Diffusion-controlled growth of wollastonite rims between quartz and calcite: comparison between nature and experiment

Growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0.1 and 1 GPa and temperatures from 850 to 1200 °C. Rim growth follows a parabolic rate law indicating that this reaction is diffusion‐controlled. From the rate constants, the D′δ‐values of the rate‐limiting species were derived, i.e. the product of grain boundary diffusion coefficient D′ and the effective grain boundary width, δ. In dry runs at 0.1 GPa, wollastonite grew exclusively on quartz surfaces. From volume considerations it is inferred that (D′_CaOδ)/(D′_SiO2δ)≥1.33, and that SiO₂ diffusion controls rim growth. D′_SiO2δ increases from about 10^?25 to 10^?23 m³ s^?1 as temperature increases from 850 to 1000 °C, yielding an apparent activation energy of 330±36 kJ mol^?1. In runs at 1 GPa, performed in a piston‐cylinder apparatus, there were always small amounts of water present. Here, wollastonite rims always overgrew calcite. Rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion‐advection mechanism. Rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion, but it is not clear whether CaO or SiO₂ diffusion is rate‐limiting. D′δ increases from about 10^?21 to 10^?20 m³ s^?1 as temperature increases from 1100 to 1200 °C. D′_SiO2δ or D′_CaOδ in rims on calcite is c. 10 times larger than D′_SiO2δ in dry rims at the same temperature. Growth structures of the experimentally produced rims are very similar to contact‐metamorphic wollastonite rims between metachert bands and limestone in the Bufa del Diente aureole, Mexico, whereby noninfiltrated metacherts correspond to dry and brine‐infiltrated metacherts to water‐bearing experiments. However, the observed diffusivities were 4 to 5 orders of magnitude larger during contact‐metamorphism as compared to our experimental results.     

Dependence of reaction kinetics on H2O activity as inferred from rates of intergranular diffusion of aluminium

Quantitative constraints on the accelerative effects of H₂O on the kinetics of metamorphic reactions arise from a comparison of rates of intergranular diffusion of Al in natural systems that are fluid‐saturated, hydrous but fluid‐undersaturated, and nearly anhydrous. Widths of symplectitic reaction coronas around partially resorbed garnet crystals in the contact aureole of the Makhavinekh Lake Pluton, northern Labrador, combined with time–temperature histories from conductive thermal models, yield intergranular diffusivities for Al from ～700–900 °C under nearly anhydrous conditions. Those rates, when extrapolated down temperature, are approximately three orders of magnitude slower than rates derived from re‐analysis of garnet resorption coronas formed under hydrous but fluid‐undersaturated conditions near 575 °C in rocks of the Llano Uplift of central Texas, which are in turn approximately four orders of magnitude slower than rates at comparable temperatures derived from numerical simulations of prograde garnet growth in fluid‐saturated conditions in rocks from the Picuris Range of north‐central New Mexico. Thus, even at constant temperature, rates of intergranular diffusion of Al – and corresponding length scales and timescales of metamorphic reaction and equilibration – may vary by as much as seven orders of magnitude across the range of H₂O activities found in nature.     

High-temperature 'clockwise'P-T paths and melting in the development of regional migmatites: an example from southern Brittany, France

The Southern Brittany Migmatite Belt (SBMB), which evolved through the metamorphic peak between c. 400 Ma and c. . 370 Ma ago, consists of a heterogeneous suite of high-grade gneisses and anatectic migmatites, both metatexites and diatexites. Rare garnet-cordierite gneiss layers record evidence of an early prograde P-T path. In these rocks, growth-zoned garnet cores and a sequence of included mineral assemblages in garnet, from core to rim, of Qtz + Ilm + Ky, Pl + Ky + St + Rt + Bt and Pl + Sil + St + Rt + Bt constrain a prograde evolution during which the reactions Ilm + Ky + Qtz→ Aim + Rt, Ms + Chl→ St + Bt + Qtz + V and St + Qtz→ Grt + Sil + V were crossed. Parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types. In all rock types, garnet has reverse zoned rims, and garnet replacement by cordierite and/or biotite and plagioclase suggests the following reactions have occurred: Grt + Sil + Qtz→ Crd → Hc ± Ilm, Bt + Sil + Qtz → Crd ± Hc → Ilm → Kfs + V and (Na + Ca + K + Ti) + Grt → Bt + Pl + Qtz. Microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained 'clockwise' P–T path for the SBMB. The high-temperature part of the path has a steep dT/dP slope characteristic of near isothermal decompression. It is proposed that the P-T path followed by the SBMB is the result of the inversion, by overthrusting, of a back-arc basin and that such a tectonic setting may be applicable to other high-temperature migmatite terranes. The near isothermal decompression is at least partly driven by the upward (diapiric) movement of the diatexite/anatectic granite core of the SBMB.     

超高温麻粒岩中尖晶石成分、结构特征和矿物反应的指示意义:以阿尔泰造山带超高温尖晶石-斜方辉石-石榴石麻粒岩为例

超高温变质作用的研究已成为世界上热门的地球科学领域之一,其中斜方辉石+夕线石+石英、假蓝宝石+石英、尖晶石+石英、大隅石+石英以及富铝斜方辉石等标型矿物组合和特征可以指示岩石经历了超高温变质作用(温度>900℃),这对查明地球深部下地壳的物质组成和构造演化具有重要意义。初次对产于世界上不同地区超高温麻粒岩尖晶石+石英共生等的标型矿物组合、不同赋存状态的尖晶石ZnO含量、XZn[Zn/(Mg+Fe2++Zn)]、XMg等进行分析和对比,认为不同地区不同赋存状态的尖晶石,其ZnO含量、XZn和XMg有所不同,但总的来说,超高温麻粒岩尖晶石中ZnO质量分数多数低于2.5%,但个别特殊的有高达近15%。尖晶石中XZn较低且尖晶石与石英共生,可以作为超高温变质的证据之一。通过对阿尔泰南缘超高温尖晶石-斜方辉石-石榴石麻粒岩中尖晶石成分及ZnO含量分析,得出与石英共生的尖晶石ZnO质量分数约为2.50%,且有与XFe成正比的变化趋势,而与钛铁矿共生的尖晶石ZnO质量分数约为1.85%,虽然两者含量稍有不同,但与世界上超高温麻粒岩中多数尖晶石ZnO含量相比都相近。结合其他特征矿物组合,可以确定该岩石中尖晶石与石英共生组合可以作为阿尔泰地区发生超高温变质作用的证据。通过对阿尔泰南缘与尖晶石-石英有关的变质反应期次划分等的初步研究,认为变质期次可划分为峰期变质和退变质两期,推测峰期的温度条件可达到1000℃,且该超高温麻粒岩可能经历了顺时针p-T演化轨迹。该超高温麻粒岩在阿尔泰造山带的出现,可能与古生代时期发生的古亚洲洋俯冲、西伯利亚板块和哈萨克斯坦—准噶尔板块碰撞造山作用过程有关。     

The effect of pre‐tectonic reaction and annealing extent on behaviour during subsequent deformation: insights from paired shear zones in the lower crust of Fiordland,New Zealand

Granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow (<20 mm) felsic dykes, in Fiordland, New Zealand, forming ~10–80 mm wide garnet reaction zones. The metamorphic reaction changed the abundance of minerals, and their shape and grain size distribution. The extent of reaction and annealing (temperature‐related coarsening and nucleation) is greatest close to the dykes, whereas further away the reaction is incomplete. As a consequence, grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres. The aspect ratios of clusters of S1 pyroxene and pargasite in the orthogneiss, which are variably reacted to post‐S1 garnet, decrease from high in the host, to near equidimensional close to the dyke. Post‐reaction deformation localized in the fine‐grained partially reacted areas. This produced a pattern of ‘paired’ shear zones located at the outer parts of the garnet reaction zone. Our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction. The weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage.