Thomas Bullen

Bio: Thomas Bullen is an academic researcher from University of California, Santa Cruz. The author has contributed to research in topics: Oceanic crust & Volcanic rock. The author has an hindex of 1, co-authored 1 publications receiving 1759 citations.

Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas

Abstract: The composition of chromian spinel in alpine-type peridotites has a large reciprocal range of Cr and Al, with increasing Cr# (Cr/(Cr+Al)) reflecting increasing degrees of partial melting in the mantle. Using spinel compositions, alpine-type peridotites can be divided into three groups. Type I peridotites and associated volcanic rocks contain spinels with Cr# 0.60, and Type II peridotites and volcanics are a transitional group and contain spinels spanning the full range of spinel compositions in Type I and Type II peridotites. Spinels in abyssal peridotites lie entirely within the Type I spinel field, making ophiolites with Type I alpine-type peridotites the most likely candidates for sections of ocean lithosphere formed at a midocean ridge. The only modern analogs for Type III peridotites and associated volcanic rocks are found in arc-related volcanic and intrusive rocks, continental intrusive assemblages, and oceanic plateau basalts. We infer a sub-volcanic arc petrogenesis for most Type III alpine-type peridotites. Type II alpine-type peridotites apparently reflect composite origins, such as the formation of an island-arc on ocean crust, resulting in large variations in the degree and provenance of melting over relatively short distances. The essential difference between Type I and Type III peridotites appears to be the presence or absence of diopside in the residue at the end of melting.

The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

Abstract: Compositional fields for spinels from a wide variety of mafic– a wide range of conditions from mafic and ultramafic ultramafic igneous rock types and tectonic environments have been magmas and, in the case of chromites, are often among determined from a global database of over 26 000 analyses. the first phases to crystallize. They also exhibit a wide These fields are defined using contoured data density plots based range of solid solution, the thermodynamics of which has on the spinel prism, and plots of TiO2 vs ferric iron, for mantle been studied extensively (O’Neill & Wall, 1987; Mattioli xenoliths, ophiolitic rocks, continental layered intrusions, alkalic & Wood, 1988; Wood, 1990; Sack & Ghiorso, 1991; and lamprophyric rocks, tholeiitic basalts, Alaskan ultramafic Poustovetov, 2000). They are relatively refractory and complexes and komatiites. Several trends appear regularly in the resistant to alteration, particularly compared with other various environments: a trend of widely variable Cr/(Cr + Al) high-temperature igneous minerals such as olivine. They at low Fe/(Mg + Fe) (the Cr–Al trend); increasing Fe, occur in a high proportion of terrestrial mafic and ultraFe/(Mg + Fe) and TiO2 at constant Cr/(Cr + Al) mafic rocks, and a very large volume of microprobe data (Fe–Ti trend); a trend found primarily in kimberlites, similar is available on their compositions. to Fe–Ti but at constant Fe/(Mg + Fe); and an unusual Publications on spinels (particularly chromites) have trend of increasing Al found only in layered intrusions. The routinely used compositional fields based on the spinel Cr–Al and Fe–Ti trends are both found to varying degrees in prism to compare populations of analyses. Some of tholeiitic basalts. The Cr–Al trend is prevalent in rocks that these fields have remarkable longevity, particularly those have equilibrated over a range of pressures, whereas the Fe–Ti defined for layered intrusions and Alpine peridotites by trend is dominantly due to low-pressure fractionation. The most Irvine (1967, 1977). Many new data have become availCr-rich chromites found in nature occur in boninites, diamondable in the last 20 years, however, and it has become bearing kimberlites, some komatiites and ophiolitic chromitites. apparent that a new compilation of data is necessary to Exceptionally reduced chromites are found in some komatiites reflect this. The prime purpose of this paper is to analyse and in ophiolitic chromitites. Detrital chromites from the a global database of about 26 000 spinel analyses from Witwatersrand conglomerates are of komatiitic provenance. terrestrial igneous and metamorphosed igneous rocks, and to extract from this database the characteristic compositional fields of spinels from the wide variety of magma types and tectonic environments in which they occur.

Factors Controlling Chemistry of Magmatic Spinel: an Empirical Study of Associated Olivine, Cr-spinel and Melt Inclusions from Primitive Rocks

Abstract: Compositions of ~2500 spinel-olivine pairs and 400 melt inclusion-spinel pairs have been analysed from 36 igneous suites from oceanic, arc and intraplate tectonic settings. Our data confirm that Cr-spinel mg-number is largely controlled by melt composition, but also influenced by octahedral site substitutions, and rate of cooling. Lavas quenched in submarine environments tend to have higher mg-number at a given cr-number than slowly cooled subaerial lavas and peridotites. Unlike mg-number, Cr-spinel Al2O3 and TiO2 contents show good correlations with melt composition, with only limited post-entrapment modifications. Out data suggest that increased activity of Al2O3 decreases the partitioning of TiO2 into spinels. The Al2O3 content of Cr-spinel is a useful guide to the degree of partial melting of mantle peridotites; however, this same relationship is obscured in volcanic rocks. Al2O3 contents of volcanic Cr-spinels are mostly determined by melt composition rather than mantle source composition. The data also suggest that most spinels from residual mantle peridotites can be readily differentiated from those hosted in volcanic rocks. Mantle peridotite spinel tend to have lower TiO2 and higher Fe2+/Fe3+ ratios than spinel from volcanic rocks. The spinel compositions in our database can be subdivided on the basis of tectonic setting and mode of occurrence using an Al2O3 vs TiO2 diagram. A total of seven fields can be distinguished with varying degrees of overlap. This diagram can then be used to determine the tectonic setting of spinel from altered mafic igneous rocks such as serpentinites or meta-basalts, or detrital spinel in sandstones.

High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle

Abstract: Synthetic spinel harzburgite and lherzolite assemblages were equilibrated between 1040 and 1300° C and 0.3 to 2.7 GPa, under controlled oxygen fugacity (fO2). fO2 was buffered with conventional and open double-capsule techniques, using the Fe−FeO, WC-WO2-C, Ni−NiO, and Fe3O4−Fe2O3 buffers, and graphite, olivine, and PdAg alloys as sample containers. Experiments were carried out in a piston-cylinder apparatus under fluid-excess conditions. Within the P-T-X range of the experiments, the redox ratio Fe3+/ΣFe in spinel is a linear function of fO2 (0.02 at IW, 0.1 at WCO, 0.25 at NNO, and 0.75 at MH). It is independent of temperature at given Δlog(fO2), but decreases slightly with increasing Cr content in spinel. The Fe3+/ΣFe ratio falls with increasing pressure at given Δlog(fO2), consistent with a pressure correction based on partial molar volume data. At a specific temperature, degree of melting and bulk composition, the Cr/(Cr+Al) ratio of a spinel rises with increasing fO2. A linear least-squares fit to the experimental data gives the semi-empirical oxygen barometer in terms of divergence from the fayalite-magnetite-quartz (FMQ) buffer: $$\Delta log (f_{O_2 } )^{FMQ} = 0.27 + 2505/T - 400P/T - 6 log(X_{Fe}^{olv} ) - 3200(1 - X_{Fe}^{olv} )^2 /T + 2 log(X_{Fe^{2 + } }^{sp} ) + 4 log(X_{Fe^{3 + } }^{sp} ) + 2630(X_{Al}^{sp} )^2 /T.$$