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Oldest reliable terrestrial 40 Ar‐ 39 Ar age from pyrite crystals at Isua west Greenland
Author(s) -
Smith Patrick E.,
Evensen Norman M.,
York Derek,
Moorbath Stephen
Publication year - 2005
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2005gl024066
Subject(s) - pyrite , geology , radiogenic nuclide , geochemistry , sedimentary rock , metamorphic rock , hadean , banded iron formation , volcanic rock , archean , crust , isotopes of argon , precambrian , metasomatism , mineral , mineralogy , volcano , argon , chemistry , mantle (geology) , organic chemistry
Pyrite grains can act as K‐Ar time capsules, by armoring the minute inclusions hosted within the pyrite, thus preventing loss of their radiogenic argon. This pyrite clock provides the means to apply single‐crystal laser 40 Ar‐ 39 Ar dating to rocks that typically resist straightforward dating, notably sedimentary rocks. We have applied the technique to a banded iron formation from Earth's oldest sequence of sedimentary and volcanic rocks, the Isua greenstone belt of west Greenland. We find that, in spite of having been subjected to several metamorphic and metasomatic events, the mineral inclusions in pyrites from these rocks give remarkably well‐defined and reproducible ages at 3.86 ± 0.07 Ga, in agreement with previous determinations of the age of their host rocks. Moreover, the isotopic data from these pyrites yield distinct indications of inclusions giving an age of 4.31 ± 0.06 Ga, possibly providing the first direct evidence for a much earlier crust at Isua.