Potassium isotopic composition of Australasian tektites

Humayun, M.; Koeberl, Christian

doi:10.1111/j.1945-5100.2004.tb00125.x

Cited by 47 publications

(45 citation statements)

References 28 publications

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“…Thus, it has often been assumed that the abundance of Na and other alkali elements in impact melts will not always closely match the composition of the target material (e.g., Delano et al, 1981). Whereas we do not dispute this argument, it is important to note that the behavior of alkalis during impact melting is often complex, and that significant losses of these elements do not follow a simple pattern, as shown by many examples of terrestrial (e.g., Humayun and Koeberl, 2004) or lunar impact glasses (e.g., Wentworth et al, 1994).…”

Section: Origin Of the Unusual Trace Element Patterns Of The High-k Smentioning

confidence: 80%

Trace element geochemistry of K-rich impact spherules from howardites

Barrat

Yamaguchi

Greenwood

et al. 2009

Geochimica et Cosmochimica Acta

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The howardite-eucrite-diogenite (HED) achondrites are a group of meteorites that probably originate from the asteroid Vesta. Howardites are complex polymict breccias that sometimes contain, in addition to various rock debris, impact melt glasses which show an impressive range of compositions. In this paper we report on the geochemistry and O isotopes of a series of 6 Saharan polymict breccias (4 howardites and 2 polymict eucrites), and on the trace element abundances of high-K impact spherules found in two of them, Northwest Africa (NWA) 1664 and 1769, which are likely paired.The high-K impact spherules found in the howardites NWA 1664 and NWA 1769 display remarkable trace element patterns. Compared to eucrites or howardites, they all show prominent enrichments in Cs, Rb, K, Li and Ba, strong depletion in Na, while the REE and other refractory elements are unfractionated. These features could not have been generated during impact melting of their host howardites, nor other normal HED target materials. The involvement of Na-poor rocks, and possibly rocks of granitic composition, appears likely.Although these lithologies cannot be well constrained at present, our results demonstrate that the surface of Vesta is certainly more diverse than previously thought. Indeed, despite the large number of available HED meteorites (about 1000 different meteorites), the latter are probably not sufficient to describe the whole surface of their parent body. ACCEPTED MANUSCRIPT3

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Section: Origin Of the Unusual Trace Element Patterns Of The High-k Smentioning

confidence: 80%

Trace element geochemistry of K-rich impact spherules from howardites

Barrat

Yamaguchi

Greenwood

et al. 2009

Geochimica et Cosmochimica Acta

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“…Additionally, these authors confirmed previous studies [51] that had observed higher K contents in terrestrial basalts and martian shergottites (typically greater than 1000 ppm), relative to mare basalts (300-750 ppm) or eucrites (160-300 ppm). The lack of isotopic fractionation of 41 K/ 39 K in basalts formed by high-temperature igneous processes, or in tektites formed by dry melting during hypervelocity impacts, indicates lack of K fractionation, either during melting, or by partial evaporation [69]. Recasting the K isotopic data suggests variable and slight heavy isotopic enrichment in mare basalts relative to terrestrial or martian igneous rocks (figure 3c).…”

Section: (C) Potassiummentioning

confidence: 99%

Section: (C) Potassiummentioning

confidence: 99%

“…The work of Humayun and others reports 41 K/ 39 K ratios as δ 41 K relative to Suprapur KNO 3 [68,69]. Limited work on K has been done in the past 20 years, although a study of K in terrestrial tektites revealed no discernible isotopic fractionation [69]. Given that potassium is moderately volatile and has a 50% condensation temperature of 1006 K, a lack of isotopic fraction led Humayun & Clayton [68] to conclude that K and other moderately volatile alkalis (e.g.…”

Section: (C) Potassiummentioning

confidence: 99%

“…Tektites are terrestrial natural glasses that are produced during hypervelocity impacts or airbursts of a projectile into terrestrial target rocks or soils [78,79]. Some tektites are extremely water-poor (less than 0.02 wt% H 2 O) [69,80,81]. In contrast, some tektites are enriched in the heavy isotopes of Zn compared to upper continental crust [74], and Zn abundance is negatively correlated with δ 66 Zn, indicating that isotopic fractionation occurred during evaporation under the high-temperature conditions of tektite formation (above 2273 K; figure 4).…”

Section: Evidence For a Non-ideal Rayleigh Regime During Isotopic Framentioning

confidence: 99%

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Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon

Day

Moynier

2014

Phil. Trans. R. Soc. A.

108

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The Moon is depleted in volatile elements relative to the Earth and Mars. Low abundances of volatile elements, fractionated stable isotope ratios of S, Cl, K and Zn, high μ ( 238 U/ 204 Pb) and long-term Rb/Sr depletion are distinguishing features of the Moon, relative to the Earth. These geochemical characteristics indicate both inheritance of volatile-depleted materials that formed the Moon and planets and subsequent evaporative loss of volatile elements that occurred during lunar formation and differentiation. Models of volatile loss through localized eruptive degassing are not consistent with the available S, Cl, Zn and K isotopes and abundance data for the Moon. The most probable cause of volatile depletion is global-scale evaporation resulting from a giant impact or a magma ocean phase where inefficient volatile loss during magmatic convection led to the present distribution of volatile elements within mantle and crustal reservoirs. Problems exist for models of planetary volatile depletion following giant impact. Most critically, in this model, the volatile loss requires preferential delivery and retention of late-accreted volatiles to the Earth compared with the Moon. Different proportions of late-accreted mass are computed to explain present-day distributions of volatile and moderately volatile elements (e.g. Pb, Zn; 5 to >10%) relative to highly siderophile elements (approx. 0.5%) for the Earth. Models of early magma ocean phases may be more effective in explaining the volatile loss. Basaltic materials (e.g. eucrites and angrites) from highly differentiated airless asteroids are volatile-depleted, like the Moon, whereas the Earth and Mars have proportionally greater volatile contents. Parent-body size and the existence of early atmospheres are therefore likely to represent fundamental controls on planetary volatile retention or loss.

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Methods for determination of the age of Pleistocene tephra, derived from eruption of Toba, in central India

et al. 2011

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Tephra, emplaced as a result of Pleistocene eruption of the Indonesian 'supervolcano' Toba, occurs at many localities in India. However, the ages of these deposits have hitherto been contentious; some workers have argued that these deposits mark the most recent eruption (eruption A, ca 75 ka), although at some sites they are stratigraphically associated with Acheulian (Lower Palaeolithic) artefacts. Careful examination of the geochemical composition of the tephras, which are composed predominantly of shards of rhyolitic glass, indicates that discrimination between the products of eruption A and eruption D (ca 790 ka) of Toba is difficult. Nonetheless, this comparison favours eruption D as the source of the tephra deposits at some sites in India, supporting the long-held view that the Lower Palaeolithic of India spans the late Early Pleistocene. In principle, these tephra deposits should be dateable using the K-Ar system; however, previous experience indicates contamination by a small proportion of ancient material, resulting in apparent ages that exceed the true ages of the tephras. We have established the optimum size-fraction in which the material from Toba is concentrated, 53-61 μm, and have considered possible origins for the observed contamination. We also demonstrate that Ar-Ar analysis of four out of five of our samples has yielded material with an apparent age similar to that expected for eruption D. These numerical ages, of 809 ± 51, 714 ± 62, 797 ± 45 and 827 ± 39 ka for the tephras at Morgaon, Bori, Gandhigram and Simbhora, provide a weighted mean age for this eruption of 799 ± 24 ka (plus-or-minus two standard deviations). However, these numerical ages are each derived from no more than 10-20% of the argon release in each sample, which is not ideal. Nonetheless, our results demonstrate that it is feasible, in principle, to date this difficult material using the Ar-Ar technique; future follow-up studies will therefore be able to refine our preparation and analysis procedures to better optimize the dating.

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Potassium isotopic composition of Australasian tektites

Cited by 47 publications

References 28 publications

Trace element geochemistry of K-rich impact spherules from howardites

Trace element geochemistry of K-rich impact spherules from howardites

Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon

Methods for determination of the age of Pleistocene tephra, derived from eruption of Toba, in central India

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