Geochemical discrimination of different magma series and their differentiation products using immobile elements

Tectonic setting of basic volcanic rocks determined using trace element analyses

http://www.usu.edu/geo/shervais/Shervais%20-%20My%20Articles/Shervais1982.pdf

Ti-V plots and the petrogenesis of modern and ophiolitic lavas

This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.

The genesis of basaltic magmas

Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath ocean ridges, and contain much information concerning melt formation, melt migration and heterogeneity within the upper mantle. MORB major element chemical systematics can be divided into global and local aspects, once they have been corrected for low pressure fractionation and interlaboratory biases. Regional average compositions for ridges unaffected by hot spots ("normal" ridges) can be used to define the global correlations among normalized Na20, FeO, TiO2 and Si02 contents, CaO/Al 203 ratios, axial depth and crustal thickness. Back-arc basins show similar correlations, but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships between FeO, Na 20 and depth over distances of 1000 km. Local variations in basalt chemistry from slowand fast-spreading ridges are distinct from one another. On slow-spreading ridges, correlations among the elements cross the global vector of variability at a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the elements are distinct from both global and slow-spreading compositional vectors, and involve two components of variation. Spreading rate does not control the global correlations, but influences the standard deviations of axial depth, crustal thickness, and MgO contents of basalts. Global correlations are not found in very incompatible trace elements, even for samples far from hot spots. Moderately compatible trace elements for normal ridges, however, correlate with the major elements. Trace element systematics are significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions of the MAR are very depleted in REE, with little variability; hot spots cause large long wavelength variations in REE abundances. Normal EPR basalts are significantly more enriched than MAR basalts from normal ridges, and still more enriched basalts can erupt sporadically along the entire length of the EPR. This leads to very different histograms of distribution for the data sets as a whole, and a very different distribution of chemistry along strike for the two ridges. Despite these differences, the mean Ce/Sm ratios from the two ridges are identical. Existing methods for calculating the major element compositions of mantle melts [Klein and Langmuir, 1987; McKenzie and Bickle, 1988; Niu and Batiza, 1991] are critically examined. New quantitative methods for mantle melting and high pressure fractionation are developed to evaluate the chemical consequences of melting and fractionation processes and mantle heterogeneity. The new methods rely on new equations for partition coefficients for the major elements between mantle minerals and melts. The melting calculations can be used to investigate the chemical compositions produced by small extents of melting or high pressures of melting that cannot yet be determined experimentally. Application of the new models to the observations described above leads to two major conclusions: (1) The global correlations for normal ridges are caused by variations in mantle temperature, as suggested by Klein and Langmuir [1987] and not by mantle heterogeneity. (2) Local variations are caused by melting processes, but are not yet quantitatively accounted for. On slower spreading ridges, local variations are controlled by the melting regime in the mantle. On the EPR, local variations are predominantly controlled by ubiquitous, small scale heterogeneites. Volatile content may be an important and as yet undetermined factor in affecting the observed variations in major elements. We propose a hypothesis, similar to one proposed by Allegre et al [1984] for isotopic data, to explain the differences between the Atlantic and Pacific local trends, and the trace element systematics of the two ocean basins, as consequences of spreading rate and a different distribution of enriched components from hot spots in the two ocean basins. In the Atlantic, the hot spot influence is in discrete areas, and produces clear depth and chemical anomalies. Ridge segments far from hot spots do not contain enriched

/pdf/petrological-systematics-of-mid-ocean-ridge-basalts-3ozaymo0kf.pdf

Petrological systematics of mid-ocean ridge basalts: Constraints on melt generation beneath ocean ridges

Basalts from the Northern Part of the Rift Zone of the Mid-Atlantic Ridge

Basalts from the Northern Part of the Mid-Atlantic Ridge

The picrite-basalts of Kilauea, [Part] 1 of Contributions to the petrology of Hawaiian basalts

The chemical characters of the lava members intermediate between mugearite and trachyte, trachybasalt and trachyte, in the oceanic basalt-trachyte association are reassessed in the light of researches recently reported (Chayes 1963). The several types of assemblages that fall within the limits of minimal frequency are recognized, and two assigned specific rank, the one more sodic — benmorite, the other more potassic — tristanite. Feldspathoidal variants may be associated with these near saturated type assemblages.

Intermediate Members of the Oceanic Basalt-Trachyte Association

The compositions of pyroxene pairs in pyroxenite and gneiss from Scourie, Scotland, metamorphosed picrite basalt from Kilauea, hornfels from the Oslo district, and granulite from Ardnamurchan, Scotland, have been analyzed chemically and inferred optically. The tie-lines of these pairs have been ploted in the diagram of the system wollastonite-enstatite- ferrosilite, and the trends are compared with those found for plutonic igneous associations.

Cecil Edgar Tilley

Papers

Basalts from the Northern Part of the Rift Zone of the Mid-Atlantic Ridge

Basalts from the Northern Part of the Mid-Atlantic Ridge

The picrite-basalts of Kilauea, [Part] 1 of Contributions to the petrology of Hawaiian basalts

Intermediate Members of the Oceanic Basalt-Trachyte Association

The compositions of coexisting pyroxenes in metamorphic assemblages