Degree Of Silica Saturation Research Articles

Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio.

Modal classifications of common volcanic rocks are expensive, difficult, or impossible to attain. As a consequence, these rocks are classified using the chemical composition. However, existing classificatory diagrams are unable to identify all 16 families of common volcanic rocks; the most used is the total alkali-silica (TAS) diagram that identifies six families. Rocks not in the TAS diagram are misclassified with other rock names; their names are evolving to extinction, e.g. latite and rhyodacite. Some diagrams use Na2O, which is a complicating element rather than discriminant. Na2O is present both in alkali feldspar and plagioclase making difficult the separation of the amount associated to either feldspar. Silicon, potassium, and calcium are the three major elements with highest variations among volcanic rocks. They are selected for use in two new diagrams confronting CaO/K2O ratio with K2O content (KCK diagrams). One diagram is designed for saturated (intermediate) rocks (quartz <5 vol.%) and the other for oversaturated (acid) volcanic rocks (quartz >5%). These diagrams are tested using compositions of volcanic rocks from Uatumã magmatism in the Amazon Craton and also by plotting compositions of rocks from type-localities and the world averages. The proposed limits between rock families agree with plotted rock compositions. The KCK diagrams are an alternative to existing diagrams to classify volcanic rocks.

The genesis of actively growing siliceous stromatolites: Evidence from Lake Specchio di Venere, Pantelleria Island, Italy

This study documents the attributes of siliceous stromatolites growing in the Lake Specchio di Venere, on the volcanic island of Pantelleria, Italy, in a setting characterized by very shallow cold waters and pools and by scattered hydrothermal activity, which exhales mainly CO 2, at emission point temperatures of 34 to 58 °C. The saturation indexes indicate that the lake waters are saturated with respect to tridymite, cristobalite, chalcedony and quartz, and slightly undersaturated with respect to amorphous silica. Common roughly laminated and poorly lithified stromatolites show scanning electron microscope (SEM) evidence for silicified microbial mat structures, including biofilms, filamentous and coccoid cells, and extracellular polymeric substances (EPS). The screening of bacterial 16S rRNA libraries indicates that autotrophic and heterotrophic bacterial communities colonize surface and core levels of the stromatolites. Locally the stromatolites show a granular non-porous fabric, where filaments and silica sheets are not apparent. Inhomogeneity in the stromatolite fabric corresponds with varying DNA content and different structures of the colonizing bacterial communities, non-porous stromatolite levels being microbially colonized to a lesser extent. Based on Fourier Transform (FT)–Raman and FT-infrared investigations, the laminated stromatolite contains early diagenetic tridymite in addition to amorphous silica (opal-A), whereas the non-porous stromatolite shows an essentially amorphous character. The laminated stromatolite is thought to form at relatively low rates of silica precipitation and with a possible microbial mediation in terms of microbial cells and their EPS accelerating the onset of amorphous silica nucleation. It is suggested that high porosity probably favoured a consistent flux of silica-rich fluids that triggered the opal-A to tridymite transformation, thus strengthening the preservation of biosignals. Non-porous stromatolite growth could reflect temporal or localised changes in environmental conditions that caused variations in the degree of silica saturation in the lake waters inducing abiotic silica accumulation. Accelerated opal-A deposition could have obscured primary filamentous fabrics and limited the flux of pore fluid needed for sustaining the process of silica maturation.

Experimental study of clinopyroxenite partial melting and the origin of ultra-calcic melt inclusions

Ultra-calcic melt inclusions (UCMI: CaO>13.5 wt% and/or CaO/Al2O3>0.9) are magnesian and near-primary liquids trapped in volcanic phenocrysts from mid-ocean ridges, arcs, back-arcs, and ocean islands. UCMI can be subdivided into two classes based on tectonic association and degree of silica saturation: those from arcs are nepheline normative and those from all other localities (silicic UCMI) are hypersthene normative. Silicic UCMI share a number of common features, including primitive host minerals, low alkali contents, and variable ratios of K2O/TiO2 ranging to high values. Their compositions are not easily derived by partial melting of mantle lherzolite. Accordingly, we have performed a series of partial melting experiments on three clinopyroxenite compositions at 1.0 to 2.0 GPa to investigate the role of partial melting of clinopyroxene-rich lithologies in silicic UCMI genesis. Estimated solidus temperatures for all three compositions are similar to those of normal peridotites, but 1.0 GPa isobaric melt productivities are higher for clinopyroxenite than for peridotite. High degree partial melts of the clinopyroxenites are ultra-calcic and have similarities to silicic UCMI, but the experiments produce ultra-calcic liquids only at melt fractions greater than 30% and temperatures higher than 1,350 °C at 1.0 GPa. Such temperatures are higher than those likely to be prevailing beneath normal mid-ocean ridges, which suggests that some or all silicic UCMI may originate by a process other than simple partial melting of clinopyroxene-rich lithologies. We consider a possible role for partial melting of depleted harzburgite in the genesis of silicic UCMI.

Neodymium isotopes in basalts of the southwest basin and range and lithospheric thinning during continental extension

The chemical compositions, ages, and Nd and Sr isotopic compositions of Miocene basaltic volcanic rocks from the southwestern Basin and Range Province were measured. The data presented, and other data obtained from the literature show that temporal variations in the Nd and Sr isotopic compositions are typically correlated with the timing of local crustal extension. The isotopic variations cannot be accounted for by crustal contamination, and are best interpreted as indicating the involvement of subcontinental lithospheric mantle as a major source of basalt magma before and during the early stages of extension, giving way to asthenospheric sources as extension proceeds. It is inferred that the basalt isotopic and age data provide information about the amount and timing of changes of the overall lithosphere thickness during extension. If this inference is valid, the inferred lithosphere thickness vs. time histories can be compared to the amount and timing of upper crustal extension to evaluate competing models for lithospheric strain during continental extension. The depth of the lithosphere–asthenospshere boundary is inferred primarily from the isotopic data, but can be refined by accounting for differences in the depth of origin of the basalts as suggested by differences in the degree of silica saturation (nepheline normative vs. hypersthene normative). The transition between silica saturated and undersaturated magmas is dependent on many factors, but for the conditions likely to apply to the Basin and Range Miocene volcanism, it corresponds to a depth of origin of 50±10 km. The data presented show strong support for the inferred mantle geochemical stratigraphy, and the inferred modern depths to the lithosphere–asthenosphere boundary are consistent with available seismic refraction and structural data. In the highly extended regions near the eastern and western margins of the southwestern Basin and Range, temporal changes in basalt isotopic compositions indicate that the subjacent lithospheric mantle thinned contemporaneously with the upper crust, as would be predicted for pure shear. These areas are also characterized by relatively high (although still low) basalt extrusion rates. In contrast, upper crustal extension in the Death Valley area was not accompanied by significant thinning of the subjacent lithospheric mantle, which implies that the location of lithospheric thinning was offset from the location of upper crustal extension as in a simple shear model. The total amount of lithospheric thinning is generally less than would be expected from the magnitude of upper crustal extension and is more evenly distributed; this suggests that there is also a component of distributed shear.

U-Series Isotope Studies of Atlantic OIB: Links Between Inferred Rates and Degrees of Partial Melting, and the Influence of (Lithospheric) Source Composition

Partial melting processes beneath ocean islands are arguably less well understood than those beneath mid-ocean ridges (MOR), not least because the source composition variations are demonstrably greater, and the thickness of the lithospheric lid influences the depth at which melting ceases. For MORB the average degrees and depths of melting increase with increasing temperature and decreasing ridge depth (Klein and Langmuir, 1987). The degree of 23~ disequilibrium also appears to be greater in MORB from shallower ridge depths, and this has been attributed to more of the melt zone being within the garnet stability field in areas of higher mantle temperatures (Bourdon et al, 1996). By contrast, in OIB, the degree of 23~ disequilibrium tends to be less, and the melt fraction lower, in those OIB associated with high buoyancy flux, i.e. high mantle potential temperature (Chabaux and All~gre, 1994; Sims et al., 1995). Thus, the controls on 23~ disequilibrium appear to be different in MORB and OIB. In intraplate settings partial melting should start deeper in regions of high buoyancy (i.e. high Tp), and the height of the melt zone is controlled by the thickness of the overlying lithosphere, which in turn limits the total amounts of melting in any area. However, in general links between the inferred rates and degrees of partial melting are not well established, and this contribution reviews the results of a number of detailed studies on Atlantic Ocean OIB. U-Th isotope analyses of young volcanic rocks from the Azores (Turner et al., 1997), the Canary Islands, and the Cape Verde Islands, indicate that the majority are out of 23~ equilibrium with (23~ in the range 1.08?1.35. All are associated with relatively low buoyancy mantle plumes, and in contrast to the results from Hawaii (Sims et al., 1995) there is no simple link between the degree of 23~ disequilibria and degree of silica saturation (Fig. 1), although the range in (23~ is greater in the silica undersaturated rocks. The Hawaiian data suggests that there may be a negative correlation between the degree of partial melting and the amount of U-Th disequilibria (Sims et al., 1995). However, there are no obvious negative trends within OIB from the central Atlantic region, despite variations in lithospheric thickness from 20 to 125 km and lithospheric age from 7 to 175 Ma. In detail, the inferred degrees of melting on Lanzarote, for example, range from 1 to 4% and yet there is no correlated change in the degree of 23~ disequilibria. Overall, the Canary Islands, Cape Verdes and Azores plumes have similar buoyancy fluxes (1.0, 1.6 and 1.1 Mg s -1 respectively) and exhibit large 23~ isotope disequilibria consistent with the argument that low buoyancy plumes result in magmas with significant U-Th disequilibria. These plumes also appear to have similar low melt production rates of 0.02, 0.03 and 0.01 km 3 y-1 respectively, despite the location of the Azores close to the axis of the mid-Atlantic ridge. Plumes may rise beneath lithosphere of any age, and so the rates of melt production, and hence 23~ disequilibria, appear largely independent of lithosphere thickness. In contrast, there is a broad negative correlation between lithosphere thickness and the integrated degrees of melting (Haase, 1996). It follows that there should be no link between the 23~ disequilibria, and hence the inferred rates of melt generation, and the integrated degrees of melting, and that is what is can be observed in the Atlantic OIB

Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe, Tanzania

This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle.

Volcanism in an arc-transform transition zone: the stratigraphy of the St. Clare Creek volcanic field, Wrangell volcanic belt, Yukon, Canada

The St. Clare Creek volcanic field in the southwestern Yukon overlies a tectonic transition in the Wrangell volcanic belt between subduction to the northwest in Alaska and transform faulting along the Duke River fault in the southeast. Two large polygenetic volcanic centres dominated the Miocene landscape of the St. Clare Creek field: the 18–16 Ma Wolverine centre and the 16–10 Ma Klutlan centre. The Wolverine centre evolved from a small alkaline shield volcano at 18 Ma, from which alkaline basalts, hawaiites and mugearites erupted, to a larger composite volcano between 18 and 16 Ma composed of transitional basalt, basaltic trachyandesite, trachyte and rhyolite lavas, and pyroclastic rocks. The youngest Wolverine lavas are calc-alkaline basaltic andesites, andesites, and hybrid lavas (transitional–calc-alkaline). This temporal progression from alkaline through transitional to calc-alkaline volcanism is accompanied by a systematic increase in the degree of silica saturation and decrease in Fe/Si, Nb/Y, and P/Y ratios. Klutlan lavas have lower Nb/Y and P/Y ratios and are characterized by an opposite eruption sequence. The earliest Klutlan lavas (16–13 Ma) erupted from a composite volcano and include calc-alkaline andesite, rhyolite, and hybrid trachyandesite lavas, followed by transitional basaltic trachyandesites, trachyandesites, trachytes, and rhyolites. Klutlan vulcanism between 13 and 11 Ma was dominated by basaltic fissure eruptions on the southern flanks of the earlier centre and include early mildly alkaline basalts followed by more voluminous transitional basalts. Volcanism reverted to a more central type of activity between 11 and 10 Ma and includes calc-alkaline dacite lava followed by transitional basaltic trachyandesite, trachyandesite, and trachyte lavas.The volcanic stratigraphy of the St. Clare Creek field and 40Ar/39Ar geochronological data provide the basis for understanding the origin of St. Clare magmas in a regional tectonic context. Early Wolverine alkaline volcanism largely reflects leaky transform faulting, whereas subsequent transitional and calc-alkaline lavas record the onset of subduction-related volcanism at the margins of the then active Wrangell arc. The opposite eruption sequence at the Klutlan centre records the demise of subduction-related volcanism between 16 and 13 Ma, due to northwestward migration of the subducted plate. Upwelling of asthenospheric mantle in place of the subducted slab led to the generation of transitional basalts between 13 and 11 Ma, which resulted in more evolved lavas between 11 and 10 Ma.

Crystallite Size of Diagenetic Chert in Carbonate Rocks--Probable Indicator of Prelithification Porosity: ABSTRACT

Silicification and dolomitization in seven constituent lithofacies of the Meramecian carbonate sequence in central eastern Missouri are genetically related and are facies dependent. In the arenite facies with variable lime-mud matrix (0-30%), both were probably caused by phreatic freshwater and seawater mixing. Silicification is characterized by microcrystalline quartz-replaced (MCQ-replaced) bioclasts and pore-occluding fibrous chalcedony (CHY); the latter is common as micronodules. In these facies-specific, silica-replaced bioclasts, MCQ has abnormally large crystallite size in parts of the profile characterized by blocky low-matrix cement. In contrast, the matrix-rich facies, characterized by extensively dolomitized matrix, have smaller crystalline size in the MCQ-replaced bioclasts. The conventional model of thermal annealing does not adequately explain the observed, intermittently anomalous, large crystallite size of the diagenetic chert in the Meramecian arenite facies, considering the regional thermal and burial history of the rocks. Also, the chert crystallite size inversely correlates with the matrix mud content in the facies. Therefore, the authors propose that the observed variation in the size of the chert crystallites reflects the degree of silica saturation in the pore water. The degree of silica saturation is a function of permeability controlled by the effective prelithification porosity in these carbonates. Preliminary data suggest that chertmore » crystallite size can be used to map the distribution of prelithification porosity in carbonate rocks that were substantially modified in texture during diagenesis. Oxygen isotopic composition of these cherts may help delineate the fluid-migration paths in these rocks. This information is useful in exploring for energy resources.« less

Aeromagnetics, geology and ore environments, I. Magnetite in igneous, sedimentary and metamorphic rocks: An overview

Aeromagnetic (AM) surveys continue to play a prominent role in mineral exploration. The function of AM surveys is to map the distribution of magnetic minerals in the bedrock. Rock magnetic properties originate from the opaque minerals, of which the most abundant is magnetite. Interpretation is the drawing of inferences about the geology and ore potential of a given region from AM survey data; but it is hampered by a lack of clearly stated principles relating magnetic minerals to geology and to ore environments. This article, the first of a series, is a culling of materials from the geological literature which relate to the production and annihilation of magnetite in various geological environments. It begins with a description of the FeTi oxide minerals (the primary opaque oxide minerals in all rocks) and the factors which determine their induced and remanent magnetizations. The role of deuteric oxidation is of particular importance: rocks which have had different cooling histories will also have different magnetic properties, even if they have similar chemical compositions. The magnetic properties of rocks are determined by the partitioning of iron between the oxides of iron and titanium on the one hand, and (Fe, Mg) silicates on the other. The partitioning is controlled primarily by oxygen fugacity and temperature, and only secondarily by bulk chemistry. Biotite plays a special role in determining the amount of magnetite that can form during magmatic emplacement and also during subsequent metamorphism. Primary basaltic magmas differentiate along two different trends, depending upon whether oxygen levels are maintained externally, or whether they are allowed to drop. Magmas which follow the first trend produce primary Fe;Ti oxides and silica-rich end members. These give the calc-alkaline series, which are relatively deficient in magnetite. Thr second differentiation trend leads to primary Fe-silicates and FeO-rich end members which are silica-deficient. Rocks which follow this trend are relatively magnetic. The two trends correspond with quite different tectonic environments. The highest magnetic susceptibility values in either series should be found in rocks of intermediate composition approximating that of dacite or diorite, unless there has been alteration. During progressive regional metamorphism secondary magnetite can be produced at all stages, but mostly it develops under rather low-grade conditions by the conversion of hematite, or under high-grade conditions from the breakdown of hydrous (Fe, Mg) silicates such as biotite and hornblende into simpler orthosilicates. The magnetite-producing capacity of a rock during metamorphism is determined by attributes which are largely inherited from the premetamorphic state, of which the most important are total iron content, oxidation state, and degree of silica saturation. Other influencing factors include magnesium, aluminum and carbon content. Magnetite is destroyed during granitization and by low-temperature alteration. In interpreting AM survey data, close attention should be paid to all of the local factors that might have an influence upon magnetite, including mineralogy, chemical composition and metamorphic grade. Magnetite in metamorphic terrains often reflects the premetamorphic lithostratigraphy; hence magnetic interpretation maps should not be constrained to resemble maps of the existing geology.

A classification of volcanic and plutonic rocks using R1R2-diagram and major-element analyses — Its relationships with current nomenclature

The R1R2 chemical variation diagram, which includes all of the major cations, a mineralogical network, the degree of silica saturation, and the combined changes in Fe(Fe+Mg) and (Ab+Or)An ratios in igneous rocks, is proposed where:X or R1 = 4Si − 11(Na+K) − 2(Fe+Ti) and Y or R2 = 6Ca+ 2Mg+ AlR1 and R2 are parameters calculated either from chemical analyses (oxide percentages converted to millications) or modal data.Statistical distributions of “current rock names” are given for three large geochemical files: CLAIR, PETROS and RKFNSYS. A single classification grid, applicable to both volcanic and plutonic rocks, is proposed which is consistent with basic petrology. Successive belts spread from two fields, “peridotic” and “granitic or rhyolitic”, respectively, with a poorly-defined intersection in the region of intermediate rock compositions. The radial subdivisions crossing the successive belts are consistent with igneous differentiation suites. On the grid, isovalue lines of silica contents clearly cut across the boundaries between successive natural belts, except for the calc-alkaline series. Classifications based on silica contents are thus inconsistent with the current use of nomenclature. The grid has been applied to redefine the mean chemical compositions of 46 principal igneous rock types.

Geochemical investigations of basalts and associated rocks from the ocean floor and their implications

Variations in trace element contents and inter-element ratios of deep-sea basalts are much more marked than variations in major element contents. This paper explores possible reasons for the variations which have been discovered. Inadequacy of sampling techniques may be responsible for some reported differences, but variations due to this cause are unlikely to approach the magnitude of reported variations. Some variation in samples from restricted areas of the ocean floor can be correlated with variation in the degree of silica saturation of the basalts. Submarine alteration of lavas by reaction with sea water is another possible cause of variation. Studies of metamorphosed deep-sea basalts suggest that very low-grade metamorphism may cause some, though slight, elemental migration. Studies on ultrabasic rocks show variations in trace element contents which, to some degree, appear to complement the variations encountered in basalts, suggesting that the extent of partial melting in the mantle during basaltic genesis influences the trace element contents of the products of melting. However, when such possible explanations have been considered, there remain variations in trace element contents of otherwise comparable basalts from different parts of the ocean floor, which appear to represent real variations in the trace element contents of the erupted basaltic magmas. In view of the difficulty of explaining such differences by contamination of magmas on their way to the surface, it is suggested that variations exist in the trace element contents of mantle material at the levels of basaltic genesis. Geochemical provinces exist in oceanic areas just as they do in continental regions.