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Recycled Ancient Ghost Carbonate in the Pitcairn Mantle Plume

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Recycled Ancient Ghost Carbonate in the Pitcairn Mantle Plume Recycled ancient ghost carbonate in the Pitcairn mantle plume Xiao-Jun Wanga, Li-Hui Chena,1, Albrecht W. Hofmannb,1, Takeshi Hanyuc, Hiroshi Kawabatad, Yuan Zhonga, Lie-Wen Xiee, Jin-Hua Shia, Takashi Miyazakic, Yuka Hiraharac,2, Toshiro Takahashic,3, Ryoko Sendac,4, Qing Changc, Bogdan S. Vaglarovc, and Jun-Ichi Kimurac aState Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, 210023 Nanjing, China; bAbteilung Klimageochemie, Max-Planck-Institut für Chemie, D-55128 Mainz, Germany; cDepartment of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, 237-0061 Yokosuka, Japan; dFaculty of Science and Technology, Kochi University, 780-8520 Kochi, Japan; and eState Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China Edited by Richard W. Carlson, Carnegie Institution for Science, Washington, DC, and approved July 17, 2018 (received for review November 9, 2017) The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn stable isotope tracer, magnesium, which undergoes strong iso- Island basalts have been labeled enriched mantle 1 (EM1), character- topic fractionation during precipitation of carbonate from izing them as one of the isotopic mantle end members. The EM1 seawater/pore fluids (12), producing distinctively low δ26Mg origin has been vigorously debated for over 25 years, with interpre- values in marine carbonates. Because of this, it appears to be an tations ranging from delaminated subcontinental lithosphere, to excellent tracer of a more specific recycled component than, for recycled lower continental crust, to recycled oceanic crust carrying example, oxygen isotopes. ancient pelagic sediments, all of which may potentially generate the There are many models for the creation of chemical hetero- requisite radiogenic isotopic composition. Here we find that δ26Mg geneity in the source regions of OIBs that invoke the recycling of ratios in Pitcairn EM1 basalts are significantly lower than in normal oceanic crust and lithologically different sediments through the mantle and are the lowest values so far recorded in oceanic basalts. A mantle (5, 13–15). The existing Mg isotope data show that marine global survey of Mg isotopic compositions of potentially recycled sediments and altered oceanic crust have variable Mg isotopic components shows that marine carbonates constitute the most com- signatures that differ significantly from those of normal peridotitic mon and typical reservoir invariably characterized by extremely low mantle (Fig. 1). In particular, marine carbonates stand out by their δ26Mg values. We therefore infer that the subnormal δ26Mg of the extremely light Mg isotopes, reaching values of δ26Mg as low as Pitcairn EM1 component originates from subducted marine carbon- −5‰. This provides the opportunity to use anomalous Mg isotopic ates. This, combined with previously published evidence showing ex- signatures in erupted OIBs as tracers of their sources. A recent Mg ceptionally unradiogenic Pb as well as sulfur isotopes affected by isotopic study on Hawaii and Louisville basalts presented a first mass-independent fractionation, suggests that the Pitcairn EM1 com- test of this proposition, showing small but significant differences in ponent is most likely derived from late Archean subducted carbonate- δ26Mg (16). However, the current Mg isotope database for OIBs is bearing sediments. However, the low Ca/Al ratios of Pitcairn still very small, and the Mg isotope characteristics of the several lavas are inconsistent with experimental evidence showing high classical mantle species, i.e., HIMU (high μ = 238U/204Pb), EM1, Ca/Al ratios in melts derived from carbonate-bearing mantle and enriched mantle 2 (3), remain to be evaluated. Here we focus sources. We suggest that carbonate–silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the Significance isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ulti- Lavas from Pitcairn Island are the best candidates for exploring mately became the Pitcairn plume source. the origin of the enigmatic EM1 component found in some mantle plumes because they show the most extreme isotopic Pitcairn mantle plume | EM1 | magnesium isotopes | ancient compositions of Sr, Nd, Hf, and Pb that define the EM1 com- carbonate-bearing sediments ponent. We find that these lavas have the lowest δ26Mg values so far recorded in oceanic basalts. Subducted late Archean he geochemical and isotopic diversity of ocean island basalts dolomite-bearing sediments are the most plausible source of T(OIBs) erupted at hot spots is generated by partial melting of the low-δ26Mg feature of Pitcairn lavas. This requires that an heterogeneous mantle sources that contain recycled crustal and/ ancient, originally sedimentary component has been emplaced or lithospheric mantle materials (1, 2). Specific recycled compo- near the core–mantle boundary to ultimately become part of nents generally show radiogenic isotope ratios distinct from the the Pitcairn plume source. ambient mantle, and radiogenic isotopes are therefore widely used tools for tracing them. However, this tool can be compromised by Author contributions: X.-J.W. and L.-H.C. designed research; X.-J.W., L.-H.C., A.W.H., T.H., the fact that similar radiogenic isotope features can appear in 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. different geological reservoirs. Thus, the exceptionally unradio- wrote the paper. genic Pb isotopes observed in intraplate enriched mantle 1 [EM1 The authors declare no conflict of interest. (3)] lavas have been variously attributed to recycled ancient pe- This article is a PNAS Direct Submission. – lagic sediments (4 7), subducted oceanic plateau (8), recycled Published under the PNAS license. lower continental crust (LCC) (9), delaminated subcontinental 1To whom correspondence may be addressed. Email: [email protected] or albrecht. lithospheric mantle (10), and recycled mafic sediments (11). [email protected]. Fortunately, a combination of stable and radiogenic isotopes 2Present address: Chiba Institute of Technology, 275-0016 Narashino, Japan. can sometimes be used to discriminate the nature of recycled 3Present address: Department of Geology, Niigata University, 950-2181 Niigata, Japan. materials less ambiguously. Recently, Delavault et al. (7) 4Present address: Faculty of Social and Cultural Studies, Kyushu University, 819-0395 demonstrated the presence of sulfur isotopes affected by mass- Fukuoka, Japan. independent fractionation in sulfides contained by Pitcairn This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. lavas, which are likely to have been introduced into the Pitcairn 1073/pnas.1719570115/-/DCSupplemental. source by subduction of ancient sediments. Here we use a Published online August 13, 2018. 8682–8687 | PNAS | August 28, 2018 | vol. 115 | no. 35 www.pnas.org/cgi/doi/10.1073/pnas.1719570115 Downloaded by guest on September 25, 2021 basalts”; δ26Mg = −0.57 to −0.45‰ (22, 23)] in northeast China Peridotite xenoliths(N=160) (Fig. 3). The δ26Mg values of Pitcairn and Rarotonga EM1 basalts, taken together, exhibit significant negative correlations with 87Sr/86Sr 206 204 e e Abyssal peridotites(N=32) and positive correlations with Pb/ Pb, Nd,and Hf (Fig. 3). Inclusion of the intracontinental EM1 basalts strengthens these correlations and further extends their range to even lower δ26Mg Altered oceanic values. A common heritage of oceanic and continental EM1 crust(N=86) 26 carbonate-rich clay-rich magmas is further substantiated by the correlation between δ Mg and CaO/Al2O3 (Fig. 4). Oceanic EM1 lavas exhibit the lowest Subducting CaO/Al2O3 ratios within the global OIB association (24), and the sediments(N=95) carbonate-rich silicate-rich intracontinental EM1 basalts have even lower CaO/Al2O3 ratios. Therefore, we consider all these EM1-type intraplate lavas as a calcite-rich dolomite-rich group to explore the origin of the characteristic EM1 component. Marine carbonate(N=221) Discussion Subnormal δ26Mg of the EM1 Mantle Sources. The analyzed OIB - - - - - samples are fresh, and no secondary carbonates can be observed (SI 5 4 3 2 10 1Appendix 26 ,Fig.S4). Their loss on ignition (LOI) values are very low δ Mg (‰) (mostly <1.0 wt %; Dataset S2), and there is no correlation between δ26Mg and LOI, precluding any obvious chemical weathering. Fig. 1. Mg isotopic compositions of mantle rocks and some potentially Seawater has lower δ26Mg [−0.83‰ (21)] value than reported recycled components. The average δ26Mg value, the SD (1 SD), and the values of OIBs (16, 21); thus, seawater contamination might po- number of samples (N) are given for each reservoir. For peridotite xenoliths tentially modify the Mg isotopic composition of OIBs. However, and abyssal peridotites, the error bars are covered by the symbols for the δ26 seawater has much lower Mg concentrations [0.128 wt % (25)] than average values. The vertical gray bar represents the Mg range of peri- δ26 dotitic mantle [−0.23 ± 0.04‰ (20)]. See SI Appendix, Part 3 for data sources. our basalt samples with low Mg values, and more than 90% of seawater would have to be added to the basalt to shift the δ26Mg of basalt from −0.25 to −0.40‰. This can be ruled out because of the 26 on the EM1 end member by presenting whole-rock Mg isotopic low LOI of our samples. Hence, the low δ Mg values of Tedside analyses of Rarotonga and Pitcairn Island lavas. The combination basalts are unlikely to be the result of postmagmatic alteration. of Mg isotopes with Sr–Nd–Hf–Pb isotopes provides critical evi- Fractionation of olivine, pyroxene, and plagioclase dominate > dence for the return of ancient, subducted dolomite-derived the major element variations of Pitcairn lavas with MgO 4wt%, – magnesium in EM1 lavas erupted at Earth’s surface.
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