Possible isotopic fractionation of argon in source obsidians and archeological artifacts from Kulkuletti, Ethiopia

Brown, Francis H.; Reid, Charles; Negash, Agazi

doi:10.1016/j.jas.2009.05.008

Cited by 14 publications

(15 citation statements)

References 8 publications

(26 reference statements)

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“…Whilst obsidian has been successfully used to produce geologically meaningful 40 Ar/ 39 Ar ages (Flude et al, 2010;Morgan et al, 2009;Vogel et al, 2006), it is known to be problematic for reasons that are only just starting to become clear, and it is at least qualitatively possible that these poorly-understood processes can result in over-estimation of 40 Ar/ 39 Ar age spectra and isochron ages. Brown et al (2009) and Morgan et al (2009) suggested that the Ar-isotope composition of obsidians in Ethiopia had been affected by kinetic isotope fractionation of atmospheric gas either prior to or during absorption, while Flude et al (in prep) concluded that kinetic isotope fractionation during magmatic degassing produced heterogeneously distributed excess 40 Ar due to preferential loss of 36 Ar during degassing. As already discussed, when distributed heterogeneously, excess 40 Ar may be difficult to detect via age spectra and isotope correlation diagrams (Kuiper, 2002;Sherlock and Arnaud, 1999), and this may be exacerbated when step-heating aliquots of crushed obsidian which may mix small-scale isotope reservoirs and destroy any naturally occurring isotope profiles that might be detected by stepped-heating of a single fragment (i.e.…”

Section: Discussionmentioning

confidence: 99%

40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals

Flude

Storey

2016

Quaternary Geochronology

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Co-magmatic granitoid clasts erupted as part of the Rotoiti Ignimbrite (Rotoehu Tephra) contain euhedral K-feldspar and biotite crystals that protrude into miarolytic cavities and show textural evidence for growth in super-cooled conditions, thus are interpreted as growing during eruption. 40 Ar/ 39 Ar stepped heating experiments on single K-feldspar crystals reveal the presence of heterogeneously distributed excess 40 Ar, preferentially released at lower temperature steps (most likely from fluid/melt inclusions), which cannot reliably be characterised by, or corrected for using isotope correlation diagrams due to mixing between three reservoirs of 40 Ar (radiogenic, atmospheric and excess). This excess 40 Ar component is common, but not ubiquitous, and an age population unmixing algorithm applied to single-crystal fusion data identifies a younger group of K-feldspar and biotite crystals that appear to be largely unaffected by excess 40 Ar. This population gives a statistically robust weighted mean age of 47.4 ± 1.5 ka (1σ, n = 13) and an indistinguishable inverse isochron age of 50 ± 3 ka for this historically difficult to date eruption. The weighted mean age is significantly younger than previous age estimates of the Rotoiti eruption obtained by K/Ar and 40 Ar/ 39 Ar dating of bracketing lavas, but is indistinguishable from recent 14 C and (U-Th)/He dates and estimates based on orbital tuning and sedimentation rates constrained by 14 C ages.

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Section: Discussionmentioning

confidence: 99%

40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals

Flude

Storey

2016

Quaternary Geochronology

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“…However, data for this paper are limited to the sources that are well documented within the Kenyan side of the Lake Turkana Basin. Data from southern Ethiopia, a likely source area for obsidian found in the Turkana Basin, are under study (Brown et al . 2009).…”

Section: Methodsmentioning

confidence: 99%

“…Despite extensive archaeological research on the Holocene in the Lake Turkana Basin, few sources of obsidian have been characterized and published (but see Merrick and Brown 1984; Merrick et al . 1994; Brown et al . 2009).…”

Section: Introductionmentioning

confidence: 99%

Transport and Subsistence Patterns at the Transition to Pastoralism, Koobi Fora, Kenya

et al. 2011

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The Turkana Basin in Kenya has an extensive record of Holocene activities relating to mobility and economy of foraging and herding communities. Obsidian is only known from a few key localities in northern Kenya. As such, the use of obsidian as a toolstone material, commonly used during the mid-Holocene, provides one way to trace exchange, interaction and population movements during the transition to pastoralism. We employ X-ray fluorescence to characterize obsidian artefacts from four Pastoral Neolithic assemblages. Data reveal a highly mobile and diversified population that used watercraft to access and transport obsidian resources. Specifically, the use of the North Island obsidian source in Lake Turkana indicates that boat use was significant during this transitional period. The incorporation of watercraft transport and aquatic resources in our analyses of Pastoral Neolithic sites affords a greater understanding of subsistence, mobility and economy in this important period in East African prehistory.

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“…Here, we summarize the elemental, isotopic, and geochronological exploratory research into successful discrimination of these important Southwestern sources of archaeological obsidian in order to provide a database and strategy to deal with this problem, one that is present in other volcanic provinces worldwide (e.g., Argote‐Espino, Solé, López‐Garcia, & Stepone, ; Brown, Reid, & Negash, ; Chataigner & Gratuze, ; Glascock, ; Morgan, Renne, Taylor, & WoldeGabriel, ; Poidevin, ; Poupeau et al., ; Sahle, Morgan, Braun, Atnafu, & Hutchings, ; Shackley & Sahle, ; Vogel, Nomade, Negash, & Renne, ; Weisler & Woodhead, ). In order to provide clarity beyond XRF, we have acquired Sr, Pb, and Nd isotopic data along with 40 Ar/ 39 Ar ages from sample splits for these sources (see Supplementary Document 1 for laboratory and instrumental methods for XRF, the isotopic analysis, and 40 Ar/ 39 Ar dating).…”

Section: Introductionmentioning

confidence: 99%

Elemental, isotopic, and geochronological variability in Mogollon‐Datil volcanic province archaeological obsidian, southwestern USA: Solving issues of intersource discrimination

2017

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Solving issues of intersource discrimination in archaeological obsidian is a recurring problem in geoarchaeological investigation, particularly since the number of known sources of archaeological obsidian worldwide has grown nearly exponentially in the last few decades, and the complexity of archaeological questions asked has grown equally so. These two parallel aspects of archaeological investigation have required more exacting understanding of the geological relationship between sources and the more accurate analysis of these sources of archaeological obsidian. This is particularly the case in the North American Southwest where the frequency of archaeological investigation is some of the highest in the world, and the theory and method used to interpret that record has become increasingly nuanced. Here, we attempt to unravel the elemental similarity of archaeological obsidian in the Mogollon‐Datil volcanic province of southwestern New Mexico where some of the most important and extensively distributed sources are located and the elemental similarity between the sources is great even though the distance between the sources is large. Uniting elemental, isotopic, and geochronological analyses as an intensive pilot study, we unpack this complexity to provide greater understanding of these important sources of archaeological obsidian.

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Possible isotopic fractionation of argon in source obsidians and archeological artifacts from Kulkuletti, Ethiopia

Cited by 14 publications

References 8 publications

40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals

40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals

Transport and Subsistence Patterns at the Transition to Pastoralism, Koobi Fora, Kenya

Elemental, isotopic, and geochronological variability in Mogollon‐Datil volcanic province archaeological obsidian, southwestern USA: Solving issues of intersource discrimination

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