Admissions > PhD by research > Research Projects > Developing elemental and isotopic thermometers for primitive magmas

Developing elemental and isotopic thermometers

for primitive magmas

Supervisors: Mike Walter and Tim Elliott

Constraining the temperature of primitive magmas is a major goal of petrology and is key to understanding the thermal structure of the Earth’s interior. A direct, compositional thermometer that can be used on early-crystallising, magmatic minerals is highly desirable for this task. A long established thermometer compares the Fe/Mg compositions of olivines with the Fe/Mg of their host magmas (1) and has received renewed recent interest (2). Unfortunately, primitive olivines are often xenocrystic and so the reliability of this approach can be questionable. However, primitive olivines frequently contain chromian spinels and so a temperature-sensitive element or isotopic exchange between these two phases would provide the sought after thermometer. The most obvious choice, Fe/Mg exchange, is indeed temperature sensitive e.g. (3) but undergoes rapid, diffusive re-equilibration during cooling e.g. (4). Thus a system that involves less rapidly diffusing species is required. Wan et al (5) recently presented some experiments that calibrated Al exchange between olivines and spinels. This thermometer shows considerable promise and is analytically accessible using laser abalation mass-spectrometery measurements of Al in olivine. We wish to explore this result in greater detail, using a wider range of elements in experiments more directly suited to the high Cr spinels of primitive igneous liquids (e.g. higher Fe3+ contents). Moreover, isotopic systems also show some potential as some elements (e.g. Mg) show contrasting co-ordination between spinel and olivine which can lead to significant isotopic fractionations(6). The later process is also temperature sensitive and with high precision analyses possible by modern multi-collector inductively coupled plasma mass spectrometry, e.g. (7), it may be possible to develop a sufficiently sensitive isotopic thermometer.

Figure: Image of primitive Hawaiian olvine
(~1mm) with series of spinel inclusions towards rim

This project will provide training in both experimental petrology (producing charges with co-existing olivine and chromium spinel over a range of temperature and oxygen fugacity conditions) and analytical geochemistry (laser ablation and analyses and MC-ICPMS isotopic measurements). The first part of the project will be to hone the success of the work of Wan et al (5) for a more specific set of magmatic conditions. Subsequent work will explore new elemental and isotopic systems. Finally the experimental calibration will be applied to suites of oceanic basalts already available at Bristol and potentially additional suites to be collected as part of the project.






References

  1. P. L. Roeder, R. F. Emslie, Olivine-liquid equilibrium. Contrib. Mineral. Petrol. 29, 275 (1970, 1970).
  2. K. D. Putirka, M. Perfit, F. J. Ryerson, M. G. Jackson, Ambient and excess mantle temperatures, olivine thermometry, and active vs. passive upwelling. Chem. Geol. 241, 177 (Jul 15, 2007).
  3. H. S. O'Neill, V. J. Wall, The olivine ortho-pyroxene spinel oxygen geobarometer, the nickel precipitation curve, and the oxygen fugacity of the Earth's upper mantle. J. Petrol. 28, 1169 (Dec, 1987).
  4. H. P. Liermann, J. Ganguly, Diffusion kinetics of Fe2+ and Mg in aluminous spinel: Experimental determination and applications. Geochim. Cosmochim. Acta 66, 2903 (Aug, 2002).
  5. Z. Wan, L. A. Coogan, D. Canil, Experimental calibration of aluminum partitioning between olivine and spinel as a geothermometer. Am. Miner. 93, 1142 (Jul, 2008).
  6. E. D. Young, E. Tonui, C. E. Manning, E. Schauble, C. A. Macris, Spinel-olivine magnesium isotope thermometry in the mantle and implications for the Mg isotopic composition of Earth. Earth Planet. Sci. Lett. 288, 524 (Nov 15, 2009).
  7. P. A. E. Pogge von Strandmann et al., Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths. Feochim. Cosmochim. Acta 75, 5247 (2011).

 

Last updated: 24/11/11