| 1. |
- Godoy, Benigno, et al.
(författare)
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Boron isotope variations in a single monogenetic cone : La Poruña (21°53′S, 68°30′W), Central Andes, Chile
- 2023
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Ingår i: Lithos. - : Elsevier. - 0024-4937 .- 1872-6143. ; 440
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Tidskriftsartikel (refereegranskat)abstract
- La Poruña is a monogenetic volcano located within the Altiplano-Puna Volcanic Complex (21°-24°S) in the Central Andean subduction zone. Since crustal contamination of Andean magmas is ubiquitous, and because extensive geochemical data exist for La Poruña, we employ this volcano as a case study to examine the behavior of boron isotopes during crustal assimilation. We present whole-rock boron concentration and 11B/10B ratios (as δ11B values) for La Poruña lava samples that were prepared as nano-particulate pressed pellets. La Poruña B contents range from 14 to 20 μg/g and δ11B values range from −1.39 ± 0.54 ‰ (2σ) to +0.94 ± 0.30 ‰ (2σ), which overlap with the range of available whole-rock data for Central Andean lavas. Moreover, La Poruña δ11B values correlate negatively with 87Sr/86Sr ratios from the same samples. Since 87Sr/86Sr is a proxy for crustal contamination at La Poruña, the data lead us to suggest that La Poruña magmas assimilated a low-δ11B, high 87Sr/86Sr component such as Andean continental crust. Mixing models based on B and Sr isotopes support a broadly two-step magma evolution for La Poruña. In step 1, mantle-derived primary melts interacted with boron-rich slab-derived fluids with high δ11B values, which yielded subduction-modified parental magmas with ca. 3 μg/g B and relatively high δ11B values. In step 2, the high δ11B parental magmas ascended through the crust where they assimilated up to 20% crustal material, which further modified their δ11B values and 87Sr/86Sr ratios. In comparison to available regional values for B and δ11B, it appears that La Poruña and nearby volcanic centers shared a similar source and magmatic history, whereas volcanoes south of 23°S differ. We stress, however, that deconvolving the roles of various subduction and crustal inputs in the Central Andes would require further studies on individual volcanoes along the arc.
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| 2. |
- Hogmalm, Johan, 1979, et al.
(författare)
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In situ Rb-Sr and K-Ca dating by LA-ICP-MS/MS: an evaluation of N2O and SF6 as reaction gases
- 2017
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Ingår i: Journal of Analytical Atomic Spectrometry. - : Royal Society of Chemistry (RSC). - 0267-9477 .- 1364-5544. ; 32:2, s. 305-313
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Tidskriftsartikel (refereegranskat)abstract
- In situ dating of K-rich minerals, e.g. micas and K-feldspar, by the Rb–Sr isotopic system is a new development made possible by the ICP-MS/MS technique. Online chemical separation of Rb and Sr is possible in an O2-filled reaction cell, wherein a portion of the Sr reacts to SrO+ while simultaneously no RbO+ is formed. O2 reactions provide stable analytical conditions sufficient for precise and accurate determination of Rb/Sr and Sr/Sr isotopic ratios using 80 micron laser ablation spots. However, to date <10% of the Sr reacts with O2 as reaction gas, leaving room for improvement using more potent reaction gases. With a more efficient reactive transfer, it should be possible to obtain similar results with a smaller laser spot size, hence gaining higher spatial resolution. In this study, we have evaluated N2O and SF6 as reaction gases since they have previously been shown to react strongly with Sr+, without affecting Rb+. Analytical conditions, including cell parameters and reaction gas flow rate were optimized while ablating NIST SRM 610. The main reaction product is SrO+ for N2O reaction and SrF+ for SF6 reaction. Both gases show significantly higher reaction product formation compared to O2 with >85% of Sr reacting with N2O and >70% Sr reacting with SF6; Rb does not react with either gas. As a result, the sensitivity for Sr reaction products is ∼10 times higher with N2O and ∼8 times higher with SF6 compared to O2. With these more reactive gases, the error of mica isochron ages, calibrated against a newly developed nano-particulate pressed powder tablet of mica–Mg, is ∼1% using a 50 μm laser spot. Our tests show that both N2O and SF6 form interfering reaction products, e.g., SrOH (N2O), SiF3 and TiF3 (SF6) that are difficult to handle using single mass spectrometer instruments, but which can be overcome using MS/MS. Using SF6 combined with H2, it is possible to measure 40Ca+ as 40Ca19F+, free from interference of 40Ar+ and 40K+. This facilitates the dating of micas by the K–Ca isotopic system; we present the first in situ K–Ca age determination.
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| 3. |
- Hogmalm, Johan K., et al.
(författare)
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In situ Rb–Sr and K–Ca dating by LA-ICP-MS/MS: an evaluation of N2O and SF6 as reaction gases
- 2017
-
Ingår i: Journal of Analytical Atomic Spectrometry. - 0267-9477 .- 1364-5544. ; 32, s. 305-313
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Tidskriftsartikel (refereegranskat)abstract
- In situ dating of K-rich minerals, e.g. micas and K-feldspar, by the Rb–Sr isotopic system is a new development made possible by the ICP-MS/MS technique. Online chemical separation of Rb and Sr is possible in an O2-filled reaction cell, wherein a portion of the Sr reacts to SrO+ while simultaneously no RbO+ is formed. O2 reactions provide stable analytical conditions sufficient for precise and accurate determination of Rb/Sr and Sr/Sr isotopic ratios using 80 micron laser ablation spots. However, to date <10% of the Sr reacts with O2 as reaction gas, leaving room for improvement using more potent reaction gases. With a more efficient reactive transfer, it should be possible to obtain similar results with a smaller laser spot size, hence gaining higher spatial resolution. In this study, we have evaluated N2O and SF6 as reaction gases since they have previously been shown to react strongly with Sr+, without affecting Rb+. Analytical conditions, including cell parameters and reaction gas flow rate were optimized while ablating NIST SRM 610. The main reaction product is SrO+ for N2O reaction and SrF+ for SF6 reaction. Both gases show significantly higher reaction product formation compared to O2 with >85% of Sr reacting with N2O and >70% Sr reacting with SF6; Rb does not react with either gas. As a result, the sensitivity for Sr reaction products is ∼10 times higher with N2O and ∼8 times higher with SF6 compared to O2. With these more reactive gases, the error of mica isochron ages, calibrated against a newly developed nano-particulate pressed powder tablet of mica–Mg, is ∼1% using a 50 μm laser spot. Our tests show that both N2O and SF6 form interfering reaction products, e.g., SrOH (N2O), SiF3 and TiF3 (SF6) that are difficult to handle using single mass spectrometer instruments, but which can be overcome using MS/MS. Using SF6 combined with H2, it is possible to measure 40Ca+ as 40Ca19F+, free from interference of 40Ar+ and 40K+. This facilitates the dating of micas by the K–Ca isotopic system; we present the first in situ K–Ca age determination.
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| 4. |
- Karlsson, Andreas KO, 1986, et al.
(författare)
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Nano-powder tablets of mineral standards as matrix-matched reference materials for Rb-Sr dating
- 2016
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Ingår i: Bull. Geol. Soc. Finland. - 0367-5211. ; Special volume (Abstracts to the 32nd NGWM)
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Konferensbidrag (refereegranskat)abstract
- Currently there is no established homogeneous matrix-matched reference material for Rb-Sr dating. As of now the K-feldspar NBS SRM607 is the only mineral-based material used as a standard for isotopic determinations of Rb/Sr ratios and Sr isotopes. However Nebel & Mezger (2006) demonstrated that the NBS SRM607 standard is heterogeneous with respect to Rb/Sr ratios. To circumvent this problem they proposed to plot Rb/Sr ratios and Sr isotopes on an Rb-Sr model isochron. As an alternative approach to overcome inherent heterogeneities of mineral standards, we are using the method described by Garbe-Schönberg & Müller (2014) to produce several nano-powder tablets of several widely available mineral standards. Compared to NBS SRM607 the Mica-Fe and Mica-Mg reference materials are available from various suppliers for more affordable prices. The Rb/Sr ratio and Sr isotopic composition of Mica-Fe and Mica-Mg have previously been determined by Govindaraju (1979), however we will re-analyse these as nano-powders by MC-ICP-MS with the method described in Nebel & Mezger (2006). One long-term motivation is recent advancements in LA-ICP-MS/MS enabling in situ Rb-Sr dating by reaction cell online chemical separation as outlined by Zack (2015, this conference). The nano-powder tablets we will produce have the advantage of being well suited for this novel technique.
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