Rock-forming Processes Research Articles

Patterns of bubble desorption during the solidification of a multicomponent melt

The desorption of bubbles during solidification of a melt occurs in processes as diverse as the making of ice cubes, the formation of igneous rocks and the casting of metals. In both the metal casting and rock formation processes, careful observation of the final solid suggests that the desorbed bubbles often form regular spatial patterns. Understanding and quantifying the mechanisms by which such patterns arise is important. In the geological context, comparison between field measurements and the predictions of a model will allow geologists to estimate in-situ magma properties. In the metal casting context, engineers would like to be able to specify mould geometries and cooling conditions to ensure that the distribution of bubbles will not compromise the strength of critical sections of the casting.In the present study, we develop a detailed mathematical model to predict the distribution of desorbed bubbles in a solidified melt. Our new model builds upon previous knowledge on this phenomenon in the geological context (Toramaru et al. 1996, 1997). We describe desorption of a dissolved gas in a semi-infinite melt, solidified by a one-dimensional heat flux. In the absence of convection, the transfer of heat and solute occurs mainly by a diffusive mechanism and the crystallization proceeds most rapidly near the cooled boundary. The crystals formed contain almost no dissolved gas and hence the concentration of gas dissolved in the melt increases progressively towards the cooled boundary. Diffusion of dissolved gas from the crystallizing zone is slow and, as a result, the local melt becomes supersaturated and gas bubbles desorb. The full equations for this coupled solidification and desorption processes are solved numerically.We find that bubbles desorb forming a sequence of layers parallel to the cooled boundary. The spacing between these bubble layers increases geometrically from the cooled boundary. We give a physical interpretation for this geometric pattern and analyse the effect of physical parameters on the layer spacing. We show that our theoretical model captures the important physical mechanisms involved in the solidification and desorption processes by comparing its predictions with available measurements from a geological formation.

Anwendung der geologischen Gefügekunde zur Charakterisierung natürlicher Werksteine - Fallbeispiel: Kauffunger Marmor / Application of geological fabric analyses for the characterization of natural building stones. - Case study: Kauffung marble

Abstract In geology, methods of fabric analyses are largely applied in order to reconstruct the rock-forming processes and define the environments in which they formed. Often, rocks are characterized by polyphase imprints, which occured over a long time span under varying conditions (pressure, temperature, chemical environmentj. In these cases, the final fabrics are usually rather complex. The Kauffung marble is used as an example to demonstrate, how the methods of geological fabric analyses can be applied to the characterization of natural building stones. A correlation with petrophysical analyses reveals that the most important fabric parameters are crystallographic-preferred orientation (texture), the shape-preferred orientation and the microcrack populations, which are usually responsible for a distinct anisotropy. They mainly control the directional dependence of the material properties. These anisotropic properties should be considered during the cutting and later use of natural building stones.

Carbonate precipitation and oxygen stratification in late Archean seawater as deduced from facies and stratigraphy of the Gamohaan and Frisco formations, Transvaal Supergroup, South Africa

The correlative 2521+ or -3 Ma Gamohaan and Frisco formations, Transvaal Supergroup, South Africa, consist of peritidal and subtidal carbonate lithofacies that pass conformably upward into the deep subtidal Kuruman and Penge iron-formations. The stratigraphic setting and lithofacies transitions demonstrate that the Gamohaan and Frisco formations were deposited in open marine environments during a transgression that resulted in drowning of the underlying Campbellrand-Malmani carbonate platform. The Gamohaan and Frisco formations contain complex microbial structures associated with abundant sea floor-encrusting and void-filling calcite. In a 40 m section of the Gamohaan Formation, more than 35 percent of the rock consists of marine calcite that precipitated as crystals directly on the sea floor or in primary voids. Individual beds of precipitated, sea floor-encrusting calcite are up to 30 cm thick and are laterally continuous for the entire 7000 km 2 of good stratigraphic control. The abundance of precipitated carbonate and the lateral continuity of individual beds demonstrate that deep subtidal seawater was supersaturated with respect to calcite, that carbonate precipitation was controlled by regional seawater chemistry, and that in situ calcite precipitation directly on the sea floor was an important rock-forming process in late Archean oceans. Transitions from the Gamohaan Formation to basinal equivalents laterally and to the Kuruman Iron Formation vertically show a progressive change from precipitated calcite to shale to siderite-facies iron-formation mixed with oxide-facies iron-formation deposition. This facies succession can be attributed to an increase in [Fe (super 2+) ] with depth: As [Fe (super 2+) ] increases, calcite precipitation slows, and siderite becomes supersaturated resulting in a change from limestone to iron-formation accumulation. This gradient probably ranges from several 100 mu mol/1 Fe (super 2+) in deep seawater to a few mu mol/1 Fe (super 2+) in the mixed zone of the oceans. The presence of any Fe (super 2+) in the mixed layer of late Archean oceans requires low atmospheric [O 2 ].

The tin-bearing topaz-quartz porphyry, Yangbin area, Taishun County, Zhejiang Province—a fluid inclusion study

The tin-bearing topaz-quartz porphyry in the Yanbin area is subvolcanic in origin, derived from granitic residual magma through strong crystallization differentiation. The rock contains various types of inclusions (hydroxyl-melt, vapor-rich, halite-bearing multiphase, and liquid-rich), which permits us to trace the evolutionary path of the fluid. It is suggested that immiscibility took place in a pulsating manner between melts and fluids during the rock-forming process with the homogenization temperatures ranging from 580°C to 180°C and the salinities varying from low through high to low (5.1–10. 5»6. 9-21.4»30-48»16-22»2-10 in wt% NaCl). The hydroxyl-melt inclusions are considered as evidence of magma-hydrothermal transition.

SPM study on the distribution of Carlin-type gold mineral deposit in China

Trace element distributions provide indications for the reconstruction of the processes of rock formation and evolution. A new technique, which combines PIXE, PIGE method with microanalysis technique, has been developed to measure the element distributions of Carlin-type gold mineral samples. Au is found to be positively correlated with As, S, Ag, Fe, while F is uncorrelated with Au.

Mössbauer effect and TEM in mineralogy

Mossbauer effect (ME) provides useful information on oxidation state, co-ordination number, co-ordination state, site occupancies, and magnetic properties of Fe-bearing phases. The information gained by ME together with the information provided by other conventional techniques is used to extract temperature, pressure, and kinetics of rockforming processes. Nevertheless, ME requires that the phases studied are homogeneous over an extremely large volume and that Fe is a major component of the system. Transmission electron microscopy (TEM), on the other hand, provides similar information over a very small volume for a system of any component. However, present TEM spectrometers do not provide sufficient resolution to detect the mixed oxidation state. A complete characterisation of phases in rocks requires, therefore, that conventional techniques be combined with TEM.

Analysis of permeability controls: a new approach

AbstractBy modelling a range of rock-forming processes such as compaction and various styles of cementation, a new understanding of how they affect pore-system geometry, and hence permeability, has been gained. For example, almost identical permeability-porosity trends result from progressive compaction or grain overgrowth cementation in a clean sandstone, and these trends are curvilinear on the traditional log-linear plot. The steepening which defines the curve marks the onset of pore-throat blocking. Other cement styles, such as pore-filling carbonates or grain-rimming clays, show different porosity-permeability trends. This new understanding can be used predictively (predicting permeability from predictions of grain size and diagenetic style), or as a tool for identifying the important permeability controls in a set of field data. This latter application is presented and illustrated using data from a variety of sandstone types. This approach has important advantages over commonly used multivariate statistical analysis approaches. It can quickly provide a good understanding of what are (and are not) the important controls on permeability. It also provides a basis for more focused and meaningful statistical analysis to quantify these controls.

Petrographic Classification of Crystal Morphology

The terms "euhedral," "subhedral," and "anhedral" have limited value as an expression of crystal morphology relating to conditions of crystal formation and rock-forming processes. The basis of this traditional nomenclature is the presence or absence of crystal faces, implying that if a crystal has faces its overall shape is likely to reflect crystal structure. Experimental reproduction of growth, dissolution, and melting textures, along with an ever-increasing awareness of the importance of incomplete and malformed crystals in natural systems, is the impetus for presentation here of a more detailed morphological classification of crystals at the petrographic scale. The classification is binary, based on the degree to which crystals are an expression of crystal structure and on the degree of completeness. Unlike the traditional scheme, the proposed classification is sensitive to faceted crystals that do not have "ideal" shape and to incomplete crystals variously described as skeletal, dendritic, cellular, spongy, and sieve.

Layered rhyolite bands in a thick North Mountain Basalt flow: the products of silicate liquid immiscibility?

AbstractThe upper 35 m of a thick (≤175 m) Early Jurassic North Mountain Basalt flow at KcKay Head contains 25 cm thick differentiated layers that are separated by 130 cm sections of basalt. The lower layers are mafic, pegmatitic, and contain thin (2 cm), fine-grained 'rhyolite' bands. Evidence that the rhyolite represents a Si-rich immiscible liquid includes: (1) textures such as fiine-grained globules of Ferich pyroxene (once Fe-rich liquid) bordering pegmatite feldspar grains; (2) structureless, microcrystalline, interstitial, polygonal patches of Si-rich minerals and similar areas of Fe-rich stilpnomelane surrounding skeletal Fe-Ti oxide grains, with bulk chemical compositions (to a first approximation), relative proportions and total modat percentages suggesting they were once Si-rich and Fe-rich glasses respectively; (3) basalt and pegmatite compositions (particularly their Fe, and Ti contents) similar to rocks known to contain immiscible liquids; (4) rhyolite major element compositions generally consistent with formation from an immiscible Si-rich liquid; (5) mineral compositions and temperature of pegmatite formation compatible with immiscibility; (6) the inability of mass balance calculations (crystal fractionation) to explain rhyolite formation unless mesostasis stilpnomelane (representing the Fe-rich liquid) is included in the caculations. If, as we suggest, these rocks are the result of immiscibility, they shed light on the incipient formation of granophyres in mafic intrusions and support liquid immiscibility as an important rock-forming process.

Realistic Expectations for Rock Identification

Successful hand-specimen rock identification for beginning students is dependent upon providing representative samples with clearly observable properties and insuring student understanding of both rock-forming processes and the construction of the classification scheme being used. Minerals classification can be clear cut and definitive because minerals are compounds that have definite internal structures and definite chemical compositions (or compositions that vary within limits as, for example, in the plagioclase feldspars). In contrast, rock-classification schemes have boundaries that are necessarily arbitrary because rocks are mixtures of minerals that are formed by gradational processes. Classification of specimens that happen to fall near boundaries is often subjective; one person's basalt may be another person's gabbro. This seemingly obvious point is not trivial and when overlooked can lead to frustration and anxiety for students who believe they must make definitive decisions about samples which t...

Recent Advances In The Analysis And Interpretation Of C-O-H-N Fluids By Application Of Laser Raman Microspectroscopy

One of the major goals of a geologist is to infer, from his field and laboratory observations, the temperature, pressure, and compositional parameters of the ambient system during rock formation or ore deposition. Because fluids are actively involved in many rock-forming processes, fluid inclusions trapped within minerals are studied as evidence of the pertinent system parameters. Individual minerals may contain several generations of fluid inclusions due to the complex histories of many geological environments. Micro-analytical techniques for the measurement of small quantities of materials in individual inclusions on the order of a few micrometers in diameter therefore are required.The classical technique is microthermometry, which involves monitoring phase changes in an inclusion during controlled cooling and heating on a microscope stage. Given appropriate phase diagrams (not available in many cases) for the chemical system under study, the phase changes at recorded temperatures can be interpreted in terms of a specific fluid composition and density.

Narragansett Basin Argillite: Lithology, Chronology, and Prehistoric Tool Manufacture

This article examines the use of one low-grade raw material, Narragansett Basin argillite, by prehistoric peoples in southeastern New England. Petrographic sections analyzed by Dr. Dan Murray (Geologist, University of Rhode Island) have provided a detailed account of the lithology of this material. Rock formation processes and lithic sources are also discussed. Artifact analysis has provided data pertaining to the prehistoric periods when Narragansett Basin argillite was most often used by prehistoric populations in Rhode Island and southeastern Massachusetts. An examination of tool manufacturing techniques has brought to light striking similarities between argillite and quartz Small Stemmed points further supporting the hypothesis of an in situ development of the Small Stemmed Point tradition in New England.

Granitization - fact or fiction?

Abstract Augen gneisses, formed by granitization of meta-andesites near Loftahammar, are described. Grantization was mainly isochemical within the parental rock suite, metarhyolites and meta-andesites. The process seems to have been triggered by influx of a vapour phase (water). The augen gneisses formed are similar to the ‘melanocratic Loftahammar granites’ found in the surroundings. The granitization process is compared with other rock-forming metasomatic processes, such as fenitization. It is concluded that the lack of unusual mineral assemblages formed during granitization makes recognition of the process difficult. However, that we do not recognize a process does not mean that it does not occur. In analogy with fenitization, it is suggested that granitization may well be an important rock-forming process.

The statistics of Pearce element diagrams and the Chayes closure problem

Pearce element ratios are defined as having a constituent in their denominator that is conserved in a system undergoing change. The presence of a conserved element in the denominator simplifies the statistics of such ratios and renders them subject to statistical tests, especially tests of significance of the correlation coefficient between Pearce element ratios. Pearce element ratio diagrams provide unambigous tests of petrologic hypotheses because they are based on the stoichiometry of rock-forming minerals. There are three ways to recognize a conserved element: 1. The petrologic behavior of the element can be used to select conserved ones. They are usually the incompatible elements. 2. The ratio of two conserved elements will be constant in a comagmatic suite. 3. An element ratio diagram that is not constructed with a conserved element in the denominator will have a trend with a near zero intercept. The last two criteria can be tested statistically. The significance of the slope, intercept and correlation coefficient can be tested by estimating the probability of obtaining the observed values from a random population of arrays. This population of arrays must satisfy two criteria: 1. The population must contain at least one array that has the means and variances of the array of analytical data for the rock suite. 2. Arrays with the means and variances of the data must not be so abundant in the population that nearly every array selected at random has the properties of the data. The population of random closed arrays can be obtained from a population of open arrays whose elements are randomly selected from probability distributions. The means and variances of these probability distributions are themselves selected from probability distributions which have means and variances equal to a hypothetical open array that would give the means and variances of the data on closure. This hypothetical open array is called the Chayes array. Alternatively, the population of random closed arrays can be drawn from the compositional space available to rock-forming processes. The minerals comprising the available space can be described with one additive component per mineral phase and a small number of exchange components. This space is called Thompson space. Statistics based on either space lead to the conclusion that Pearce element ratios are statistically valid and that Pearce element diagrams depict the processes that create chemical inhomogeneities in igneous rock suites.

The last interglacial high sea level in the granitic Seychelles, Indian ocean

In the granitic Seychelles islands, widely scattered remnants of marine limestones of Late Pleistocene age are found over a range of elevations within about 9 m with respect to present sea level. These exposures can be subdivided into coralgal buildups and rubbly conglomerates. 230Th/ 234U dating from corals indicates that the limestones were deposited during the last interglacial stage (140-120 kyr B.P.). In order to understand the sea level history for this time range, various fabrics related to sequences of rock-forming processes (i.e. frame building, bioerosion, internal sedimentation, cementation, other depositional and diagenetic processes) have been identified and used as possible indicators of paleosea level datum at their time of formation. Detailed examination of the rock fabric sequences, in combination with radiometric dating, reveals that sea level overstepped its present position around 140 kyr ago and reached its maximum altitude 135 kyr ago before retreating. There were presumably two distinct events in sea level rise, separated by a gap of about 10 kyr. Comparison between elevations of high sea stands of a similar age on a regional scale supports the contention that the Seychelles area has been relatively instable since at least the Late Pleistocene. Either hydro-isostatic uplift of the Seychelles granitic bank or volcanic subsidence of nearby atolls are assumed to be responsible for such differences in paleosea level records.

Electron beam analytical instruments and the determination of modes, spatial variations of minerals and textural features of rocks in polished section

Point counts based on X-ray intensities collected with an automated electron beam analytical instrument can be used to estimate the accurate modes required by modern physical-chemical modelling of rock-forming processes. In addition, the spatial distributions of mineral species and mineral compositions can be extracted from the coordinates of the point counting grid. Statistical tests of whether the mineral and porosity distributions are uniform can be performed by comparing the results from subareas of the polished section. The separate and composite contributions of the mineral phases to the texture and structure of the rock can be visually represented with mineral distribution maps. Qualitative differences in grain size can be recognized because the nonzero, finite electron beam diameter can detect grain boundaries. Spots on the minerals are identified by comparing the ranking of X-ray intensities with the ranking of intensities on a set of standards. For example, this method identified more than 88% of the spots on the surface of a polished section of an alkali olivine basalt. A small additional number of spots, approximately 1%, were identified because their spectra were dominated by the X-ray intensities of two characteristic elements. This method was particularly efficient in identifying phases with a small grain size and simple chemistry, e.g. apatite. The modal amounts of an alkali olivine basalt obtained with a microprobe are within approximately one standard deviation of the results obtained by point counting the same areas of the polished section under reflected and transmitted light if the minerals are major constituents and occur only in the groundmass. Accessory and phenocryst phases agree to within approximately three standard deviations.

A process-response model for the formation of pedogenic calcretes

Summary The compositional and fabric evolution of pedogenic calcretes is viewed as a process-response model involving inorganic and organic soil-forming processes on the one hand, and diagenetic, rock-forming processes on the other; the resulting battle being expressed as six evolutionary stages: (1) preparation of host material; (2) soil formation and horizon differentiation; (3) accumulation of calcium carbonate; (4) profile development; (5) induration; and (6) reworking. In nature, some stages may be repeated or reversed; others may be omitted. Emphasis is given to the role of organic activity which determines largely the morphology of pedogenic calcrete profiles and which controls the direction and extent of profile development.

Experimental determination of rare earth fractionation patterns in partial melts from peridotite in the upper mantle

Rare earth element patterns of partial melts of a natural peridotite with a chondrite-like rare earth pattern as starting material have been experimentally determined at high pressure. The experimental data are combined with major element data on the same partial melts to test the applicability of models of volatile-free melting of peridotite to rock-forming processes in the upper mantle. Alkali basalts could not have been formed by partial melting of peridotite in the absence of volatiles because the major and trace element abundances produced experimentally are not compatible with those of natural alkali basalts. Abyssal tholeiite without light rare earth element depletion can be derived from volatile-free peridotite by direct partial melting. Suites of abyssal tholeiite and picritic basalt may have been formed by a two-stage melting process from a common peridotite source. The peridotite residue must have lost clinopyroxene and garnet at the time of formation of picritic melt. Abyssal tholeiites with depletion of light rare earth elements could have been derived only from a peridotite source that has been depleted of the same elements prior to the melting event yielding the tholeiite.

U-Pb age determinations on zircons and monazites from a migmatite area on Bjørneøer, Scoresby Sund, East Greenland

During the 1969 geological expedition to Scoresby Sund, a small migmatite area on the south-west peninsula of island no. 1 of the Bjørneøer (71°08'44"N/25°20' 56"W - see fig. 1) was mapped and sampled in detail. More than 2000 kg of rock were collected for petrographic and isotope geochemical investigations to give an insight into the nature of rock-forming processes induced by high-grade regional metamorphism. The restriction to a very limited sampling area (700 m × 200 m) provided rock samples which were subjected to very similar P-T conditions for at least part of their history. This report presents preliminary results obtained by the comparison of U-Pb systematics of accessory minerals from various rock phases.

Variations in Surface Textures of Quartz Sand using Electron Microscopy

Examination of sand grain surfaces from early Paleozoic sandstones by electron microscopy reveals a variety of secondary effects caused by rock-forming processes after final deposition of the sand. Detailed studies were conducted on both coarse (≥0.71mm) and fine (=0.25mm) fractions of St. Peter Sandstone, a widespread sand deposit underlying much of the U.S. Central Interior and used in the glass industry because of its remarkably high silica purity.The very friable sandstone was disaggregated and sieved to obtain the two size fractions, and then cleaned by boiling in HCl to remove any iron impurities and rinsed in distilled water. The sand grains were then partially embedded by sprinkling them onto a glass slide coated with a thin tacky layer of latex. Direct platinum shadowed carbon replicas were made of the exposed sand grain surfaces, and were separated by dissolution of the silica in HF acid.