Jay B. Thomas

Determination of zircon/melt trace element partition coefficients from SIMS analysis of melt inclusions in zircon

Partition coefficients ( zircon/melt DM) for rare earth elements (REE) (La, Ce, Nd, Sm, Dy, Er and Yb) and other trace elements (Ba, Rb, B, Sr, Ti, Y and Nb) between zircon and melt have been calculated from secondary ion mass spectrometric (SIMS) analyses of zircon/melt inclusion pairs. The melt inclusion-mineral (MIM) technique shows that DREE increase in compatibility with increasing atomic number, similar to results of previous studies. However, DREE determined using the MIM technique are, in general, lower than previously reported values. Calculated DREE indicate that light REE with atomic numbers less than Sm are incompatible in zircon and become more incompatible with decreasing atomic number. This behavior is in contrast to most previously published results which indicate D 1 and define a flat partitioning pattern for elements from La through Sm. The partition coefficients for the heavy REE determined using the MIM technique are lower than previously published results by factors of 15 to 20 but follow a similar trend. These differences are thought to reflect the effects of mineral and/or glass contaminants in samples from earlier studies which employed bulk analysis techniques. DREE determined using the MIM technique agree well with values predicted using the equations of Brice (1975), which are based on the size and elasticity of crystallographic sites. The presence of Ce 4 in the melt results in elevated DCe compared to neighboring REE due to the similar valence and size of Ce 4 and Zr 4 . Predicted zircon/melt D values for Ce 4 and Ce 3 indicate that the Ce 4 /Ce 3 ratios of the melt ranged from about 10 3 to 10 2 . Partition coefficients for other trace elements determined in this study increase in compatibility in the order Ba Rb B Sr Ti Y Nb, with Ba, Rb, B and Sr showing incompatible behavior (DM 1.0), and Ti, Y and Nb showing compatible behavior (DM 1.0). The effect of partition coefficients on melt evolution during petrogenetic modeling was examined using partition coefficients determined in this study and compared to trends obtained using published partition coefficients. The lower D REE determined in this study result in smaller REE bulk distribution coefficients, for a given mineral assemblage, compared to those calculated using previously reported values. As an example, fractional crystallization of an assemblage composed of 35% hornblende, 64.5% plagioclase and 0.5% zircon produces a melt that becomes increasingly more enriched in Yb using the DYb from this study. Using DYb from Fujimaki (1986) results in a melt that becomes progressively depleted in Yb during crystallization. Copyright

Melt inclusions in zircon

Silicate melt inclusions (MI) are small samples of melt that are trapped during crystal growth at magmatic pressures and temperatures. The MI represent a sample of the melt that was isolated from the bulk melt during host crystallization. Thus, MI preserve the composition of the melt that was present during crystal growth and record the P-T growth conditions. As such, MI provide a valuable tool for constraining the magmatic history of igneous systems. Melt inclusions may be composed of a single-phase glass or they may contain multiple phases (vapor bubbles ± crystals) that nucleated from the melt within the inclusion during cooling, or were produced by devitrification of the glass following trapping. Heating and homogenization techniques applied to multiphase MI produce a glass suitable for microanalysis, and may also provide information regarding the temperature of MI and crystal formation (Roedder 1984). Many workers have used MI in major rock-forming minerals to constrain igneous processes, such as crystal fractionation and magma degassing (e.g., Anderson et al. 1989, Hervig and Dunbar 1992, Sobolev and Shimizu 1993, Sobolev and Danyushevsky 1994, Lu 1991), but there have been far fewer studies of MI in accessory minerals (e.g., Chupin et al. 1998, Spandler et al. 2000, Sokolov 2002, Thomas et al. 2002). Melt inclusions in zircons of igneous origin are common, and have been identified in a wide range of igneous rock types, including quartz diorite and rhyolite, and in detrital zircon grains from heavy mineral sands (Fig. 1⇓). Figure 1. Transmitted light photographs of crystalline and glass melt inclusions (MI) in zircons. The zircons commonly contain solid inclusions of apatite (labeled “ap”), feldspar (labeled “fsp”) and less commonly, oxide minerals. The zircons in the photographs are from: the 56 Ma quartz dioritic Quottoon Igneous Complex in …

Structure and Properties of Silica Glass Densified in Cold Compression and Hot Compression.

Silica glass has been shown in numerous studies to possess significant capacity for permanent densification under pressure at different temperatures to form high density amorphous (HDA) silica. However, it is unknown to what extent the processes leading to irreversible densification of silica glass in cold-compression at room temperature and in hot-compression (e.g., near glass transition temperature) are common in nature. In this work, a hot-compression technique was used to quench silica glass from high temperature (1100 °C) and high pressure (up to 8 GPa) conditions, which leads to density increase of ~25% and Young’s modulus increase of ~71% relative to that of pristine silica glass at ambient conditions. Our experiments and molecular dynamics (MD) simulations provide solid evidences that the intermediate-range order of the hot-compressed HDA silica is distinct from that of the counterpart cold-compressed at room temperature. This explains the much higher thermal and mechanical stability of the former than the latter upon heating and compression as revealed in our in-situ Brillouin light scattering (BLS) experiments. Our studies demonstrate the limitation of the resulting density as a structural indicator of polyamorphism, and point out the importance of temperature during compression in order to fundamentally understand HDA silica.

A study of cathodoluminescence and trace element compositional zoning in natural quartz from volcanic rocks: Mapping titanium content in quartz

This article concerns application of cathodoluminescence (CL) spectroscopy to volcanic quartz and its utility in assessing variation in trace quantities of Ti within individual crystals. CL spectroscopy provides useful details of intragrain compositional variability and structure but generally limited quantitative information on element abundances. Microbeam analysis can provide such information but is time-consuming and costly, particularly if large numbers of analyses are required. To maximize advantages of both approaches, natural and synthetic quartz crystals were studied using high-resolution hyperspectral CL imaging (1.2-5.0 eV range) combined with analysis via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Spectral intensities can be deconvolved into three principal contributions (1.93, 2.19, and 2.72 eV), for which intensity of the latter peak was found to correlate directly with Ti concentration. Quantitative maps of Ti variation can be produced by calibration of the CL spectral data against relatively few analytical points. Such maps provide useful information concerning intragrain zoning or heterogeneity of Ti contents with the sensitivity of LA-ICPMS analysis and spatial resolution of electron microprobe analysis.

Application of the Ti-in-quartz thermobarometer to rutile-free systems. Reply to: a comment on: ‘TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz’ by Thomas et al.

The premise of the Wilson et al. comment is that the Ti-in-quartz solubility calibration (Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010) is fundamentally flawed. They reach this conclusion because P–T estimates using the Ti-in-quartz calibration differ from their previous interpretations for crystallization conditions of the Bishop and Oruanui rhyolites. If correct, this assertion has far-reaching implications, so a careful assessment of the Wilson et al. reasoning is warranted. Application of the Ti-in-quartz calibration as a thermobarometer in rutile-free rocks requires an estimation of TiO2 activity in the liquid (

Calibrating Ti concentrations in quartz for SIMS determinations using NIST silicate glasses and application to the TitaniQ geothermobarometer

a_{{{\text{TiO}}_{ 2} }}

Pressure-enabled phonon engineering in metals

(liquid–rutile); referenced to rutile saturation) and an independent constraint on either P or T to obtain the crystallization temperature or pressure, respectively. The foundation of Wilson et al.’s argument is that temperature estimates obtained from Fe–Ti oxide thermometry accurately reflect crystallization conditions of quartz in the two rhyolites discussed. We maintain that our experimental approach is sound, the thermodynamic basis of the Ti-in-quartz calibration is fundamentally correct, and our experimental results are robust and reproducible. We suggest that the reason Wilson et al. obtain implausible pressure estimates is because estimates for T and