Mutual replacement reactions in alkali feldspars II: trace element partitioning and geothermometry     

Ian Parsons  Charles W. Magee  Charlotte M. Allen  J. M. G. Shelley  Martin R. Lee 《Contributions to Mineralogy and Petrology》2009,157(5):663-687

Perthitic alkali feldspar primocrysts in layered syenites in the Klokken intrusion in South Greenland, underwent dissolution–reprecipitation reactions in a circulating post-magmatic aqueous fluid at ~450°C, and are to a large degree pseudomorphs. These ‘mutual replacement’ reactions provide a perfect natural experiment with which to study trace element partitioning between sodium and potassium feldspars growing simultaneously. The reactant ‘phase’ was a cryptoperthitic feldspar consisting of low albite and low microcline in a coherent sub-μm ‘braid’ intergrowth and the product phases were ‘strain-free’ incoherent subgrains of low albite and low microcline forming microporous patch perthites on scales up to 200 μm. The driving force for the reaction was reduction of coherency strain energy. The mechanisms of this process are described in Part I. Five mixed braid perthite–patch perthite crystals were analysed for major and trace elements using laser ablation-inductively coupled plasma mass spectrometry with a 19 μm beam diameter. This gave bulk analyses of the braid texture, which were in the range Ab_73–54Or_45–27An_4.3–0.8, but could resolve Ab- and Or-rich patches in patch perthite. The major element bulk compositions of the crystals were retained during the replacement reactions. Major components in patches plot on tielines in the Ab–Or–An ternary system that pass through or very close to the parent braid perthite composition and indicate local equilibrium on the scale of a few tens of mm. Many trace elements, including REE, were lost to the fluid during the deuteric reactions, but the effect is large only for Fe and Ti. Cs, Pb and Sr were added to some crystals. Plots of log distribution coefficient D for Rb, Ba, Pb, Eu²⁺, La and Ce between Or- and Ab-rich patches against ionic radius are straight lines, assuming eightfold coordination, and to a first approximation are independent of ionic charge. K also lies on these lines, and the smaller ions Na and Ca lie close to them. The best linear fits were obtained using ionic radii for ^[8]K and ^[8]Ca, but there is ambiguity as to whether ^[7]Na or ^[5]Na is most appropriate. The linear relationship shows that the listed trace elements are in the feldspar M-site rather than in inclusions. Tl is in M although an exact D could not be obtained. The very large Cs ion partitions strongly into the Or-rich phase but its D value appears to be less than predicted by extrapolation. The near-linearity arises because partitioning is occurring between two solids into sites which have similar Young’s moduli, so that the parabolas that normally represent trace element partitioning between crystals and liquids (which have negligible shear strength) approximately cancel out. Ga and Be are in T-sites, as well as some of the Fe and Ti present, although part is in oxide inclusions. The site of Sc is unclear, but if structural it is likely to be T. Partitioning on M-sites is a potential geothermometer but because the effective size of the irregular M-site is defined by its K and (Na + Ca) contents, which are controlled by ternary solvus relationships, its calibration is not independent of conventional two-feldspar geothermometers. Trace elements may however provide a useful means of confirming that feldspar pairs are in equilibrium, and of recognising feldspar intergrowths produced by non-isochemical replacement rather than exsolution. Two-feldspar geothermometry for the ternary phases in the low-albite microcline patch perthites gives temperatures above the stability range of microcline, markedly so if a correction is made for Si–Al ordering. This is probably because current geothermometers are too sensitive to low concentrations of An in ordered Or-rich feldspars. This interpretation is supported by two-feldspar assemblages growing at known temperatures in geothermal systems and sedimentary basins. This paper and the earlier Part I are dedicated in the memory of J. V. Smith and W. L. Brown, both of whom died in 2007, in acknowledgement of their unrivalled contributions to the study of the feldspar minerals over more than half a century. An erratum to this article can be found at  相似文献   

A TEM and microprobe study of a two-perthite alkali gabbro: Implications for the ternary feldspar system     

I. Parsons  W. L. Brown 《Contributions to Mineralogy and Petrology》1983,82(1):1-12

An unusual magmatic three-feldspar syenogabbro occurs 3 m inside the contact of the Klokken gabbro-syenite stock. It contains plagioclase, mesoperthite and cryptoperthite, together with a low temperature symplectite intergrowth. Textural relationships have been investigated by cathodoluminescence, bulk chemistry by microprobe, and exsolution microtextures and intracrystalline boundaries by TEM. The mesoperthite has a bulk composition around Or₂₆Ab₅₂An₂₂, well outside accepted limits of ternary feldspar solid solution. It is a mainly coherent, lamellar intergrowth of sodic andesine and orthoclase with incipient Mtwinning. The cryptoperthite, bulk composition around Or₆₁Ab₃₃An₆, is a coherent lamellar intergrowth of orthoclase and sodic low oligoclase. The compositions of the exsolved phases have been estimated. The meso- and cryptoperthite crystals have sharp boundaries with each other. Plagioclase (zoned An 55-35, with 2–3% Or) defines a solidus fractionation path. It behaved as an inert phase during crystallization of the perthites, which grew as homogeneous monoclinic phases in equilibrium on the strain-free ternary solvus, defining a solvus isotherm at ～ 950° C. Both of the monoclinic phases exsolved on reaching the coherent ternary spinodal at temperatures estimated to be close to 800–850° C and 700–830° C respectively. Lamellar periodicities (529±149 nm and 148±18 nm respectively) are considerably finer scale than predicted by coarsening experiments, suggesting that An and/or Al-Si order greatly inhibit coarsening. The symplectite is a coarse, incoherent intergrowth of sodic andesine and nearly pure K-feldspar, probably produced by simultaneous crystallization at <400° C. The new data and literature analyses are used to construct the geometry of the ternary feldspar system. Solvus isotherms implied by existing experimental data approach the Ab apex too closely at high temperature. The critical solution curve becomes slightly more albitic after leaving the binary Ab-Or join, and then turns sharply towards the An-Or join. It intersects the proposed new limit of feldspar solid solution near Or₃₆Ab₄₄An₂₀ at 1,060° C. This probably approximates to the highest temperature on the ternary critical curve at 1 bar.  相似文献   

The nature of potassium feldspar,exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars     

William L. Brown  Ian Parsons 《Contributions to Mineralogy and Petrology》1984,86(4):335-341

Transmission electron microscope data on the morphology of exsolution lamellae, the nature of the potassium feldspar and the development of dislocations at lamellar interfaces in coherent cryptoperthites and fine microperthites are reviewed. Dislocations have been reported previously in only two crystals, and periodic dislocations noted in only one, an Or-rich microperthite. Periodic dislocations (spacing 100–150 nm) are here described from a ternary mesoperthite (Or₂₆ Ab₅₂ An₂₂). Small crystallites (<30 nm) of other phases have sometimes nucleated on the dislocations. The 020 lattice fringes of the feldspar phases have been imaged; the difference in 020 spacings can be almost entirely accommodated by the regular dislocations, so that the boundaries may be termed nearlyperfectly semicoherent.Dislocations have been found so far only in cryptoperthites with lens-shaped or straight lamellae, either in Or-rich feldspars or in Ab-rich ternary ones. In intermediate compositions with wavy or zig-zag albite lamellae, or lozengeshaped albite areas (braid microperthites) dislocations have not been observed. Strain reduction in intermediate compositions occurs by migration of lamellar interfaces from (¯601) to near (¯6¯61) as microcline forms in the diagonal association. In Ab-rich ternary feldspars the relatively high Ancontent blocks interface migration, and strain reduction occurs by nucleation of dislocations; the Or-rich feldspar phase is tweed orthoclase. In Or-rich bulk compositions the low volume of albite exerts insufficient stress to promote microcline formation, and tweed orthoclase develops. Interfaces do not migrate, and dislocations again develop. Fields in which different potassium feldspar polymorphs occur and in which the different exsolution textures are developed are summarized on a ternary diagram.  相似文献   

Feldspar evolution in the Roxby Downs Granite,host to Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam,South Australia     

《Ore Geology Reviews》2017

The textural relationships and geochemistry of feldspars from least-altered to sericite-hematite altered and mineralised ~ 1.595 Ga Roxby Downs Granite (RDG) at Olympic Dam, South Australia, were examined. The sample suite is representative of RDG both distal (> 5 km) and proximal (< 1 km) to the hydrothermal breccias of the Olympic Dam Breccia Complex (ODBC), which host Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam. Microscopic observations and quantitative analyses indicate that a range of feldspar reactions have taken place within the RDG hosting the Olympic Dam deposit. An early phase of igneous plagioclase (~ An_27–34) is recognised, along with a more abundant, less-calcic plagioclase (~ An_12–20) both displaying rapakivi and anti-rapakivi textures with alkali feldspar. Alkali feldspars (~ Or₅₅Ab₄₃An₂) record post-magmatic evolution from cryptoperthite to patch perthite. Subsequent patch perthite is overprinted by highly porous, near end-member albite and K-feldspar, while plagioclase undergoes replacement by albite + sericite ± Ba-rich K-feldspar. In sericite-hematite altered and mineralised RDG along the margin of the ODBC, sericite replaces all plagioclase, whereas red-stained, Fe-rich K-feldspar persists. Sulphide-uranium-rare earth element mineralisation is observed in association with hydrothermal feldspars, and increases in abundance with proximity to the orebody. Petrographic observations and whole-rock geochemistry illustrate the transformation of plagioclase and alkali feldspar from igneous to hydrothermal processes, and indicate that hydrothermal albite and K-feldspar formed within the RDG without the need for an external source of alkalis. Feldspar geothermometry indicates a minimum crystallisation temperature of 765 °C at 2.2 kbar for alkali feldspar (pressure estimate obtained using plagioclase-amphibole geobarometry) followed by a range of lower temperature transformations. Late-stage magma mixing/contamination is postulated from supportive temperature and pressure estimates along with feldspar and mafic mineral relationships.  相似文献   

河北武安坦岭多斑斜长斑岩的成因:冻结岩浆房活化机制   总被引：5，自引：3，他引：2  

刘璐璐  苏尚国  侯建光  谢玉淳 《岩石学报》2017,33(1):204-220

流变学实验表明,当岩浆中晶体体积分数达到约50vol%时,岩浆体实际上处于冻结状态,不再具有整体迁移的能力。但在自然界中仍存在含大量斑晶的浅成火成岩和火山岩。因此,富晶体岩浆的上升过程和侵位机制是近年来地球科学领域关注的热点之一。目前,冻结岩浆房的活化机制主要有二种:升温活化机制和流体活化机制。河北武安坦岭地区新发现的多斑斜长斑岩为揭示冻结岩浆房的活化提供了契机。野外观察和晶体粒度分布(CSD)分析表明,坦岭斜长斑岩中斜长石斑晶高达70vol%,基质为显微晶质结构。斜长石斑晶粒径分布均一,大小约为3.1×1.7mm;显微镜观察和背散射图像揭示,斜长石斑晶具环带结构,由宽广的斜长石核部+宽度可变的条纹长石边部组成,且无熔蚀现象;电子探针成分剖面分析表明,斑晶核部成分为更长石(An_(27)Ab_(71)Or_2),幔部为更长石(An_(13)Ab_(83)Or_4),边部为条纹长石。边部条纹长石的成分有一定变化,从内侧到外侧,主晶钠长石成分由Ab_(53)Or_(47)变为Ab_(99)Or_1,客晶钾长石成分由Ab_(48)Or_(51)变为Ab3Or97。斑晶斜长石核部存在细长条状或斑点状钾长石,且越靠近中心,钾长石斑点的数量越少。这些特点表明,边部条纹长石为交代成因。稀土和微量元素分析则显示,边部条纹长石具弱正Eu异常,相对富集LREE和K、Rb、Ba、Sr等大离子亲石元素,亏损Th、Zr、Nb的特点。CSD相关图解及以上特征表明,斜长石斑晶形成于稳定,封闭的结晶环境,并受到晚期碱交代作用的改造。基质主要由微粒钙质角闪石,条纹长石,石英,钾长石和钠长石组成,含少量自形-半自形磁铁矿和钛铁矿、磷灰石、榍石、金红石和锆石等11种矿物组成。11种矿物相和结构特征暗示基质形成于极端不稳定的结晶环境,与斜长石斑晶形成条件鲜明对照。根据基质的矿物组成,推测形成基质的岩浆具有富含K、Na、Fe、Si和挥发分的特征。这种特征与上述关于条纹长石环边形成条件的判断一致。据此,本文认为:产生斜长石斑晶的岩浆曾经在地壳深部作过长时间滞留,导致了斜长石的稳定结晶,增加了岩浆的粘度和密度,使岩浆处于冻结状态;富碱高铁熔体-流体流的注入大幅降低了岩浆的总粘度,并提高了岩浆的浮力,从而促使冻结岩浆房迅速活化和上升侵位;同时,富碱高铁熔体-流体流强烈交代了先存的斜长石斑晶,使其边部形成条纹长石;这种熔体-流体流则在快速排气,冷却过程中迅速结晶,形成了具有不平衡矿物组合的显微晶质基质。在岩浆侵入体较深部位,富碱高铁熔体-流体经历了很缓慢的固结过程,而相分离产生的流体有可能萃取携带岩浆中的铁质,形成富Fe流体流,后者可能对区内"铁矿浆"型铁矿的形成具有重要的贡献。  相似文献   

Time–temperature evolution of microtextures and contained fluids in a plutonic alkali feldspar during heating     

Ian Parsons  John D. Fitz Gerald  James K. W. Lee  Tim Ivanic  Ute Golla-Schindler 《Contributions to Mineralogy and Petrology》2010,160(2):155-180

Microtextural changes brought about by heating alkali feldspar crystals from the Shap granite, northern England, at atmospheric pressure, have been studied using transmission and scanning electron microscopy. A typical unheated phenocryst from Shap is composed of about 70 vol% of tweed orthoclase with strain-controlled coherent or semicoherent micro- and crypto-perthitic albite lamellae, with maximum lamellar thicknesses <1 μm. Semicoherent lamellae are encircled by nanotunnel loops in two orientations and cut by pull-apart cracks. The average bulk composition of this microtexture is Ab_27.6Or_71.8An_0.6. The remaining 30 vol% is deuterically coarsened, microporous patch and vein perthite composed of incoherent subgrains of oligoclase, albite and irregular microcline. The largest subgrains are ~3 μm in diameter. Heating times in the laboratory were 12 to 6,792 h and T from 300°C into the melting interval at 1,100°C. Most samples were annealed at constant T but two were heated to simulate an ⁴⁰Ar/³⁹Ar step-heating schedule. Homogenisation of strain-controlled lamellae by Na↔K inter-diffusion was rapid, so that in all run products at >700°C, and after >48 h at 700°C, all such regions were essentially compositionally homogeneous, as indicated by X-ray analyses at fine scale in the transmission electron microscope. Changes in lamellar thickness with time at different T point to an activation energy of ~350 kJmol⁻¹. A lamella which homogenised after 6,800 h at 600°C, therefore, would have required only 0.6 s to do so in the melting interval at 1,100°C. Subgrains in patch perthite homogenised more slowly than coherent lamellae and chemical gradients in patches persisted for >5,000 h at 700°C. Homogenisation T is in agreement with experimentally determined solvi for coherent ordered intergrowths, when a 50–100°C increase in T for An₁ is applied. Homogenisation of lamellae appears to proceed in an unexpected manner: two smooth interfaces, microstructurally sharp, advance from the original interfaces toward the mid-line of each twinned, semicoherent lamella. In places, the homogenisation interfaces have shapes reflecting the local arrangements of nanotunnels or pull-aparts. Analyses confirm that the change in alkali composition is also relatively sharp at these interfaces. Si–Al disordering is far slower than alkali homogenisation so that tweed texture in orthoclase, tartan twinning in irregular microcline, and Albite twins in albite lamellae and patches persisted in all our experiments, including 5,478 h at 700°C, 148 h at 1,000°C and 5 h at 1,100°C, even though the ensemble in each case was chemically homogeneous. Nanotunnels and pull-aparts were modified after only 50 min at 500°C following the simulated ⁴⁰Ar/³⁹Ar step-heating schedule. New features called ‘slots’ developed away from albite lamellae, often with planar traces linking slots to the closest lamella. Slot arrays were often aligned along ghost-like regions of diffraction contrast which may mark the original edges of lamellae. We suggest that the slot arrays result from healing of pull-aparts containing fluid. At 700°C and above, the dominant defects were subspherical ‘bubbles’, which evolved from slots or from regions of deuteric coarsening. The small degree of partial melting observed after 5 h at 1,100°C was often in the vicinity of bubbles. Larger micropores, which formed at subgrain boundaries in patch perthite during deuteric coarsening, retain their shape up to the melting point, as do the subgrain boundaries themselves. It is clear that modification of defects providing potential fast pathways for diffusion in granitic alkali feldspars begins below 500°C and that defect character progressively changes up to, and beyond, the onset of melting.  相似文献   

Retrograde metamorphic reactions in deforming granites and the origin of flame perthite     

L. L. PRYER  P.-Y. F. ROBIN 《Journal of Metamorphic Geology》1995,13(6):645-658

Abstract Microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram: orthoclase (Or_92–97), albite and oligoclase-andesine. The albite occurs as grains in the matrix, as flame-shaped lamellae in orthoclase, and in patches within plagioclase grains. We present a metamorphic model for albite flame growth in the K-feldspar in these rocks that is related to reactions in plagioclase, rather than alkali feldspar exsolution. Flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions. Reaction 1 is a continuous or discontinuous (across the peristerite solvus) reaction in plagioclase, in which the An component forms epidote or zoisite. Most of the albite component liberated by Reaction 1 stays to form albite in the host plagioclase, but some Na migrates to form the flames within the K-feldspar. Reaction 2 is the exchange of K for Na in K-feldspar. Reaction 3 is the retrograde formation of muscovite (as ‘sericite’) and has all of the chemical components of a hydration reaction of K-feldspar. The Si and Al made available in the plagioclase from Reaction 1 are combined with the K liberated from the K-feldspar, to produce muscovite in Reaction 3. The muscovite forms in the plagioclase, rather than the K-feldspar, as a result of the greater mobility of K relative to Al. The composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction. Using an initial plagioclase composition of An₂₀, the total reaction can be summarized as: 20 oligoclase + 1 K-feldspar + 2 H₂O = 2 zoisite + muscovite + 2 quartz + 15 albite_plagioclase+ 1 albite_flame. This model does not require that any additional feldspar framework be accreted at replacement sites: Na and K are the only components that must migrate a significant distance (e.g. from one grain to the next), allowing Al to remain within the altering plagioclase grain. The resulting saussuritization is isovolumetric. The temperature and extent of replacement depends on when, and how much, water infiltrates the rock. The fugacity of the water, and therefore the pressure of the fluid, may have been significantly lower than lithostatic during flame growth.  相似文献   

Ordering and exsolution processes in Or-rich alkali feldspar megacrysts from the Eldzhurtinskiy granite (Caucasus)   总被引：3，自引：0，他引：3  

G. Witt-Eickschen  C. Evangelakakis  H. A. Seck  H. Kroll  A. G. Gurbanov 《Contributions to Mineralogy and Petrology》1996,124(1):71-81

 The extremely young (2.5 Ma) I-type Eldzhurtinskiy granite complex (Central Caucasus) is uniform with respect to modal composition, major and trace element chemistries of bulk rocks and mineral phases. In contrast, it reveals two types of alkali feldspar megacrysts differing in tetrahedral Al-content (2t₁) and exsolution microtextures: 1. Alkali feldspar megacrysts (Or₇₀An₂Ab₂₈) from the top of the body consist of ideally coherent intergrowths of fine-scale regular Or- and Ab-rich lamellae. The exsolved K-feldspar host is monoclinic (2t₁=0.7), the exsolved Na-rich phase consists of Albite- and/or Pericline-twinned albite. 2. Megacrysts from greater depths have the same bulk composition, but the exsolved Ab-rich phase occurs in the form of optically visible, broad lamellae and patches of low albite. In addition, the K-rich host yields a higher degree of (Al, Si) ordering (2t₁=0.8). The evolution of the distinct types of megacrysts reflects differences in the cooling history within the upper and lower part of the granite body. The occurrence of the coherent lamellae in the megacrysts from the top of the body is attributed to exsolution under dry conditions during fast cooling, whereas coarsening of lamellae and formation of albite patches in the megacrysts from the lower part are caused by fluid-feldspar interaction. The transition zone in the body between the two types of megacrysts is sharp (in a depth interval of 100–200 m) and not related to shear zones. Received: 12 June 1995 / Accepted: 29 January 1996  相似文献   

Twinning and exsolution in an antiperthite     

H. Ried  M. Korekawa 《Physics and Chemistry of Minerals》1978,3(3):263-270

An antiperthite feldspar (composition of the main part An_27.2 Ab_69.2Or_3.6) has been studied by x-rays and transmission electron microscopy. Complex twinning and exsolution on very fine scale are described for the first time for this compositional range. Evidence is given for a distinct intermediate region between the plagioclase and the potash feldspar. The formation of the crystal probably involves partial replacement, at least two step exsolution, and transformation of monoclinic plagioclase to triclinic plagioclase.  相似文献   

Peristerite compositions in quartzofeldspathic schists, Franz Josef-Fox Glacier Area, New Zealand     

RODNEY GRAPES  MASAYUKI OTSUKI 《Journal of Metamorphic Geology》1983,1(1):47-61

Plagioclase compositions vary from An_0.1–2.5 to An₃₂ with increasing grade in chlorite zone to oligoclase zone quartzofeldspathic schists, Franz Josef-Fox Glacier area, Southern Alps, New Zealand. This change is interrupted by the peristerite composition gap in rocks transitional between greenschist and amphibolite facies grade. Oligoclase (An_20-24) and albite (An_0.1–0.5) are found in biotite zone schists below the garnet isograd. With increasing grade, the plagioclase compositions outline the peristerite gap, which is asymmetric and narrows to compositions of An₁₂ and An₆ near the top of the garnet zone. In any one sample, oligoclase is the stable mineral in mica-rich layers above the garnet isograd, whereas albite and oligoclase exist in apparent textural equilibrium in adjacent quartz-plagioclase layers. The initial appearance of oligoclase in both layers results from the breakdown of epidote and possibly sphene. Carbonate is restricted to the quartz-plagioclase rich layers and probably accounts for the more sodic composition of oligoclase in these layers. The formation of more Ca-rich albite and more Na-rich oligoclase near the upper limit of the garnet zone coincides with the disappearance of carbonate and closure of the peristerite gap. Garnet appears to have only a localized effect on Ca-enrichment of plagioclase in mica-rich layers within the garnet zone. The Na-content of white mica increases sympathetically with increasing Ca-content of oligoclase and metamorphic grade. Comparison of the peristerite gap in the Franz Josef-Fox Glacier schists and schists of the same bulk composition in the Haast River area, 80 km to the S, indicates that oligoclase appears and epidote disappears at lower temperatures, and that the composition gap between coexisting albite and oligoclase is narrower in the Franz Josef-Fox Glacier area. It is suggested that a higher thermal gradient (38-40°C/km) and variations in Si/Al ordering during growth of the plagioclases between the two areas may account for these differences. In the Alpine schists the peristerite gap exists over a temperature and pressure interval of about 370-515°C and 5.5-7 kbar (550-700 MPa) P_H2O.  相似文献   

Paragenesis and chemical characteristics of the celsian−hyalophane−K-feldspar series and associated Ba-Cr micas in barite-bearing strata of the Mesoarchaean Ghattihosahalli Belt,Western Dharwar Craton,South India     

Michael M. Raith  Tadasore C. Devaraju  Beate Spiering 《Mineralogy and Petrology》2014,108(2):153-176

The upper greenschist - lower amphibolite facies, argillaceous to chemical-exhalative metasedimentary sequence of the Mesoarchaean Ghattihosahalli Schist Belt (GHSB), southern India, has been examined with a special focus on the paragenesis and solid solution characteristics of barian feldspars and associated dioctahedral Ba-Cr-bearing micas. Barian feldspars occur as untwinned porphyroblasts in a recrystallized finely banded matrix of barite, quartz and minor white mica. Idioblastic celsian (Cls_98-76Or_2-20Ab_1-8) and hyalophane (Cls_55-39Or_35-51Ab₁₀) predate the greenschist-facies foliation, whereas xenoblastic hyalophane (Cls_44-35Or_45-59Ab_8–17) and mantles on celsian (Cls_45-35Or_42-60Ab_13-5) as well as xenoblastic barian K-feldspar (Cls₆Or₉₀Ab₂) postdate the last fabric-defining event. The preservation of extremely complex zoning patterns down to the micron-scale shows that diffusional homogenization did not operate at fluid-present low to medium-grade conditions (350–550 °C, 3–5 kb). Microstructures indicate that at these conditions barian feldspars deform exclusively by brittle fracturing and do not undergo recrystallization. Barian feldspar compositions confirm the positive correlation of Na-content with temperature and the existence of a narrow asymmetric compositional gap (Cls_90-85?Cls₅₅, ~350 °C) which probably closes at lower amphibolite facies conditions (X_c ~Cls_75; T_c ~550 °C). White micas are solid solutions of the end-members muscovite, ganterite (Ba_0.5?K_0.5)Al₂(Al_1.5Si_2.5)O₁₀(OH)₂, paragonite, celadonite with a significant substitution of ^[VI]Al by Cr. Zoning is a common feature with cores being enriched in Ba. The data document extensive Ba substitution for K from muscovite to ganterite, exclusively controlled by the coupled substitution ^[XII]K + ^[IV]Si ? ^[XII]Ba + ^[IV]Al and strongly dependent on bulk composition. The extent of solid solution from (Ms+Gnt) towards paragonite and celadonite end-members is controlled by the miscibility gap in the (Ms+Gnt)–Pg–Cel pseudoternary, with the Pg-substitution depending on temperature and the Cel-substitution on pressure. ^[IV]Si values between 3.1 and 3.3 in Ba-poor micas indicate minimum pressures of chemical equilibration in the order of 3–5 kbar, while the most sodian compositions of low-celadonite micas provide an upper temperature estimate of ~550 °C, consistent with P-T estimates for assemblages of metapelites (500–550 °C, 4–5 kb).  相似文献   

Mutual replacement reactions in alkali feldspars I: microtextures and mechanisms   总被引：5，自引：1，他引：4  

Ian Parsons  Martin R. Lee 《Contributions to Mineralogy and Petrology》2009,157(5):641-661

Intracrystal microtextures formed by a process of mutual replacement in alkali feldspars record fluid–rock reactions that have affected large volumes of the Earth’s crust. Regular, ≤1 μm-scale ‘strain-controlled’ perthitic microtextures coarsen, by up to 10³, by a dissolution–reprecipitation process, producing microporous patch or vein perthites on scales >100 μm. We have developed earlier studies of such reactions in alkali feldspar cm-scale primocrysts in layered syenites from the Klokken intrusion, South Greenland. We present new hyperspectral CL, SEM images, and laser ICPMS analytical data, and discuss the mechanism of such replacement reactions. The feldspars grew as homogeneous sodic sanidines which unmixed and ordered by volume diffusion during cooling into the microcline field at ~450°C, giving regular, fully coherent ‘braid’ cryptoperthite. At ≤450°C the crystals reacted with a circulating post-magmatic aqueous fluid. The braid perthite behaved as a single reactant ‘phase’ which was replaced by two product phases, incoherent subgrains of low albite and microcline, with micropores at their boundaries. The driving force for the reactions was coherency strain energy, which was greater than the surface energy in the subgrain mosaic. The external euhedral crystal shapes and bulk major element composition of the primocrysts were unchanged but they became largely pseudomorphs composed of subgrains usually with the ‘pericline’ and ‘adularia’ habits (dominant {110} and subordinate {010} morphology) characteristic of low T growth. The subgrains have an epitactic relationship with parent braid perthite. Individual subgrains show oscillatory zoning in CL intensity, mainly at blue wavelengths, which correlates with tetrahedral Ti. Regular zoning is sometimes truncated by irregular, discordant surfaces suggesting dissolution, followed by resumption of growth giving regular zoning. Zones can be traced through touching subgrains, of both albite and microcline, for distances up to ~500 μm. At ≤340°C, the microcline subgrains underwent a third stage of unmixing to give straight lamellar film perthites with periodicities of ~1 μm, which with further cooling became semicoherent by the development of spaced misfit dislocations. Sub-grain growth occurred in fluid films that advanced through the elastically strained braid perthite crystals, which dissolved irreversibly. Braid perthite was more soluble than the strain-free subgrain mosaics which precipitated from the supersaturated solution. Some volumes of braid texture have sharp surfaces that suggest rapid dissolution along planes with low surface energies. Others have complex, diffuse boundaries that indicate a phase of coherent lamellar straightening by volume diffusion in response to strain relief close to a slowly advancing interface. Nucleation of strain-free subgrains was the overall rate-limiting step. To minimise surface energy subgrains grew with low energy morphologies and coarsened by grain growth, in fluid films whose trace element load (reflected in the oscillatory zoning) was dictated by the competitive advance of subgrains over a range of a few tens of mm. The cross-cutting dissolution surfaces suggest influxes of fresh fluid. Removal of feldspar to give 2 vol% porosity would require a feldspar:fluid ratio of ~1:26 (by wt). The late reversion to strain-controlled exsolution in microcline subgrains is consistent with loss of fluid above 340°C following depressurization of the intrusion. A second paper (Part II) describes trace element partitioning between the albite and microcline subgrains, and discusses the potential of trace elements as a low-T geothermometer. This paper and the Part II are dedicated in memory of J.V. Smith and W.L. Brown, both of whom died in 2007, in acknowledgement of their unrivalled contributions to the study of the feldspar minerals over more than half a century.  相似文献   

Ternary feldspar thermometry in granulites from the Oaxacan Complex,Mexico     

C. I. Mora  J. W. Valley 《Contributions to Mineralogy and Petrology》1985,89(2-3):215-225

Coexisting feldspars from across 2,000 km² of the granulite facies Oaxacan Complex, southern Mexico exhibit variable amounts of solid solution from nearly binary (Ab-An and Ab-Or) to substantially ternary (Ab-An-Or). Reintegrated analyses of 21 coarsely exsolved perthite (AF)-plagioclase (PL) pairs yield AF=Or_30–63 Ab_30–56An_2–15 and PL=Or_1–2Ab_70–84An_11–28. These data have been used to test existing two feldspar geothermometers for this extended composition range.For all compositions, temperature estimates show relatively little spread in value (660° to 795° C, 7 kbar) using the Haselton et al. (1983) calibration (HHHR). These temperatures are in fair agreement with estimates of 750±40° C for feldspar pairs with nearly binary compositions using the Stormer (1975) thermometer (STO). However, STO temperatures increase significantly (to 990° C) with increasing ternary solid solution in AF, suggesting that thermometers derived for binary systems are inaccurate for ternary compositions. Isotherms drawn from HHHR which take into account variable anorthite solution in alkali feldspar show that estimated temperature decreased by 50–100° C for each 5 mole percent anorthite in alkali feldspar.Experimentally determined solvus relations (Seck 1971) require feldspars with significant ternary solid solution to have crystallized or to have equilibrated at higher temperature than feldspars with more binary compositions. However, petrographic and field relations of ternary and binary feldspars in the Oaxacan Complex suggest they were all equilibrated at similar metamorphic pressures and temperatures and do not support a model where ternary feldspars have preserved higher premetamorphic temperatures. The composition of coexisting feldspars from other Precambrian granulite-facies terranes are also inconsistent with Seck's (1971) results. Hence, thermometers which fit Seck's solvus relations may not yield accurate temperatures in high grade metamorphic terranes. Parallel tie-lines for ternary and binary feldspars in the Oaxacan Complex and the consistency of inferred temperatures (HHHR) for many granulite terranes suggest that estimation of temperature using tie-line slopes rather than solvus width may yield more accurate results for these samples.Peak metamorphic conditions in the Oaxacan Complex are inferred to have been 730±50° C, 7±1 kbar. Pressure estimates from four garnet-plagioclase barometers show good agreement. Results of feldspar thermometry are consistent with diopside-forsterite equilibria in marbles which restrict T=720–765° C at P=7 kbar.  相似文献   

Geochemistry, petrogenesis and radioactivity of El Hudi I-type younger granites, South Eastern Desert, Egypt     

Ashraf Emam  Nasser M. Moghazy  Anas M. El-Sherif 《Arabian Journal of Geosciences》2011,4(5-6):863-878

The present study deals with geochemical characteristics and petrogenesis of three younger granite varieties (coarse-grained biotite-muscovite granites (CBG), garnetiferous muscovite granites (GMG) and Abu Aggag biotite granites (AAG)) in El-Hudi area, east of Aswan, southeastern desert of Egypt. Mineral chemistry and whole rock chemistry data revealed that all granites have high SiO₂ (70.8-74.7 wt.%), Al₂O₃ (12.8-14.3 wt.%), Na₂O and K₂O (>3.2 wt.%) contents with high Na₂O/K₂O ratios (～>1). Plagioclase feldspars range in composition from albite to oligoclase (An_9-27) in CBG, oligoclase (An_13-18) in GMG and albite (An_2-6) in AAG. Potash feldspars are mainly perthitic microcline and exhibit chemical formulae as (Or_93-96 Ab_7-4 An₀) in CBG, (Or_95-98 Ab_5-2 An₀) in GMG and (Or_82-98 Ab_18-2 An₀) in AAG. Biotites from CBG and GMG are enriched in (Mg and Ti) and depleted in (Al, Fe, Mn and K) compared with those of AAG. Biotites from CBG and GMG had been derived from calc-alkaline magma, whereas those from AAG had been derived from peraluminous magma. Chlorites from CBG and GMG are Mg-Fe bearing, while that from AAG is Fe-rich chlorite (chamosite). The CBG and GMG are Mg-rich monzogranites originated from high-K calc-alkaline magma with metaluminous to mildly peraluminous nature. The AAG are Fe-rich monzogranites to syenogranites generated from high-K calc-alkaline peraluminous magma. Both CBG and GMG are late- to post-orogenic granites, while the AAG are post-orogenic granites. All three granite varieties are considered as evolved I-type granites, formed under low to moderate water pressures (～ 0.5-7 kbars) and relatively high ranges of crystallization temperatures (～700-890°C). They were generated from partial melting of crustal materials at lower (CBG >30 km depth) and intermediate (GMG & AAG ～20-30 km depth) levels. The crystal fractionation was the predominant process during differentiation of parent magmas of these granites. Geochemical characteristics manifest that AAG represent the highly fractionated member of magma cycle differs from that produced CBG and GMG. The CBG are relatively enriched in both U and Th existing only within the accessory minerals such as zircon, sphene, and allanite.  相似文献   

Calcium distribution patterns in alkali feldspar in a quartz syenite from Oki-Dozen, southwest Japan     

S. Nakano 《Mineralogy and Petrology》1998,63(1-2):35-48

Summary Microscopically alkali feldspar in a quartz syenite from Oki-Dozen, Japan, consists of clear and turbid areas. Clear areas occur in the interiors of feldspar grains and are cryptoperthitic. Film microperthites are developed with turbidity in the rims, and mosaic microperthites are developed with turbidity in the interiors and in the rims. Turbidity is attributed to the presence of abundant micropores. The chemical compositions of pristine clear feldspars are around Or₃₃Ab₆₄An₃. The interior microperthitic feldspars have lower An content than the clear feldspars. Some areas of the clear and microperthitic feldspars in the interiors are poor in calcium. In contrast, the microperthitic rims contain almost no calcium. The zonal patterns of calcium-rich cores and calcium-poor rims are very distinct in almost all feldspar grains. The overall calcium distribution patterns suggest a secondary calcium-depletion from the feldspars during hydrothermal or deuteric reactions. The development of Or-rich veins transversing feldspar grains and rim albite is consistent with this model. Calcium distribution patterns in alkali feldspar provide new and useful information on processes during geologic events.

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Calcium-Verteilung in Alkalifeldspat eines Quarz-Syenites von Oki-Dozen, Südwest-Japan

Zusammenfassung Mikroskopisch zeigt Alkalifeldspat aus einem Quarz-Syenit von Oki-Dozen, Japan, klare und trübe Bereiche. Die klaren Bereiche treten in den inneren Zonen von Feldspatkörnern auf und sind kryptoperthitisch. Die getrübten Ränder sind Film-Mikroperthite während Mosaik-Perthite Trübung in Kern-und Randbereichen zeigen. Die Trübung ist auf die Anwesenheit zahlreicher Mikroporen zurückzuführen. Die chemische Zusammensetzung der ursprünglichen klaren Feldspäte ist etwa Or₃₃Ab₆₄An₃. Die Kerne der mikroperthitischen Feldspäte haben niedrigere An-Gehalte als die klaren Feldspäte. Im Unterschied führen die Mikroperthit-Ränder kein Kalzium. Die Zonierungsmuster, d.h. Ca-reiche Kerne und Ca-arme Ränder, sind sehr typisch für alle Feldspäte. Die generelle Ca-Verteilung weist auf eine sekundäre Ca-Verarmung der Feldspäte im Zuge hydrothermaler oder deuterischer Prozesse hin. Die Bildung Orreicher Gängehen, die die Feldspatkörner durchsetzen und von Albiträndern stimmen mit diesem Modell überein. Die Ca-Verteilungsmuster in Alkalifeldspat erlauben somit Einblicke in und geben nützliche Informationen über geologische Prozesse.

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With 7 Figures  相似文献   

Electrical conductivity of alkali feldspar solid solutions at high temperatures and high pressures   总被引：1，自引：0，他引：1  

Haiying Hu  Heping Li  Lidong Dai  Shuangming Shan  Chengming Zhu 《Physics and Chemistry of Minerals》2013,40(1):51-62

The electrical conductivities of alkali feldspar solid solutions ranging in chemical composition from albite (NaAlSi₃O₈) to K-feldspar (KAlSi₃O₈) were measured at 1.0 GPa and temperatures of 873–1,173 K in a multi-anvil apparatus. The complex impedance was determined by the AC impedance spectroscopy technique in the frequency range of 0.1–10⁶ Hz. Our experimental results revealed that the electrical conductivities of alkali feldspar solid solutions increase with increasing temperature, and the linear relationship between electrical conductivity and temperature fits the Arrhenius formula. The electrical conductivities of solid solutions increase with the increasing Na content at constant temperature. At 1.0 GPa, the activation enthalpy of solid solution series shows strong dependency on the composition, and there is an abrupt increase from the composition of Or₄₀Ab₆₀ to Or₆₀Ab₄₀, where it reaches a value of 0.96 eV. According to these results in this study, it is proposed that the dominant conduction mechanism in alkali feldspar solid solutions under high temperature and high pressure is ionic conduction. Furthermore, since the activation enthalpy is less than 1.0 eV for the alkali feldspar solid solutions, it is suggested to be a model where Na⁺ and K⁺ transport involves an interstitial mechanism for electrical conduction. The change of main charge carriers can be responsible for the abrupt increase in the activation energy for Or₆₀Ab₄₀. All electrical conductivity data were fitted by a general formula in order to show the dependence of activation enthalpy and pre-exponential factor on chemical composition. Combining our experimental results with the effective medium theory, we theoretically calculated the electrical conductivity of alkali feldspar granite, alkali feldspar quartz syenite, and alkali feldspar syenite with different mineral content and variable chemical composition of alkali feldspar at high temperatures at 1.0 GPa, and the calculated results are almost in agreement with previous experimental studies on silicate rocks.  相似文献   

Strain-driven disordering of low microcline to low sanidine during partial phase separation in microperthites     

William L. Brown  Martin R. Lee  Kim A. Waldron  I. Parsons 《Contributions to Mineralogy and Petrology》1997,127(3):305-313

Microperthitic feldspar crystals containing low microcline in a braid intergrowth often have distinctive microtextures including coarse semi- to in-coherent grain-boundary pleated rims and fine coherent intracrystalline Ab- and Or-rich pleats (Lee et al. 1997). The coarser pleated rims are generally separated from the braid microtexture in the crystal interior by a coherent to semi-coherent transitional zone. Partial phase separation has occurred in the transitional zone in step with that in the Ab- and Or-rich pleats at the grain boundaries, such that Ab-rich lamellar film micro-antiperthite alternates along (010) with more Or-rich lamellar film microperthite; the microtextures and phases are those expected for the respective local bulk compositions. Lamellar microtextures contain tweed orthoclase, whereas low microcline is the only K-feldspar in the fine coherent pleats and braid microperthite. We propose that the small coherent pleats developed from the braid microtexture by interaction of the spontaneous coherency strains with discontinuities within or at the surface of the crystal, and that their initial spacing is guided by that of the braid microperthite. We infer that the transitional zone formed by straightening of the zig-zag braid microtexture above the pleat heads during coarsening and partial phase separation. We further infer that the resulting coherency shear strains induced a reversal of the K-feldspar phase transformation, involving Si, Al disordering of low microcline into low sanidine, now tweed orthoclase, although the crystal was at a T within the hydrostatic T-stability of microcline. Received: 10 June 1996 / Accepted: 12 December 1996  相似文献   

Partition of Sr,Ba, Ca,Y, Eu²⁺, Eu³⁺, and other REE between plagioclase feldspar and magmatic liquid: an experimental study   总被引：1，自引：0，他引：1  

Michael J Drake  Daniel F Weill 《Geochimica et cosmochimica acta》1975,39(5):689-712

Plagioclase feldspar/magmatic liquid partition coefficients for Sr, Ba, Ca, Y, Eu²⁺, Eu³⁺ and other REE have been determined experimentally at 1 atm total pressure in the temperature range 1150–1400°C. Natural and synthetic melts representative of basaltic and andesitic bulk compositions were used, crystallizing plagioclase feldspar in the composition range An₃₅–An₈₅. Partition coefficients for Sr are greater than unity at all geologically reasonable temperatures, and for Ba are less than unity above approximately 1060°C. Both are strongly dependent upon temperature. Partition coefficients for the trivalent REE are relatively insensitive to temperature. At fixed temperature they decrease monotonically from La to Lu. The partition of Eu is a strong function of oxygen fugacity. Under extreme reducing conditions D_Eu approaches the value of D_Sr.  相似文献   

Ternary feldspars: Kinetics and possible equilibria at 800° C     

W. Johannes 《Contributions to Mineralogy and Petrology》1979,68(2):221-230

The ternary feldspar system KAlSi₃O₈ - NaAlSi₃O₈ - CaAl₂Si₂O₈ was reinvestigated at 650 ° C and 800 ° C (P _H2O = 1 kb) using mixtures of crystalline plagioclases and alkali feldspars as starting materials. The compositions of plagioclases and alkali feldspars of the run products were determined by X-ray means. The Or-content of the feldspar phases was determined by measuring the position of the (201) X-ray peak of the unexchanged feldspars, whereas the An-content was determined by measuring the same X-ray peak of the K-exchanged feldspars. The reaction rate of a reaction leading to a more An-rich plagioclase (type II reaction) is much faster than a reaction producing a more Ab-rich plagioclase (type I). In a type II reaction run times of approximately 20 days are needed to reach new constant plagioclase and alkali feldspar compositions at 650 ° C, and 10 days are needed to reach constant compositions at 800 ° C. In a reaction of type I only the outer zone of the plagioclases reacts to more Abrich compositions. A diffuse zone with a wide range of compositions was observed in 650 ° C runs. Equilibrium could not be reached in these experiments within 45 days. At 800 ° C a new zone having a specific composition develops in 42 days. This new zone is believed to be in equilibrium with the coexisting alkali feldspar. The depth of reaction is calculated as 0.03 μm after 42 days (800 ° C, P _f= 1 kb). The reaction between the two feldspar phases could be reversed at 800 ° C. The following compositions are considered to represent equilibrium data at 800 ° C and P _t = 1 kb:

An 43 Ab 51 Or 6 coexisting with Or 79 Ab 20 An 1, and

An 40 Ab 54 Or 6 coexisting with Or 75 Ab 24 An 1.

Recent data obtained with gels of ternary feldspar composition as starting materials do not agree with the results presented in this paper. Gels obviously crystallize spontaneously forming coexisting feldspars of non - equilibrium composition - alkali feldspars too rich in Ab and plagioclases too rich in An.  相似文献   

Mineralogy of silicate inclusions of the Colomera IIE iron and crystallization of Cr-diopside and alkali feldspar from a partial melt     

Hiroshi Takeda  Weibiao Hsu  Gary R. Huss 《Geochimica et cosmochimica acta》2003,67(12):2269-2287

We studied the mineralogy, mineral chemistry, and compositions of 48 interior silicate inclusions and a large K-rich surface inclusion from the Colomera IIE iron meteorite. Common minerals in the interior silicate inclusions are Cr diopside and Na plagioclase (albite). They are often enclosed by or coexist with albitic glasses with excess silica and minor Fe-Mg components. This mineral assemblage is similar to the “andesitic” material found in the Caddo County IAB iron meteorite for which a partial melt origin has been proposed. The fairly uniform compositions of Cr diopside (Ca₄₄Mg₄₆Fe₁₀) and Na plagioclase (Or_2.5Ab_90.0An_7.5 to Or_3.5Ab_96.1An_0.4) in Colomera interior inclusions and the angular boundaries between minerals and metal suggest that diopside and plagioclase partially crystallized under near-equilibrium conditions from a common melt before emplacement into molten metal. The melt-crystal assemblage has been called “crystal mush.” The bulk compositions of the individual composite inclusions form an array between the most diopside-rich inclusion and plagioclase. This is consistent only with a simple mechanical mixing relationship, not a magmatic evolution series. We propose a model in which partly molten metal and crystal mush were mixed together by impact on the IIE parent body. Other models involving impact melting of the chondritic source material followed by growth of diopside and plagioclase do not easily explain near equilibrium growth of diopside and Na plagioclase, followed by rapid cooling. In the K-rich surface inclusion, K feldspar, orthopyroxene, and olivine were found together with diopside for the first time. K feldspar (sanidine, Or_92.7Ab_7.2An_0.1 to Or_87.3Ab_11.0An_1.7) occurs in an irregular veinlike region in contact with large orthopyroxene crystals of nearly uniform composition (Ca_1.3Mg_80.5Fe_17.8 to Ca_3.1Mg_78.1Fe_18.9) and intruding into a relict olivine with deformed-oval shape. Silica and subrounded Cr diopside are present within such K-feldspar regions. Some enrichments of the albite component have been detected at the end of curved elongated nodules of K feldspar intruded into the mafic silicates. The textural relationships suggest that a K-rich melt was present. A K-rich melt is neither the first melt of a chondritic system nor a differentiation product of a Na-rich partial melt of chondritic material. The K-rich material may have originated as a fluid phase that leached K from surrounding materials and segregated by a mechanism similar to that proposed for the Na-rich inclusions.  相似文献