Qunke Xia

Water contents of pyroxenes in intraplate lithospheric mantle

Water contents of clinopyroxene and orthopyroxene in mantle peridotites from various xenolith occurrences in intraplate settings (both oceanic and continental) were determined by Fourier Transform Infrared Spectroscopy (FTIR). Samples from the following localities were studied: Sal Island (Cape Verde Archipelago); Baker Rocks and Greene Point (Northern Victoria Land, Antarctica); Panshishan and Lianshan (Subei Basin, Eastern China). They represent well-known localities where detailed petrographical and geochemical studies have already been carried out or areas which are currently under investigation. The water incorporated in these pyroxenes is low (cpx, 37–399 ppm; opx, 9–166 ppm) or very low (as in Greene Point, Antarctica; cpx, 5–16 ppm; opx, 9–16 ppm). Within each population there is no clear correlation with melting parameters such as MgO contents in any single mineral. Results are compared with the available literature data on water contents in mantle pyroxenes, which include peridotites from on-craton (hosted by kimberlitic-type magmas) and off-craton (hosted by alkaline basic magmas), as well as subarc mantle settings. The “relatively dry” (cpx, 140–528 ppm; opx, 38–280 ppm) sub-arc mantle xenoliths (Peslier et al. , 2002) are shown to be wetter than the intraplate (off-craton) xenoliths. Cratonic mantle pyroxenes are only represented by a few determinations on garnet peridotites and eclogite from Kaapvaal and Colorado Plateau. They record the highest water contents (cpx, 342–1012 ppm; opx, 180–491 ppm) so far measured in mantle pyroxenes from various tectonic settings. Despite the limited data set, the indication that the cratonic mantle is strongly hydrated is compelling. Rehydration of cratonic mantle may be related to plate subduction ( i.e. , Colorado Plateau) or alternatively to metasomatic enrichment ( i.e. , Kaapvaal Craton). However, assuming that the water content was initially very low, metasomatic events in intraplate settings, similar to those presented here, would not be able to produce a significant increase in pyroxenes water content. According to our own data and those reported in the literature, it appears that substantial rehydration may instead occur at convergent plate margins.

Temperature dependence of IR absorption of OH species in clinopyroxene

Abstract The behavior of structural OH in clinopyroxene (augite and omphacite) during successive heating has been investigated by in situ micro-FTIR measurements under temperatures ranging from room temperature to 500 °C at 100 °C increment. The first group of OH band (3620-3640 cm-1) exhibits a systematic decrease of peak position upon successive heating, while the other two groups (3520-3535 and 3450-3465 cm-1) show only little change. Both augite and omphacite display a decrease of integral absorbance of OH fundamental stretching vibration upon successive heating. The IR spectra of OH band are reversible when the temperature decreases from 500 °C to room temperature, suggesting that changes in IR indicate changes in molecular state of OH. Based on: (1) the decreases of integral absorbance of OH bands with increasing temperature, and (2) comparison with Paterson’s relationship (Paterson 1982), we suggest that OH absorption coefficients are temperature dependent, so it is necessary to apply different absorption coefficients when determining OH content from Beer-Lambert law at different temperatures and sample temperatures should be reported in quantitative IR studies

In situ FTIR investigations at varying temperatures on hydrous components in rutile

Abstract In situ high- and low-temperature unpolarized FTIR absorption spectra of hydrous components in natural Fe-rich rutile have been recorded using a FTIR600 heating and cooling stage attached to an IR microscope. Two absorption bands (3279 and 3297 cm-1) at room temperature are ascribed to fundamental stretching bands of OH. Both OH bands shift to lower wavenumbers at elevated temperatures, but with different amplitudes. With increasing temperature, the integral absorbance of the 3279 cm-1 band increases while the integral absorbance of the 3297 cm-1 band decreases. These changes are fully reversible. The different frequency shifts of the two IR absorption bands with temperature imply two different local chemical environments of H defects. Based on the chemical composition of the rutile and the electronegativity of impurity ions, the results of the in situ high- and low-temperature FTIR measurements support the band assignment of Bromiley and Hilairet (2005): the 3279 and 3297 cm-1 bands are due to interstitial H decoupled from any substitutional defects, and H coupled to Fe3+ on the neighboring octahedral (Ti) site (Fe3+ Ti + H+i ↔ Ti4+Ti), respectively. Variations in integral absorbance of the two bands suggest the transformation between decoupled H defects and coupled H defects as a function of temperature, possible due to some statistical relationship between the proportions of coupled and decoupled interstitial H dependent on the amount of heat energy applied to the system.

Zircon SIMS ages and chemical compositions from Northern Dabie Terrain: Its implication for pyroxenite genesis

We present the results of a detailed micro-scale investigation of zircons from pyroxenites, Daoshicong Northern Dabie using a combination of SIMS and ICPMS. The SIMS measurements gave ages of 134–159 Ma. Its average of (144.5±6.2) Ma is interpreted as the best estimate of the pyroxenite intrusion. The crystallization of zircons continued for quite a long time and underwent slow cooling. The pyroxenites are products of post-collision magmatism. The REE pattern is HREE-enriched, and its HREE concentrations fall between the magmatic and metamorphic range of gneissic zircons from the Dabie area, which indicate the involvement of crust material in its mantle source.

Anomalously high δD and micro-scale hydrogen isotope heterogeneities in the mantle: Ion microprobe analysis of amiboles from peridotite xenoliths at Nushan, eastern China

Hydrogen isotopic compositions of four amphibole grains from three pieces of lherzolite xenoliths in Cenozoic basanites of Nushan, eastern China have been analyzed by ion microprobe. δD values of all analyzed points range from −94‰ to +46‰, some of which are much higher than the highest δD (+8‰) reported previously for mantle materials. The heterogeneities of D/H ratios within single grains have been observed, the variation of δD is up to 80‰ on the scale of less than 400 μm. No correlation between hydrogen isotopic ratios and hydrogen contents can be found, implying that the scatter of δD values could not result from a late shallow process such as hydrogen loss or hydrothermal alterations and should be considered as inherited from the source at depth. Chemical compositions of Nushan amphiboles are very homogeneous, excluding the fact that the scatter of δD values could arise from variable fractionation factors between a single fluid source and minerals. Therefore, metasomatic fluids responsible for the formation of Nushan amphiboles should be heterogeneous and result in the observed large variable and anomalously high δD values of amphiboles. We suggested that such metasomatic fluids could be related to magma degassing in the mantle source. Based on the D-H diffusion data and the scale of hydrogen isotope heterogeneities, it was inferred that the mantle metasomatism took place soon before the eruption of host magma.

Kinetics of deuteration in andradite and garnet

Abstract The hydrogen mobility in andradite single crystals from an iron-skarn deposit was investigated through H-D and D-H exchange experiments. Thin slices were annealed in a horizontal furnace flushed with a gas mixture of Ar/D2(10%) and Ar/H2(10%) at ambient pressure between 400 and 700 °C. FTIR analyses were performed before and after each annealing run. Between 15 and 35% of the original OH content remained in the crystal structure at the end of the deuteration experiments. This contrasts with the results of similar experiments performed in other NAMs, where all hydrogen atoms are replaced by deuterium in the structure. However, because a steady state was reached at the end of the experiments, the diffusion law for the exchange process was determined as: DH-D = D0exp[-(96 ± 11) kJ/mol/RT], with logD0 = -5.9 ± 0.7 (in m2/s). The activation energy is similar to that for hydrogen diffusion in grossular, but H diffusivity is more than two orders of magnitude faster. Our results demonstrate that, because major element composition has a major effect on H-D diffusion laws, it must be considered in any discussion of δD signatures in garnets. In andradite-rich garnets, hydrogen isotope data can only be used to record short, low-grade metamorphic or metasomatic events, at temperatures lower than 400 °C.

Water effects on the anharmonic properties of forsterite

Abstract To quantify the effects of hydration on anharmonicity of olivine thermodynamics, we have measured in situ Raman spectra of an extremely hydrous forsterite with 4500 ppm (wt) H2O at temperatures up to 1273 K. All the Raman modes in hydrous forsterite shift linearly to lower wavenumbers with increasing temperature. The calculated isobaric mode Grünesien parameters related to SiO4 internal stretching and bending vibrations are much lower than lattice vibrations. Additionally, compared with anhydrous forsterite, except for the modes at 919, 858, and 227 cm-1, water greatly reduces the isobaric mode Grüneisen parameters of the Raman modes in forsterite. Water also has a large effect on the anharmonic parameters related to lattice vibrations, whereas it has little effect on the anharmonic parameters related to SiO4 internal stretching and bending vibrations. Those results have the implications to the variations of local structure with temperature and estimation of water effects on the thermodynamics of forsterite.