Melting Of Precursors Research Articles

Mega-chondrules and large, igneous-textured clasts in Julesberg (L3) and other ordinary chondrites: vapor-fractionation, shock-melting, and chondrule formation

A petrographic-microprobe study of large, metal-poor, igneous-textured objects in Julesberg (L3) and other ordinary chondrites suggests that they can be classified into two petrographic types: mega-chondrules and large lithic clasts; and into two chemical types: Na-poor and Na-rich. Mega-chondrules show textural evidence of having solidified as freely-floating melt droplets, whereas lithic clasts formed by the fragmentation of larger objects, possibly still larger mega-chondrules. Barred-olivine or barred-olivine-pyroxene textures are most common for mega-chondrules, whereas a variety of textures occur in large lithic clasts. The two petrographic types cannot be distinguished on the basis of modal, bulk, or phase compositions. Sodium-poor objects are characterized by (1) plagioclase or glass with mainly bytownite composition (An 70-90 typical), (2) subchondritic Na/Al ratios and typically subchondritic volatile-element (Na, K, and Mn) abundances, and (3) bulk-chemical trends that resemble those expected for vapor-fractionation processes. Some Na-poor objects may have formed by the melting of precursors that formed as condensates or vaporization residues; others may have formed by the melting of precursors that formed by fractional condensation or fractional vaporization. Following vapor-fractionation, Na-poor objects or their precursors appear to have reequilibrated at lower temperatures, which raised the bulk Na content of the objects, but not to the levels seen in Na-rich objects. Enrichments of Fe 2+, Na, K, and P on the margins of some Na-poor objects suggest that they partly reacted with volatile-rich surroundings, both before and after brecciation. Sodium-rich objects are characterized by (1) plagioclase or glass with oligoclase or albite composition (An 2-25), (2) roughly chondritic Na/Al ratios and volatile-element (Na, K, Mn) abundances, and (3) bulk-chemical trends similar to those shown by melt-pocket glasses in ordinary chondrites. Sodium-rich melt objects could have formed by the shock-melting of chondritic precursors. Feldspathic compositions for some Na-rich melt objects can be explained by preferential shock-melting of feldspar or feldspathic glass in chondritic target materials. Literature data imply that the same two chemical populations of objects, Na-poor and Na-rich, occur among smaller, normal-sized chondrules in Type 3 ordinary chondrites, suggesting that the same processes that affected large melt objects also affected chondrules.

Oxygen isotope variations in Lau Basin lavas

New laser-fluorination oxygen isotope data are presented for volcanic glasses and phenocrysts from Lau and North Fiji Basin lavas. The low oxygen blank of the technique allows accurate analysis of these often scarce igneous phases. δ 18O values of volcanic glass range from 5.53‰ to 6.06‰. Oxygen isotope ratios of ferromagnesian and plagioclase phenocrysts are lower and higher than their host glasses, respectively, and coexisting phases appear to represent oxygen isotope equilibrium at magmatic temperatures. Evolved lavas from the propagating and dying rift tips in the Central Lau Basin display significant 18O-enrichment with magmatic evolution due to FeTi-oxide crystallisation and require that precursor melts had relatively low δ 18O values immediately prior to oxide saturation. Development of these precursor melts from basaltic parents involved assimilation of hydrothermally altered sheeted dykes or gabbros in recently accreted oceanic crust accompanied by crystallisation of a plagioclase-rich assemblage. The stable isotope geochemistry and petrology of Central Lau Basin lavas constrain magmatic evolution to depths between 2.5 and 5 km beneath the seafloor. A dacite from Valu Fa Ridge is slightly 18O-enriched relative to Valu Fa basalts, consistent with crystallisation of a mineral assemblage similar to mid-ocean ridge basalts. 18O/ 16O ratios of Lau Basin basaltic glasses decrease with Mg Number; this observation could be reconciled with the assimilation and fractional crystallisation model proposed for initial differentiation of the evolved lavas. However, oxygen isotope data for Central Lau Basin basalts correlate positively with Ba/La while basaltic glasses from throughout the region display a negative correlation between δ 18O and Na 8.0 suggesting that recycling of subducted oxygen and/or mantle fertility may also exert an influence on 18O/ 16O of primary melts. Presently, the lack of quantitative control over oxygen isotope fractionation in basaltic systems prohibits resolution of these effects. Detailed investigation of less complex suites of lavas, employing the precision offered by laser fluorination, may provide new insights into the behaviour of oxygen at magmatic temperatures.

Morphology of Single-Component Particles Produced by Spray Pyrolysis

ABSTRACT This work is a review of the experimental results from the literature for single-component metal and simple metal-oxide particles. Criteria for correlating particle morphology, i.e., whether the particles are solid or hollow, with process parameters and material properties during spray pyrolysis are presented and compared with the data available in the literature. The materials were classified into two categories for which the precursor: (1) melts and (2) does not melt before chemical reaction takes place, and separate criteria were used for each category based on the work of Jayanthi et al. (1993) J. Aerosol Sci. 19:478. In systems where the precursor melts before chemical reaction occurs, e.g., decomposition of nitrates of Mg, Al, Fe, Zn, Pb, Ni, Co, Pd, Mn, Cu, Sr, and Ag, the particle morphology is determined primarily by the densities and formula weights of the reactant and product compounds unless high temperature densification or puffing up of the particles due to gases evolved during the chemical reaction alter the morphology. In systems where the precursor undergoes nucleation to form a solid crust which does not melt before chemical reaction takes place, e.g., Ba(C2H3O2)2, Al2(SO4)3, Zr(C2H3O2)2, and Zn(C2H3O2)2, solubility and density of the precursor as well as the operating temperature are the main factors that affect the product particle morphology. Overall, particle morphologies predicted by the criteria were in agreement with experimental observations reported in the literature.

Major and trace element geochemistry and isotope (Sr, Nd, Pb, O) systematics of an Archaean basement involved in a 2.0 Ga very high‐temperature (1000°C) metamorphic event: In Ouzzal Massif, Hoggar, Algeria

ABSTRACTThe In Ouzzal granulitic massif is composed mainly of various meta‐igneous rocks which, in spite of Rb, U, Th, Cs and some K and Sr mobility, can be dated and generally classified according to their chemical composition as follows.Basic and ultrabasic granulites interlayered with the metasediments correspond to (1) ultrabasic cumulates from dislocated tholeiitic bodies, (2) ancient komatiitic to high‐Mg tholeiitic basalts similar to the suites found in Archaean greenstone belts and (3) calcalkaline protoliths of high‐K andesitic composition. No geochronological constraints are available apart from the depositional age of some associated sediments which is younger than 2.70 Ga detrital zircons, and the Nd model age of the andesitic granulites of c. 3.4 Ga.In spite of the high‐grade metamorphism, the acidic magmatic precursors of the charnockites can be divided in three groups. (1) The most juvenile acid orthogneisses are trondhjemitic or tonalitic in composition, being similar to the TTG suites which are classically considered to be formed by partial melting of mantle‐derived protoliths. The 3.3–3.2 Ga TDM indicates a possible age of separation from the mantle reservoir while the plutons may have been emplaced between 3.3 and 2.7 Ga (U–Pb zircon & Nd ages). (2) A group of alkaline granitic gneisses, similar in composition to rift‐related‐granites, were emplaced at 2650±10 Ma (U–Pb & Rb–Sr ages) in a thick continental crust. (3) Calcalkaline granodioritic and monzogranitic suites derived from the partial melting of continental precursors (3.5–3.3 Ga), in lower to middle levels of the continental crust. They were emplaced close to 2.5 Ga during crustal thickening.The very high‐temperature metamorphism occurred at 2002±7 Ma from the age of synfoliation intrusions and was probably related to major overthrusting. Retrogressive metamorphism is dated at 1.95 Ga from garnet‐Nd ages. In spite of the very high‐temperature conditions, partial melting during granulite facies metamorphism may be restricted to scarce cordierite‐bearing monzogranitic gneisses. The 2.0 Ga VHT metamorphism could be related to overthrusting, extensional or underplating processes.

Initial lead isotopic composition of silicate minerals from the Mulcahy layered intrusion: Implications for the nature of the Archean mantle and the evolution of greenstone belts in the Superior Province, Canada

Samples from six localities in the late Archean Mulcahy layered intrusion of the Wabigoon greenstone belt have been analysed for their U and Pb concentrations and their lead isotopic composition in order to estimate the initial isotopic composition of the intrusion. Leaching experiments have been performed on plagioclase fractions and seven out of nine residues have yielded very similar calculated initial compositions ( 206 Pb/ 204 Pb= 13.32 ± 0.05; 207 Pb/ 204 Pb= 14.44 ± 0.04). These seven plagioclas residues also define a U-Pb isochron corresponding to an age of 2764 ± 20 Ma from which initial 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios of 13.33 and 14.43, respectively, are obtained, indicating that the Mulcahy intrusion had a homogeneous initial lead isotopic composition. Plagioclase and whole-rock fractions from a titanomagnetite-rich layer near the top of the middle zone of the intrusion has yielded a Pb-Pb age of 2722 ± 4 Ma, which is only slightly younger than the reported U Pb zircon age of 2733 ± 1 Ma (MORRISON et al., 1985) for the same sample. Plagioclase, clinopyroxene, orthopyroxene, and magnetite mineral separates from the five other samples, however, yield older internal Pb Pb ages, ranging between 2.76 and 2.80 Ga, and all of the data lie on a single line in the Pb Pb isochron diagram, corresponding to an age of 2787 ± 14 Ma. This is believed to represent the crystallization age of the bulk of the Mulcahy intrusion. The 2733 Ma event may be the age of a small influx of magma intruded late after the bulk of the intrusion had crystallized or of a sill intruded within the middle zone and possibly associated to the adjacent 2732 Ma Atikwa batholith. When compared to available initial lead isotopic composition of various rocks from other greenstone belts of the Superior Province, the Mulcahy intrusion has one of the least radiogenic initial lead isotopic compositions, identical to that of komatiite lava flows from the Abitibi belt. These rocks are believed to represent melts of a late Archean depleted mantle component, which was homogeneous at that time, at the scale of greenstone belts of the Superior Province. Syn- to late-tectonic rocks in the Wawa, Wabigoon, and Abitibi belts have homogeneous and more radiogenic 207 Pb/ 204 Pb initial ratios than the depleted mantle values, indicating the melting of an enriched component of the mantle at that time, whereas post-tectonic plutons chiefly resulted from the melting of crustal precursors in a thickened lithosphere in response to the juxtaposition of greenstone belts and metasedimentary-plutonic-gneiss belts.

Initial lead isotopic composition of silicate minerals from the Mulcahy layered intrusion: Implications for the nature of the Archean mantle and the evolution of greenstone belts in the Superior Province, Canada

Samples from six localities in the late Archean Mulcahy layered intrusion of the Wabigoon greenstone belt have been analysed for their U and Pb concentrations and their lead isotopic composition in order to estimate the initial isotopic composition of the intrusion. Leaching experiments have been performed on plagioclase fractions and seven out of nine residues have yielded very similar calculated initial compositions (206Pb/204Pb= 13.32 ± 0.05; 207Pb/204Pb= 14.44 ± 0.04). These seven plagioclas residues also define a U-Pb isochron corresponding to an age of 2764 ± 20 Ma from which initial 206Pb/204Pb and 207Pb/204Pb ratios of 13.33 and 14.43, respectively, are obtained, indicating that the Mulcahy intrusion had a homogeneous initial lead isotopic composition.Plagioclase and whole-rock fractions from a titanomagnetite-rich layer near the top of the middle zone of the intrusion has yielded a Pb-Pb age of 2722 ± 4 Ma, which is only slightly younger than the reported UPb zircon age of 2733 ± 1 Ma (MORRISON et al., 1985) for the same sample. Plagioclase, clinopyroxene, orthopyroxene, and magnetite mineral separates from the five other samples, however, yield older internal PbPb ages, ranging between 2.76 and 2.80 Ga, and all of the data lie on a single line in the PbPb isochron diagram, corresponding to an age of 2787 ± 14 Ma. This is believed to represent the crystallization age of the bulk of the Mulcahy intrusion. The 2733 Ma event may be the age of a small influx of magma intruded late after the bulk of the intrusion had crystallized or of a sill intruded within the middle zone and possibly associated to the adjacent 2732 Ma Atikwa batholith. When compared to available initial lead isotopic composition of various rocks from other greenstone belts of the Superior Province, the Mulcahy intrusion has one of the least radiogenic initial lead isotopic compositions, identical to that of komatiite lava flows from the Abitibi belt. These rocks are believed to represent melts of a late Archean depleted mantle component, which was homogeneous at that time, at the scale of greenstone belts of the Superior Province. Syn- to late-tectonic rocks in the Wawa, Wabigoon, and Abitibi belts have homogeneous and more radiogenic 207Pb/204Pb initial ratios than the depleted mantle values, indicating the melting of an enriched component of the mantle at that time, whereas post-tectonic plutons chiefly resulted from the melting of crustal precursors in a thickened lithosphere in response to the juxtaposition of greenstone belts and metasedimentary-plutonic-gneiss belts.

Constraints on formation processes of two coarse‐grained calcium‐ aluminum‐rich inclusions: A study of mantles, islands and cores

Abstract— Many coarse‐grained calcium‐ aluminum‐rich inclusions (CAIs) contain features that are inconsistent with equilibrium liquid crystallization models of origin. Spinel‐free islands (SFIs) in spinel‐rich cores of Type B CAIs are examples of such features. One model previously proposed for the origin of Allende 5241, a Type B1 CAI containing SFIs, involves the capture and assimilation of xenoliths by a liquid droplet in the solar nebula (El Goresy et al., 1985; MacPherson et al., 1989). This study reports new textural and chemical zoning data from 5241 and identifies previously unrecognized chemical zoning patterns in the melilite mantle and in a SFI. These zoning patterns are identified by large‐scale elemental mapping techniques. The compositional zoning is completely independent of, and cross‐cuts individual melilite crystals in the mantle, a relation that suggests the mantle was deposited or accreted onto a preexisting core of the inclusion. Lack of correlation with individual mantle crystals also suggests that the mantle totally recrystallized at subsolidus temperatures. Sodium distribution maps demonstrate that most of the Na in 5241 was introduced during the secondary alteration process. Major‐ and trace‐element data from the SFI boundary in a second type B1 CAI, Allende 3529Z, were obtained. The boundary bisects a large fassaite crystal. If the SFI is a relict xenolith, then chemical differences are likely to be present across the boundary. Electron microprobe analysis of the fassaite crystal reveals concentric zoning of Ti, which is unrelated to the SFI boundary, as well as distinct zones enriched in Al and depleted in Ti+3. Ion microprobe analyses at the SFI boundary show no significant variation in Ba, Sc, V, Cr, Sr, Zr, Nb and REE in fassaite. There is no evidence that requires the capture of a xenolith in 3529Z. Based on chemical zoning and textural arguments, it is suggested that both of these CAIs formed by a process of partial melting of precursors, which contained either vesicles or spinel‐free grains. Allende 5241 shows evidence for vapor condensation and accretion and/or introduction of a second liquid to form the melilite mantle. Chemical zoning patterns in the mantles of the inclusions indicate that 3529Z experienced a higher degree of partial melting than 5241, but it was not high enough to melt spinel or completely melt and homogenize relict fassaite components.

Pb isotopic geochemistry of granitoids and gneisses from the late Archean Pontiac and Abitibi Subprovinces of Canada

Initial Pb isotopic compositions were determined on K-feldspars for thirty granitoid plutons and gneiss complexes in the Abitibi and the Pontiac Subprovinces of Canada. The gneisses cover a wide spectrum of initial Pb isotopic compositions and undoubtedly demonstrate that very radiogenic reservoirs were present in the Pontiac Subprovince and along its southern boundary with the Grenville Province in the late Archean. The gra nitoid rocks also define a large spectrum, covering 0.3 unit of the 207Pb/ 204Pb ratio, ranging from compositions typical of mafic mantle melts to those of sedimentary banded iron formations. The older intrusives emplaced between 2.72 and 2.68 Ga were principally derived from juvenile mantle sources. They may have been formed in a volcanic arc environment, but they still frequently record the signature of crustal precursors which may have been represented by subducted sediments or by older assimilated crustal components. A sharp change in the provenance of the granitoid magmas occurred close to 2.68 Ga, an age that also corresponds to an episode of alkaline activity and fault-related sedimentation. For the following 40 Ma, magmatism chiefly resulted from melting of crustal precursors in a thickened crustal lithosphere. The Pb isotopic compositions of these crustal melts demonstrate that the basement of the Pontiac Subprovince contains a major component of rocks older than 3.0 Ga. In comparison, the Abitibi Subprovince may not contain basement components older than 2.9 Ga, or else they represent a negligible fraction of the crust. This supports the concept that the Abitibi and Pontiac Subprovinces formed as discrete crustal segments tectonically juxtaposed. Finally, the Grenville Front tectonic zone was metamorphosed and partially melted in the late Archean and is probably the deep crustal equivalent of the Pontiac Subprovince.

Spinel‐bearing, Al‐rich chondrules in two chondrite finds from Roosevelt County, New Mexico: Indicators of nebular and parent body processes

Abstract— Two rare, spinel‐bearing, Al‐rich chondrules have been identified in new chondrite finds from Roosevelt County, New Mexico—RC 071 (L4) and RC 072 (L5). These chondrules have unusual mineralogies, dominated by highly and asymmetrically zoned, Al‐Cr‐rich spinels. Two alternatives exist to explain the origin of this zoning—fractional crystallization or metamorphism. It appears that fractional crystallization formed the zoning of the trivalent cations (Al, Cr) and caused a localized depletion in chromites around the large Al‐Cr‐rich spinels. The origin of the zoning of the divalent cations (Fe, Mg, Zn) is less certain. Diffusive exchange and partitioning of Fe and Mg between olivine and spinel during parent body metamorphism can explain the asymmetric zoning of these elements. Unfortunately, appropriate studies of natural and experimental systems to evaluate the formation of zoning of the divalent cations by fractional crystallization have not yet been conducted. The bulk compositions of the chondrules suggest affinities with the Na‐Al‐Cr‐rich chondrules, as would be expected from the abundance of Al‐Cr‐rich spinels. Melting of rare and unusual precursors produced the compositions of Na‐Al‐Cr‐rich chondrules, possibly including a spinel‐rich precursor enriched in Cr2O3 and ZnO. The two chondrules we studied have larger modal abundances of Al‐Cr‐rich spinels than reported in other Na‐Al‐Cr‐rich chondrules of similar composition, and Al‐rich chondrules even more enriched in spinel are reported in the literature. These differences indicate that factors other than bulk composition control the mineralogy of the chondrules. The most important of these factors are the temperature to which the molten chondrule was heated and the cooling rate during crystallization. These two chondrules cooled rapidly from near the liquidus, as indicated by the zoning, occurrence and sizes of spinels, radiating chondrule textures and localized chromite depletions. The range of mineralogies in other Al‐rich chondrules of similar composition reflect a range of peak temperatures and cooling rates. We see no reason to believe that this range is fundamentally different from the range of thermal histories experienced by “normal” Fe‐Mg‐rich chondrules.

Geochemistry and genesis of the angrites

The angrites Angra dos Reis, LEW86010, and LEW87051 are petrologically and compositionally similar achondrites. All angrites have high FeO MnO ratios of 80–94 and very low CI normalized Na/Sm ratios of 0.001–0.003. High abundances of oxidized Fe and low abundances of moderately volatile Na most likely resulted from parent body processes, such as magmatic outgassing, rather than nebular processes. All angrites have fractionated Ca/Al ratios, with Angra dos Reis exhibiting the most extreme ratio (3.1 × CI). This Ca/Al fractionation may be due to small amounts of residual spinel in the source region after removal of the angrite melts or to crystallization of spinel from precursor melts. For LEW86010 and LEW87051, the fractionated Ca/Al ratios are not caused by the presence of significant cumulus material. For Angra dos Reis, cumulus material may be the cause of the high Ca/Al ratio. Refractory element (Ca, Eu, Al, Sc, La, Ce, Sm, Tb, Yb, Lu, Ti, Hf, Th, and Ta) abundances of LEW86010 and LEW87051 show similar patterns, while Angra dos Reis has both greater enrichments in these elements and more fractionated patterns. Compositional and petrologic constraints indicate that LEW86010 and LEW87051 are related via olivine control. The refractory element abundances and mg# of LEW86010 can be approximated by removal of olivine from LEW87051, suggesting that LEW86010 may be a residual melt from a LEW87051-like precursor. Alternatively, LEW87051 may have formed via olivine accumulation from a LEW86010-like precursor. The origin of the unusual refractory element pattern in Angra dos Reis remains obscure. The differences between the LEW86010-LEW87051 duo and Angra dos Reis suggest that either the angrite parent body was heterogeneous or that Angra dos Reis was formed on a separate parent body. Based on FeO MnO ratios and normative mineralogies, the angrite parent body(ies) may be similar in bulk composition to one of the carbonaceous chondrite groups, particularly CI-CM-CO.

Al-rich chondrules from the Ybbsitz H4-chondrite: evidence for formation by collision and splashing

Three Al-rich chondrules (Al 2O 3 > 10 wt.%) from the H4-chondrite Ybbsitz were chemically and mineralogically studied. All three clearly crystallized from molten droplets as individual objects. They consist predominantly of olivine, spinel, and a feldspar-rich mesostasis. The three Al-rich chondrules have high concentrations of incompatible elements. The rare earth element patterns are fractionated with enrichment of light rare earths and a strong positive Eu anomaly. This type of fractionated refractory element pattern is indicative of igneous processes. Ca,Al-rich inclusions from carbonaceous chondrites also show high enrichments in refractory elements. Their patterns are, however, either flat or fractionated according to variable volatilities of refractory elements in the solar nebula (e.g. Yb anomaly). The high level of refractory siderophiles observed in Ca,Al-rich inclusions from carbonaceous chondrites is not found in the Al-rich chondrules. We therefore conclude that the Al-rich Ybbsitz chondrules formed by different processes than the Ca,Al-rich inclusions from carbonaceous chondrites. Melting of inhomogeneous precursors, generally assumed for the formation of ferromagnesian chondrules, is also unlikely to produce such extreme compositions. The possibility that metamorphism is the cause of the high content of incompatible elements can be excluded. We therefore propose a new model for the formation of the Al-rich chondrules. They may have formed by collisions of partly molten ferromagnesian chondrules with other objects (chondrules). During this process Al-rich melt fractions (residual liquids) may be ejected and form independent Al-rich chondrules. This model explains the high enrichment and the fractionated patterns of incompatible elements in Al-rich chondrules as well as their generally much smaller sizes compared with ferromagnesian chondrules. The Eu anomaly is probably due to diffusive exchange of Eu between matrix and the Al-rich chondrules. Eu is decoupled from the other REE in many ferromagnesian chondrules of equilibrated ordinary chondrites, including three ferromagnesian chondrules from Ybbsitz analyzed in this study.

Exploratory experiments in the conversion of plasticized melt spun PAN-based precursors to carbon fibers

Current carbon fiber production from acrylic fibers employ wet or dry spun PAN-based precursors that require expensive solvents and costly solvent recovery methods. Recently, it has been discovered that melt spun PAN-based fibers can be prepared by using water as a plasticizer to lower the viscosity and the melting point of PAN. Results from an exploratory investigation into the production of carbon fibers from experimental plasticized melt spun, PAN-based precursors are reported here. Structural parameters based on X-ray measurements and mechanical properties of these precursors suggest that the morphology of these fibers is similar to that of wet and dry spun PAN-based precursors. However, the precursor fibers have broken filaments as well as surface defects and internal voids, all of which hinder the development of superior properties. This investigation shows, nevertheless, that carbon fibers of reasonable strength, up to an average of 15 cN/dtex (2.5 GN/m 2), and modulus, 1080–1310 cN/dtex (173–214 GN/m 2), can be produced from plasticized melt spun PAN-based precursors. Better properties may be achieved if impurities are removed from the plasticized precursor melt, surface flaws are reduced, fiber uniformity is enhanced, and the stabilization and carbonization processes are optimized for the precursors.

On the Glass in Coal Fly Ashes: Recent Advances

ABSTRACTRecent advances in the characterization of the glassy aluminosilicate phases in coal fly ashes are reviewed and discussed in terms of the development of new models describing their mechanism of formation, composition, structural relationships and chemical reactivity. Characterization techniques such as electron microscopy, x-ray diffraction, infrared and Raman spectroscopy, nuclear magnetic resonance and Mössbauer spectroscopies, thermal analysis, acid dissolution, silanation, and density fractionation have been used as experimental probes, and reveal a range of information relating to the glass phases from the macro, through the micro, to the nano-structural levels. Complex inter- and intra-particle heterogeneity in ash is observable not only in bulk chemical and mineralogical analyses but also at the molecular level in the constituent glasses. Remarkable relationships between ash particle size, density and glass composition have been observed and lead to a new proposed mechanism for ash morphogenesis based on temperature-viscosity effects in the mineral precursor melts in the boiler flame. A glass-ceramic model for fly ash particle structure is proposed and discussed in terms of phase relations in the CaO–SiO2–Al2O3 and FeO–Fe2O3–A12O3–SiO2 systems, modification of the aluminosilicate glasses by alkali and alkaline earth metal cations, and microheterogeneity involving glass devitrification and phase separation.

Rb—Sr and Sm—Nd geochronology of lower Proterozoic granite—greenstone terrains in French Guiana, South America

Nine samples of metavolcanic rock from the lower parts of greenstone belts in central French Guiana (the Paramaca series) and 14 granitic samples from the intrusive gneisses (the Degrad Roche and Arawa gneisses) were selected for Sm—Nd and Rb—Sr analysis. The Sm—Nd results from the metavolcanic series (including two tholeiites, five peridotitic komatiites and two andesites) yield an isochron age of 2.11±0.09 (2 σ) Ga with an initial 143Nd 144Nd ratio (I Nd) of 0.51002±9 (2 σ), corresponding to ϵ Nd( T) = + 2.1 ± 1.8. This isochron is interpreted as representing the age of initial volcanism of the Paramaca series. Acid intrusives were dated by the Rb—Sr method. A whole rock Rb—Sr isochron, including data points from both the Degrad Roche and Arawa gneisses, yields an age of 2.00±0.07 (2 σ) Ga with initial 87Sr 86Sr ratio (I Sr value) of 0.7019±4 (2 σ). This result is considered to be the time of emplacement of the orthogneiss protoliths. The positive ε Nd value (+ 2.1 ± 1.8) obtained from the metavolcanic rocks of French Guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in Light Rare Earth Elements (LREE). This result confirms the possible existence of ancient LREE-depleted reservoirs within the lower Proterozoic mantle. Moreover, the high ε Nd( T) value for these rocks excludes any significant crustal contamination during magma genesis. The French Guianese orthogneisses yield a low I Sr value (0.7019±4 (2 σ)) which, together with geochemical considerations, suggests that their granitic protoliths could have originated by partial melting of short-lived crustal precursors of basaltic to granodioritic composition. The present geochronological and isotopic study suggests that the Guiana Shield may represent a major continental accretion event during the lower Proterozoic.