Disequilibrium Crystallization Research Articles

Short-Range Stacking Disorder in Mixed-Layer Compounds: A HAADF STEM Study of Bastnäsite-Parisite Intergrowths

Atomic-scale high angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging and electron diffractions are used to address the complexity of lattice-scale intergrowths of REE-fluorocarbonates from an occurrence adjacent to the Olympic Dam deposit, South Australia. The aims are to define the species present within the intergrowths and also assess the value of the HAADF STEM technique in resolving stacking sequences within mixed-layer compounds. Results provide insights into the definition of species and crystal-structural modularity. Lattice-scale intergrowths account for the compositional range between bastnäsite and parasite, as measured by electron probe microanalysis (at the µm-scale throughout the entire area of the intergrowths). These comprise rhythmic intervals of parisite and bastnäsite, or stacking sequences with gradational changes in the slab stacking between B, BBS and BS types (B—bastnäsite, S—synchysite). An additional occurrence of an unnamed B2S phase [CaCe3(CO3)4F3], up to 11 unit cells in width, is identified among sequences of parisite and bastnäsite within the studied lamellar intergrowths. Both B2S and associated parisite show hexagonal lattices, interpreted as 2H polytypes with c = 28 and 38 Å, respectively. 2H parisite is a new, short hexagonal polytype that can be added to the 14 previously reported polytypes (both hexagonal and rhombohedral) for this mineral. The correlation between satellite reflections and the number of layers along the stacking direction (c*) can be written empirically as: Nsat = [(m × 2) + (n × 4)] − 1 for all BmSn compounds with S ≠ 0. The present study shows intergrowths characterised by short-range stacking disorder and coherent changes in stacking along perpendicular directions. Knowing that the same compositional range can be expressed as long-period stacking compounds in the group, the present intergrowths are interpreted as being related to disequilibrium crystallisation followed by replacement. HAADF STEM imaging is found to be efficient for depiction of stacking sequences and their changes in mixed-layer compounds, particularly those in which heavy atoms, such as rare-earth elements, are essential components.

The Tanco Pegmatite At Bernic Lake, Manitoba. XVII. Mineralogy and Geochemistry of Alkali Feldspars

Abstract The alkali-feldspar paragenetic sequence in the Tanco pegmatite spans the entire crystallization and cooling history of the pegmatite and includes several different K-feldspar and albite varieties. Total feldspar modal abundance approaches 50% for the entire pegmatite volume. Blocky K-feldspar, aplitic albite, and albite of the cleavelandite habit together account for most of the total feldspar volume, whereas late, low-temperature feldspars in veins and cavities are mostly contained in inner zones associated with pollucite and petalite. In an inward progression through the concentrically zoned pegmatite, average bulk compositional data for blocky K-feldspar reveal steady increases in Rb 2 O (1.45–3.08 wt.%) and Cs 2 O (0.11–0.29 wt.%), increasing followed by decreasing contents of P 2 O 5 (0.34–0.47 wt.%), Li 2 O (0.014–0.095 wt.%), and Tl (105–239 ppm), decreasing Na 2 O (2.5–1.1 wt.%) and CaO (0.13–0.08 wt.%), and variable BaO (0.004–0.17 wt.%) and Ga 2 O 3 (0.006–0.010 wt.%). In aplitic albite and albite of the cleavelandite habit, CaO decreases (0.08–0.03 wt.%) and overall K 2 O and P 2 O 5 contents are variable. Late adularia feldspars from veins and clusters in pollucite exhibit extreme ranges in contents of Rb 2 O (1.80–24.84 wt.%) and Cs 2 O (0.10–1.43 wt.%) and significant P 2 O 5 (up to 0.51 wt.%). X-ray powder-diffraction data indicate near-maximum structural order of the early feldspar varieties as maximum microcline and high albite. Hydrothermal adularia crystals are essentially endmember (Or 100 ) high sanidine. The large and economically significant Tanco pegmatite represents an extreme in terms of petrologic processes involving fractional and disequilibrium crystallization from granitic melts enriched in volatiles and fluxing components (H 2 O, B, F, P), rare alkalis (Li, Rb, Cs), and high field strength elements (Ta, Nb, Zr, Hf). The alkali feldspars are the most significant carriers of P, Li, Rb, Tl, and Cs prior to the stabilization of large volumes of amblygonite–montebrasite, petalite, lepidolite, and pollucite. The rise and fall of trace-element content in feldspars, in relation to the spatial distribution of different zones and mineral assemblages, has implications in terms of the internal evolution of complex granitic pegmatites, as well as on the types and extents of subsolidus readjustments.

Interplay of irreversible reactions and deformation: a case of hydrofracturing in the rodingite\u2013serpentinite system

The interplay of irreversible reactions and deformation during regional metamorphism was analyzed in the reaction zones between rodingite and serpentinite. Rodingites are leucocratic rocks found commonly in serpentinites and are considered metasomatic products of Ca-rich fluid. Rodingites occur ubiquitously in serpentinite from the Nomo metamorphic rocks, western Kyushu, a Cretaceous accretionary complex of greenschist to epidote–amphibolite facies condition. We used the singular value decomposition (SVD) method to analyze rodingitization reactions based on mineral compositions of a rodingite sample and a possible protolith (clinopyroxene gabbro). The resultant reaction implied that the rodingite was formed due to the addition of considerable amounts of CaO and H2O to the protolith, whereas other components such as SiO2, AF (Al2O3 + Fe2O3), and FM (FeO + MgO) were conserved. The Nomo rodingites are associated with reaction zones between serpentinite. A careful study on the compositional variations of diopside and chlorite in the rodingite, serpentinite, and the reaction zones between them showed a disequilibrium crystallization of diopside in the reaction zone. Diopside shows a serrated variation in composition across the reaction zone together with disequilibrium signature within grains (composite grain consisting of Fe-rich and Fe-poor parts). These features clearly indicate that diopside crystallized in a relatively short period compared to the metamorphism by irreversible reactions. Another conspicuous feature is that perovskite occurs in some reaction zones, whereas titanite is common in rodingites; this indicates that the reaction zone is poorer in SiO2. The tremolite veins are typically derived from the reaction zone into serpentinite; showing this is a good example of hydrofracturing associated with the formation of the reaction zone. All the reactions obtained by the SVD method that contribute to the formation of the reaction zones consumed CaO and evolved H2O, strongly suggesting that the hydrofracturing is caused by a rapid increment of fluid pressure during the progression of the reactions. The reactions consumed considerable amounts of SiO2 in the reaction zone, which is consistent with the occurrence of perovskite, and the conservation of AF and FM components.

The Occurrence of Breithauptite and Nisbite-Like Sb-Ni Phases At Sindesar-Khurd, Rajasthan, India: Implications For Melt-Assisted Sulfide Remobilization

Abstract Breithauptite (NiSb) generally occurs in magmatic Ni-Cu-PGE ores and in hydrothermal veins associated with Ni and Co arsenide minerals. Its presence in the Rajpura-Dariba–Sindesar-Khurd–Bethumni metallogenic belt, Rajasthan, India, may be ascribed to metamorphic remobilization as observed elsewhere, e.g ., Sulitjelma, Norway. From the coexistence of breithauptite with probable Sb-Ni melt phases at Sindesar-Khurd, a temperature of approximately 930–970 °C is indicated from the phase relations in the Ni-Sb binary phase diagram. However, this temperature is substantially above the peak metamorphic temperature of 580–600 °C estimated from equilibrium silicate phase assemblages in the rocks around the Rajpura-Dariba ore body, which is situated six kilometers to the south of Sindesar-Khurd. We have conducted experiments at 600 °C in the Ni–Sb system in the presence of Ag to understand if Ag is responsible for such a drastic decrease of the temperature of melting. The presence of other phases including freibergite [(Ag,Cu,Fe,Zn) 12 Sb 4 S 13 ], Ag-rich tetrahedrite [(Cu,Ag,Fe,Zn) 12 Sb 4 S 13 ], ∼2FeS.Ag 2 S, and ∼CuPbSbS 3 .Ag 2 S along with breithauptite at Sindesar-Khurd indicates the selective incorporation of Ag. In a second assemblage nisbite (NiSb 2 ) instead of breithauptite was observed along with 3PbS.Ag 2 S, 2Ag 2 S.FeS, (Fe,Ag)S, a Sb-Ni-Ag-Fe-S phase, and a Ag-Sb-Ni-Cu-Fe-S phase. Nisbite is stable only below 628 °C, suggesting that these assemblages formed below this temperature, and the presence of several additional phases of variable compositions further indicates disequilibrium and probable rapid crystallization from a melt. Our results suggest that Ag significantly depresses melting temperatures in the Ni–Sb system, making possible the existence of an Ag-Sb-Ni melt at 600 °C. In the presence of S and other elements such as Fe, Pb, Cu, etc . the melting temperature would be further depressed. Thus the removal of Ag from the melt through formation of Ag-bearing phases, e.g ., 2FeS.Ag 2 S, 3PbS.Ag 2 S, 2Ag 2 S.FeS, or CuPbSbS 3 .Ag 2 S, may result in a residual metastable melt at the ambient temperature. Such a melt may quench to a fine-grained assemblage containing breithauptite and nisbite-like Sb-Ni solid phases, such as encountered at Sindesar-Khurd. The second assemblage, which is nisbite-bearing, could result from a relatively Ag-rich Ni-poor melt fraction that becomes extremely Sb-rich due to the removal of Ag.

Role of syn-eruptive plagioclase disequilibrium crystallization in basaltic magma ascent dynamics.

Timescales of magma ascent in conduit models are typically assumed to be much longer than crystallization and gas exsolution for basaltic eruptions. However, it is now recognized that basaltic magmas may rise fast enough for disequilibrium processes to play a key role on the ascent dynamics. The quantification of the characteristic times for crystallization and exsolution processes are fundamental to our understanding of such disequilibria and ascent dynamics. Here we use observations from Mount Etna's 2001 eruption and a magma ascent model to constrain timescales for crystallization and exsolution processes. Our results show that plagioclase reaches equilibrium in 1–2 h, whereas ascent times were <1 h. Using these new constraints on disequilibrium plagioclase crystallization we also reproduce observed crystal abundances for different basaltic eruptions. The strong relation between magma ascent rate and disequilibrium crystallization and exsolution plays a key role in controlling eruption dynamics in basaltic volcanism.

Hybrid genesis of Jurassic fayalite-bearing felsic subvolcanic rocks in South China: Inspired by petrography, geochronology, and Sr–Nd–O–Hf isotopes

Fayalite-bearing felsic (FBF) magmatic rocks are a special type of granitic rocks with controversial origins. A suite of fayalite- and ferrosilite-bearing subvolcanic rocks, namely, the Xishan FBF rocks in South China, is investigated in this study. The Xishan FBF rocks have high SiO2 contents of 69–70wt.%, high K2O/Na2O ratios of 1.71–1.95, and high FeOt/(FeOt+MgO) ratios of 0.88–0.89. Fayalite (Fo=7.3–9.6) and ferrosilite (Fs=74.1–76.5) minerals are found in the Xishan FBF rocks. According to the Unmix function of Isoplot, the zircon U–Pb ages and initial Hf isotope compositions are categorized into two groups with ages of 156.6Ma and 151.5Ma and εHf(t) values of −7.1 and −5.2, respectively. The minerals show δ18O values of 8.8–9.8‰ for zircon, 6.0–8.1‰ for fayalite, and 7.0–8.2‰ for ferrosilite. The oxygen isotope fractionations between ferrosilite and fayalite (ΔOpx-Ol) vary from −0.8‰ to +1.5‰, which indicates disequilibrium crystallization. Whole-rock analyses show high initial 87Sr/86Sr ratios of 0.7169 to 0.7180 and negative εNd(t) values of −7.3 to −6.8; zircon analyses show εHf(t) values of −9.1 to −3.8 and δ18O values of 8.8–9.8‰. So whole rock and zircon isotopes indicate a crustal signature. Based on these mineralogical and geochemical data, the Xishan FBF rocks were attributed to A-type granites and derived from the mixing of two batches of crustal magmas, which were all derived from the partial melting of ancient igneous protolith under the conditions of high temperature (683–893°C), moderate water (3–5wt.%), and low oxygen fugacity (lg fO2=−1.21). Such rigorous physical conditions may be common for the FBF igneous rocks all over the world, which may be the primary factors controlling occurrence of the FBF rocks in limited volume and quantity.

Cryptic mineral variation and PGE distribution in the Merensky Reef and its discontinuous mela-gabbronoritic hanging wall (“Brown Sugar Norite” - BSN) at Two Rivers Platinum Mine, Eastern Bushveld Complex, South Africa

Two Rivers Platinum Mine, situated on the southern sector of the eastern limb of the Bushveld Complex, extracts Platinum Group Elements (PGE) from the UG-2 chromitite layer. A trial operation of the Merensky Reef (MR) exposed unusual occurrences of mela-gabbronoritic lenses, ‘brown sugar norite’ (BSN in mine terminology), within the pyroxenite of the hanging wall. The BSN is a fine-grained mela-gabbronorite, which appears to only occur where the upper chromitite stringer of the Merensky Reef unit is present, and forms laterally inconsistent, flat lenses. Textural features, mineral chemistry of the various rock types, Sr analyses of plagioclase separates and whole rock geochemistry indicate multiple magma injections, fractionation and disequilibrium crystallization of the magma within the Merensky interval. Whole rock MgO and enstatite content of orthopyroxene are higher in BSN compared with the surrounding Merensky Reef pyroxenite, thus supporting the notion of the BSN having formed from a more primitive magma. Both the pyroxenite and BSN plagioclase have 87Sr/86Sr ratios representative of Critical Zone magma; that of the BSN has a lower initial 87Sr/86Sr isotope ratio relative to that of the pyroxenite, suggesting a more primitive magma source. It is suggested that the unusual, discontinuous BSN lenses within pyroxenite may have formed by magmatic erosion of a new magma injection, which affected the crystal mush of the earlier deposited BSN layer. This resulted in isolated lenses of relict, but more primitive orthopyroxene of the BSN in a residual plagioclase-rich melt from an earlier pyroxenitic MR magma. In both rock types (MR pyroxenite and BSN), the PGE occur as discrete platinum-group minerals (PGM), and in solid solution in sulphides. At Two Rivers Platinum Mine, the presence of PGE in sulphides has been identified by EMPA and LA-ICP-MS. The PGE occur mineralogically either in solid solution or as PGM inclusions of different size. Pentlandite, chalcopyrite, pyrrhotite and rare pyrite occur disseminated as individual grains with associated PGM throughout the Merensky pyroxenite interval, but more enriched near the bottom and top chromitite stringers. PGM encountered are moncheite, platarsite and irarsite, relatively enriched in Pt, but poor in Pd and Rh. Our results show that most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE. Textural features of the encountered PGM grains suggest their redistribution through late stage magmatic (probably hydrothermal) processes. The BSN contains relatively little if any base metal sulphides and PGM, indicating its formation from magma depleted in PGE.

Titanium-Rich Magnesio-Hastingsite Macrocrysts In A Camptonite Dike, Lafarge Quarry, Montreal Island, Québec: Early Crystallization In A Pseudo-Unary System

Abstract A prominent dike of camptonite cuts the Middle Ordovician Tetreauville Formation of the Trenton Group in the Montreal-Est quarry operated by Lafarge Canada Inc. The “Lafarge” dike is strikingly porphyritic, with largely anhedral macrocrysts of unzoned calcic amphibole up to 13 cm across. The macrocrysts are rimmed with ferri-kaersutite resembling the amphibole in the fine-grained matrix of the camptonite. The magnesio-hastingstite macrocrysts have virtually the same composition as the matrix; they thus grew without much of a boundary layer. The magma crystallized in a disequilibrium way as a pseudo-unary system. The macrocrysts are unusually enriched in Fe 3+ (approximately 44% of the total iron), yet locally enclose globules of immiscible sulfide melt. The magma became oxygenated owing to preferential loss of hydrogen upon the dissociation of aqueous gas bubbles. The amygdaloidal macrocrysts have a relatively high δD value because of this loss of H 2 ; the values of δ 18 O are typical of an upper mantle source. Camptonite dikes are very common on Mont Royal. Like the Lafarge dike, they likely arose by the disequilibrium crystallization of batches of the parental melt of asthenospheric origin.

In-situ Crystallization and Thermal Migration in Karaj Dam Basement Sill (Northern Iran)

Abstract: The Karaj Dam basement sill (KDBS) shows layering between upper and lower chilled margins. Geochemically, the KDBS belongs to high-K calc-alkaline series and displays the geochemical characteristics related to an active continental margin environment. The rocks of the chilled margins are gabbroic in composition and porphyritic, becoming coarse-grained toward the center of the sill. The layers are dominated by gabbro, monzogabbro, and monzodiorite, formed from the lower to the upper chilled margin, with gradational transitions between rock types. The KDBS shows an S-shaped profile of MgO concentration from the base to the top of the sill. In situ crystallization is indicated by an approximately constant modal abundance of pyroxene and plagioclase, a narrow range of major element contents in the main lithologies and variable trace element concentrations. Calculated densities of successive cumulates and estimated Rayleigh number also confirm extraction of the residual liquid by compositional convection at the crystallization front and existence of turbulent flow in the main magma chamber. Disequilibrium crystallization, caused by saturation gradient thermal diffusion and to some extent Soret effect (Thermal migration) in the marginal series, is also indicated by an unexpected overall enrichment trend in MgO concentration and average Mg# of pyroxenes from the chilled margin (olivine-bearing gabbro) to marginal gabbro.

Mineralogy of the Koktokay No. 3 pegmatite, Altai, NW China: implications for evolution and melt–fluid processes of rare-metal pegmatites

The Koktokay No. 3 pegmatite is the largest Li-Be-Nb-Ta-Cs pegmatitic rare-metal deposit of the Chinese Altai orogenic belt, and is famous for its concentric ring zonation pattern (nine internal zones) and rare-metal reserves. In this paper, we present the results of major and trace element analysis for garnet, tourmaline, and beryl from the outer to inner zones of the No. 3 pegmatite. From previous studies on tourmaline and beryl, potential tracers of the degree of pegmatite evolution could be found by comparing with accepted indicators and evolutionary trends from the outer to inner zones of this pegmatite. The Sc, Y, Li, Ta contents and Y/Ho value of garnet, together with the Ti, Ca, Na, Pb, Bi and REE contents of tourmaline, and the Fe, Ga and Rb concentrations of beryl could be possible tracers of the degree of evolution of pegmatites. The textures and internal structures (homogeneity, oscillatory pattern, heterogeneous rims, replacement structures) of minerals in the pegmatite suggest that the rock underwent (1) rapid undercooling and crystallization after emplacement, resulting in the formation of an unidirectional solidification texture (UST), and then (2) became relatively stable, forming coarse crystals in outer zones, and finally (3) experienced disequilibrium crystallization, leading to the formation of complex internal structures of minerals in the inner zones, mainly caused by exsolution of more fluid and the formation of immiscible melts with different alkali contents. The evolutionary trend and the internal structures of tourmaline and beryl indicate that the formation of the Koktokay No. 3 pegmatite was controlled mainly by fractional crystallization. The convex tetrad effect, non-CHARAC Y/Ho values, and extreme Eu depletion of garnet suggest that immiscibility, which is a significant mechanism during the formation of the No. 3 pegmatite, occurred during emplacement. The differences in composition and internal structures of the minerals between the outer and inner zones of the No. 3 pegmatite suggest that there was an abrupt change, during transition from the melt stage to the melt–fluid stage, resulting from a decompression event, which led to the exsolution of the fluid and initiated the Li-Ta-Cs deposition in the pegmatite.

Mineral chemistry and thermobarometry of Eocene monzogabbroic stocks from the Bafra (Samsun) area in Turkey: implications for disequilibrium crystallization and emplacement conditions

Monzogabbro stocks including felsic enclaves (monzosyenite) around the Bafra (Samsun) area at the western edge of the Eastern Pontides cut Eocene-aged volcanic and sedimentary units. The monzogabbros contain plagioclase, alkali feldspar, clinopyroxene, olivine, hornblende, biotite, apatite, and iron-titanium oxides, whereas the felsic enclaves contain alkali feldspar, plagioclase, hornblende, biotite, clinopyroxene, and iron-titanium oxides. Mineral chemistry data suggest that magmas experienced hydrous and anhydrous crystallization in deep and shallow crustal magma chambers. Several thermobarometers were used to estimate temperatures of crystallization and emplacement for the mafic and felsic magmas. Clinopyroxene thermobarometry yielded 1100–1232 C and 5.9–8.1 kbar for monzogabbros, and 931–1109 C and 1.8–6.9 kbar for felsic enclaves. Hornblende thermobarometry and oxygen fugacity estimates reveal 739–971°C, 7.0–9.2 kbar and 10−9.71 for monzogabbros and 681–928°C, 3.0–6.1 kbar and 10−11.34 for felsic enclaves. Biotite thermobarometry shows elevated oxygen fugacity varying from 10−18.9–10−11.07 at 632–904°C and 1.29–1.89 kbar for monzogabbros, to 10−15.99 –10−11.82 at 719–873°C and 1.41–1.77 kbar for felsic enclaves. The estimated zircon and apatite saturation temperatures are 504–590°C and 693–730°C for monzogabbros and 765–775°C and 641–690°C for felsic enclaves, respectively. These data imply that several phases in the gabbroic and syenitic magmas did not necessarily crystallize simultaneously and further indicate that the mineral compositions may register intervals of disequilibrium crystallization. Besides, thermobarometry contrasts between monzogabbro and felsic enclave may be partly a consequence of extended interactions between the mafic and felsic magmas by mixing/mingling and diffusion. Additionally, the hot felsic magma was close to liquidus conditions (crystallinity < 30%) when injected into cooler mafic magma (crystallinity > 50%), and thus, the monzogabbro stocks reflect hybrid products from the mingling and incomplete mixing of these two magmas.

Micas, feldspars and columbite-tantalite minerals from the zoned granitic lepidolite-subtype pegmatite at Namivo, Alto Ligonha, Mozambique

The Namivo granitic lepidolite-subtype pegmatite is concentrically zoned. Textural and chemical studies show at the crystal scale and within each zone that muscovite evolved to lithian muscovite and the latter to “mixed form” in the outer and inner intermediate zones and core. “Mixed form” also evolved to lepidolite in the inner intermediate zone and core. Each of these micas represents an evolution from the pegmatite outer intermediate zone to the inner intermediate zone and core. Some crystals are progressively zoned while others are reversely zoned, but overgrowths and replacements also occur. Albite and K-feldspar evolved from the wall zone to the core and feldspar thermometry records a decrease in temperature. Columbite-(Mn) shows a compositional trend from the outer intermediate zone to the core typical of this mineral from a lepidolite-subtype granitic pegmatite. The chemical zonation of this pegmatite is derived from crystallization of an undercooled granitic melt in which a probably local constitutional zone refining of fluxing and incompatible elements contributes to the textural and chemical changes from the wall zone to the core. The unexpected occurrence of lepidolite and Fe/Mg-containing polylithionite, the richest micas in Si, Li calc . and F, in the outer intermediate zone is attributed to disequilibrium crystallization from an undercooled melt. They are associated with tantalite, and Ta precipitation may have been caused by the local decrease of Li and F in the melt. These minerals are uncommon in this zone of granitic pegmatites.

Petrology, age, and polychronous sources of the initial magmatism of the Imandra-Varzuga paleorift, Fennoscandian shield

This paper reports new geochemical and isotope data on the volcanogenic complexes of the Arvarench sequence of the Imandra-Varzuga paleorift structure of the Fennoscandian shield. It was established that these complexes are made up of komatiites, basalts, high-Mg andesites, and dacites and occupy a Sumian chronostratigraphic position with U-Pb (SHRIMP) age of 2429 ± 6.6 Ma in the regional Early Precambrian stratigraphic scale of the Kola-Norwegian province of the Fennoscandian shield, thus constraining the Sumian Subhorizon of the Lower Karelian Complex of the Northeastern Fennoscandian shield within 2450–2430 Ma. The high negative eNd, LREE enrichment, and the presence of different-age Archean zircons with REE patterns indicative of disequilibrium crystallization suggest that the parental dacitic melts were derived by anatectic melting of polychronous (3.2, 2.9, 2.8, 2.7 Ga) lithological complexes of the Archean continental crust of the Kola-Norwegian province of the Fennoscandian shield. Numerical petrological-geochemical modeling of generation and evolution of primary melts of the metavolcanic rocks made it possible to establish that the isotope-geochemical peculiarities of the komatiites, basalts, and basaltic andesites can be best described by fractional crystallization of primary komatiite melt contaminated by ∼ 2% of the Archean crustal material of tonalitic composition. The mantle protolith of primary melt in terms of its isotope-geochemical parameters was similar to the “enriched” mantle source of the Paleoproterozoic (2430–2450 Ma) mafic-ultramafic layered intrusions of the Kola-Norwegian province and Sumian metavolcanic rocks of the Fennoscandian shield. The high-Mg andesites of the Arvarench sequence were derived by fractionation of crustally contaminated (∼ 2%) magnesian basalts with elevated Al content (Al2O3 ∼ 15.6 wt %) in equilibrium with 40–50% Cpx40-Ol20-Opx10-Pl10-Mag20 assemblage at P < 1 GPa. Obtained isotope-geochemical data and modeling results could be interpreted by off-subduction geodynamic model of the evolution of mantle plume and its interaction with the Archean continental lithosphere at the early stage of intracratonic rifting.

APATITE TEXTURES AND COMPOSITIONS AS RECORDS OF CRYSTALLIZATION PROCESSES IN THE ANIMIKIE RED ACE PEGMATITE DIKE, WISCONSIN, USA

Unidirectional and skeletal textures in the thin inward-crystallizing Animikie Red Ace (ARA) pegmatite dike, in northeastern Wisconsin, suggest rapid, disequilibrium crystallization of the outer zones as the pegmatite-forming magma came in contact with much cooler country-rocks. The rock textures support the cooling and solidification of the dike in less than 50 days in conditions of strong to moderate undercooling, as estimated in a previous fluid- and melt-inclusion study and modeling of the conductive cooling. The magmatic disequilibrium textures are accompanied by a suite of replacement textures resulting from the aggressive reaction of assemblages of first-generation minerals deposited on the colder walls with a liquid derived from the hotter inner zones. Apatite habit, frequency, cathodoluminescence, and chemical composition fluctuate systematically within the ARA pegmatite from the outer to the inner zones. Apatite of distinctive Mn, Cl, and Fe contents occurs in the border zone, wall zone, and core-margin zone; the intervening intermediate zones and main core zones do not contain any apatite. Manganese concentrations alternate several times from very low in apatite-(CaF) to as much as 12.7 wt% MnO in manganoan apatite-(CaF). The jigsaw-like pattern of apatite occurrence and chemical composition is interpreted as a consequence of rhythmic evolution of melt composition in boundary layers formed ahead of solidifying fronts during the rapid, inward crystallization of the dike.

Mineralogical and Geochemical Characterization of Beryl‐Bearing Granitoids, Eastern Desert, Egypt: Metallogenic and Exploration Constraints

AbstractMineral chemistry and geochemical characteristics of beryl‐bearing granitoids in Eastern Desert of Egypt, were examined in order to identify the metallogenetic processes of the host granitoids. The investigated Be‐bearing granitoids and type occurrences are classified into two groups: (i) peraluminous, Ta ≥ Nb + Sn + Be ± W‐enriched, Li‐albite granite (e.g. Nuweibi and Abu Dabbab); and (ii) metasomatized, Nb &gt;&gt; Ta + Sn + Be ± W ± Mo‐enriched alkali feldspar granite (i.e. apogranite; e.g. Homr Akarem, Homr Mikpid and Qash Amir). In these two groups, beryl occurs as stockwork greisen veins, greisen bodies, beryl‐bearing cassiterite ± wolframite quartz veins, dissemination, and miarolitic pegmatites. Beryl of the Be‐granitoids, particularly those of miarolitic pegmatites, contains appreciable contents of Fe, Na, and H2O. An important feature of the Be‐apogranites is the occurrence of white mica as the sole mafic mineral in the unaltered alkali feldspar granite in lower zones. Presence of white mica as volatile‐rich pockets suggests that the melt underwent disequilibrium crystallization, rapid nucleation rates, and exsolving and expulsion of volatiles.

Origin of melt pockets in mantle xenoliths from southern Patagonia, Argentina

Peridotite mantle xenoliths collected north of Gobernador Gregores, Patagonia, affected by cryptic and modal metasomatism bear melt pockets of unusually large size. Melt pockets consist of second generation olivine (ol2), clinopyroxene (cpx2) and spinel (sp2) ± relict amphibole (amph) immersed in a yellowish vesicular glass matrix. Amphibole breakdown was responsible for melt pocket generation as suggested by textural evidence and proved by consistent mass-balance calculations: amph → cpx2 + ol2 + sp2 + melt. Composition of calculated amphibole in amphibole-free melt pockets is very similar to that measured in amphibole-bearing melt pockets from the same xenolith, i.e. amphibole was consumed in the melt pocket generation process. In melt pockets devoid of relict amphibole, mass-balance calculations show remarkable differences between the calculated amphibole and the measured amphibole compositions in melt pockets from the same xenolith. The participation of minor proportions of a consumed reactant phase could be a reasonable explanation. In some samples the calculated phase proportion of glass is in excess compared to modal estimations based on backscattered electron images, probably because a portion of the generated melt was able to migrate out of the melt pockets. Compositional inhomogeneity of cpx2 and variable Ti Kd in cpx2 vs. glass in the same melt pocket reflect fast nucleation and growth and disequilibrium crystallisation, respectively. This and the difference between forsterite content in calculated equilibrium olivine and second generation olivine, suggest that mineral equilibrium was inhibited by rapid quenching of melt pockets.

Pegmatite genesis: state of the art

No one universally accepted model of pegmatite genesis has yet emerged that satisfactorily explains all the diverse features of granitic pegmatites. Genesis from residual melts derived from the crystallization of granitic plutons is favoured by most researchers. Incompatible components, fluxes, volatiles and rare elements, are enriched in the residual melts. The presence of fluxes and volatiles, which lower the crystallization temperature, decrease nucleation rates, melt polymerization and viscosity, and increase diffusion rates and solubility, are considered to be critical to the development of large crystals. A number of new concepts have shed light on problems related to pegmatite genesis. Cooling rates calculated from thermal cooling models demonstrate that shallow-level pegmatites cool radically more rapidly than previously believed. Rapid cooling rates for pegmatites represent a quantum shift from the widely held view that the large crystals found in pegmatites are the result of very slow rates of cooling and crystal growth. Experimental and field evidence both suggest that undercooling and disequilibrium crystallization dominate pegmatite crystallization. London’s constitutional zone refining model of pegmatite evolution involves disequilibrium crystallization from an undercooled, flux-bearing granitic melt. The melt is not necessarily flux–rich and the model does not require the presence of an aqueous vapor phase. Experimental studies of volatile- and flux-rich melts and fluid inclusion studies suggest that volatile-rich silicate melts may persist to temperatures well below 500 °C and even down to 350 °C. Studies of melt inclusions and fluid inclusions have led some researchers to suggest that the role of immiscible fluids must be considered in any model regarding pegmatite genesis. Fluid saturation is thought to occur early in the crystallization history of pegmatites. Two types of melt inclusions along with primary fluid inclusions have been found coexisting in pegmatite minerals. Advances by Petr Cerný in pegmatite classification are in wide use and the fractionation trends of Nb, Ta and other HFSE and K, Rb, Cs, Li, Ga and Tl are now well understood. How pegmatitic melts are produced, the types of source rocks involved and how melt generation relates to plate tectonic models are challenging areas for future investigations. Also, the roles of regional zoning, anatexis, and chemical quenching in pegmatite genesis are areas for future pegmatite research.

Rapid crystallization of the Animikie Red Ace Pegmatite, Florence county, northeastern Wisconsin: inclusion microthermometry and conductive-cooling modeling

We evaluated the crystallization regime of a zoned pegmatite dike and the degree of magma undercooling at the onset of crystallization by analyzing coeval fluid and melt inclusion assemblages. The liquidus temperature of the pegmatite magma was ~720°C, based on re-melting of crystallized-melt inclusions in heating experiments. The magma crystallized sequentially starting with a thin border zone, which formed in less than one day at an average temperature of ~480°C based on primary fluid inclusions, meaning 240°C undercooling. The primary inclusions from the outer zones were postdated by secondary inclusions trapped between 580 and 720°C, representing fluid exsolved from hotter, still crystallizing inner pegmatite units. The huge temperature contrast between the pegmatite’s inner and outer zones was simulated by conductive-heat numerical modeling. A 2.5 m wide dike emplaced in 220°C rocks cools entirely to <400°C in less than 50 days. Unidirectional and skeletal textures also indicate rapid, disequilibrium crystallization.

Petrogenesis of olivine-phyric shergottite Yamato 980459, revisited

Primitive magmas provide critical information on mantle sources, but most Martian meteorites crystallized from fractionated melts. An olivine-phyric shergottite, Yamato 980459 (Y-980459), has been interpreted to represent a primary melt, because its olivine megacrysts have magnesian cores (Fo84–86) that appear to be in equilibrium with the Y-980459 whole-rock composition based on Fe–Mg partitioning. However, crystal size distribution (CSD) plots for Y-980459 olivines show a size gap, suggesting a cumulus origin for some megacrysts. Because melting experiments using the Y-980459 whole-rock composition have been used to infer the thermal structure and volatile contents of the Martian mantle, the interpretation that this rock is primitive should be scrutinized. We report major, minor and trace element compositions of Y-980459 olivines and compare them with results from melting experiments (both hydrous and anhydrous) and thermodynamic calculations. Cores of the olivine megacrysts have major and minor element contents identical to those of the most magnesian olivines from the experiments, but they differ slightly from those of thermodynamic calculations. This is probably because the Y-980459 whole-rock composition lies near the limit of the range of liquids used to calibrate these models. The megacryst cores (Fo80–85) exhibit minor and trace element (Mn–Ni–Co–Cr–V) characteristics distinct from other olivines (megacryst rims and groundmass olivines, Fo < 80), implying that the megacryst cores crystallized under more reduced conditions (∼IW + 1). Y-980459 contains pyroxenes with orthopyroxene cores mantled by pigeonite and augite. We also found some reversely zoned pyroxenes that have augite cores (low-Mg#) mantled by orthopyroxenes (high-Mg#), although they are uncommon. These reversely zoned pyroxenes are interpreted to have grown initially as atoll-like crystals with later crystallization filling in the hollow centers, implying disequilibrium crystallization at a moderate cooling rate (3–7 °C/h). The calculated REE pattern of a melt in equilibrium with normally zoned pyroxene is parallel to those of glass and the Y-980459 whole-rock as well as other depleted olivine-phyric shergottites, suggesting that Y-980459 was derived from a depleted mantle reservoir. Considering the CSD patterns of Y-980459 olivines, we propose that the olivine megacrysts are cumulus crystals which probably formed in a feeder conduit by continuous melt replenishment, and the parent melt composition was indistinguishable from the Y-980459 whole-rock with 0–2 wt% of H 2O and 0–5 wt% of CO 2. The final magma pulse entrained these cumulus olivines and then crystallized groundmass olivines and pyroxenes. Although Y-980459 contains small amounts of cumulus olivine (<∼6 vol%), we conclude that the Y-980459 whole-rock composition closely approximates a Martian primary melt composition.

40Ar/ 39Ar age and geochemistry of the post-collisional Miocene Yamadağ volcanics in the Arapkir area (Malatya Province), eastern Anatolia, Turkey

The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a 40Ar/ 39Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give 40Ar/ 39Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.