Water in coesite: Incorporation mechanism and operation condition,solubility and P-T dependence,and contribution to water transport and coesite preservation

A series of coesite,coexisting with or without a liquid phase,was synthesized in the nominal system SiO2-H2O at800-1450℃and 5 GPa.Micro-Raman spectroscopy was used to identity the crystalline phase,electron microprobe and LA-ICP-MS were employed to quantity some major and trace elements,and unpolarized FTIR spectroscopy was applied to probe the different types of hydrogen defects,explore water-incorporation mechanisms and quantify water contents.Trace amounts of A1 and B were detected in the coesite.Combining our results with the results in the literatures,we have found no positive correlation between the Al contents and the"Al"-based hydrogen concentrations,suggesting that previously proposed hydrogen-incorporation mechanism H^++Al^3+■Si^4+does not function in coesite.In contrast,we have confirmed the positive correlation between the B contents and the B-based hydrogen concentrations.The hydrogen-incorporation mechanism H^++B3^+■Si^4+readily takes place in coesite at different P-T conditions,and significantly increases the water content at both liquid-saturated and liquid-undersaturated conditions.For the SiO2-H2O system,we have found that type-Ⅰhydrogarnet substitution plays a dictating role in incorporating water into coesite at liquid-saturated condition,type-II hydrogarnet substitution contributes significantly at nearly dry condition,and both operate at conditions in between.The water solubility of coesite,as dictated by the type-Ⅰhydrogarnet substitution,positively correlates with both P and T,cH2O=-105(30)+5.2(32)×P+0.112(26)×T,with cH2O in wt ppm,P in GPa and T in℃.Due to its low water solubility and small fraction in subducted slabs,coesite may contribute insignificantly to the vertical water transport in subduction zones.Furthermore,the water solubility of any coesite in exhuming ultra-high pressure metamorphic rocks should be virtually zero as coesite becomes metastable.With an adequately fast waterdiffusion rate,this metastable coesite should be completely dry,which may have been the key factor to the partial preservation of most natural Coe.As a byproduct,a new IR experimental protocol for accurate water determination in optically anisotropic nominally anhydrous minerals has been found.Aided with the empirical method of Paterson(1982)it employs multiple unpolarized IR spectra,collected from randomly-orientated mineral grains,to approximate both total integrated absorbance and total integrated molar absorption coefficient.Its success relies on a high-level orientation randomness in the IR analyses.     

Metamorphic evolution of the coesite-bearing ultrahigh-pressure terrane in the North Qaidam, Northern Tibet, NW China

Widespread evidence for ultrahigh‐pressure (UHP) metamorphism is reported in the Dulan eclogite‐bearing terrane, the North Qaidam–Altun HP–UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K‐feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO₂ lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites are P = 2.9–3.2 GPa, and T = 631–687 °C; the eclogite shows a near‐isothermal decompression P–T path suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites: (1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn at P = 2.9–3.3 GPa and T = 729–746 °C; (2) a high‐pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24–29) + Scp at P = 1.9–2.0 GPa, T = 873–948 °C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep/Czo at P = 0.7–0.9 GPa and T = 660–695 °C. The clockwise P–T path of the SDB eclogites is different from the near‐isothermal decompression P–T path from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub‐belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north‐western China.     

中温榴辉岩中柯石英和金刚石形成的一种地震成因模型

中温榴辉岩中超高压相矿物柯石英和金刚石的发现引起了地质学界的广泛关注，这不仅来自其本身的岩石学意义，而且也来源于对板块构造的过程和造山带的演化提出了限制条件。因此进一步研究这两种矿物可能的成因十分必要，为此本文提出柯石英和金刚石形成的一种新的模型─地震有关的成因模型．本文的论述将主要来自目前我们对大别一苏鲁地区含柯石英金刚石榴辉岩的了解。     

Pressure dependence of hydroxyl solubility in coesite

 The solubility of hydroxyl in coesite was investigated in multianvil experiments performed at 1200 °C over the nominal pressure range 5–10 GPa, at an f _O2 close to the Ni-NiO buffer. The starting material for each experiment was a cylinder of pure silica glass plus talc, which dehydrates at high P and T to provide a source of water and hydrogen (plus enstatite and excess SiO₂). Fourier-transform infrared (FTIR) spectra of the recovered coesite crystals show five sharp bands at 3606, 3573, 3523, 3459, and 3299 cm⁻¹, indicative of structurally bonded hydrogen (hydroxyl). The concentration of hydrogen increases with pressure from 285 H/10⁶ Si (at 5 GPa) to 1415 H/10⁶ Si (at 10 GPa). Assuming a model of incorporation by (4H)_Si defects, the data are fit well by the equation C _OH=Af ² _H2<\INF>Oexp(−PΔV/RT), with A=4.38 H/10⁶ Si/GPa, and ΔV=20.6 × 10⁻⁶ m³ mol⁻¹. An alternative model entailing association of hydrogen with cation substitution can also be used to fit the data. These results show that the solubility of hydroxyl in coesite is approximately an order of magnitude lower than in olivines and pyroxenes, but comparable to that in pyropic garnet. However, FTIR investigations on a variety of ultrahigh pressure metamorphic rocks have failed in all cases to detect the presence of water or hydrogen in coesite, indicating either that it grew in dry environments or lost its hydrogen during partial transformation to quartz. On the other hand, micro-FTIR investigations of quartz crystals replacing coesite show that they contain varying amounts of H₂O. These results support the hypothesis that preservation of coesite is not necessarily linked to fast exhumation rates but is crucially dependent on limited fluid infiltration during exhumation. Received: 23 August 1999 / Accepted: 10 April 2000