant in the present-day mantle in spite of large differences in the degree of source depletion or enrichment. As it appears improbable that these ratios could be both constant and non-primitive, we propose that they are representative of the primitive mantle and of the present-day crust-mantle system. We explain this uniformity of relative abundances as follows: the mantle is depleted by subtraction of a mobile phase such as a partial melt or an aqueous fluid. In either case, a significant amount of the mobile phase remains in the residue. Ba, Rb and Cs are among the most highly incompatible elements. Therefore the mobile phase cannot fractionate these elements relative to one another but retains the source ratios of Ba/Rb and Cs/Rb. Also, the amount of mobile phase remaining in the residue is enough to dominate the Ba, Rb and Cs concentrations in the residue. Consequently, neither the mobile phase nor the residue, nor any other portion of the mantle that may be enriched by addition of the mobile phase, will be changed in their relative abundances of Ba, Rb and Cs, even though the absolute abundances of these elements may change by orders of magnitude.The primitive Ba/Rb = 11.3 and Cs/Rb = 12.6 x 10-3 lead to the following estimates for the primitive mantle: Ba = 6.9 ppm (taken from Jagoutz et al. [1]). Rb = 0.61 ppm and Cs = 7.7 ppb.Assuming the earth has a chondritic Sr/Ba ratio of 3.08, we calculate a Rb/Sr ratio of 0.029 for the earth. This corresponds to a present-day 87Sr/ 86Sr of 0.7045. This value lies near the lower limit of the ratios estimated from the correlation of Nd and Sr isotopic abundances in oceanic basalts.The Cs/Rb ratios is about a factor of ten lower than the Cl-chondritic ratio and a factor of three lower than the lunar ratio. This low terrestrial Cs/Rb ratio should be matched by similar values in the continental crust. However, the large range of Cs/Rb ratios found in the crust prevent us from obtaining a meaningful mass balance.

Download Full-text

The Mechanism and Dynamics of N-S trends normal faults in Tibetan Plateau: Insight From Thermochronology, Magnetotellurics, Magmatism and GPS Measurements

10.5194/egusphere-egu2020-14987 ◽

2020 ◽

Author(s):

Han-Ao Li ◽

in-Gen Dai ◽

Le-Tian Zhang ◽

Ya-Lin Li ◽

Guang-Hao Ha ◽

...

Keyword(s):

Tibetan Plateau ◽

Lower Crust ◽

Normal Faults ◽

The Tibetan Plateau ◽

Gps Measurements ◽

Southern Tibet ◽

Magnetotelluric Data ◽

Deep Process ◽

Lower Crustal Flow ◽

Lower Crustal

The N-S trends normal faults are widespread through the whole Tibetan Plateau. It records key information for the growth and uplift of the Tibetan Plateau. Numerous models are provided to explain the causes of rifting in the Tibetan Plateau based on the low-temperature thermochronology1. With the developments of the geophysical and magmatic geochemistry methods and its applications on the Tibetan Plateau, we could gain more profound understanding on the sphere structure of the Tibetan Plateau. This would give us more clues on how the deep process affect the formation and evolution of the shallow normal faults. However, few researchers pay attention on this and the relationship between the surface evolution and deep process of these faults. In order to solve these puzzles, we collected the published thermochronology data, magnetotelluric data, faults-related ultrapotassic, potassic and the adakitic rocks ages and present-day GPS measurements. We find that the distribution of the N-S trends normal faults are closely related to the weak zones in the middle to lower crust (15-50 km) revealed by the magmatism and magnetotelluric data2. Besides, the present-day GPS data show that the E-W extension rates match well with the eastward movements speeds interior Tibetan Plateau3. Combined with the thermochronology data (25-4 Ma), we concluded that 1.The weak zone in the middle to lower crust influence the developments and evolution of the N-S trends normal faults. 2. The material eastward flow enhance the N-S normal faults developments. 3. The timing of the middle to lower crustal flow may begin in the Miocene.Key words: N-S trends normal faults; Thermochronology; Magnetotellurics; Magmatism; GPS Measurements; middle to lower crustal flowReferences:1Lee, J., Hager, C., Wallis, S.R., Stockli, D.F., Whitehouse, M.J., Aoya, M. and Wang, Y., 2011. Middle to Late Miocene Extremely Rapid Exhumation and Thermal Reequilibration in the Kung Co Rift, Southern Tibet. Tectonics, 30(2).2Pang, Y., Zhang, H., Gerya, T.V., Liao, J., Cheng, H. and Shi, Y., 2018. The Mechanism and Dynamics of N-S Rifting in Southern Tibet: Insight from 3-D Thermomechanical Modeling. Journal of Geophysical Research: Solid Earth.3Zhang, P.-Z., Shen, Z., Wang, M., Gan, W., B&#252;rgmann, R., Molnar, P., Wang, Q., Niu, Z., Sun, J., Wu, J., Hanrong, S. and Xinzhao, Y., 2004. Continuous Deformation of the Tibetan Plateau from Global Positioning System Data. Geology, 32(9).Acknowledgements:We thank Shi-Ying Xu, Xu Han, Bo-Rong Liu for collecting data. Special thanks are given to Dr. Guang-Hao Ha and Professors Jin-Gen Dai, Le-Tian Zhang&#65292;Ya-Lin Li and Cheng-Shan Wang for many critical and constructive comments.

Download Full-text