Abstract
We have developed the first laboratory for (U-Th-Sm)/He thermochronology measurements in South America. Helium is measured using a high-sensitivity magnetic sector mass spectrometer (GVI-Helix-SFT) and a double focusing single collector inductively coupled plasma mass spectrometer (Thermo Scientific ELEMENT2) is used for U, Th and Sm determinations. Repeated analyses of fragments of Durango fluorapatite crystals yields an average age of 31.6 ± 1.6 Ma (2σ) (n=62). This overlaps the long-term average of Durango fluorapatite measured in laboratories worldwide. The analysis of multiple single apatite crystals of a Precambrian basement sample from Serra do Mar, southeastern Brazil, yields an average He age (60.5 ± 8.7 Ma; n=8) that overlaps that measured in the SUERC laboratory (59.6 ± 3 Ma; n=3). This confirms that the UNESP laboratory is capable of routinely measuring the (U-Th-Sm)/He ages of single apatite crystals.
Highlights
(U-Th-Sm)/He dating of minerals relies on the production of 4He from 235U, 238U, 232Th, and 147Sm via alpha decay
The radiogenic 4He diffuses at relatively low temperatures (
This may be apparent as a positive correlation between He age and eU (Flowers et al 2009, Gautheron et al 2009) or crystal size (Reiners and Farley 2001). Apatite He thermochronology has been used to study the exhumation-induced cooling history of samples in tectonically active and passive margins regions (House et al 1998, Farley et al 2002, Persano et al 2002, Ehlers and Farley 2003), and is powerful when combined with apatite fission track and zircon (U+Th)/He thermochronology (e.g. Kirstein et al 2006, Merten et al 2010, Macaulay et al 2014)
Summary
The effect of radiation damage and grain size is most pronounced in rocks that have spent significant lengths of time at temperatures close to the closure temperature This may be apparent as a positive correlation between He age and eU (Flowers et al 2009, Gautheron et al 2009) or crystal size (Reiners and Farley 2001). Helium abundances are measured using a Helix-SFT magnetic sector mass spectrometer (GV Instruments) (Figure 1c). It is equipped with a modified Nier-type electron bombardment source with x and z focusing. Low resolution and electrostatic peak jumping mode ensure high sensitivity and adequate measurement precision to isotope ratios. 238U sensitivity is checked using a tuning solution with a concentration of 1 μg/L which usually yields 1.4 to 2.0 x 106 cps (counts per second)
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