Noble gas composition, cosmic-ray exposure age, 39Ar-40Ar, and I-Xe analyses of ungrouped achondrite NWA 7325

Jens Hopp, Natalie Schröter, Olga Pravdivtseva, Hans Peter Meyer, Mario Trieloff, U. Ott

Research output: Article

1 Citation (Scopus)

Abstract

Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large-degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of 40Ar/39Ar and 129I-129Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. 21Ne and 38Ar cosmic-ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This 38Ar age is in reasonable agreement with a cosmic-ray exposure age of 17.5 Ma derived in our 40Ar/39Ar dating study. Due to the low K-content of 19 ± 1 ppm and high Ca-content of approximately 12.40 ± 0.15 wt%, no reliable 40Ar/39Ar age could be determined. The integrated age strongly depends on the choice of an initial 40Ar/36Ar ratio. An air-like component is dominant in lower temperature extractions and assuming air 40Ar/36Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K-Ar system. Results of 129I-129Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest 129Xe*/128XeI ratio as equivalent to a lower age limit, we calculate an I-Xe age of about 4536 Ma. In addition, elevated 129Xe/132Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I-Xe system.

Original languageEnglish
JournalMeteoritics and Planetary Science
DOIs
Publication statusAccepted/In press - jan. 1 2018

Fingerprint

achondrites
achondrite
noble gas
gas composition
cosmic ray
rare gases
cosmic rays
dating
rocks
air
meteorites
disturbances
isotopes
heating
melt
exposure
Africa
interactions
rock
meteorite

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science

Cite this

Noble gas composition, cosmic-ray exposure age, 39Ar-40Ar, and I-Xe analyses of ungrouped achondrite NWA 7325. / Hopp, Jens; Schröter, Natalie; Pravdivtseva, Olga; Meyer, Hans Peter; Trieloff, Mario; Ott, U.

In: Meteoritics and Planetary Science, 01.01.2018.

Research output: Article

Hopp, Jens ; Schröter, Natalie ; Pravdivtseva, Olga ; Meyer, Hans Peter ; Trieloff, Mario ; Ott, U. / Noble gas composition, cosmic-ray exposure age, 39Ar-40Ar, and I-Xe analyses of ungrouped achondrite NWA 7325. In: Meteoritics and Planetary Science. 2018.
@article{7465585bedb24bba9b4f7ede1a50394f,
title = "Noble gas composition, cosmic-ray exposure age, 39Ar-40Ar, and I-Xe analyses of ungrouped achondrite NWA 7325",
abstract = "Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large-degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of 40Ar/39Ar and 129I-129Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. 21Ne and 38Ar cosmic-ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This 38Ar age is in reasonable agreement with a cosmic-ray exposure age of 17.5 Ma derived in our 40Ar/39Ar dating study. Due to the low K-content of 19 ± 1 ppm and high Ca-content of approximately 12.40 ± 0.15 wt{\%}, no reliable 40Ar/39Ar age could be determined. The integrated age strongly depends on the choice of an initial 40Ar/36Ar ratio. An air-like component is dominant in lower temperature extractions and assuming air 40Ar/36Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K-Ar system. Results of 129I-129Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest 129Xe*/128XeI ratio as equivalent to a lower age limit, we calculate an I-Xe age of about 4536 Ma. In addition, elevated 129Xe/132Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I-Xe system.",
author = "Jens Hopp and Natalie Schr{\"o}ter and Olga Pravdivtseva and Meyer, {Hans Peter} and Mario Trieloff and U. Ott",
year = "2018",
month = "1",
day = "1",
doi = "10.1111/maps.13062",
language = "English",
journal = "Meteoritics and Planetary Science",
issn = "1086-9379",
publisher = "The University of Arkansas Press",

}

TY - JOUR

T1 - Noble gas composition, cosmic-ray exposure age, 39Ar-40Ar, and I-Xe analyses of ungrouped achondrite NWA 7325

AU - Hopp, Jens

AU - Schröter, Natalie

AU - Pravdivtseva, Olga

AU - Meyer, Hans Peter

AU - Trieloff, Mario

AU - Ott, U.

PY - 2018/1/1

Y1 - 2018/1/1

N2 - Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large-degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of 40Ar/39Ar and 129I-129Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. 21Ne and 38Ar cosmic-ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This 38Ar age is in reasonable agreement with a cosmic-ray exposure age of 17.5 Ma derived in our 40Ar/39Ar dating study. Due to the low K-content of 19 ± 1 ppm and high Ca-content of approximately 12.40 ± 0.15 wt%, no reliable 40Ar/39Ar age could be determined. The integrated age strongly depends on the choice of an initial 40Ar/36Ar ratio. An air-like component is dominant in lower temperature extractions and assuming air 40Ar/36Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K-Ar system. Results of 129I-129Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest 129Xe*/128XeI ratio as equivalent to a lower age limit, we calculate an I-Xe age of about 4536 Ma. In addition, elevated 129Xe/132Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I-Xe system.

AB - Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large-degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of 40Ar/39Ar and 129I-129Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. 21Ne and 38Ar cosmic-ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This 38Ar age is in reasonable agreement with a cosmic-ray exposure age of 17.5 Ma derived in our 40Ar/39Ar dating study. Due to the low K-content of 19 ± 1 ppm and high Ca-content of approximately 12.40 ± 0.15 wt%, no reliable 40Ar/39Ar age could be determined. The integrated age strongly depends on the choice of an initial 40Ar/36Ar ratio. An air-like component is dominant in lower temperature extractions and assuming air 40Ar/36Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K-Ar system. Results of 129I-129Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest 129Xe*/128XeI ratio as equivalent to a lower age limit, we calculate an I-Xe age of about 4536 Ma. In addition, elevated 129Xe/132Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I-Xe system.

UR - http://www.scopus.com/inward/record.url?scp=85043603794&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85043603794&partnerID=8YFLogxK

U2 - 10.1111/maps.13062

DO - 10.1111/maps.13062

M3 - Article

AN - SCOPUS:85043603794

JO - Meteoritics and Planetary Science

JF - Meteoritics and Planetary Science

SN - 1086-9379

ER -