Toward the feldspar alternative for cosmogenic 10Be applications
Introduction
Terrestrial Cosmogenic Nuclides (TCN) are produced through nuclear interactions between minerals of the upper Earth's crust surface and the energetic cosmic rays secondary particles. Offering the possibility to determine surface exposure durations to the cosmic ray derived secondary particles, they are useful to quantify the evolution of many geomorphological features (e.g. landslides, moraines, fault-scarps, terraces) (e.g. Gosse and Phillips, 2001). They may also provide quantitative constraints on denudation rates at a single location or at watershed scales (e.g. Brown et al., 1995, von Blanckenburg, 2005). Over the last decades, significant efforts and progress have been made not only to improve the analytical procedures (e.g. Schimmelpfennig et al., 2009, Bromley et al., 2014, Corbett et al., 2016), but also the accuracy and precision of in situ-production rates (e.g. Braucher et al., 2011, Braucher et al., 2013, Fenton et al., 2011, Blard et al., 2013b, Lifton et al., 2014, Kelly et al., 2015, Martin et al., 2015, Borchers et al., 2016, Delunel et al., 2016).
In situ-produced 10Be is one of the most commonly used TCN in quantitative geomorphology due to the fact that its production rate is relatively well constrained in the ubiquitous quartz mineral whose integrity minimizes the possibility of contamination by meteoric 10Be. Easily decontaminated from meteoric 10Be, it is in addition reliably measured using the Accelerator Mass Spectrometry technique for which its detection limit is lower than 104 at.g−1 (e.g. Arnold et al., 2010). However, volcanic or mafic areas are generally quartz free, which hamper the routine use of 10Be. In these geological settings, other nuclide-mineral couples are used in routine, such as 3He in olivine-pyroxene (e.g. Kurz, 1986), or 36Cl in Ca or K rich feldspars (e.g. Schimmelpfennig et al., 2009). Nevertheless, in some cases, the applicability of these TCNs may be complicated by several limitations such as pre-exposure inheritance in the case of stable TCN (i.e. a cosmogenic component produced in an earlier exposure period) and/or large corrections for non-cosmogenic components in the case of 3He (e.g. Gosse and Phillips, 2001, Blard and Farley, 2008, Amidon et al., 2009, Athanassas et al., 2016). Additionally, when production pathways are multiple such as for the 36Cl, a precise knowledge of the chemical mineral composition is necessary (Phillips et al., 2001, Dunai et al., 2007) which requires measuring specific major and trace elements. Especially in the case of 36Cl, despite recent important progress (Marrero et al., 2016) the problem of the scattering of the different elemental production rates in the literature (e.g. spallation from Ca and from K) is still not completely understood (see discussion in Marrero et al., 2016) and it remains a possible source of significant uncertainties.
In the case of a quartz poor lithology, an alternative possibility is to rely on 10Be - feldspars. Two preliminary studies (Kober et al., 2005, Blard et al., 2013a) already provided promising results, demonstrating that the decontamination protocol classically applied to quartz (Brown et al., 1991) efficiently removes all the meteoric 10Be contamination from the feldspar grains. These studies also suggest that the total production rate of 10Be in feldspar is 8–10% lower than that in quartz. However, only two samples were analyzed in both studies. In order to better constrain the 10Be in situ-production rate within feldspars, the number of samples analyzed needs to be increased. Moreover, it is important to further investigate to what extent the 10Be in situ-production rate within feldspar depends on the chemical composition of the analyzed minerals.
In this study, we explore and demonstrate the potential of feldspar to accurately determine the concentration of in situ-produced cosmogenic 10Be, thus substituting quartz in mafic areas. For this, we developed a new chemical protocol for the 10Be extraction from these matrices, and to cross-calibrate the total 10Be in situ-production rate in feldspar (P10fsp) against the total 3He production rate in pyroxene (P3px). The cosmogenic 3He and 10Be concentrations were measured, respectively, in pyroxene and feldspar extracted from eight samples of ignimbrite boulders from a giant landslide located between 800 and 2500 m in the high central Andes of Southern Peru. This area is ideally located, since two studies have already determined the local total 3He production rate in pyroxene on the nearby Altiplano, above 3000 m (Blard et al., 2013b, Delunel et al., 2016). Starting from the chemical protocol classically used for the extraction of the in situ-produced 10Be from quartz, we propose a slightly updated procedure to extract the in situ-produced 10Be from feldspar. This adapted chemical procedure allows overcoming difficulties specific to feldspars, such as the precipitation of fluorite salts during HF substitution, and also prevents from the saturation of the resin columns during cation and anion exchange chromatography.
Section snippets
Material: geomorphological and geological settings of the sampling site
The giant Caquilluco landslide is located in southern Peru (Fig. 1A), in the northern part of the Atacama Desert, about forty kilometers north of the city of Tacna (Fig. 1B). The instability has developed along the western flank of the Peruvian Andes, an area characterized by steep slopes and extremely dry climate for the last million years (Placzek et al., 2010). A preliminary geomorphological description of this exceptional site can be found in Audin and Bechir (2006) and Crosta et al. (2015)
Sample treatment - pyroxene and feldspar separation
Thin sections were realized in each rock sample in order to ensure that enough pyroxene and feldspars were available and also to check the crystal sizes (>200 μm). Samples were then crushed, washed and sieved to select the 200–800 μm fractions. Successive magnetic separations were performed using a Frantz© magnetic separator to separate the magnetic from the non-magnetic fraction bearing pyroxene and feldspars, respectively. For efficiency, and because the magnetic fraction dominates in the
(U–Th–Sm)/4He ages and nucleogenic 3He correction
Helium isotopic data and derived calculation results related to this paragraph are presented in Table 2. The major and trace element concentrations measured both in the pyroxene phenocrysts and in the bulk ignimbrite are available in the Supplementary Material file (Supp. Table 1). The 8 obtained (U–Th–Sm)/4He pyroxene crystallization ages range between 8 ± 3 and 17 ± 6 Ma. It is worth noting that these ages should be considered as minimum ages because: (i) as the pyroxene were etched, an
3Hepx - 10Befsp production ratios
The ten 3Hepx - 10Befsp concentration couple values, measured respectively in co-genetic pyroxene and feldspar, include eight samples from boulders of the Caquilluco landslide (this study) and two samples from boulders of the Tunupa volcano (Blard et al., 2013a). Their distribution in a plot of 3He versus 10Be nuclide concentrations are highly linearly correlated (R2 = 0.99) as highlighted on Fig. 2. This indicates that, at the first order, the concentrations of 3He in pyroxene and 10Be in
Conclusion
In this paper we investigate the suitability of feldspar minerals for 10Be dating. The strategy was to cross-calibrate the total production rate of 10Be in feldspar against the total production rate of 3He in pyroxene in the same rock sample. This was performed by measuring the concentration of cosmogenic 3He and 10Be in pyroxenes and feldspars respectively, of eight samples of ignimbrite boulders from a giant paleo-landslide located in the high central Andes of Southern Peru (Lat. 18°S). The
Acknowledgments
This publication is part of the convenio IRD-INGEMMET. The work has been supported by INSU SYSTER, the ANR Jeunes Chercheurs GALAC project (ANR-11-JS56-011-01), the CNES and a grant from Labex OSUG@2020 (Investissements d'avenir – ANR10 LABX56). We warmly thank François Senebier, Sylvain Campillo, Valerie Magan, Nathaniel Findling and Catherine Chauvel for ICP-MS measurements, geochemical analysis and fruitful discussions. The measurements were performed at the ASTER AMS national facility
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ASTER Team: George Aumaître and Karim Keddadouche.