Development Of Zonation Research Articles

Magmatic–hydrothermal evolution of the Koktokay No. 3 pegmatite, Altai, northwestern China: Constraints from in situ boron isotope and chemical compositions of tourmaline

Lithium–Cs–Ta-enriched (LCT-type) pegmatites are characterized by zoning texture and extreme enrichment of rare-metals. The role of fluids in zonation development and rare-metal mineralization remains a topic of debate. The Koktokay No. 3 pegmatite in northwestern China represents a typical LCT-type pegmatite characterized by economically significant Li–Be–Nb–Ta–Cs mineralization. This pegmatite exhibits a complete evolutionary sequence from outer to inner zones, with diverse rare-metal mineralization. Tourmaline is ubiquitously present in all textural zones of LCT-type pegmatites. In the Koktokay No. 3 pegmatite, the tourmalines exhibit a range of compositions from dravite and schorl to elbaite and rossmanite. The tourmalines in this pegmatite are characterized by significantly enriched light B isotope compositions (δ11B = −14.8‰ to −8.1‰), similar to those typically observed in most pegmatites derived from continental crust sources. The tourmalines in the outer zones have less variable δ11B values (−13.8‰ to −11.5‰) and are interpreted to have crystallized in a viscous magma. The tourmalines in the inner zones exhibit larger variations in δ11B values (−14.8‰ to −8.1‰) than those from the outer zones. The decreasing δ11B values of tourmalines from the outer to inner zones are attributed to the separation of an immiscible fluid phase that preferentially incorporated heavy B isotopes. The occurrence of hydrothermal tourmaline as veinlets indicates the exsolution of fluids in inner zones, with high δ11B values ranging from −10.2‰ to −9.3‰. Therefore, the hydrothermal fluid can be saturated when the magma has been strongly evolved. In addition to fractional crystallization, exsolved fluids play a significant role in the formation of internal zones and rare-metal mineralization in LCT-type pegmatites. The rare-metals are gradually enriched through fractional crystallization of magma, and the fluid contains mobile elements such as Cs that facilitate precipitation of rare-metal minerals.

Whole Rock, Mineral Chemistry during Skarn Mineralization-Case Study from Tongshan Cu-Mo Skarn Profile

Studying the activation, migration and precipitation processes of ore-forming elements is essential for understanding the genesis and mechanisms of skarn deposits. A typical skarn profile formed by the intrusion of Yanshanian granodiorite into the Carboniferous carbonate strata was studied. The profile is highly consistent with the classic skarn profile, ranging from the intrusion, weak alteration belt, skarn belt (inner and outer skarn belt) and mineralization belt (mainly characterized by Cu mineralization) to the surrounding marble without being affected by late-stage low-temperature or supergene weathering alteration. Whole-rock data show that the major and trace elements exhibit relatively small changes in the granodiorite and inner skarn, but huge variation in the boundary between the inner and outer skarn; Na, Al, Ti and Sr show significant decreases, while Fe, Mg, Zn, V and Ni show significant increases. The elemental content in the outer skarn is 10–100 times or more higher than that in the marble, but the elements such as Ca, Sr and Cs diluted from the marble. During the migration process from the inner skarn to the outer skarn, some elements (such as K, Rb and Ba) were depleted in the inner, but not enriched in the outer, indicating that they may migrate to farther locations. Grossularite developed in the inner skarn, with light rare earth element (LREE) depletion and heavy REE enrichment, as well as positive and negative anomalies of Eu (δEu = 0.42–3.95). Andradite developed in the outer skarn, with zonation development, light REE enrichment, and heavy REE depletion and a positive Eu anomaly (δEu = 0.36–46.83). Some negative Eu anomalies appear at the edges of garnets in the outer skarn, indicating fluctuations in fO2 during the late skarn process. A positive correlation between Fe3+ and REE3+ in the garnets from the inner skarn, as well as between Al3+ and REE3+ from the outer skarn indicated that there are different YAG substitution mechanisms of REE between the inner and outer skarn. Low garnet REE contents and highly variable Y/Ho ratios in outer skarn suggest that the significant fluctuations in REEs may be primarily controlled by water-rock interactions. Considering the whole-rock major and trace element contents, as well as the trace element features of garnet, we found that whole-rock Na, Al, Ti and Sr elements, garnet Ti, Zr and Nb elements exhibit significant differences between the inner and outer skarn. These characteristics can be used to distinguish the boundary between the rock body and carbonate during the skarnification process.

Experimental study of high to intermediate temperature alteration in porphyry copper systems and geological implications

Porphyry copper systems, which provide most of the world’s copper resource, are commonly associated with characteristic concentric zonation of alteration and mineralization. In-depth knowledge of the distribution and transport mechanism of elements in the alteration zones is essential for understanding the ore-forming processes. We employed flow-reaction apparatus to simulate the fluid-rock interactions during porphyry ore formation so as to investigate the mechanisms that govern the transport of elements and the development of zonation. The results indicate more heterogeneous distribution of elements in the experimental products at 450°C compared to those at lower temperatures, which implies a crucial role of temperature in controlling elements redistribution in hydrothermal systems. Heating advances potassic alteration and Ca leaching of wall rocks. To achieve the same degree of sodic alteration, it requires a higher concentration of Na+ in the fluid toward higher temperature. Temperature also facilitates the incorporation of Ti, Sr and Pb into silicate minerals through cation substitution. We infer from experimental results that from the center of intermediate to mafic volcanic wall rocks toward periphery, the contents of K and Ti should decrease and the contents of Ca, Zn and Mn should increase, whereas the trend for Si and Na could be non-monotonic. This study provides experimental and theoretical insights into a variety of vital geological observations, including anhydrite formation and the widespread development of potassic rather than sodic alteration in porphyry copper deposits.

Germination of Monocolpate Angiosperm Pollen: Evolution of the Actin Cytoskeleton and Wall during Hydration, Activation and Tube Emergence

The monocolpate pollen grain of Narcissus pseudonarcissus L. has two preferred sites for tube emergence, one at each end of the colpus. While the cellulosic microfibnls of the inner layer of the intine are disposed circumferentially in the centre of the grain, the microfibrils in these terminal sites are shorter and randomly oriented Soon after the beginning of hydration, inclusions of the vegetative cell begin movement, firstly in a rotatory manner, and then in a pattern focused on one or both germination sites, where the intine bulges as hydration progresses. These changes are associated with the evolution of the actin cytoskeleton. Actin is present in the unactivated grain in the form of fusiform bodies. During hydration these dissociate to form finer fibrils, initially randomly disposed. Then, correlated with the change of the pattern of movement in the vegetative cell, the actin fibril system becomes polarized towards the germination sites, where shorter fibrils accumulate. Callose, absent from the ungerminated grain, is deposited within the cellulosic wall in these locations, forming a shallow dome which eventually develops into an annulus subtending the inner callose lining of the emerging tube. The transition to cylindrical growth is associated firstly with the development of zonation in the cytoplasm of the vegetative cell, with the tip occupied by a population of wall precursor bodies (P-particles) and a dense aggregate of short actin fibrils; and then with the establishment of the ‘inverse fountain’ pattern of movement characteristic of the apical part of the extending tube.

Polybaric Geochemical Evolution of Two Shield Volcanoes from Santorini, Aegean Sea, Greece: Evidence for Zoned Magma Chambers from Cyclic Compositional Variations

Petrographic, mineralogic, and geochemical data are reported for lavas from two of the major shield volcanoes of the Santorini volcanic complex (Skaros and Micro Profitis Ilias), both of which were active prior to the well-known Minoan eruption with associated caldera collapse. Field work and whole-rock chemical analyses indicate four cycles of eruptive activity within the Skaros sequence and three within the Micro Profitis Ilias (M P1) sequence. SiO2 and LIL-element contents decrease from the base to the top of all cycles except for the uppermost cycle of Skaros. Chemical variations within cycles are interpreted to result from eruption from compositionally and thermally zoned magma chambers. Major oxide data and the results of least-squares, mass balance modeling indicate that fractional crystallization played an important role in the development of the observed chemical variations. However, observed systematic variations in groundmass compositions within each cycle, observed irregular variations in total phenocryst content and the results of density calculations require that generation of the chemical zonation did not involve crystal settling but reflects unstable density stratification, probably resulting from sidewall crystallization. Some of the primitive lavas erupted on Santorini preserve phenocryst and xenocryst evidence for a stage of high-pressure fractional crystallization (involving removal of olivine, clinopyroxene, orthopyroxene, and Cr-spinel). Trace element data combined with petrographic data (i.e. the occurrence of abundant phenocrysts with resorption textures) is taken as evidence that magma mixing was also important in the development of cyclic variations. Most basaltic andesites from Skaros appear to be hybrids derived by mixing of basalt and andesite/dacite. Mineralogic data demonstrate that mixing was also important in the development of zonation in the chambers beneath MPI, but trace element data cannot be explained by combined fractionation and mixing alone. Specifically, incompatible, and compatible element abundances are lower than predicted if fractionation and mixing occurred and it is suggested that the anomalous trace element behaviour of especially LIL elements reflects the simultaneous operation of assimilation, for which there is support from isotopic studies. It is concluded that inter- cyclic chemical variations are explicable in terms of fractionation, mixing and assimilation. The LIL element and highly compatible element concentrations in the most primitive lavas erupted in each cycle of Skaros and MPI increase with time, indicating that mixing became more important with time.

AN INVESTIGATION OF THE MORPHOGENETIC MECHANISMS WHICH CONTROL THE DEVELOPMENT OF ZONATION IN SEEDLING SHOOT APICAL MERISTEMS

The development of zonation in the shoot apical meristems of 28 species of cacti was examined. At germination the embryonic apex may have one of three types of organization: 1) tunica/corpus; 2) tunica/central mother cells/corpus; 3) tunica/central mother cells/peripheral zone/pith‐rib meristem. Apices of the third type have complete zonation and undergo little or no further structural development. Both of the other types develop the missing zones. First, the apices become mitotically active, and peripheral zone characters develop in the outer corpus. Simultaneously, or slightly later, the central mother cells differentiate if they are not yet present. The final step is the formation of the pith‐rib meristem. The sequence of appearance of the zones was constant throughout all species examined, but the time of appearance of only one zone could be correlated with any other morphogenetic process: the development of peripheral zone characteristics in the outer corpus occurs with or before the beginning of leaf production. The development of zonation appears to be independent of apical size, shape, or age, either real age or plastochron age. This has been interpreted to indicate that the metabolic mechanism controlling the development of zonation in shoot apical meristems is largely autonomous and independent of other aspects of morphogenesis occurring in the seedling. Also, the fact that leaf initiation and shoot axis production can both occur before the development of either the central mother cells or the pith‐rib meristem indicates that neither of these zones is essential for these two apical morphogenetic activities.

Effect of Late Holocene sea-level fall on atoll development

Abstract A net fall of 2.4 m since 2760 yrs B.P., the local date for the Flandrian Transgression Maximum, has been of major importance in reef and atoll islet growth in the Gilbert and Ellice Islands. The islets have been built from sand taken from the shallow margins of the lagoonal floor as a direct result of this fall, and it may be no coincidence that the earliest Micronesian and Polynesian settlements date from about the sea level maximum. Later second-order transgressions have almost certainly been important in the development of zonation and increased width of the low-tide platform.

Histochemistry and ultrastructure of adrenal cortical development in the golden hamster

Adrenal glands were collected from fetuses of golden hamsters at 12 1 2 , 14 1 2 , and 15 1 2 days' gestation and from newborn animals. Samples were prepared for light microscopy, histochemical analyses, and electron microscopy. Ultrastructural differentiation of the fetal adrenal was characterized by the development of zonation and typical steroid-producing cells. At 12 1 2 days' gestation, cortical cells had smooth borders, many polyribosomes, and contained mitochrondria with lamellar cristae. Smooth endoplasmic reticulum was not evident. Signs of functional zonation into deep and superficial zones were evident by 14 1 2 days' gestation. Cortical cells of the superficial zone contained scattered lipid droplets, small Golgi areas, and mitochrondria with lamellar cristae. This zone later developed into the adult zona glomerulosa. The cortical cells of the deep zone were characterized by mitochondria containing tubulovesicular cristate, well-developed Golgi areas, vesicular smooth endoplasmic reticulum, and lysosomes. At 15 1 2 days' gestation and in newborn animals the zonation was more prominent, and desmosomes and tight junctions were present between the deep cortical cells. High levels of acid phosphatase, alkaline phosphatase, and esterase were present in the deep cortex, but lower activity occurred in the superficial zone. The activity of 3β-hydroxysteroid dehydrogenase was low in all specimens.