Header
Accueil du site > Publications > Dating post-Archean lithospheric mantle : Insights fromRe-Os and Lu-Hf (...)

Dating post-Archean lithospheric mantle : Insights fromRe-Os and Lu-Hf isotopic systematics of the CameroonVolcanic Line peridotites

Liu, J. ; Graham Pearson, D. ; Shu, Q. ; Sigurdsson, H. ; Thomassot, E. ; Alard, O., GCA

Dating post-Archean lithospheric mantle : Insights fromRe-Os and Lu-Hf isotopic systematics of the CameroonVolcanic Line peridotites

Liu, J. ; Graham Pearson, D. ; Shu, Q. ; Sigurdsson, H. ; Thomassot, E. ; Alard, O.

Geochimica et Cosmochimica Acta, 2020, 278, 177-198

Abstract :

Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to theincreasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in datingpost-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is impor-tant to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peri-dotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes,highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyosin the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized bylow to moderate olivine Fo contents (88.9–91.2) and low spinel Cr# (8.4–19.3), together with moderate to high whole-rockAl2O3contents (2.0–3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20%partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of meltdepletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyrox-ene with143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration,most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenescannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recentmetasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget ofthese rocks and has176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rockLu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron fromortho- and clinopyroxenes gives an age of 2.01 ± 0.18 Ga (1r), and when olivine and spinel are considered, is 1.82± 0.14 Ga (1r). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2–2.4 Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0 Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at-2.0 Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that iscomplementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archeanperidotites. The recognition of-2.0 Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyosregion was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleopro-terozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin ofthe CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source ofthe CVL basalts.

Voir en ligne : https://doi.org/10.1016/j.gca.2019....




publié mardi 5 mai 2020