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Huaiwei Ni 《中国地球化学学报》2014,33(2):125-130
Oxygen isotope fractionation between coexisting minerals in slowly cooled rocks conveys information about their cooling history. By using the fast grain boundary （FGB） model to simulate closed-system diffusive ex- change of oxygen isotopes between coexisting minerals, I show that the apparent equilibrium temperatures （Tae） by the mineral pair with the largest isotopic fractionation （PLIF） always lies between the closure temperatures （To） of those two minerals. Therefore, when the rate of oxygen diffusion and hence Tc for the PLIF chance to be comparable （such as in the case of quartz and magnetite）, Tae will serve as a good approximation of To regardless of variation in mineral proportions. The specialty of the PLIF in constraining Tac within their Tc range can be generalized to other stable isotope systems and element partitioning. By approximating Tc with Tac and inverting Dodson＇s equation, the cooling rate of plutonic or metamorphic rocks can be inferred. 相似文献
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Rapid cooling and geospeedometry of granitic rocks exhumation within a volcanic arc: A case study from the Central Slovakian Neovolcanic Field (Western Carpathians) 
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下载免费PDF全文 U–Pb Sensitive High‐Resolution Ion MicroProbe (SHRIMP) dating of zircon in combination with (U–Th)/He dating of zircon and apatite is applied to constrain the emplacement and exhumation history of the youngest granitic rocks in the Western Carpathians collected in the Central Slovakian Neovolcanic Field. Two samples of diorite from the locality Banky, and granodiorite from Banská Hodru?a yield the U–Pb zircon concordia ages of 15.21 ±0.19 Ma and 12.92 ±0.27 Ma, respectively, recording the time of zircon crystallization and the intrusions’ emplacement. Zircon (U–Th)/He ages of 14.70 ±0.94 (Banky) and 12.65 ±0.61 Ma (Banská Hodru?a), and apatite (U–Th)/He ages of 14.45 ±0.70 Ma (diorite) and 12.26 ±0.77 Ma (granodiorite) are less than 1 Myr younger than the corresponding zircon U–Pb ages. For both diorite and granodiorite rocks their chronological data thus document a simple cooling process from magmatic crystallization/solidification temperatures to near‐surface temperatures in the Middle Miocene, without subsequent reheating. Geospeedometry data suggest for rapid cooling at an average rate of 678 ±158 °C/Myr, and the exhumation rate of 5 mm/year corresponding to active tectonic‐forced exhumation. The quick cooling is interpreted to record the exhumation of the studied granitic rocks complex that closely followed its emplacement, and was likely accompanied by a drop in the paleo‐geothermal gradient due to cessation of volcanic activity in the area. 相似文献
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A model that relates the characteristic diffusion length and average cooling rate to peak temperature was developed for chemical diffusion in spherical geometries on the basis of geospeedometry principles and diffusion theory. The model is quantitatively evaluated for cation diffusion profiles in garnet. Important model parameters were calibrated empirically using diffusion zoning of Ca in garnet from the Pikwitonei Granulite Domain, a terrane for which the thermal history has been well characterized. The results are used: (i) to empirically test diffusion parameters for Mg and Fe(II) and (ii) to develop a tool that uses the diffusion zoning of these cations in garnet to constrain peak temperature conditions for garnet‐bearing rocks. The thermometric approach was externally tested by applying it to garnet crystals from various metamorphic terranes worldwide and comparing the results to published peak temperature estimates. The results overlap within uncertainties in all cases, but result that are based on Fe(II) and Mg chemical‐diffusion profiles are up to three times more precise than those acquired by conventional methods. The remarkable consistency of the results implies that the model is robust and provides a reliable means of estimating peak temperatures for different types of high‐grade metamorphic rock. The tool could be of particular advantage in rocks where critical assemblages for conventional thermometry do not occur or have been replaced during retrogression. 相似文献
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Previous studies suggest that the metamorphic evolution of the ultrahigh‐pressure garnet peridotite from Alpe Arami was characterized by rapid subduction to a depth of c. 180 km with partial chemical equilibration at c. 5.9 Gpa/1180 °C and an initial stage of near‐isothermal decompression followed by enhanced cooling. In this study, average cooling rates were constrained by diffusion modelling on retrograde Fe–Mg zonation profiles across garnet porphyroclasts. Considering the effects of temperature, pressure and garnet bulk composition on the Fe–Mg interdiffusion coefficient, cooling rates of 380–1600 °C Myr?1 for the interval from 1180 to 800 °C were obtained. Similar or even higher average cooling rates resulted from thermal modelling, whereby the characteristics of the calculated temperature‐time path depend on the shape and size of the hot peridotite body and the boundary conditions of the cooling process. The very high cooling rates obtained from both geospeedometry and thermal modelling imply extremely fast exhumation rates of c. 15 mm yr?1 or more. These results agree with the range of exhumation rates (16–50 mm yr?1) deduced from geochronological results. It is suggested that the Alpe Arami peridotite passively returned towards the surface as part of a buoyant sliver, caused as a consequence of slab breakoff. 相似文献
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Compositional zoning in biotite–garnet pairs in metamorphic rocks from eastern Finland has been studied. The Mg profiles in the garnet side of biotite–garnet crystal pairs have been interpreted by means of Lasaga's theory (geospeedometry). However, the binary interdiffusion equations are first reformulated by starting from a ternary system and using the lattice fixed frame of reference. This frame of reference gives the fluxes directly by means of the numbers of diffusing ions, which helps to check the 1-dimensionality of the analysis assumed in Lasaga's theory. It is also shown that the recently argued effect of the third cation Ca is negligible in our samples. We were able to investigate satisfactory profiles in three samples from different areas. The values for the cooling rate are a few degrees per million years if the diffusion data obtained by Freer are adopted. The cooling rates are in agreement with recent estimates based on the K–Ar ages on biotite in the same areas. 相似文献
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Laurence A. Coogan Simone A. Kasemann Sumit Chakraborty 《Earth and Planetary Science Letters》2005,240(2):415-424
Episodic emplacement and cooling of lavas and dikes at mid-ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity. However, the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood. We have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling. We have calibrated this Li-geospeedometer using new high-temperature experiments to determine both the temperature dependence of the partitioning of Li between plagioclase and clinopyroxene and the diffusion coefficient for Li in clinopyroxene. Application of this method to lavas and dikes from ODP Hole 504B shows that cooling rates vary dramatically with depth in the upper oceanic crust. Extremely rapid cooling rates (> 450 °C hr− 1) in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column. 相似文献
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An eclogite sample from the Grossgockner region of the Hohe Tauern, Austria contains garnet with a pronounced compositional discontinuity between a Mn‐rich core and an Fe‐rich rim. This jump in composition was caused by a garnet‐consuming reaction followed by growth of the garnet rim + omphacite and marks the prograde transition from epidote–amphibolite to eclogite facies metamorphism. Garnet growth ended at peak metamorphic conditions of 570 °C, 17 kbar, but intracrystalline diffusion continued until about 450 °C, 4 kbar on the retrograde path. This garnet overgrowth texture represents a natural diffusion couple and a time span of 1 Myr was calculated from the diffusion profile developing out of the original sharp compositional step. For typical crustal densities, this time corresponds to a minimum average velocity in the range 4.6–7.4 cm yr?1 (for vertical movement), which is one of the fastest exhumation rates reported. The diffusion of all divalent cations of four profiles was modelled, both analytically and numerically. Both approaches gave comparable results, but the times computed for each element were always discrepant up to a factor of 2. Variations of diffusion coefficients within 2 in analytical calculations remedied this and gave consistent upper time limits. Numerical modelling does not require the simplifications introduced in the analytical approach. On the other hand, error propagation was computationally unfeasible with this method. 相似文献
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Two approaches to determining the high-temperature (1000°Cto 600°C) cooling rate of the lower oceanic crust and uppermantle are presented and critically evaluated. The first isbased on the down-temperature diffusive exchange of Ca betweenolivine and clinopyroxene. The second, less well-constrained,approach is based on the down-temperature diffusive exchangeof Mg and Fe between olivine and spinel. Cooling rates basedon olivine–spinel geospeedometry are approximately anorder of magnitude faster than those from Ca-in-olivine geospeedometry.In contrast, cooling rates derived from thermochronology andremanent magnetism are approximately an order of magnitude slowerthan those derived by Ca-in-olivine geospeedometry; this isprobably because they record cooling at lower temperatures.Using the Ca-in-olivine geospeedometer, the cooling rate ofsamples from the lower oceanic crust and upper oceanic mantleformed in the Oman ophiolite and in the three main ocean basinshas been determined. Samples from the lower oceanic crust formedat fast-spreading ridges show a large decrease in cooling ratebetween the top and base of the gabbroic section, with mostof the variation occurring within the upper kilometre. Thisis consistent with vertical heat loss (within the crustal frameof reference) dominating the thermal evolution at fast-spreadingridges. Samples from Ocean Drilling Program Hole 735B, whichformed at the slow-spreading Southwest Indian Ridge, show novariation in cooling rate over 1500 m depth range and cooledsubstantially faster than rocks from the deeper portion of thegabbros in the Oman ophiolite, where the change in cooling ratewith depth is limited. These observations are consistent withheat loss from small plutons emplaced in cool lithosphere atthe slow-spreading ridge. Alternatively, they could be explainedby cooling through the Ca-in-olivine closure interval duringuplift towards the surface. KEY WORDS: geospeedometry; lower oceanic crust; Hess Deep; Pito Deep; ODP Hole 735B; ODP Leg 153 相似文献
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Garnet from a kinzigite, a high-grade gneiss from the central Black Forest (Germany), displays a prominent and regular retrograde diffusion zoning in Fe, Mn and particularly Mg. The Mg diffusion profiles are suitable to derive cooling rates using recent datasets for cation diffusion in garnet. This information, together with textural relationships, thermobarometry and thermochronology, is used to constrain the pressure–temperature–time history of the high-grade gneisses. The garnet–biotite thermometer indicates peak metamorphic temperatures for the garnet cores of 730–810 °C. The temperatures for the outer rims are 600–650 °C. Garnet–Al2 SiO5 –plagioclase–quartz (GASP) barometry, garnet–rutile–Al2 SiO5 –ilmenite (GRAIL) and garnet–rutile–ilmenite–plagioclase–quartz (GRIPS) barometry yield pressures from 6–9 kbar. U–Pb ages of monazite of 341±2 Ma date the low- P high- T metamorphism in the central Black Forest. A Rb/Sr biotite–whole rock pair defines a cooling age of 321±2 Ma. The two mineral ages yield a cooling rate of about 15±2 °C Ma−1 . The petrologic cooling rates, with particular consideration of the f O2 conditions for modelling retrograde diffusion profiles, agree with the geochronological cooling rate. The oldest sediments overlying the crystalline basement indicate a minimum cooling rate of 10 °C Ma−1 . 相似文献
10.
程昊 《矿物岩石地球化学通报》2005,24(2):148-152
对石榴子石冷却速率计中的几个重要参数误差分析结果表明，在慢速冷却的情况下，由测量数据误差所致的偏差可以忽略不计。但当冷却速率较大时，误差对模拟结果的影响则非常显著。然而，若采用现代分析仪器，由定位导致的模拟误差也可以完全忽略。空间测量的偏差往往使模拟得到的岩石冷却速率比实际值小得多；初始温度估计的偏差会使模拟结果产生很大的变化；扩散系数的选择对模拟结果影响也颇大。因此．具体情况下需仔细选取那些实验条件与所研究的具体地质情况最为相似的实验数据得到的模拟结果才是合理的。只有在合理的计算和选取参数以及可靠的测试技术的情况下，得到的模拟结果才具有地质意义。 相似文献
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