New SIMS reference materials for measuring water in mantle minerals — American Mineralogist 2017

 

One-sentence summary:

This paper describes a set of standards and methods for making measurements of very small amounts of water in mantle minerals.

At a glance:

In order to understand and quantify how water affects mineral properties, we have to be able to measure the very small amounts of water locked up inside the structures of mantle minerals. That's where this study comes in - it's all about a method for measuring these small amounts of water using secondary ion mass spectrometry, or SIMS. We also look a bit at how water, fluorine, and phosphorus divide between orthopyroxene and clinopyroxene.

Abstract:

Trace amounts of water in the nominally anhydrous minerals of the upper mantle can dramatically affect their thermodynamics and rheological properties. Secondary ion mass spectrometry (SIMS) has become a mainstream technique for quantifying small amounts of water in these minerals, but depends on standards with known concentrations of water. The current standards in use for mantle minerals are well-characterized (Hauri et al. 2002; Koga et al. 2003; Aubaud et al. 2007; Mosenfelder and Rossman 2013a, 2013b), but a lack of extra material has limited the spread of this technique to other laboratories.

We present new SIMS measurements on natural mantle xenolith pyroxenes that re suitable for use as calibration reference materials. They are calibrated off of the pyroxene standards currently in use at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington (Koga et al. 2003; Aubaud et al. 2007). They have homogeneous water contents, defined as a standard deviation of <10% data-preserve-html-node="true" for analyses across multiple grains. Reference materials for H2O cover ranges from 52 to 338 ppm and from 9 to 559 ppm in orthopyroxene and clinopyroxene, respectively, covering most of the observed range of mantle water contents. The samples are evenly distributed over those ranges. The orthopyroxene reference materials can also be used to measure water in olivine based on previous observations that these two minerals have similar calibration slopes.

The new pyroxene reference materials can also be used to calibrate fluorine and phosphorus at low concentrations. We found that fluorine in particular was homogeneous in both orthopyroxene and clinopyroxene, with concentrations of 3 to 50 ppm in orthopyroxene and 0.5 to 118 in clinopyroxene. Phosphorus ranges from below detection up to 19 ppm in orthopyroxene and up to 72 ppm in clinopyroxene, but was more heterogeneous within some samples. Most of the reference materials have concentrations at the lower end of the ranges for fluorine and phosphorus in this study, with only a few samples showing higher concentrations.

 

Links: Article - American Mineralogist

Kumamoto, K.M., J.M. Warren, and E.H. Hauri, 2017, New SIMS reference materials for measuring water in mantle minerals, American Mineralogist, vol. 102, pp. 537-547.

 
Katie Kumamoto