Frequency Bands and Gaps of Magnetospheric Chorus Waves Generated by Resonant Beam/Plateau Electrons

In this paper, the modifications of the whistler dispersion characteristics are investigated which arise if resonant electrons are taken into account. The following chain of processes is emphasized: Generation of whistler waves propagating at different angles to the magnetic field and their nonlinear interaction with resonant electrons result in the appearance of modulated electron beams in the background plasma. As a result, the dispersion characteristics of waves in this new plasma might be significantly changed. By analysing the modified dispersion characteristics these changes are discussed. Supported by particle simulations and space observations, it is assumed that in the electron distribution function at the resonance velocity a plateau-like beam is formed. Because of the weakness of the beam, the term “beam/plateau population (b/p)” is used. By solving the kinetic dispersion relation of whistler waves in electron plasmas with b/p populations, the associated modifications of the whistler dispersion characteristics are presented in diagrams showing, in particular, the frequency versus propagation angle dependence of the excited waves. It is important to point out the two functions of the b/p populations. Because of the bi-directional excitation of whistler waves by temperature anisotropy, one has to distinguish between up- and downstream populations and accordingly between two b/p modes. The interaction of the beam-shifted cyclotron mode ω= Ωe + k⋅Vb (V bVb is the b/p velocity, Ωe: electron cyclotron frequency) with the whistler mode leads to enhanced damping at the ω-k point where they intersect. This is the origin of the frequency gap at half the electron cyclotron frequency (ω~Ωe/2) for quasi-parallel waves which are driven by temperature anisotropy. Furthermore, it is shown that the upstream b/p electrons alone (in the absence of temperature anisotropy) can excite (very) oblique whistler waves near the resonance cone. The governing instability results from the interaction of the beam/plateau mode ω= k⋅Vb (Vb > 0) with the whistler mode. As a further remarkable effect, another frequency gap at ω~Ωe/2 in the range of large propagation angles may arise. It happens at the triple point where both b/p modes and the whistler mode intersect. Our investigation shows that the consideration of resonant electrons in form of beam/plateau populations leads to significant modifications of the spectrum of magnetospheric whistler waves which are originally driven by temperature anisotropy. Relations to recent and former space observations are discussed.

An Experimental Study of Partial Melting of Metafelsic Rocks:Constraints on the Feature of Anatectic Melts and the Origin of Garnets in Collisional Orogens

Crustal anatexis in continental subduction zones has great bearing on chemical differentiation of the continental crust at convergent plate boundaries.This was experimentally investigated for ultrahigh-pressure(UHP)metafelsic rocks at 0.5-3.0 GPa and 650-900℃.The results show that partial melting begins at about 750℃ when pressure drops from 3.0 to 2.0 GPa,corresponding to decompressional exhumation of the deeply subducted continental crust.As the pressure further decreases to 1.0 GPa,the partial melting degree reaches the maximum of~25%at 900℃.Partial melts produced in these experiments are rich in silica and alkali,and poor in iron,manganese and magnesium.As the degree of partial melting increases,the composition of partial melts gradually converges toward homogeneous one.In the absence of free water,the partial melting of metafelsic rocks were triggered by the breakdown of hydrous minerals.At low temperatures of~750℃at 1.0-2.0 GPa,phengite dehydration melting occurs at first,giving rise to small amounts of felsic melts and peritectic K-feldspar.As the temperature rises up to 850-900℃,biotite begins to break down and gives rise to large amounts of felsic melts and peritectic minerals such as garnet,K-feldspar and orthopyroxene.It is noted that peritectic garnet is much different from anatectic garnet crystallized from anatectic melts and metamorphic garnet formed through metamorphic dehydration reaction under subsolidus conditions.The peritectic garnet is characterized not only by anhedral shapes with many multiphase crystal inclusions but also by compositions poor in spessartine and grossular but rich in almandine and pyrope.On the other hand,the anatectic garnets are characterized not only by euhedral shapes with few inclusions but also by compositions rich in grossular and spessartine but poor in almandine and pyrope.These observations provide experimental constraints on the origin of garnets in UHP metamorphic rocks,which have great bearing on understanding of anatectic metamorphism in collisional orogens.

Lithium Isotope Analytical Methods and Implications for Rare-Metal Mineralization in Granite-Pegmatite Systems:An Overview

The origin of highly-fractionated granite-pegmatite systems and their associated rare metal mineralization has been widely studied,but there is still ongoing debate.Prevailing hypotheses suggest that pegmatite formation and the associated rare metal mineralization are closely related to aqueous fluid processes.Lithium(Li)isotope analysis has been widely applied to trace granite-pegmatite evolution.This is because lithium is widely present in various minerals(e.g.,mica,tourmaline)that record the melt and fluid compositions,and lithium isotopes are sensitive to magmatic-hydrothermal processes.We briefly review the methodology of Li isotope analyses,the mechanisms of Li isotopic fractionation,and,in particular,Li isotope fractionation in granite-pegmatite system based on Li isotope data we have collected and the latest developments in Li isotope geochemistry.With the development of analytical technology,high-precision measurement of the Li content and isotopic compositions have facilitated a series of scientific breakthroughs in understanding the magmatic-hydrothermal evolution of rareelement ore deposits.Li isotope analyses on bulk mineral separates have demonstrated their ability to trace various hydrothermal processes.In situ Li isotope analysis methods has been enhanced by the development of new,homogeneous mineral reference materials.In situ SIMS and LA-MC-ICP-MS Li isotope measurements on minerals(e.g.,tourmaline)will likely become more important in studying the fluid-rock interactions in magmatic,metamorphic,and hydrothermal processes,as well as on pegmatite petrogenesis and rare-metal mineralization.