In-situ oxygen isotope and trace element geothermometry of rutilated quartz from Alpine fissures

Abstract

Finely acicular rutile intergrown with host quartz (rutilated quartz) is commonly found in hydrothermal veins, including the renown cleft mineral locations of the Swiss Alps. These Alpine cleft mineralizations reportedly formed between ∼13.5 and 15.2 Ma (based on ages of rare hydrothermal monazite and titanite) at temperatures (T) of ∼150-450 °C (based on fluid inclusions and bulk quartz-mineral oxygen isotope exchange equilibria), and pressures (P) of 0.5-2.5 kbar (estimated from a geothermal gradient of 30 °C/km). The potential of rutilated quartz as a thermochronometer, however, has not been harnessed previously. Here, we present the first results of age and T determinations for rutilated quartz from six locations in the Swiss Alps with vein country rocks that cover peak-metamorphic conditions between ∼600 and <350 °C. Samples were cut and mounted in epoxy disks to expose rutile (∼30 to 1400 µm in diameter) and its host quartz. Cathodoluminescence (CL) and backscattered electron (BSE) imaging of host quartz and rutile inclusions, respectively, shows internal zonations, which are nevertheless isotopically homogeneous. Newly developed secondary ionization mass spectrometry (SIMS) oxygen isotopic analysis protocols for rutile were combined with those established for trace elements (including Zr) and U-Pb ages in rutile, and Ti abundances in the host quartz. U-Pb rutile ages average 15.1 ± 1.7 Ma (2σ), in excellent agreement with previous accessory mineral geochronometers. Pressure-independent T estimates, calibrated for low-temperature conditions, from oxygen isotope fractionation between rutile and quartz in touching pairs are 310-576 °C. Individual rutile needles vary in Zr abundances beyond analytical uncertainties, but average Zr-in-rutile inversely correlates with oxygen isotopic fractionation between quartz and rutile. Linear regression of the data yields:with x = Zr ppm and R = 0.008314 (uncertainties scaled by the square root of the mean square of weighted deviates MSWD = 11; n = 9). This relationship supports previously recognized temperaturedependent Zr uptake in rutile, although widely used Zr-in-rutile thermometer calibrations based on high-T experiments are at variance with oxygen isotope exchange temperatures. By contrast, Ti-inquartz lacks systematic relations with oxygen isotope temperatures. The discrepancy between low-T Ti-in-quartz thermometry on one side, and oxygen isotope and Zr-in-rutile thermometry on the other, suggests that Ti-in-quartz thermometry should be applied with caution for low-T (<500 °C) rocks.

Melt Impregnation of Mantle Peridotite Facilitates High‐Temperature Hydration and Mechanical Weakening: Implications for Oceanic Detachment Faults

“…The temperature dependence of the substitution of Zr for Ti in rutile was determined in several studies (Ferry & Watson, ; Shulaker et al, ; Tomkins et al, ; Watson et al, ). We found rutile in one sample (1271B‐5R‐1, 12–14 cm); its Zr contents range from 505 to 1,987 μg/g.…”

Two billion years of evolution enclosed in hydrothermal rutile: Recycling of the São Francisco Craton Crust and constraints on gold remobilisation processes

“…A C C E P T E D M A N U S C R I P T metamorphic conditions or low-grade metamorphic conditions, respectively. Hart et al (2016) demonstrate how the use of mineral inclusions can help constrain pressure, and thus assist in temperature estimates, while Shulaker et al (2015) presented a new calibration at lower temperature ranges combined with oxygen isotopes. Regardless, this has been a useful tool in complementing P-T-t constraints and more studies are needed to tackle these existing difficulties.…”

Assessment of crystallographic orientation effects on secondary ion mass spectrometry (SIMS) analysis of cassiterite

“…The documented occurrence of large crystallographic orientation effects in rutile (Li et al 2010Taylor et al 2012;Schmitt & Zack 2012;Shulaker et al 2015) but not the isostructural cassiterite (this study) indicates that a fundamental difference unrelated to crystallographic structural parameters may contribute to crystallographic orientation effects, During SIMS analyses, the interaction between an incoming ion and a target sample is highly complex, involving numerous variables including the type of primary ion, impact angle and energy, and the composition, bond strength and structure of the sample matrix. Many models for the origin of orientation effects focus predominantly on the crystallographic structure of the target minerals (Huberty et al 2010;Taylor et al 2012;Schmitt & Zack 2012).…”

“…Previous studies demonstrated that rutile displays some of the largest effects that have been observed so far (Li et al 2010(Li et al , 2011Taylor et al 2012;Schmitt & Zack 2012;Shulaker et al 2015). Taylor et al (2012) showed that the crystallographic orientation of rutile affects the total signal as measured on the reference mass (Ti3O3 + ), as well as the measured UO2 + /UO + ratio.…”

“…3 isotope studies of cassiterite by secondary ionisation mass spectrometry (SIMS) have been reported. Caution is warranted, however, because previous studies have demonstrated large crystallographic orientation effects on SIMS analyses of rutile (Li et al 2010(Li et al , 2011Taylor et al 2012;Schmitt & Zack 2012;Shulaker et al 2015), which is isostructural with cassiterite, as well as for other simple structured oxides and sulphides including baddeleyite (Wingate & Compston, 2000;Li et al 2010;Schmitt et al 2010), magnetite (Lyon et al 1998;Huberty et al 2010;Kita et al 2011), sphalerite and galena .…”