TY - JOUR
T1 - 40Ar/39Ar laser-probe dating of diamond inclusions from the Premier kimberlite
AU - Phillips, D.
AU - Onstott, Tullis C.
AU - Harris, J. W.
PY - 1989/1/1
Y1 - 1989/1/1
N2 - INCLUSIONS encapsulated by diamonds at the time of their formation provide a means for determining diamond crystallization ages and the chemistry of the surrounding upper mantle at that time. Sm-Nd studies of peridotitic inclusions, from Cretaceous-age kimberlites in southern Africa, suggest that the diamonds formed 3.3 Gyr ago. By contrast, eclogite-suite inclusions generally yield younger ages, sometimes approaching the time of kimberlite eruption. Here we report the results of 40Ar/39Ar laser-probe analyses of individual eclogitic clinopyroxene inclusions from Premier diamonds, which yield a mean age of 1,198 ± 14 Myr. This age agrees well with Sm-Nd (ref. 2) and 40Ar/39Ar (ref. 3) analyses on similar Premier inclusions, and is indistinguishable from the inferred time of emplacement of the host kimberlite (1,150-1,230 Myr)2, which implies that diamond formation was essentially synchronous with kimberlite generation. The extrapolated non-radiogenic 40Ar/36Ar ratio of 334 ± 102 is similar to the present-day atmospheric composition. This value is inconsistent with Sr and Nd isotopic signatures from Premier eclogite inclusion, which suggest a depleted mantle source (40Ar/36Ar > 20,000). Pre-entrap-ment equilibration of the inclusions with an 36Ar-rich fluid is the most probable explanation for the low non-radiogenic (40Ar/36Ar) composition.
AB - INCLUSIONS encapsulated by diamonds at the time of their formation provide a means for determining diamond crystallization ages and the chemistry of the surrounding upper mantle at that time. Sm-Nd studies of peridotitic inclusions, from Cretaceous-age kimberlites in southern Africa, suggest that the diamonds formed 3.3 Gyr ago. By contrast, eclogite-suite inclusions generally yield younger ages, sometimes approaching the time of kimberlite eruption. Here we report the results of 40Ar/39Ar laser-probe analyses of individual eclogitic clinopyroxene inclusions from Premier diamonds, which yield a mean age of 1,198 ± 14 Myr. This age agrees well with Sm-Nd (ref. 2) and 40Ar/39Ar (ref. 3) analyses on similar Premier inclusions, and is indistinguishable from the inferred time of emplacement of the host kimberlite (1,150-1,230 Myr)2, which implies that diamond formation was essentially synchronous with kimberlite generation. The extrapolated non-radiogenic 40Ar/36Ar ratio of 334 ± 102 is similar to the present-day atmospheric composition. This value is inconsistent with Sr and Nd isotopic signatures from Premier eclogite inclusion, which suggest a depleted mantle source (40Ar/36Ar > 20,000). Pre-entrap-ment equilibration of the inclusions with an 36Ar-rich fluid is the most probable explanation for the low non-radiogenic (40Ar/36Ar) composition.
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U2 - 10.1038/340460a0
DO - 10.1038/340460a0
M3 - Article
AN - SCOPUS:0024569627
SN - 0028-0836
VL - 340
SP - 460
EP - 462
JO - Nature
JF - Nature
IS - 6233
ER -