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Edited by Richard W. Carlson, Carnegie Institution for Science, Washington, DC, and approved July 17, 2018 (received for review November 9, 2017)
Significance
Lavas from Pitcairn Island are the best candidates for exploring the origin of the enigmatic EM1 component found in some mantle plumes because they show the most extreme isotopic compositions of Sr, Nd, Hf, and Pb that define the EM1 component. We find that these lavas have the lowest δ26Mg values so far recorded in oceanic basalts. Subducted late Archean dolomite-bearing sediments are the most plausible source of the low-δ26Mg feature of Pitcairn lavas. This requires that an ancient, originally sedimentary component has been emplaced near the core–mantle boundary to ultimately become part of the Pitcairn plume source.
Abstract
The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate–silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.
Footnotes
- ↵1To whom correspondence may be addressed. Email: chenlh{at}nju.edu.cn or albrecht.hofmann{at}mpic.de.
↵2Present address: Chiba Institute of Technology, 275-0016 Narashino, Japan.
↵3Present address: Department of Geology, Niigata University, 950-2181 Niigata, Japan.
↵4Present address: Faculty of Social and Cultural Studies, Kyushu University, 819-0395 Fukuoka, Japan.
Author contributions: X.-J.W. and L.-H.C. designed research; X.-J.W., L.-H.C., A.W.H., T.H., H.K., Y.Z., L.-W.X., J.-H.S., T.M., Y.H., T.T., R.S., Q.C., B.S.V., and J.-I.K. performed research; X.-J.W., L.-H.C., A.W.H., T.H., and H.K. analyzed data; and X.-J.W., L.-H.C., A.W.H., and T.H. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1719570115/-/DCSupplemental.
Published under the PNAS license.
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Recycled ancient ghost carbonate in the Pitcairn mantle plume
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