Eight-phase alkali feldspars: low-temperature cryptoperthite, peristerite and multiple replacement reactions in the Klokken intrusion

Eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the Klokken intrusion. The processes leading to the formation of the first four phases have been described previously. The feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition, between 750 and 600 °C, depending on bulk composition, to give fully coherent, strain-controlled braid cryptoperthites with sub-μm periodicities. Below ~500 °C, in the microcline field, these underwent a process of partial mutual replacement in a deuteric fluid, producing coarse (up to mm scale), turbid, incoherent patch perthites. We here describe exsolution and replacement processes that occurred after patch perthite formation. Both Or- and Ab-rich patches underwent a new phase of coherent exsolution by volume diffusion. Or-rich patches began to exsolve albite lamellae by coherent nucleation in the range 460–340 °C, depending on patch composition, leading to film perthite with ≤1 μm periodicities. Below ~300 °C, misfit dislocation loops formed, which were subsequently enlarged to nanotunnels. Ab-rich patches (bulk composition ~Ab₉₁Or₁An₈), in one sample, exsolved giving peristerite, with one strong modulation with a periodicity of ~17 nm and a pervasive tweed microtexture. The Ab-rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at ~450 °C. This is the first time peristerite has been imaged using TEM within any perthite, and the first time peristerite has been found in a relatively rapidly cooled geological environment. The lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion. All the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity. We term this material ultra porous late feldspar (UPLF). It is composed predominantly of regions of microporous very Or-rich feldspar (mean Ab_2.5Or_97.4An_0.1) associated with very pure porous albite (Ab_97.0Or_1.6An_1.4) implying replacement below 170–90 °C, depending on degree of order. In TEM, UPLF has complex, irregular diffraction contrast similar to that previously associated with low-temperature albitization and diagenetic overgrowths. Replacement by UPLF seems to have been piecemeal in character. Ghost-like textural pseudomorphs of both braid and film parents occur. Formation of patch perthite, film perthite and peristerite occurred 10⁴–10⁵ year after emplacement, but there are no microtextural constraints on the age of UPLF formation.     

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

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.     

Low-temperature coherent exsolution in alkali feldspars from high-grade metamorphic rocks of Sri Lanka

In the alkali feldspars of the amphibolite- and granulite-facies rocks of Sri Lanka, a late-stage, final exsolution event is observed which produced film lamellae and fine-scale spindles. These were investigated by optical, microprobe, single-crystal, transmission electron microscopy and atomic resolution microscopy techniques. The lamellae and spindles exsolved below the coherent solvus at temperatures as low as 300 to 350° C. Precession photographs and ARM micrographs show that the intergrowth is perfectly coherent. In sections (010) the rhombic section of the Pericline twins corresponds to analbite or high albite. The albite lamellae and spindles nucleated and grew at low temperatures in a metastable disordered structural state within a tweed-orthoclase matrix and became periodically twinned analbite or high albite, which subsequently developed only a slight increase in Al, Si order. The relationship between twin periodicity and lamellar width, predicted for coherent intergrowths by Willaime and Gandais (1972), is obeyed. In Or-rich grains, in which coherent exsolution is the only exsolution event, the film lamellae tend to be restricted to the rim, the fine-scale spindles to the centre of the grains. The films nucleated heterogeneously at grain boundaries and grew towards the grain centres. Fine-scale spindles probably nucleated homogeneously in the interior part of grains. Heterogeneous nucleation and coherent growth are not mutually exclusive.     

Exsolution lamellae in a clinopyroxene megacryst aggregate from Cenozoic basalt,Leizhou Peninsula,South China: petrography and chemical evolution

Petrographic investigations and electron microprobe analyses have been performed on a rare aggregate of clinopyroxene megacrysts collected from Cenozoic basalts in Yinfengling, Leizhou Peninsula of South China. The aggregate, composed of several clinopyroxene megacrysts, shows abundant exsolution lamellae of garnet (Grt) and orthopyroxene (Opx), and granular texture. Cr- and Ti-poor spinels are also present in this sample. They occur predominantly as Sp–Opx–Grt clusters (Cr# = 0.025–0.034) at the interspace between different megacrysts, and subordinately as bleb-shaped (Cr# = 0.025–0.034) or thin-lamella crystals (Cr# = 0.006–0.021) in clinopyroxene. Three different assemblages of exsolution are identified, namely (1) Sp (high Cr/Al) and Opx; (2) Grt–Opx; (3) Sp (low Cr/Al) and Opx. In addition, some garnets were likely developed as response to breakdown of the high-Cr/Al Sp. The homogeneous compositions in all constituent minerals and the good agreement between calculated Cpx/Grt partition coefficients (K _d’s) for trace elements and reference data strongly suggest a chemical equilibrium among coexisting minerals, probably attained by diffusion after the exsolution. Thermobarometric calculation based on exsolved assemblage yields a temperature of 900 ± 30°C and a pressure of 12 ± 2.2 kbar, corresponding to the present-day thermal gradient in the region. Much higher P–T estimates (T = 1,210 ± 30°C, P = 16.2 ± 3.5 kbar) are obtained for the reconstructed composition of cpx prior to exsolution. The contrast in thermal state before and after the exsolution might reflect the thermal evolution of the lithosphere beneath South China during the Cenozoic. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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

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.     

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

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.     

Exsolved garnet-bearing pyroxene megacrysts from some South African kimberlites

Clinopyroxene and orthopyroxene megacrysts containing garnet lamellae up to 1.2 mm thick as an exsolved phase are found rarely in kimberlites from Frank Smith and Bellsbank. Chemically the clinopyroxenes are characteristically subcalcic, being within the range of 100 Ca/Ca + Mg + Fe = 27 to 36, and the orthopyroxenes are characterized by high Al₂O₃ and Cr₂O₃. Immediately after crystallization during very slow cooling, clinopyroxene and orthopyroxene exsolve wide-spaced orthopyroxene and clinopyroxene phases parallel to (100) of the host phases, respectively, then both host and exsolved phases exsolve garnet lamellae. Topotactic relations between pyroxenes and garnet are determined by X-ray for the first time. Partitioning of major and minor elements among the coexisting clinopyroxene, orthopyroxene and garnet in pyroxene megacrysts is the same as that of the granular-type garnet peridotite xenoliths in Lesotho and South African kimberlies. Mineralogy and chemistry indicate that subcalcic clinopyroxene and orthopyroxene megacrysts contain respectively about 10 and 3 mole % of the garnet molecule in solid solution.     

TEM evidence of perovskite–brownmillerite coexistence in the Ca(AlxFe1−x)O2.5 system with minor amounts of titanium and silicon

Observations by transmission electron microscopy (TEM) of the submicrometer phases present in calcium aluminate cements have shown that Ca-Al-Fe oxides coexist in two forms with brownmillerite (b) and perovskite (p) structures, respectively. Homogeneous single crystals of both brownmillerite and perovskite have been observed but exsolved lamellae also occur on the scale of tens of nanometers. Perovskite lamellae in brownmillerite exhibit coherent interfaces with an almost perfect [1 0 1]_b = {1 0 0}_p topotactic relationship. Energy-dispersive X-ray spectroscopy (EDXS) measurements show that perovskite lamellae are enriched in Ti and Si relative to the brownmillerite lamellae. The perovskite phase may accommodate up to 0.17 Si atoms per formula unit, but the exsolution process seems mainly to concern the Ti content. It is estimated that the solvus width ranges between concentrations of 0.06 < Ti < 0.13 atoms per formula unit. O K and Fe L _2,3 edges collected by electron energy loss spectrometry (EELS) confirm that both phases are mainly composed of Fe³⁺, requiring that the perovskite is highly oxygen-deficient. Al K and Si K EELS spectra have features comparable with those of fourfold-co-ordinated Al and Si sites, suggesting that they are probably located close to oxygen-vacant sites. Received: 23 June 1999 / Accepted: 18 February 2000     

Microtextures and crystal chemistry in P21/c pigeonites

Summary ?A single-crystal X-ray investigation was performed on crystals of P2₁/c natural pigeonite with varying Ca and Fe^* ( = Fe²⁺ + Mn²⁺) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae. The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated. The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples. In particular, it is shown that Ca and Fe^* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and decrease the difference between the M2–O3 distances. Received October 18, 2001; revised version accepted February 15, 2002     

Palaeoproterozoic A-type felsic magmatism in the Khetri Copper Belt, Rajasthan, northwestern India: petrologic and tectonic implications

Summary A number of small Palaeoproterozoic granitoid plutons were emplaced in the Khetri Copper Belt, which is an important Proterozoic metallogenic terrane in the northeastern part of Aravalli mountain range. Contiguous Biharipur and Dabla plutons are located about 15 km southeast of Khetri, close to a 170 km long intracontinental rift zone. The plutons are composed of amphibole-bearing alkali-feldspar granites, comprising microcline-albite granite, albite granite and late-stage microgranite. The albite granite in Biharipur is confined to the margins of the pluton, and shows extensive commingling with the synchronous mafic plutonics. Geochemically, the albite granites are characterised by low K₂O (∼0.5 wt.%) and elevated Na₂O (∼7.0 wt.%) abundances. By contrast, the microcline-albite granite does not show any significant mafic-granite interactions and shows normal concentrations of alkali elements. The granitoids display high concentrations of the rare earth (except Eu) and high field strength elements, high values of Ga/Al (>2.5), agpaitic index and Fe^*-number. These features together with their alkaline metaluminous and ferroan nature classify the rocks as typical A-type within-plate granites. All the granitoid facies display similar REE and incompatible element profiles indicating their cogenetic nature. These granitoids were emplaced in a shallow crustal chamber under relatively low pressures, high temperature (≥850 °C) and relatively oxidising conditions. The oxidised nature, HFSE concentrations and Nd isotope data (ɛNd = −1.3 to −2.9) favour derivation of these granitoid rocks from crustal protoliths. The generation of albite granite is attributed to the replacement of alkali feldspar and plagioclase of the original granite by pure albite as a consequence of pervasive infiltration of a high Na/(Na + K) fluid at the late-magmatic stage. This model may have wider significance for the generation of albite granites/low-K granites or albitites in other areas. The A-type plutonism under consideration seems to be an outcome of ensialic rifting of the Bhilwara aulacogen.     

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

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     

Development of perthite microstructures in the Storm King granite,N.Y.

The Storm King granite at Bear Mountain, New York contains coarse alkali feldspar mesoperthite. The microstructure of these perthites grades continuously from lamellae to isolated blebs. The K rich phase in all samples has a nerely constant composition (Õr₉₇Ab₃), but the Na-rich phase ranges from An_3.8 (lamellae) to An₂₁ (blebs). It is suggested that the more calcium-rich feldspars exsolved at a higher temperature and thus experienced a longer time at higher temperature, during which the microstructure became more bleby or spherical in response to minimization of the interfacial energy. Lack of perfect correlation of the microstructure with bulk composition, as well as variation in the Al/Si ordering, may be due to additional factors, of which water activity or concentration is the most likely.     

Subcalcic diopsides from kimberlites: Chemistry,exsolution microstructures,and thermal history

Twenty-six subcalcic diopside megacrysts (Ca/(Ca+ Mg)) = 0.280–0.349, containing approximately 10 mol% jadeite, from 15 kimberlite bodies in South Africa, Botswana, Tanzania, and Lesotho, have been characterized by electron microprobe analysis, X-ray-precession photography, and transmission electron microscopy. Significant exsolution of pigeonite was observed only in those samples for which Ca/(Ca+Mg)0.320. The exsolution microstructure consists of coherent (001) lamellae with wavelengths ranging from 20 to 31 nm and compositional differences between the hosts and lamellae ranging from 10 to 30 mol% wollastonite. These observations suggest that the exsolution reaction mechanism was spinodal decomposition and that the megacrysts have been quenched at various stages of completion of the decomposition process.Annealing experiments in evacuated SiO₂ glass tubes at 1,150° C for 128 hours failed to homogenize microstructure, whereas, at 5 kbar and 1,150° C for only 7.25 hours, the two lattices were homogenized. This pressure effect suggests that spinodal decomposition in the kimberlitic subcalcic diopside megacrysts can only occur at depths less than 15 km; the cause of the effect may be the jadeite component in the pyroxene. Apparent quench temperatures for the exsolution process in the megacrysts range from 1,250° C to 990° C, suggesting that decomposition must have commenced at temperatures of more than 1,000° C.These P–T limits lead to the conclusion that, in those kimberlites where spinodal decomposition has occurred in subcalcic diopside megacrysts, such decomposition occurred at shallow levels (<15 km) and, at the present erosion level, temperatures must have been greater than 1,000° C.     

Opx–Cpx exsolution textures in lherzolites of the Cretaceous Purang Ophiolite (S. Tibet,China), and the deep mantle origin of Neotethyan abyssal peridotites

ABSTRACT

We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle–melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al₂O₃ = 3.85–4.90 wt%, CaO = <3.77 wt%, Cr₂O₃ = 0.85–3.82 wt%) are characteristic of abyssal peridotites. Host clinopyroxenes (Cpx1) have higher Mg#s and Na₂O, with lower TiO₂ and Al₂O₃ contents than Cpx2 exsolution lamellae in Opx, and show variable LREE patterns. Pyroxene compositions of the lherzolites indicate 10–15% partial melting of a fertile mantle protolith. P–T estimates (1.3–2.3 GPa, 745–1067°C) and the trace element chemistry of pyroxenes with exsolution textures suggest crystallization depths of ~75 km in the upper mantle, where the original pyroxenes became decomposed, forming exsolved structures. Further upwelling of lherzolites into shallow depths in the mantle resulted in crystal–plastic deformation of the exsolved pyroxenes. Combined with the occurrence of microdiamond and ultrahigh-pressure (UHP) mineral inclusions in chromites of the Purang peridotites, the pyroxene exsolution textures reported here confirm a multi-stage partial melting history of the Purang lherzolites and at least three discrete stages of P-T conditions in the course of their upwelling through the mantle during their intra-oceanic evolution.     

Melting of albite and dehydration of brucite in H2O–NaCl fluids to 9 kbars and 700–900°C: implications for partial melting and water activities during high pressure metamorphism

 The melting reaction: albite_(solid)+ H₂O_(fluid) =albite-H₂O_(melt) has been determined in the presence of H₂O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H₂O–CO₂ fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X _(H2O) ) melting relations in the two systems are almost coincident. By contrast, H₂O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 ^°C higher in H₂O–NaCl brines than in H₂O–CO₂ fluids for X _(H2O) =0.8 and ∼100 ^°C higher for X _(H2O) =0.5. The melting temperature of albite in H₂O–NaCl fluids of X _(H2O)=0.8 is ∼100 ^°C higher than in pure water. The P–T curves for albite melting at constant H₂O–NaCl show a temperature minimum at about 5 kbar. Water activities in H₂O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H₂O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a _(H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996     

Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites

 Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation. Feldspar rims have higher ⁸⁷Sr/⁸⁶Sr_i and lower ɛ_Nd(i) than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high ⁸⁷Sr/⁸⁶Sr, low ɛ_Nd(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale. Received: 28 September 1998 / Accepted: 29 December 1999     

Cooling kinetics of garnet websterites from the Freychinède orogenic lherzolite massif,French Pyrenees

Clinopyroxene and orthopyroxene megacrysts with lamellar intergrowths of pyroxenes and garnet rarely survive in pyroxenite layers from the exposed spinel-lherzolite massifs because of the emplacement history into the crust. Such features are remarkably preserved in some thick bands (up to 1 m) from the Freychinède ultramafic body (Ariège, French Pyrenees). These bands display a symmetrical zoning from the edges to the centre due to the concurrent decrease of orthopyroxene/clinopyroxene and spinel/garnet modal ratios. Textural and chemical data suggest that the present pyroxenite parageneses resulted from subsolidus recrystallization of magmatic assemblages composed of Al-rich orthopyroxene and clinopyroxene with minor spinel. These primary assemblages were changed by subsolidus recrystallization connected with an isobaric cooling at upper-mantle depth (45–50 km) from solidus temperature (1250°C) down to steady equilibrium temperature (950° C). The primary Al-rich ortho-and clinopyroxenes behaved differently on cooling. In a first stage, orthopyroxene exsolved concomitant Al-rich clinopyroxene and garnet, whereas clinopyroxene exsolved only Al-rich orthopyroxene. The garnet exsolution in clinopyroxene host is delayed to lower temperatures. This multistage process could account for the contrasting shapes of diffusion gradients adjacent to exsolved garnet, which tend to be flat in host-orthopyroxene and steep in host-clinopyroxene. An independent thermal modelling, together with available Al-diffusion data in clinopyroxene, allows us to define a fast magmatic cooling followed by a two-stage subsolidus cooling (35° C/year^-6 from 1250° C to 1050° C and 9° C/year^-6 to 900° C). This matches the contrasted exsolution sequences observed in the pyroxene megacrysts.     

Luna 20 pyroxenes: exsolution and phase transformation as indicators of petrologic history

Pyroxenes from our sample of Luna 20 soil are predominantly orthopyroxene with subordinate pigeonite. The orthopyroxenes are chromium-rich bronzites and contain submicroscopic lamellae of augite in a twinned orientation exsolved on (100). These lamellae have a composition close to the diopside-hedenbergite join. Asymmetric diffuse streaks parallel to $\text{a}{}^1$ indicate stacking faults parallel to (100) and possibly very thin (10–20 Å) lamellae of clinobronzite parallel to (100). Pigeonite crystals are very complex crystallographically and chemically, with optically visible (001) augite exsolution lamellae and two sets of chromite exsolution lamellae. In addition, there are submicroscopic (100) augite lamellae and a second generation of clinohypersthene lamellae which appear to have exsolved from the (001) augite lamellae. The clinohypersthene host, which has a large number of stacking faults parallel to (100), has partially inverted to hypersthene of the same composition. The hypersthene occurs as very fine lamellae (less than 1000 Å) parallel to the (100) plane of the clinohypersthene. X_D^Fe-Mg values for five host-lamellae pairs in pigeonite K-4 indicate a significant amount of subsolidus readjustment. We tentatively conclude that many of the bronzite and pigeonite crystals were derived from rocks crystallized from a high level magma chamber in the lunar highland crust.     

Chemistry of Zircon in Rare Metal Granitoids and Associated Rocks, Eastern Desert, Egypt

Typological study, including paragenic, morphological, textural, and chemical characteristics of zircon from nine rare metal granitic stocks and associated greisens, was carried out in order to identify the metallogenic processes of their host granitoids. The investigated zircon‐bearing granitoids and type occurrences can be categorized into magmatically and metasomatically specialized types. The magmatic type includes: (i) peralkaline, Zr + Nb‐enriched, A₁‐granite (e.g. Um Hibal); (ii) metaluminous, Nb + Zr + Y‐enriched, A₂‐type alkali granite (e.g. Hawashia and Ineigi); and (iii) peraluminous, Ta ≥ Nb + Sn + Be ± W‐enriched, Li‐albite granite (e.g. Nuweibi, Igla and Abu Dabbab). The metasomatized granites are Nb>>Ta + Sn + Zr + Y + U ± Be ± W‐enriched and hydrothermally altered alkali feldspar granite (i.e. apogranite; e.g. Um Ara, Abu Rusheid, and Um Naggat). Zircon of peralkaline granite is characteristically equant with well‐developed pyramidal faces and short prisms (i.e. pseudo‐octahedral form) with length/width ratios in the range of 2:1–1:1. It is of Zr_0.990Hf_0.007SiO₄ composition and is associated with hypersolvus assemblage consisting of alkali feldspar, quartz, aegirine and minor reibeckite. Zircon of metaluminous alkali granites is of Zr_0.99Hf_0.01SiO₄ composition and is associated with sub‐ to transolvus assemblage of K‐feldspar, quartz, plagioclase and annite‐siderophyllite mica. It is prismatic with length/width ratios in the range of 5:1–3:1, doubly terminated with small pyramidal faces. Compositionally, zircon of Li‐albite granite ranges between Zr_0.925Hf_0.075SiO₄ and Zr_0.705Hf_0.295SiO₄. It is idiomorphic with a simple combination of prism and bipyramidal terminations with a length/width ratio of 3:1–2:1. This zircon commonly exhibits a normal zoning with rims consistently higher in Hf than cores. The higher Hf content, of this zircon coupled with its association with topaz, tantalite and lithian micas (e.g. zinnwaldite and Li‐white mica), indicates a higher solubility of Hf‐fluoride complexes and their more stabilized state at lower temperature in Li‐ and F‐rich sodic melts. Zircon of apogranite association ranges in composition between Zr_0.967Hf_0.013SiO₄ in the lower unaltered alkali feldspar granite zone and Zr_0.805Hf_0.064(Y, U, Th, heavy rare‐earth elements) [HREE])_0.125SiO₄ in the apical metasomatized (i.e. microclinized, albitized, and greisenized) apogranite zones. This compositional change appears to reflect a roofward increasing in μ_KF, μ_NaF, and μ_HF of the exsolved fluids. Columbite, xenotime, thorite, cassiterite, beryl and fluorite are common associates of this zircon. This zircon is of bipyramidal to typical octahedral form with complete absence of prism concurrently with conspicuous development of pyramid, thus the zircon crystals have a length/width ratio of 1:1–0.5:1. The neoformed metasomatic zircon commonly exhibits either normal or reverse zoning with rims consistently different in Hf, U, Y, and HREE than cores, reflecting disequilibrium conditions (e.g. sudden change in P, T, salinity, and pH) between the growing crystals and the exsolved fluids.     

P-T-X-controlled element transport through granulite-facies ternary feldspar from Lofoten,Norway

Fluid transport on the grain-scale controls many rock properties and governs chemical exchange. Charnockites from Lofoten indicate fluid penetration into ternary alkali feldspars controlled by their microtextures. In a process of fluid infiltration at granulite-facies conditions (∼600°C and 8–11 kbar), tiny pyroxenes enclosed in alkali feldspar reacted to amphiboles, which are always spatially connected to perthitic albite. Investigation of these microtextures by TEM imaging of Focused Ion Beam (FIB) prepared foils revealed that pyroxenes in contact with albite lamellae show dissolution features. An amorphous Fe- and Cl-bearing material interpreted to be a residuum of the percolating fluid was found within albite lamellae. Textures and mineral compositions indicate that a Cl-rich aqueous fluid attacked the lamellae, which then provided pathways for further fluid flow. A correspondence was found between feldspar compositions, their microtexture and their degree of alteration as a result of their permeability for fluids at specific temperatures. Hence, in addition to pressure and temperature, small variations of feldspar composition can strongly influence the fluid permeability of feldspathic rocks under lower crustal conditions.