HOW LOW CAN YOU GO? DETECTING TRACE ELEMENTS IN NATURAL QUARTZ CRYSTALS BY SECONDARY ION MASS SPECTROMETRY (SIMS)

Measuring diverse trace elements in quartz may be useful for determining temperature of formation, provenance, and indicate relative activities of other components. LA-ICP-MS has been used in many recent studies of trace elements in quartz (e.g., [1]). Here we explore using SIMS for these measurements [e.g., 2]. SIMS is a much slower analysis technique, but may be less prone to consuming small, unwanted inclusions due to its small sampled volume.

Samples from the Lower Bandelier Tuff and the Huckleberry Ridge Tuff were analyzed using a Cameca ims 6f at Arizona State University. Using an O^- primary beam, positive secondary ions, at a lateral resolution of ~30 um, we contrasted high mass resolution (HMR) techniques with conventional energy filtering (CEF) for a wide variety of elements.

Our results show that the simple technique of CEF gives the same results as HMR for only a limited range of elements (Li, B, Al, Ti, Fe). It appears that signals from rare, high energy molecular ions can drown out the (commonly sub-ppm) signal coming from the trace elemental ion. And some interferences are from doubly-charged ions that are not removed by CEF. Using HMR allows reliable determination of Be, Mg, P, Mn, Ge, and perhaps Zr in addition to the five elements mentioned above. Na, K, and Ca are detectable but continually decreased during these analyses.

Elements we tested that are either undetectable or very close to this threshold in high-T quartz include Sc, V, Cr, Ga, Rb, Sr, Y, Nb, Mo, Cs, and Ba. For these elements the issues for successful detection are a combination of low abundance and/or unresolvable interfering species (and Cs has a background signal from use of a Cs primary ion source).

Besides the common application of the Ti-in-quartz thermometer, collecting such information on quartz crystals can be combined with analyses of trapped melt inclusions to derive partition coefficients. The Mg/(Mg+Fe) value for quartz is accessible as an indicator of fractionation. Direct measurements for H in quartz are also possible, but require special sample preparation (epoxy-free). Because of the small amounts of trace elements, a precise (but not necessarily accurate) H content can be determined using an assumption of charge balance (H⁺ = Al³⁺ + Fe³⁺ - Li⁺ - Na⁺). [1] Müller, A. et al. Econ Geol 110 (2015) 1737. [2] Lehmann, K. et al., Min Mag 73 (2009) 633.