Oscillatory-zoned Crystals Research Articles

Výrazně zonální tetraedrit-tennantit z Kramolína, rudní revír Michalovy Hory (Česká republika)

The crystals of significantly zonal tetrahedrite-tennantite were found in the mine dump material of the Grubenwall 42 mine, Kramolín, the Michalovy Hory ore district, western Bohemia (Czech Republic). Tetrahedrite-tennantite forms layer of tetrahedral, partly corroded crystals up to 1 mm in size on a crust of crystalline quartz in association with chalcopyrite and cerussite. Individual zones in oscillatory zoned crystals are represented by three members of tetrahedrite group minerals - tetrahedrite-(Zn), tennantite-(Zn) and rare tennantite-(Fe). The observed range of AsSb-1 substitution is unusual within a single crystal and indicates high variability of the As/Sb ratio in the hydrothermal fluids.

Pyroxenites and Megacrysts From Alkaline Melts of the Calatrava Volcanic Field (Central Spain): Inferences From Trace Element Geochemistry and Sr-Nd Isotope Composition

Alkaline volcanic rocks from explosive monogenetic centers often carry an unusual cargo of crystals and rock fragments, which may provide valuable constraints on magma source, ascent and eruption. One of such examples is the Cenozoic Calatrava Volcanic Field in central Spain, a still poorly explored area to address these issues. Clinopyroxene, amphibole and phlogopite appear either as megacryst/phenocrysts or forming fine-grained cumulates (pyroxenite enclaves s.l.) in some eruptive centers of this volcanic field. They have previously been interpreted as cogenetic high-P minerals formed within the upper lithospheric mantle. The presence of Fe-Na-rich green and Mg-Cr-rich colorless clinopyroxene types as phenocryst cores or as oscillatory zoned crystals in pyroxenite enclaves points to a complex evolution of mineral fractionates from petrogenetically related magmas. In trace element chemistry all studied clinopyroxene types show parallel rare earth element patterns irrespective of whether they are megacrysts, colorless or green core phenocrysts, or zoned crystals within pyroxenite cumulates. This similarity indicates a genetic relationship between all the fractionated minerals. This is in agreement with the overlapping of initial 143Nd/144Nd and 87Sr/86Sr ratios of pyroxenite enclaves (0.512793–0.512885 and 0.703268–0.703778) that is within the chemical field of the host magmas and the Calatrava volcanics. The initial 143Nd/144Nd and 87Sr/86Sr ratios of megacrystic clinopyroxene, amphibole and phlogopite show a more restricted range (0.512832–0.512890 and 0.703217–0.703466), also falling within the isotopic composition of the Calatrava volcanic rocks. Deep magmatic systems beneath monogenetic volcanic fields involve several stages of melt accumulation, fractionation and contamination at variable depths. Trace element and isotope mineral chemistry are powerful tools to understand the history of ascent and stagnation of alkaline basaltic magmas and discriminate between magma mixing, wall-rock contamination and closed magmatic system evolution. In our study, we establish a cogenetic origin for green and colorless clinopyroxene as high-pressure precipitates from liquids of different fractionation degrees (up to 80%, for the highly evolved melts equilibrated with the green clinopyroxene), originated from a highly solidified front of silica-undersaturated alkaline magmas at mantle reservoirs.

Mt. Etna plumbing system revealed by combined textural, compositional, and thermobarometric studies in clinopyroxenes

Coupled textural and in situ geochemical studies of clinopyroxene (cpx) phenocrysts, from both historical and recent eruptions of Mt. Etna volcano, provide a means to investigate the processes occurring in the deepest portion of the feeding system (>10 km depth). Five distinct textures were recognized: (1) normal oscillatory zoning, (2) normal zoning with Fe-rich rim, (3) sieve-textured core, (4) reverse oscillatory zoning, and (5) dusty rim. Electron microprobe analyses indicate an almost constant diopside–augite composition, with a slight enrichment in the enstatite for more recent erupted cpx. Core-to-rim compositional profiles, performed along the cpx, reveal distinct compositional characteristics. Normal oscillatory zoning is often characterized by a sharp increase in FeO (Δ ~ 2 wt%) accompanied by a drop in Al2O3 on the outermost 30 μm. Reverse oscillatory zoning, by contrast, exhibits a drop in FeO, Al2O3 (Δ ~ 2 wt%), and a remarkable crystal rim increase in MgO (up to 5 wt%). Similar compositional changes are evident in dusty-textured rims, which are characterized by dissolution edges and overgrowth containing glass pockets and channels. No significant compositional variations have been observed across crystals with sieve-textured cores. Trace element concentrations show enrichments in Sr, La, Zr, and REE, together with a decreasing La/Yb ratio (from ~7 to ~4) in rims of normally zoned crystals. Cpx with reverse zoning and dusty rims has low Sr, La, Zr, and REE contents toward crystal rims. Thermometers and barometers, based on equilibrium cpx-melt pairs, suggest that cpx cores start nucleating at 720 MPa, with the majority of them forming between 600 and 400 MPa but continuing to crystallize until very shallow depths (<100 MPa). Normal oscillatory-zoned phenocrysts surrounded by rims form at pressures shallower than 400 MPa, while reverse zoning and dusty rims occur between 400 and 500 MPa. Coupled petrologic and thermobarometric studies on both clinopyroxenes and plagioclases, associated with detailed textural and in situ geochemical analyses, are promising tools to reconstruct the entire magma ascent path beneath open-system volcanoes. At Mt. Etna, two distinct processes could account for the observed textures: Fe-rich rims in normal oscillatory-zoned crystals can be related to decompression-induced crystallization, while reverse zoning and dusty rims can be produced by mixing with a more basic magma at 400–500 MPa (i.e., ~10 km). Textural features are not restricted to a particular evolutionary phase of the volcano, which suggest that the deep feeding system has not changed significantly since the first alkaline volcanic phase.

A case example of the importance of multi-analytical approach in deciphering carbonatite petrogenesis in South Qinling orogen: Miaoya rare-metal deposit, central China

The South Qinling orogen in central China hosts carbonatites occurring as stocks associated with syenites and collectively regarded as the Miaoya intrusive complex. The complex hosts economic resources of rare-earth elements (REE) and Nb. The Miaoya syenites are strongly metasomatized at the contact with the carbonatites and cross-cut by carbonate and felsic veinlets. Small oscillatory-zoned crystals of zircons from the syenites give a concordant U–Pb age of 147±0.5Ma, which differs significantly from the ages of both large magmatic zircon grains from the syenites and primary monazite from the carbonatites (766Ma and 234Ma, respectively). To account for the possibility that the Miaoya syenites are coeval and cogenetic with the carbonatites, the trace-element budget of both rock types was examined in detail. The Miaoya carbonatite contains primary REE-rich fluorapatite and monazite, which precipitated earlier than the rock-forming REE-poor calcite, indicating that the primary carbonatitic magma was rich in REE. The compositions of the parental syenitic and carbonatitic magmas, calculated on the basis of the trace-element composition of primary fluorapatite in the two rock suites, show that the carbonatitic magma contained higher Sr and REE (La–Tb), but lower Ba, Pb, Th, U, Nb and Ta levels in comparison with the syenitic melt. These differences are inconsistent with derivation of the Miaoya rocks from a homogeneous carbonate–silicate melt by immiscibility or crystal fractionation. It is therefore concluded that the carbonatitic magma at Miaoya was generated directly in the mantle. Emplacement of the carbonatites in the South Qinling orogen marked transition to a postorogenic regime, and was preceded by oceanic crust subduction and closure of the Mianlue Ocean in the Triassic. Our models show that melting of the Mianlue crust and up to 10wt.% of sediments cannot produce the levels of REE enrichment observed in the Miaoya carbonatites. More complex models, involving recycling of the Mianlue oceanic crust and a REE-rich carbonate liquid from an old deep-seated mantle source are required to explain the observed trace-element characteristics of the Miaoya carbonatites.

Oscillatory-zoned crystals of pyrochlore-group minerals from the Guaniamo kimberlites, Venezuela

Oscillatory-zoned crystals of pyrochlore-group minerals are found in the La Ceniza diamondiferous kimberlites, Guaniamo region, Venezuela. This is the first known occurrence of the pyrochlore-group minerals in kimberlites worldwide. The minerals occur in the kimberlite groundmass in association with olivine, zoned spinel, phlogopite, carbonates (dolomite, calcite, magnesite), apatite, tetraferriphlogopite, Fe–Ni–Cu-sulfides and serpentine. They form discrete octahedral crystals or intergrowths (up to 50 μm) with a red-brown core and light brown rim. Individual zones in the cores are remarkably enriched in TiO 2 (up to 14.3 wt.%), Ta 2O 5 (up to 10.1 wt.%), ZrO 2 (up to 23.7 wt.%), ThO 2 (up to 22.3 wt.%) and UO 2 (up to 8.0 wt.%) at moderate Nb 2O 5 contents, whereas the outer regions contain significant CaO (up to 17.5 wt.%), Na 2O (up to 5.7 wt.%), Nb 2O 5 (up to 58.9 wt.%) and F (up to 4.6 wt.%), approaching the ideal formula (Na,Ca) 2Nb 2O 6F. Following the IMA recommendations and the “50% rule”, the interior is classified as uranoan Zr-rich thoriobetafite/thoriopyrochlore or thorian Zr-rich betafite/pyrochlore, and the outer part as pyrochlore or thorian pyrochlore. Strong compositional zoning is consistent with two substitutions: 2(Th,U) 4+ + □ + (Ti,Zr) 4+ + O 2− → 3Ca 2+ + Nb 5+ + (F,OH) − and 2(Th,U) 4+ + □ + 2(Ti,Zr) 4+ + O 2− → 2Ca 2+ + Na + + 2Nb 5+ + (F,OH) −. The zoning patterns and the absence of perovskite indicate the dominant role of F among fluid components and relatively high alkalinity during crystallization of the silicate–carbonate groundmass of the La Ceniza kimberlite. The composition and zoning of the La Ceniza pyrochlore-group minerals are very specific, but in general strongly resemble those of some pyrochlores from carbonatites.

Apatite in the Ilímaussaq alkaline complex: Occurrence, zonation and compositional variation

Apatite from the augite syenite and the roof sequence of layered nepheline syenites (pulaskite, foyaite, sodalite foyaite and naujaite) of the Ilímaussaq alkaline complex has been analysed by electron microprobe for Ca, P, Na, Si, selected rare earth elements (REE), F and Cl. In the augite syenite, chilled facies and middle zone, euhedral apatite crystals are relatively abundant as inclusions in other liquidus phases. The REE content is ca. 1 wt.% REE 2O 3. The same level is found in interstitial apatite in the coarse-grained innermost facies. REE-enriched intercumulus apatite overgrowths are common and can have up to ca. 18 wt.% REE 2O 3. Apatite is a common phase in the pulaskite and foyaite, whereas it is rare in the sodalite foyaite and naujaite. Based on compositional variation and zoning, it is possible to distinguish four generations of apatite in the layered roof sequence: 1) primary apatite belonging to the high-temperature mineral assemblage. This apatite occurs in the pulaskite, foyaite and sodalite foyaite as oscillatory-zoned crystals, whereas only remnants are found in the naujaite. The REE enrichment is dominated by the Ca 2+ + P 5+ = REE 3+ + Si 4+ substitution and maximum REE 2O 3 exceeds 20 wt.%. 2) irregularly zoned apatite occurs as rim zones on the primary apatite in the foyaite and as the dominant type of apatite in the sodalite foyaite and naujaite. Most irregularly zoned grains are altered primary apatite. During the dissolution and recrystallisation process the composition gradually changed towards a lower REE content. 3) overgrowths of homogeneous apatite on the irregularly zoned apatite in the naujaite. 4) Na- and REE-rich apatites occur in the foyaite, sodalite foyaite and naujaite either as overgrowths on earlier generations of apatite or as individual crystals. Calculated chondrite normalised patterns show that the later generations of apatite are relatively enriched in LREE and depleted in HREE. The most pronounced evolution is found in Na- and REE-rich apatite. Apatite/melt partition coefficients for La, Ce, Nd, Sm, Dy, Y and Yb are calculated for the augite syenite and nepheline syenites of the roof sequence. The coefficients show the characteristic convex shape with a maximum around Nd and Sm. Using the calculated partition coefficients and the chemical composition of the Na- and REE-rich apatite, the REE content of a late aqpaitic melt in the roof sequence is estimated. The shape of the chondrite normalised pattern is similar to that of a late arfvedsonite lujavrite dyke and steeper than the patterns of the primary magma.

Minor- and trace-element zoning in plagioclase: implications for magma chamber processes at Parinacota volcano, northern Chile

Textural and compositional zoning in plagioclase phenocrysts in a sample from Parinacota volcano (Chile) was investigated using backscattered electron images and electron microprobe analysis of major and trace elements. Large (2 mm) oscillatory zoned crystals (type I) with resorption surfaces of moderate An discontinuities (⩽10% An) and decreasing trace-element contents (Sr, Mg, Ti) towards the rim reflect melt differentiation and turbulent convection in the main magma body. Early recharge with a low-Sr mafic magma is seen in the core. Small-scale Sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years. Smaller crystals (type II) with low trace-element/An ratio reflect the influence of an H2O-rich melt probably from a differentiated boundary layer. Closed-system in-situ crystallisation, mafic magma recharge and the role of a water-rich differentiated boundary layer can be distinguished from the An–trace element relationships. Crystals apparently move relatively freely between different parts and regimes in the magma chamber, evidence for "convective crystal dispersion". High-Sr type II crystals indicate an earlier input of Sr-rich mafic magma. Recharge of two distinct mafic magma types is thus identified (high-Sr and low-Sr), which must have been present – at increasing recharge rates with time – in the plumbing system throughout the volcano's history.

High-resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images: new constraints on oscillatory zoning

Oscillatory zoning in plagioclase is investigated at small scale (≤10 µm) using accumulated backscattered electron (BSE) images as high-resolution imaging method. Combined with electron microprobe quantitative analysis, gray-value profiles across these images can be calibrated for An-content with a resolution of 0.5 mol% An. Applied to oscillatory-zoned crystals, this new application of BSE imaging allows better characterization of zoning patterns along a profile and quantification of wavelength, amplitude, and shape of the oscillations. We also obtain high-resolution information on the morphology of growth zones boundaries. This approach allows us to better classify the different types of "oscillations" and concentric zoning. Dissolution is more frequent than usually recognized. Major resorption surfaces crosscut several growth zones. Irregular 5–10-µm saw-tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small-scale oscillations (≤1–3 µm) with straight boundaries. This suggests at least two mechanisms for the formation of these zoning patterns: faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics.

Spatiotemporal dynamics in oscillatory zoned magmatic plagioclase

This work presents a quantitative analysis of spatiotemporal patterns that form during the growth of oscillatory zoned plagioclase. These patterns were imaged using Nomarski interference contrast imaging of etched polished surfaces. Analysis of these imaged patterns yields new information and constraints on how patterns form, and consequently on how oscillatory zoned crystals may develop in magmatic systems.Utilizing mathematical techniques recently developed for the analysis of dynamical systems, we show that the irregular nature of oscillatory zoning is not compatible with random or stochastic processes. The analysis of our observations suggests that the plagioclase‐liquid system is a deterministic nonlinear system whose attractor has a relatively low dimension. Finally, comparison of our empirical results with computer models of other dynamical systems implies that as few as two variables may be sufficient to describe the dynamics of the plagioclase‐liquid system.

Zoning in columbite-tantalite crystals from the granitic pegmatites of the Eräjärvi area, southern Finland

Electron microprobe analyses of zoned columbite-tantalite crystals from the granitic pegmatites of the Eräjärvi area in Orivesi, southern Finland indicate wide compositional variation within the series FeNb 2O 6-FeTa 2O 6-MnNb 2O 6-MnTa 2O 6, especially in specimens from thin pegmatite dikes. Most crystals show gentle progressive zoning characterized by small-scale variations in the major elements. Where the compositional variation is large, backscattered electron images indicate oscillatory or patchy zoning or various replacement textures. The zones in oscillatory zoned crystals are usually 1–50 μm in width, exceptions reaching 50–120 μm. The wider zones often consist of a group of very narrow subzones of only slightly different composition. Zoning is due mainly to the compositional variation in Ta and Nb. In two of these crystals, the oscillations in Mn follow the strongest oscillations in Nb content. Patchy zoned crystals exhibit corroded cores of early columbite-tantalite, surrounded by later zones enriched in Ta. The mottled appearance of such crystals results from two or more successive replacements. Replacement tongues or network-like replacement textures are typical in the rims of some crystals. The zoning of the columbite-tantalite is related to the complex crystallization history of the pegmatite dikes. The main factors controlling oscillatory zoning are considered to be the growth dynamics of the crystals, the concentration and diffusion of the main elements, and the successive flows of the magma in an intrusion channel. The generation of a corrosive supercritical vapor phase at the end of magmatic crystallization caused resorption, patchy zoning and the replacement of the columbite-tantalite.