A new LA-ICP-MS method for Ti-in-Quartz: Implications and application to HP rutile-quartz veins from the Czech Erzgebirge

Abstract:

Experimental determination of the pressure and temperature controls on Ti solubility in quartz provide a calibration of the Ti-in-quartz (TitaniQ) geothermometer applicable to geologic conditions up to ~20 kbar (Thomas et al. (2010) Contrib Mineral Petrol 160, 743-759). One of the greatest limitations to analyzing Ti in metamorphic quartz by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is the lack of a suitable matrix-matched reference material. Typically LA-ICP-MS analyses of Ti in minerals use ⁴⁹Ti as a normalizing mass because of an isobaric interference from ⁴⁸Ca, which is present in most well characterized reference glasses, on ⁴⁸Ti. The benefit of using a matrix-matched reference material to analyze Ti in quartz is the opportunity to use ⁴⁸Ti (73.8 % abundance) as a normalizing mass, which results in an order of magnitude increase in signal strength compared to the less abundant isotope ⁴⁹Ti (5.5 % abundance), thereby increasing the analytical precision. Here we characterize Ti-bearing SiO₂ glasses from Heraeus Quarzglas and natural quartz grains from the Bishop Tuff by cathodoluminescence (CL) imaging, electron probe microanalysis (EPMA), and LA-ICP-MS, in order to determine their viability as reference materials for Ti in quartz. Titanium contents in low-CL rims in the Bishop Tuff quartz grains were determined to be homogenous by EPMA (41 ± 2 µg/g Ti, 2σ), and are a potential natural reference material.

We present a new method for determining ⁴⁸Ti concentrations in quartz by LA-ICP-MS at the 1 µg/g level, relevant to quartz in HP-LT terranes. We suggest that natural quartz such as the homogeneous low-CL rims of the Bishop Tuff quartz are more suitable than NIST reference glasses as an in-house reference material for low Ti concentrations because matrix effects are limited and Ca isobaric interferences are avoided, thus allowing for the use of ⁴⁸Ti as a normalizing mass. Titanium concentration from 33 analyses of low-temperature quartz from the Czech Erzgebirge is 0.9 ± 0.2 µg/g (2σ) using ⁴⁸Ti as a normalizing mass and the Bishop Tuff quartz rims as a reference material. The 2σ average analytical uncertainty for analyses of ⁴⁸Ti is 8 % for 50 µm spots and 7 % for 100 µm spots, which offers much greater precision than the 35 % uncertainty (2σ) incurred from using ⁴⁹Ti as a normalizing mass.