Xun Wei

Plume-lithosphere interaction in the generation of the tarim large igneous Province, NW China: Geochronological and geochemical constraints

The magmatism in the early Permian Tarim large igneous province (TLIP) in NW China is represented by basaltic lava flows in Keping and ultramafic-mafic-felsic intrusions and mafic dikes in Bachu, northwestern Tarim Craton. This paper reports new Ar-Ar dating results and chemical compositions of Keping basalts and Bachu dikes, with aims of better characterizing the timing of and mantle/crustal contribution to the TLIP. The Keping basalts yield two well-defined 40Ar/39Ar plateau ages of 287.3 ± 4.0 Ma and 287.9 ± 4.1 Ma, which, together with age data from the literature, define a magmatic event at ∼289 Ma. The intrusions and dikes in Bachu are believed to have formed at ∼279 Ma based on screened literature data. Thus, they together define two magmatic episodes. The Keping basalts, representing the earlier episode, have alkaline affinity (SiO2 = 44.0-47.9 wt.%, Na2O + K2O = 3.7-4.9 wt.%), low MgO (4.3-5.9 wt.%) and high TiO2 (3.8-5.1 wt.%) contents, showing fractionated chondrite-normalized LREE and nearly flat HREE patterns [(La/Yb)N = 6.27-7.71; (Dy/Yb)N = 1.36-1.48] with noticeable negative Nb and Ta anomalies in the primitive mantle-normalized trace element diagram. They have negative and relatively uniform εNd(t) (−2.3 to −3.8) and low (206Pb/204Pb)i (17.43-17.57). We argue that these “crustal signatures” cannot be attributed to crustal assimilation because neither εNd(t) nor (206Pb/204Pb)i correlates with SiO2; rather they are more likely derived from a sub-continental lithospheric mantle (SCLM) source metasomatized by subduction-related processes. The Bachu dikes, representing the later episode and confined to the margins of the Tarim Craton, have similar MgO (3.6-5.4 wt.%) and TiO2 (3.1-4.7 wt.%) contents to the Keping basalts, and display more fractionated REE patterns [(La/Yb)N = 10.1-14.0; (Dy/Yb)N = 1.79-1.99]. They have variable isotope compositions [εNd(t) = −0.3-4.8, (206Pb/204Pb)i = 17.50-18.11] and display OIB-like trace element signatures. Correlations between isotopic and trace element ratios indicate that some dikes with low εNd(t) and low initial Pb isotope ratios could have been subjected to crustal assimilation. We propose a model involving plume-lithosphere interaction to account for the two discrete magmatic episodes with distinct mantle sources in the TLIP. The earlier episode was formed in response to the impact of a sub-lithospheric mantle plume at the base of the SCLM. Partial melting of the metasomatized lithospheric mantle was triggered by temperature increase due to conductive heating of the impregnating mantle plume. The later episode was generated by decompression melting of the mantle plume, as a result of deflection of the plume towards the margins of the Tarim Craton with thinner lithosphere.

Rapid loss of the plasma sheet energetic electrons associated with the growth of whistler mode waves inside the bursty bulk flows

During the interval similar to 07:45:36-07:54:24UT on 24 August 2005, Cluster satellites (C1 and C3) observed an obvious loss of energetic electrons (similar to 3.2-95keV) associated with the growth of whistler mode waves inside some bursty bulk flows (BBFs) in the midtail plasma sheet (X-GSM similar to-17.25 R-E). However, the fluxes of the higher-energy electrons (128keV) and energetic ions (10-160keV) were relatively stable in the BBF-impacted regions. The energy-dependent electron loss inside the BBFs is mainly due to the energy-selective pitch angle scatterings by whistler mode waves within the time scales from several seconds to several minutes, and the electron scatterings in different pitch angle distributions are different in the wave growth regions. The plasma sheet energetic electrons have mainly a quasi-perpendicular pitch angle distribution (30 degrees<<150 degrees) during the expansion-to-recovery development of a substorm (AE index decreases from 1677nT to 1271nT), and their loss can occur at almost all pitch angles in the wave growth regions inside the BBFs. Unlike the energetic electrons, the low-energy electrons (similar to 0.073-2.1keV) have initially a field-aligned pitch angle distribution (0 degrees 30 degrees and 150 degrees 180 degrees) in the absence of whistler mode waves, and their loss in field-aligned directions is accompanied by their increase in quasi-perpendicular directions in the wave growth regions, but the loss of the low-energy electrons inside the BBFs is not obvious in the presence of their large background fluxes. These observations indicate that the resonant electrons in an anisotropic pitch angle distribution mainly undergo the rapid pitch angle scattering loss during the wave-particle resonances.

New technique to calculate electron Alfven layer and its application in interpreting geosynchronous access of PS energetic electrons

The magnetospheric particle drift theory in (U, B) coordinates provides a convenient tool for analyzing particle drift trajectories from plasma sheet to the inner magnetosphere. However, in this drift theory, the Alfven layer for a realistic asymmetric magnetic field model cannot be obtained due to numerical difficulty. In this paper, we developed a new technique to calculate electron Alfven layer for Volland-Stern electrical potential model and dipole+T89 geomagnetic field model. The results show that for realistic asymmetric field models, the dawn and dusk separators are not in the dawn-dusk meridian but asymmetric and tilt toward the dayside. The stagnant point of the electron Alfven layer is always located in the duskside separator. Compared with the electron Alfven layers calculated from Volland-Stern electric field model and dipole magnetic field model, the new electron Alfven layer becomes closer to the Earth and meanwhile rotates clockwise. This rotation makes the point closest to the Earth in the Alfven layer shift from dawnside to nightside. The newly calculated geosynchronous Alfven layer crossings cannot only explain the observed geosynchronous access of plasma sheet thermal (0.913 and 2.783keV) electrons as previous studies did but also the geosynchronous access of plasma sheet energetic (10.472 and 31.039keV) electrons. This new technique can be used to calculate Alfven layer for any asymmetric field models and may become an important compliment to the classical magnetospheric particle drift theory in the (U, B) coordinates.

Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi wehrlite, Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes

ABSTRACT The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.

Parallel-dominant and perpendicular-dominant components of the fast bulk flow: Comparing with the PSBL beams†

Utilizing multipoint observations by the Cluster satellites, we investigated the ion distributions of the fast bulk flows (FBFs) in the plasma sheet. Simultaneous observation by C1 and C3 revealed that parallel-dominant and perpendicular-dominant components of the flows coexist and correspond to B-x-dominant and B-z-dominant magnetic field regions within the FBFs, respectively. In both cases, the ions distributions are characterized by a single-beam/crescent shape. In particular, no reflected ions are found within the FBFs. Statistical analysis showed that within the FBFs, the strength of the B-x component is typically less than 5 nT for B-z-dominant regions and above 10 nT for B-x-dominant regions. To distinguish between the parallel-dominant component of the FBFs and the field-aligned beams in the plasma sheet boundary layer (PSBL), we further statistically analyzed the tailward parallel flows (TPF) with positive B-z in the plasma sheet. The results indicated that the FBFs tend to have higher velocity, weaker B, and higher magnetic tilt angle (theta(MTA)) than the TPFs/PSBL beams. Statistically, in the region of B > 30 nT (theta(MTA) > 10 degrees), only PSBL beams can be observed, while in the region of B 30 degrees), the FBFs are dominant. In the intermediate region (10 degrees < theta(MTA) < 30 degrees) of the plasma sheet, the FBFs and the PSBL beams cooccur. These Cluster observations suggest that the X line can produce both perpendicular flow in central plasma sheet and parallel flow in the PSBL. In addition, the parallel-dominant component of the FBFs could be an important origin for the PSBL beams.

B isotopes of Carboniferous-Permian volcanic rocks in the Tuha basin mirror a transition from subduction to intraplate setting in Central Asian Orogenic Belt

Controversies remain as to the Permian tectonic setting in the Central Asian Orogenic Belt (CAOB), in particular, regarding the triggering of Permian magmatism. To address this issue, we carried out a geochemical study on Dananhu volcanics from the Tuha basin, southwestern CAOB. 40Ar/39Ar analyses for feldspar separates yield a Carboniferous age of 321.2 ± 9.8 Ma for the andesites and a Permian age of 278.9 ± 4.2 Ma for the basalts. Both the andesites and basalts geochemically resemble subduction-related magmas in aspects of their trace element and Sr-Nd-Pb isotopic composition, but they may differ in petrogenesis. The andesites are typical adakites. Their high Cr and Ni contents, and low heavy rare earth element contents, and positive correlation between B/Nb ratios and δ11B values suggest that they were generated by interaction between slab-derived melts/fluids and the mantle wedge. The Permian basalts are subdivided into two subgroups: alkali and tholeiitic basalts. High Ba/La and Ba/Zr ratios indicate that the source of the alkali basalts has been metasomatized by fluids/melts derived from altered oceanic crust. In contrast, a volatile-free source metasomatized by sediment-derived fluids/melts is inferred for the tholeiitic basalts given their high Th/Zr and Th/Ce ratios. Negative correlation between B/Nb and δ11B for the Permian basalts imply that the arc signatures were imposed by previous subduction events, and no direct slab-derived fluids/melts participated in their petrogenesis. These geochemical characteristics alongside regional geologic records collectively suggest a tectonic shift in the southwestern CAOB from subduction-related setting during the Late Carboniferous to an intraplate setting during the Early Permian.