Xiaolin Xiong

Experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition, with applications to TTG genesis

Abstract TiO2 solubility in rutile-saturated felsic melts and coexisting minerals was determined at 1.5-3.5 GPa, 750-1250 °C, and 5-30 wt% H2O. TiO2 solubility in the melt primarily increases with temperature and melt basicity; it increases slightly with water content in the melt, and it decreases with pressure. A general TiO2 solubility model was obtained and is expressed as: ln(TiO2)melt = ln(TiO2)rutile + 1.701 - (9041/T) - 0.173P + 0.348FM + 0.016H2O, where TiO2 and H2O are in wt%, T is in Kelvin, P in GPa, and FM is the melt composition parameter given by FM = (1/Si)·[Na + K + 2(Ca + Fe + Mn + Mg)]/ Al, in which the chemical symbols represent cation fractions. TiO2 solubility in amphibole, garnet, and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived. These data were used to assess the protolith TiO2 content required for rutile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition. The results show that only 0.8-1.0 wt% TiO2 is required for rutile saturation during low-degree (<20%) melting. Rutile is stable up to ~1150 °C with 1.6 wt% TiO2 in the protolith and 30-40% melting for dehydration melting and up to ~1050 °C and 50-60% melting for fluid-present melting. The data also show that 0.7-0.8 wt% TiO2 in the protolith is needed for rutile saturation during subsolidus dehydration. Therefore, nearly all basaltic protoliths in deep-crustal settings and subduction zones will be saturated with rutile during subsolidus dehydration and low-degree melting at hydrous conditions. Archean tonalites-trondhjemites-granites (TTG) are widely accepted to be the products of lowdegree melting of metabasalts at the amphibolite to eclogite transition, with rutile being present in the residue. Comparison of natural TTG compositions with our experimental rutile solubility data indicates that the dominant TTG magmas were produced at temperatures of 750-950 °C, which requires that the partial melting occurred at hydrous conditions. Models involving melting at the base of oceanic plateaus are inadequate to explain TTG genesis because the plateau root zones are likely dominated by anhydrous cumulates. A slab-melting model satisfies the requirement of a hydrous metabasalt, which during subduction would melt to produce voluminous TTG melts under high Archean geothermal gradients. The geothermal gradients responsible are estimated to be between 10 and 19 °C/km based on a pressure range of 1.5-2.5 GPa for the amphibolite to eclogite transition.

Large volume collapse observed in the phase transition in cubic PbCrO3 perovskite

When cubic PbCrO3 perovskite (Phase I) is squeezed up to ∼1.6 GPa at room temperature, a previously undetected phase (Phase II) has been observed with a 9.8% volume collapse. Because the structure of Phase II can also be indexed into a cubic perovskite as Phase I, the transition between Phases I and II is a cubic to cubic isostructural transition. Such a transition appears independent of the raw materials and synthesizing methods used for the cubic PbCrO3 perovskite sample. In contrast to the high-pressure isostructural electronic transition that appears in Ce and SmS, this transition seems not related with any change of electronic state, but it could be possibly related on the abnormally large volume and compressibility of the PbCrO3 Phase I. The physical mechanism behind this transition and the structural and electronic/magnetic properties of the condensed phases are the interesting issues for future studies.

Crust-mantle interaction and its contribution to the Shizhuyuan superlarge tungsten polymetallic mineralization

The Qianlishan granite, which is closely related to the Shizhuyuan tungsten polymetallic mineralization, is aluminous alkali-type granite. The intimate temporal and spatial association among the basaltic, syenite and granitic rocks in the mining area, and their major and trace elements, and Nd, Pb, Sr, O isotopic compositions indicate that the crustal-mantle interaction probably was an important constraint on, and participated in, the formation of the superlarge ore deposit.

Structural properties of PbVO3 perovskites under hydrostatic pressure conditions up to 10.6 GPa.

High-pressure synchrotron x-ray powder diffraction experiments were performed on PbVO(3) tetragonal perovskite in a diamond anvil cell under hydrostatic pressures of up to 10.6 GPa at room temperature. The compression behavior of the PbVO(3) tetragonal phase is highly anisotropic, with the c-axis being the soft direction. A reversible tetragonal to cubic perovskite structural phase transition was observed between 2.7 and 6.4 GPa in compression and below 2.2 GPa in decompression. This transition was accompanied by a large volume collapse of 10.6% at 2.7 GPa, which was mainly due to electronic structural changes of the V(4+) ion. The polar pyramidal coordination of the V(4+) ion in the tetragonal phase changed to an isotropic octahedral coordination in the cubic phase. Fitting the observed P-V data using the Birch-Murnaghan equation of state with a fixed [Formula: see text] of 4 yielded a bulk modulus K(0) = 61(2) GPa and a volume V(0) = 67.4(1) Å(3) for the tetragonal phase, and the values of K(0) = 155(3) GPa and V(0) = 58.67(4) Å(3) for the cubic phase. The first-principles calculated results were in good agreement with our experiments.

SHRIMP U-Pb zircon geochronology of Yangfang aegiriteaugite syenite in Wuyi Mountains of South China and its tectonic implications

In addition to the wide spread peraluminous granites, some alkaline or alkali-rich intrusive rocks were recently observed in the Wuyi Mountains. The Yangfang aegiriteaugite syenites in the Wuyi Mountains are strongly enriched in light rare earth elements (LREE), K, Ba, Ga, Zr and Y. What is more, differentiations of REE are obvious. In addition, the Yangfang aegiriteaugite syenites have also some characteristics similar to A-type granite. Zircons are selected from the Yangfang aegiriteaugite syenites for Sensitive High-Resolution Ion Microprobe (SHRIMP-II) U-Pb geochronology study. 15 analyses give concordant206Pb/238U ages in a narrow range of 233–249 Ma, which correspond to a single age population with a weighted mean206Pb/238U age of 242 ± 4 Ma, which records the crystallization time of the aegiriteaugite syenites. It is suggested that: (1) there were early Triassic peralkaline intrusive rocks in South China; (2) the early Indosinian extension event was probably active in local area, South China. This discovery provides a new insight for farther understanding of “Indosinian orogeny” in South China.

A new cubic perovskite in PbGeO3 at high pressures

Abstract A new cubic perovskite polymorph of PbGeO3 (Phase II) was synthesized by laser heating in the diamond-anvil cell (DAC) at the pressure of 36 GPa. Fitting the Birch-Murnaghan equation of state against its observed P-V data yields a bulk modulus K0 of 196(6) GPa and the volume V0 of 56.70(13) Å3 when K0′ is assumed being 4. After the pressure is released, the PbGeO3 Phase II changes gradually into an amorphous phase, which contains mainly fourfold-coordinated germanium. It indicates that the PbGeO3 Phase II with a GeO6 octahedron framework transforms to a GeO4 tetrahedron network during the amorphization. The existence of PbGeO3 cubic perovskite Phase II at high pressures indicates that the polarized character of the Pb2+ ion induced by its 6s2 lone pair electrons would be totally reduced in the environment of major silicate perovskites inside the lower mantle, and thus the Pb atom would substitute the Ca atom to enter the CaSiO3 perovskite.

Nb/Ta Fractionation by Amphibole in Hydrous Basaltic Systems: Implications for Arc Magma Evolution and Continental Crust Formation

To understand fully the role of amphibole in the fractionation of Nb/Ta during arc magma evolution, we conducted experiments with mid-K and high-K basalts to determine amphibole/melt Nb, Ta and Ti partition coefficients (DNb, DTa and DTi) at variable conditions of bulk TiO2, P, T, H2O and fO2. The experimental results show that, at crustal pressures, amphibole is the most important crystalline phase in hydrous basaltic systems. The amphibole/melt Nb, Ta, and Ti partitioning results are 0 16–0 90 for DNb, 0 13–0 68 for DTa, 1 81–10 63 for DTi and 0 76–2 81 for DNb/DTa. Bulk TiO2 and fO2 show no observable effects. T and H2O, in addition to the compositions of amphibole and melt, are the main affecting factors. DNb, DTa, DTi and DNb/DTa increase with decreasing temperature, amphibole Mg# and melt H2O content and increasing melt polymerization. During cooling and crystallization of arc magmas at crustal pressures, amphibole Mg# decreases and melt polymerization increases, leading to significant increase in amphibole/melt DNb, DTi and DNb/DTa. Nb/Ta fractionation in evolved melts will thus be enhanced with crystallization progress. Meanwhile, melt H2O content will increase with the degree of crystallization, which slows down the increase in these D values. Therefore, the trend and extent of Nb/Ta fractionation in the melt by amphibole critically depends on temperature and melt H2O content. Only low temperatures or low H2O contents at high temperatures lead to high D values. For arc magmas with an average H2O of 3 9 wt %, DNb and DNb/DTa are in general >0 40 and >1 20, respectively, which explains why amphibole fractionation results in lower Nb/Ta ratios in evolved arc magmas. The bulk Nb/Ta fractionation trend during arc magma evolution appears to be generally controlled by fractional crystallization of amphibole. Experimental and modeling results suggest that amphibole is a main fractionating phase during arc magma evolution and continental crust formation.

Au-Te deposits associated with alkali-rich igneous rocks in China

Four Au-Te or Au(Te) deposits are discussed with respect to their geological and geochemical characteristics. All of them are rich in tellurides but low in sulfide contents. The Dongping Au(Te) deposit was formed by interaction between fluids derived from an alkaline syenite magma and hydrothermal origin. The Guilaizhuang Au-Ag (Te) deposit is a quartz-fluorite-adularia type epithermal deposit associated with high-K and shoshonitic series igneous rocks. The Dashuigou deposit may be a seldom giant tellurium-dominant Te(Au) deposit and is possibly related to the Ermeishan basaltic Large Igneous Province. The Detiangou Au-Te-Ag deposit is a hydrothermal deposit, controlled by ductile shear zone.

The recognition of adakite-type gneisses in the North Dabie Mountain and its implication to ultrahigh pressure metamorphic geology

Adakite[1] is an igneous rock generated by the partial melting of subducting ocean slab in an island arc setting. Meanwhile, residuals of the slab-melting are transformed to eclogite or garnet-amphibolite during a slab subduction process. This note reports the adakite-type grey gneisses discovered in the North Dabie Mountain. These adakitic rocks are characterized by high Al2O3,(14.97%–17.56%), Sr (545–941 gm/g), Sr/Y (44.71–218.98), La/Yb (16.70–97.50), and low Y (3.32–12.19 μg/g), Yb (0.31–1.34 μg/g), Sc (0.92–4.2 μg/g), as well as positive Sr anomaly but absent or positive Eu anomaly. They are different from the low aluminum grey gneisses with low Al2O3 (13.73%–14.38%), Sr (262–409 μg/g), Sr/Y (6.46–15.97) and La/Yb (1.90–22.60), high Y(21.79–36.24 μg/g), Yb (2.00–5.59 μg/g), Sc (8.30%–12.70%), and negative Sr, Eu anomaly in the North Dabie Mountain. The adakite-type grey gneisses in the North Dabie Mountain were most probably formed by the partial melting of subducting ocean slab before the Indo-Chinese epoch. We suggest that the subducting of an ocean slab took place prior to the incorporation between the Yangtze and North China plates in the Indo-Chinese epoch, resulting in formation of adakite-type igneous rocks and some ultrahigh pressure eclogites related to the subducted ocean slab. Therefore, besides the ultrahigh pressure metamorphism of the Indo-Chinese epoch, some ultrahigh pressure ecoglits were most likely formed prior to the Indo-Chinese epoch in the Dabie Mountain. In addition, the adakite-type grey gneisses with arc origin occur in the North Dabie Mountain, inferring that the Yangtze plate subducted down below the North China plate and the main suture zone between the two plates was likely along the southern side to the North Dabie Mountain.

First-order character of the displacive structural transition in BaWO4

Nearly all displacive transitions have been considered to be continuous or second order, and the rigid unit mode (RUM) provides a natural candidate for the soft mode. However, in-situ X-ray diffraction and Raman measurements show clearly the first-order evidences for the scheelite-to-fergusonite displacive transition in BaWO4: a 1.6% volume collapse, coexistence of phases, and hysteresis on release of pressure. Such first-order signatures are found to be the same as the soft modes in BaWO4, which indicates the scheelite-to-fergusonite displacive phase transition hides a deeper physical mechanism. By the refinement of atomic displacement parameters, we further show that the first-order character of this phase transition stems from a coupling of large compression of soft BaO8 polyhedrons to the small displacive distortion of rigid WO4 tetrahedrons. Such a coupling will lead to a deeper physical insight in the phase transition of the common scheelite-structured compounds.