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1.
In situ X-ray diffraction study on KAlSi3O8 has been performed using the cubic type high pressure apparatus, MAX90, combined with synchrotron radiation. We determined the phase relations of sanidine, the wadeite-type K2Si4O9+kyanite (Al2SiO5)+coesite (SiO2) assemblage, and hollandite-type KAlSi3O8, including melting temperatures of potassic phases, up to 11 GPa. Our data on subsolidus phase boundaries are close to the recent data of Yagi and Akaogi (1991). Melting relations of sanidine are consistent with the low pressure data of Lindsley (1966). The breakdown of sanidine into three phases reduces melting temperature, and wadeite-type K2Si4O9 melts first around 1500° C in three phase coexisting region. Melting point of hollandite-type KAlSi3O8 is between 1700° C and 1800° C at 11 GPa. If these potassic phases host potassium in the earth's mantle, the true mantle solidus temperature will be much lower than the reported dry solidus temperature of peridotite.  相似文献   

2.
Phase relations in the system KAlSi3O8-NaAlSi 3O8 have been examined at pressures of 5–23 GPa and temperatures of 700–1200° C. KAlSi3O8 sanidine first dissociates into a mixture of wadeite-type K2Si4O9, kyanite and coesite at 6–7 GPa, which further recombines into KAlSi3O8 hollandite at 9–10 GPa. In contrast, NaAlSi3O8 hollandite is not stable at 800–1200° C near 23 GPa, where the mixture of jadeite plus stishovite directly changes into the assemblage of calcium ferrite-type NaAlSiO4 plus stishovite. Phase relations in the system KAlSi3O8-NaAlSi3O8 at 1000° C show that NaAlSi3O8 component gradually dissolves into hollandite with increasing pressure. The maximum solubility of NaAlSi3O8 in hollandite at 1000° C was about 40 mol% at 22.5 GPa, above which it decreases with pressure. Unit cell volume of the hollandite solid solution decreases with increasing NaAlSi3O8 component. The hollandite solid solution in this system may be an important candidate as a host mineral of K and Na in the uppermost lower mantle  相似文献   

3.
KAlSi3O8 sanidine dissociates into a mixture of K2Si4O9 wadeite, Al2SiO5 kyanite and SiO2 coesite, which further recombine into KAlSi3O8 hollandite with increasing pressure. Enthalpies of KAlSi3O8 sanidine and hollandite, K2Si4O9 wadeite and Al2SiO5 kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 ± 7.4 kJ mol–1 for sanidine wadeite + kyanite + coesite and 99.3 ± 3.6 kJ mol–1 for wadeite + kyanite + coesite hollandite. The isobaric heat capacity of KAlSi3O8 hollandite was measured at 160–700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi3O8 hollandite above 298 K as Cp=3.896 × 102–1.823 × 103T–0.5–1.293 × 107T–2+1.631 × 109T–3 (Cp in J mol–1 K–1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite hollandite intersects with the coesite–stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase–equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO4 kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi3O8 determined in this study put some constraints on the stability range of KAlSi3O8 hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.  相似文献   

4.
The heat capacity of synthetic, stoichiometric wadeite-type K2Si4O9 has been measured by DSC in the 195≤T(K)≤598 range. Near the upper temperature limit of our data, the heat capacity observed by DSC agrees with that reported by Geisinger et al. (1987) based on a vibrational model of their infrared and Raman spectroscopic data. However, with decreasing temperature, the Cp observed by DSC is progressively higher than that predicted from the vibrational model, suggesting that the standard entropy of K2Si4O9 is likely to be larger than 198.9 ± 4.0 J/K · mol computed from the spectroscopic data. A fit to the DSC data gave: Cp(T) = 499.13 (±1.87) − 4.35014 · 103(±3.489 · 101) · T −0.5, with T in K and average absolute percent deviation of 0.37%. The room-temperature compressibilities of kalsilite and leucite, hitherto unknown, have been measured as well. The data, fitted to the Murnaghan equation of state, gave K o = 58.6 GPa, K o ′ = 0.1 for kalsilite and K o = 45 GPa, K o ′ = 5.7 for α-leucite. Apart from the above mentioned data on the properties of the individual phases, we have also obtained reaction-reversals on four equilibria in the system K2O-Al2O3-SiO2. The Bayesian method has been used simultaneously to process the properties of 13 phases and 15 reactions between them to derive an internally consistent thermodynamic dataset for the K2O-Al2O3-SiO2 ternary. The enthalpy of formation of K2Si4O9 wadeite is in perfect agreement with its revised calorimetric value, the standard entropy is 232.1 ± 10.4 J/K · mol, ∼15% higher than that implied by vibrational modeling. The phase diagram, generated from our internally consistent thermodynamic dataset, shows that for all probable P-T trajectories in the subduction regime, the stable pressure-induced decomposition of K-feldspar will produce coesite + kalsilite rather than coesite + kyanite + K2Si4O9 (cf. Urakawa et al. 1994). Received: 11 June 1997 / Accepted: 2 December 1997  相似文献   

5.
Liquidus phase equilibria have been determined in the system CaAl2Si2O8-NaAlSi3O8-KAlSi3O8-NaAlSiO4-KAlSiO4 (An-Ab-Or-Ne-Ks) at a pressure of water of 5 kb, for low anorthite contents. The main effects of increasing anorthite content on phase relationships in the system Ab-Or-Ne-Ks include the expansion of the plagioclase stability field towards the potassium-rich part of the system, and an accompanying contraction of the alkali feldspar, leucite, nepheline and kalsilite stability fields; and an increase in liquidus temperatures throughout most of the compositional range. Two quaternary invariant points have been identified in the system, one a reaction point between the fields of alkali feldspar, plagioclase, nepheline and kalsilite at approximately An4, and the other probably a quaternary eutectic between the fields of alkali feldspar, plagioclase, leucite and kalsilite at approximately An6. A shallow minimum trough in liquidus temperatures occurs on the two-feldspar surface, and this would be expected to control the paths of liquids cooling under equilibrium conditions. Phase relationships in this quaternary system have been applied to the interpretation of the histories of the potassium-rich rocks of the Roman Volcanic Region, Italy. Differentiation of the phonolitic series in this region may have occurred by two-feldspar fractionation.  相似文献   

6.
The low-temperature heat capacity (C p ) of KAlSi3O8 with a hollandite structure was measured over the range of 5–303 K with a physical properties measurement system. The standard entropy of KAlSi3O8 hollandite is 166.2±0.2 J mol−1 K−1, including an 18.7 J mol−1 K−1 contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. The entropy of K2Si4O9 with a wadeite structure (Si-wadeite) was also estimated to facilitate calculation of phase equilibria in the system K2O–Al2O3–SiO2. The calculated phase equilibria obtained using Perple_x are in general agreement with experimental studies. Calculated phase relations in the system K2O–Al2O3–SiO2 confirm a substantial stability field for kyanite–stishovite/coesite–Si-wadeite intervening between KAlSi3O8 hollandite and sanidine. The upper stability of kyanite is bounded by the reaction kyanite (Al2SiO5) = corundum (Al2O3) + stishovite (SiO2), which is located at 13–14 GPa for 1,100–1,400 K. The entropy and enthalpy of formation for K-cymrite (KAlSi3O8·H2O) were modified to better fit global best-fit compilations of thermodynamic data and experimental studies. Thermodynamic calculations were undertaken on the reaction of K-cymrite to KAlSi3O8 hollandite + H2O, which is located at 8.3–10.0 GPa for the temperature range 800–1,600 K, well inside the stability field of stishovite. The reaction of muscovite to KAlSi3O8 hollandite + corundum + H2O is placed at 10.0–10.6 GPa for the temperature range 900–1,500 K, in reasonable agreement with some but not all experiments on this reaction.  相似文献   

7.
A detailed study of the phase chemistry of selected lavas from Vico shows that the “excess silica” in leucite is a significant and widespread crystal-chemical characteristic which may be interpreted in terms of an extended solubility of KAlSi3O8 and/or differential loss of potassium from the leucite structure. Plagioclase geothermometer data suggest increasing plagioclase control in the differentiation with decreasing Mg/Mg+Fe2+ ratios for the bulk rocks. The pyroxene compositions show moderate amounts of ferri-Tschermak's and Ca-Tschermak's molecules, probably controlled by temperature and compositional variations in the cooling liquids. Both microphenocrystal and late-crystallizing micas correspond to phlogopite compositions throughout the suite. The phase chemistry is discussed in relation to the differentiation of the suite and current petrogenetic models.  相似文献   

8.
The system KAlSiO4-Mg2SiO4-SiO2-H2O includes model representatives of (1) hydrous siliceous magma from subducted oceanic crust — the eutectic liquid in KAlSi3O8-SiO2-H2O, and (2) the overlying mantle peridotite — the assemblage forsterite+enstatite (Fo+En). In a series of partly schematic isobaric isothermal sections, the products of hybridization between the model materials at pressures between 20 and 30 kbar have been determined. The liquid dissolves peridotite components with little change in composition. Hybridization is not a simple mixing process, because of the incongruent melting of peridotitic assemblages with phlogopite (Ph). Hybridization causes solidification of the liquid, with products a sequence of three mineral assemblages: Ph, Ph+quartz (Qz), and Ph+En. The products represent an absolute geochemical separation and local concentration of all potassium from the liquid. Hybridization is accompanied by H2O-saturation of melts, and evolution of aqueous fluid. Although there are significant differences between the melt composition and that of the magma rising from subducted oceanic slab, and between Fo+En and the mantle rock, extrapolation of the results suggests that the conclusions can probably be extended to mantle conditions with sodium in the melt, and jadeitic clinopyroxene included in the hybrid products.  相似文献   

9.
The pseudo-binary system CaMgSi2O6-KAlSi2O6, modeling the potassium-bearing clinopyroxene (KCpx) solid solution, has been studied at 7 GPa and 1,100–1,650 °C. The KCpx is a liquidus phase of the system up to 60 mol% of KAlSi2O6. At higher content of KAlSi2O6 in the system, grossular-rich garnet becomes a liquidus phase. Above 75 mol% of KAlSi2O6 in the system, KCpx is unstable at the solidus as well, and garnet coexists with kalsilite, Si-wadeite and kyanite. No coexistence of KCpx with kyanite was observed. Above the solidus, KAlSi2O6 content of the KCpx coexisting with melt increases with decreasing temperature. Near the solidus of the system (about 1,250 °C) KCpx contains up to 5.6 wt% of K2O, i.e. about 22–26 mol% of KAlSi2O6. Such high concentration of potassium in KCpx is presumably the maximal content of KAlSi2O6 in the Fe-free clinopyroxene at 7 GPa. In addition to the major substitution MgM1C2Al1K2, the KCpx solid solution contains Ca-Eskola and only minor Ca-Tschermack components. Our experimental results indicate that the natural assemblage KCpx+grossular-rich garnet might be a product of crystallization of the ultra-potassic SiO2-rich alumino-silicate mantle melts (>200 km).Editorial responsibility: J. Hoefs  相似文献   

10.
Mössbauer spectra of glasses of NaFeSi3O8 and 3NaAlSi2O6 · NaFeSiO4 starting compositions consist of a dominant Fe3+ and subordinate Fe2+ quadrupole-split doublet, in agreement with previous work. Fe3+ is assigned to tetrahedral coordination. Pressure-induced coordination changes are not observed in the pressure range 1 bar to 30 kbar. A gradual increase in isomer shift of the Fe3+ doublet with increase in pressure is attributed to steric effects. Raman spectra of GeO2, NaGaSi3O8 and NaGaSiO4 glasses are dominated by network structure vibrations. There is no detectable change in the nearest-neighbor coordination of Ge4+ in GeO2 from 1 bar to 14 kbar, of Ga3+ in NaGaSi3O8 from 1 bar to 28 kbar and of Ga3+ in NaGaSiO4 from 1 bar to 25 kbar. However, some structural reorganization outside of the first coordination sphere occurs in the high pressure glasses.XANES and EXAFS spectra on powdered samples of 1 bar and 25 kbar NaGaSiO4 glasses and crystalline NaGaSiO4 were obtained from K edge absorption spectra at the Stanford Synchrotron Radiation Laboratory using a double crystal monochromator equipped with Si(220) crystals. The XANES spectra indicate that Ga3+ has a similar extended coordination geometry in both glasses. The EXAFS spectra reveal that Ga3+ is four-coordinated with oxygen in all three samples with a Ga3+-O distance of about 1.83 Å. The radial distribution functions of the two glasses are virtually identical. However, both XANES and EXAFS spectra indicate significant structural differences between crystalline NaGaSiO4 (nepheline-type structure) and vitreous NaGaSiO4 beyond the first coordination shell of Ga3+. Thus, X-ray absorption spectroscopy independently confirms the Raman results on the unchanged coordination of Ga3+ in NaGaSiO4 glasses with pressures up to 25 kbar.Glass compositions were selected in anticipation that larger and/or lower charged cations would exhibit pressure-induced coordination changes at lower pressures than Al3+ and Si4+. The present null result suggests that the stabilizing features of open network structures in the liquid state (large entropy and minimized cation-cation repulsion) more than compensate for large molar volume in the pressure range accessible to experimentation. It appears that network structures in natural magmas should remain stable throughout the upper mantle. Consequently, the densities of magmas at high pressures which are calculated from compressibility data and the appropriate equation of state will be only slightly underestimated, due to the effect of minor structural changes beyond the first coordination sphere.  相似文献   

11.
The solubility of Tio2 in phlogopites has been experimentally determined in the system K2Mg6Al2Si6O20(OH)4-K2Mg4TiAl2Si6O20(OH)4-K2Mg5TiAl4Si4O20(OH)4 between 825–1300°C and 10–30 kbar under vapour absent conditions. Starting compositions lie along the join K2Mg6Al2Si6O20(OH)4-K2Mg4.5TiAl3Si5O20(OH)4 which represents a combination of the Mg[VI]2Si[IV] = Ti[VI]2Al[VI] and 2Mg[VI] = Ti[VI]□[VI] substitution mechanisms for Ti in phlogopites. The results of the experiments indicate a systematic increase in solubility of Ti with increasing temperature and decreasing pressure for given bulk Tio2 content. Under isobaric conditions high temperature Ti-saturated phlogopite breaks down to Ti-deficient phlogopite + rutile + vapour. Mass balance calculations suggest that the vapour phase may contain K2O dissolved in H2O and that the reaction is controlled by the vapour phase. Analyses of phlogopites coexisting with rutile and vapour can be represented in terms of the end-member components phlogopite [K2Mg6Al2Si6O20(OH)4], eastonite [K2Mg5Al4Si5O20(OH)4], an octahedral site deficient Ti-phlogopite (Ti-OSD) of composition K2(Mg4Ti□)Al2Si6)O20(OH)4, and Ti-eastonite [K2Mg5TiAl4Si4O20(OH)4]. With decreasing amounts of Ti in these phlogopites there is a decrease in the Ti-eastonite component and increase in the eastonite component.The general equation for the breakdown of Ti-phlogopite solid solution to Ti-free phlogopite + rutile + vapour is: 14 Ti-eastonite + 7 Ti-OSD ? 16 eastonite + 3 phlogopite + 21 rutile + 4 H2O + 2 K2O. Lack of knowledge of H2O and K2O activities in the vapour phase does not permit evaluation of thermodynamic constants for this reaction. The Ti solubility in phlogopites and hence its potential as a geothermobarometer under lower crustal to upper mantle conditions is likely controlled by common mantle minerals such as forsterite.  相似文献   

12.
Wadeite of composition Zr2K4Si6O18, synthesized at 1 atm, is stable between 12–25 kb at 800 °–1,250 ° C; conditions appropriate to those of partial melting of an anomalous K-enriched upper mantle. If published hypotheses for the generation of high potash mafic to ultramafic lavas based on partial melting of such an anomalous mantle are correct, wadeite is a possible K-bearing mineral, in addition to phlogopide and K-richterite, stable under mantle conditions. The restricted occurrence of wadeite to rocks of West Kimberley, Australia and Leucite Hills, Wyoming is believed to be due to their high K/Al and Zr contents relative to other high potash rocks. The cell constants of wadeite of Zr2K4Si6O18 composition are in agreement with those of natural Zr-rich wadeite and with the values predicted from synthetic wadeites with smaller tetravalents ions in the Zr site.  相似文献   

13.
The low-temperature heat capacity (C p) of Si-wadeite (K2Si4O9) synthesized with a piston cylinder device was measured over the range of 5–303 K using the heat capacity option of a physical properties measurement system. The entropy of Si-wadeite at standard temperature and pressure calculated from the measured heat capacity data is 253.8 ± 0.6 J mol−1 K−1, which is considerably larger than some of the previous estimated values. The calculated phase transition boundaries in the system K2O–Al2O3–SiO2 are generally consistent with previous experimental results. Together with our calculated phase boundaries, seven multi-anvil experiments at 1,400 K and 6.0–7.7 GPa suggest that no equilibrium stability field of kalsilite + coesite intervenes between the stability field of sanidine and that of coesite + kyanite + Si-wadeite, in contrast to previous predictions. First-order approximations were undertaken to calculate the phase diagram in the system K2Si4O9 at lower pressure and temperature. Large discrepancies were shown between the calculated diagram compared with previously published versions, suggesting that further experimental or/and calorimetric work is needed to better constrain the low-pressure phase relations of the K2Si4O9 polymorphs. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

14.
This work focuses on the kamafugites from Santo Antônio da Barra, Minas–Goiás Alkaline Province. These rocks contain olivine, clinopyroxene, titanomagnetite, perovskite, leucite (pseudomorphs), kalsilite, nepheline, and phlogopite.The rocks investigated are ultrabasic, with high contents of CaO, FeO, and TiO2, high to moderate contents of Al2O3, alkalis, and P2O5, and low contents of MgO. The alkaline characteristic of the rocks is reflected in TiO2, K2O, and Na2O contents and in the frequent presence of normative nepheline and leucite. K2O contents are not primary since most of the leucite was replaced by analcime.The negative K anomaly verified in the extended incompatible element distribution diagram for kamafugites seems to be mainly related to alteration. Kamafugites are characterized by a marked enrichment in incompatible and large ion lithophile elements together with other typical compatible elements.The Santo Antônio da Barra kamafugites are less enriched in titanium, niobium, zirconium, and REE than the Mata da Corda and most of the Toro-Ankole ones. San Venanzo–Cupaello rocks have much lower titanium contents.  相似文献   

15.
Suprasolidus phase relations at pressures from 4 to 7 GPa andtemperatures from 1000 to 1700C have been determined experimentallyfor a sanidine phlogopite lamproite from North Table Mountain,Leucite Hills, Wyoming. The lamproite is silica rich and hasbeen postulated to be representative of the magmas which wereparental to the Leucite Hills volcanic field. Near-liquidusphases above 5 GPa are pyrope-rich garnet and jadeite-rich pyroxene.Below 5 GPa, jadeite-poor pyroxene is the only near-liquidusphase. Near-solidus assemblages consist of clinopyroxene, titanianpotassium richterite and titanian phlogopite with either potassiumtitanian silicate above 5 GPa or potassium feldspar below 5GPa. The potassium titanian silicate is a newly recognized high-pressurephase ranging in composition from K4Ti2Si7O20 to K4TiSi8O20.It coexists with coesite at pressures above 6 GPa at 1100–1400C.A previously unrecognized K-Ba-phosphate is a common near-solidusphase. The phase relationships are interpreted to suggest thatlamproites cannot be derived by the partial melting of simplelherzolitic sources. However, it is proposed that sanidine phlogopitelamproites an derived by high degrees of partial melting ofancient metasomatic veins within a harzburgitic–lherzoliticlithospheric substrate mantle. The veins are considered to consistof phlogopite, K–Ti-richterite, K–Ba-phosphate andK–Ti-silicates. KEY WORDS: lamproilte; experimental petrology; upper mantle *Corresponding author  相似文献   

16.
Various members of the KAlSi3O8-BaAl2Si2O8 feldspar series are hydrothermally synthesized. Cellparameters of these are calculated from diffractometer patterns and found to be similar to those of Gay and Roy. A variation diagram is constructed correlating Cn-content and values of ΔFeKα(2θ(111)CaF2—2θ(004)Fsss), which gives $${\text{Mol}}\% {\text{ Cn = 229}}{\text{.83}}\Delta {\text{2}}\theta ---{\text{190}}{\text{.81}}$$ by a least square regression fitting. Phase equilibria relation in the solidus-liquidus-region for the KAlSi3O8-BaAl2Si2O8-H2O system at 1000 kg/cm2 are investigated. It is found to be a case of simple solid solution in a binary system, with reservations at the potassium-rich side of the system. Goranson (1938) gives a temperature of about 1000°C at 1000 kg/cm2 \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) for the incongruent melting of sanidine, but the authors prefer a value around 930°C at the same \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) . Reaction products of starting materials on the join KAlSi2O6-BaAl2Si2O8 and KAlSiO4-BaAl2Si2O8 gave no experimental hint for replacement of K+ by Ba++.  相似文献   

17.
The diffusivities of network-forming cations (Si4+, Al3+, Ge4+ and Ga3+) in melts of the jadeitic composition NaAl(Si, Ge)2O6 and Na(Al, Ga)Si2O6 have been measured at pressures between 6 and 20 kbar at 1400°C. The rates of interdiffusion of Si4+-Ge4+ and Al3+-Ge3+ increase with increasing pressure at constant temperature. The results are consistent with the ion-dynamics computer simulations of Jadeite melt by Angellet al. (1982, 1983). The coefficient measured for the Si4+-Ge4+ interdiffusion is between 8 × 10?10 and 2.5 × 10?8cm2sec at 6 kbar, depending on the composition of the melt, whereas at 20 kbar it is between 7 × 10?9 and 2 × 10?7cm2sec. The effect of pressure is greater for more Si-rich compositions (i.e., closer to NaAlSi2O6 composition). The coefficient measured for the Al3+-Ga3+ inter- diffusion is between 9 × 10?10 and 3 × 10?9 cm2/sec at 6 kbar and between 3 × 10?9 and 1 × 10?8cm2sec at 20 kbar. The rate of increase in diffusivity with pressure of Al3+-Ga3+ (a factor of 3–4) is smaller than that of Si4+-Ge4+ (a factor of 7–17).The Si4+-Ge4+ interdiffusion in melts of Na2O · 4(Si, Ge)O2 composition has also been measured at 8 and 15 kbar for comparison. The effect of pressure on the diffusivity in this melt is significantly smaller than that for the jadeitic melts. The increase in diffusivity of the network-forming cations in jadeitic melts with increasing pressure may be related to the decrease in viscosity of the same melt. The present results, as well as the ion-dynamics simulations, suggest that the homogenization of partial melts and mixing of magmas would be more efficient at greater depths.  相似文献   

18.
《Gondwana Research》2000,3(1):55-63
Ultrapotassic rock is reported for the first time from the polycyclic Eastern Ghats belt, India, near Borra, Visakhapatnam district, Andhra Pradesh. The rock, consisting of leucite, kalsilite, Khyphen;feldspar, graphite, apatite together with diopside, meionite and phlogopite, occurs as thin vein and veinlets in diopsidite, in close spatial association with a granulite facies carbonate ensemble of massive dolomitic carbonate rock and calc silicate granulite. It was emplaced in the midhyphen;crust along late ductile shear zones. Subsequent to its emplacement, the ultrapotassic melt with liquidus leucite interacted with the granulite wall rock, incorporating at least 40% of the crustal components mainly as Si, Al, Mg and Ca. After necessary correction of the crustal contaminant, the recalculated K2O/Na2O ratio of ∼12 (molar) and K2O/Al2O3 ratio of ∼1 (molar) in the bulk rock composition indicates that the Borra ultrapotassic melt has a lamproitic affinity. However, it is significantly modified as well, particularly being impoverished in mafic liquidus phases and depleted in incompatible (excepting Rb, Th and U) and compatible trace elements, compared to an average lamproite. Leucite later underwent subsolidus decomposition to Khyphen;feldspar + kalsilite intergrowths. The emplacement of the ultrapotassic melt posthyphen;dates an early ultra high temperature metamorphism and also the 1000 Ma Grenvillian metamorphism in the Eastern Ghats Belt and is possibly of Panhyphen;African age. The extensive Khyphen;feldspathisation in the Eastern Ghats belt could also be linked with this ultrapotassic melt.  相似文献   

19.
Examples of the three volcanic rock types, wyomingite, orendite and madupite from the Leucite Hills have been examined with the electron microprobe. The results show that leucite is non-stoichiometric as predicted byCross (1897), having an excess of potassium and silicon, and that the only feldspar found, a sanidine, contains up to 18 percent of the iron-feldspar molecule. The co-existing phlogopite, diopside and olivine together with the groundmass amphibole are all highly magnesian. Of the varied accessory minerals, priderite (K2Ti8O16) and wadeite (K4Zr2Si6O18) have been identified and analyzed together with ubiquitous apatite and perovskite, both of which contain rare earths in abundance. Comparative mineralogical data has been obtained on a few representative specimens from West Australia and on the jumillite from Spain.The new rock analyses together with the existing data from the Leucite Hills show the rare but characteristic molecular excess of potassium over aluminium; this excess is considered to account for the ahsence of the iron-titanium oxides in the orendites and wyomingites, and of course for the unusual composition and species of the minerals. Exploratory melting experiments show that these potassic lavas have a comparable melting range to magnesian basalts, and a crustal origin is thereby considered precluded. There is no evidence that sialic contamination contributed notably to the composition of the oversaturated orendites and wyomingites, and their relationship by any process of crystal fractionation to the undersaturated madupite is obscure. The generation of madupite could be achieved by crystal fractionation at high pressure of a liquid derived by partial fusion of mantle material.  相似文献   

20.
In the system CaO-MgO-Al2O3-SiO2, the tetrahedron CaMgSi2O6(di)-Mg2SiO4(fo)-SiO2-CaAl2 SiO6(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl2Si2O8(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO2-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.Department of Geosciences, University of Texas at Dallas Contribution No. 327. Hawaii Institute of Geophysics Contribution No. 814.  相似文献   

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