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LETTER Doi:10.1038/Nature10326 LETTER doi:10.1038/nature10326 An ancient recipe for flood-basalt genesis Matthew G. Jackson1 & Richard W. Carlson2 Large outpourings of basaltic lava have punctuated geological (LIPs)—volcanic provinces characterized by anomalously high rates of time, but the mechanisms responsible for the generation of such mantle melting that represent the largest volcanic events in the Earth’s extraordinary volumes of melt are not well known1. Recent geo- history—to determine whether they are associated with a primitive chemical evidence suggests that an early-formed reservoir may (albeit non-chondritic) mantle source. have survived in the Earth’s mantle for about 4.5 billion years Located in the southwestern Pacific, the Ontong Java Plateau (OJP) (ref. 2), and melts of this reservoir contributed to the flood basalt is the largest LIP on the Earth1,6,7. The average e143Nd(t) of these emplaced on Baffin Island about 60 million years ago3–5. However, lavas6,7 plots close to the BIWG lavas (Fig. 1) and within the range the volume of this ancient mantle domain and whether it has con- predicted for the non-chondritic primitive mantle. Excluding the most tributed to other flood basalts is not known. Here we show that incompatible and fluid mobile elements, the OJP lavas have relatively basalts from the largest volcanic event in geologic history—the flat primitive-mantle-normalized trace-element patterns (Fig. 2) sim- Ontong Java plateau1,6,7—also exhibit the isotopic and trace ilar to the relatively flat patterns identified in the two highest 3He/4He element signatures proposed for the early-Earth reservoir2. lavas from BIWG (Fig. 2). The flatness of the trace-element patterns18 Together with the Ontong Java plateau, we suggest that six of the led Tejada et al. (ref. 7) to suggest that the OJP mantle is ‘‘almost’’ largest volcanic events that erupted in the past 250 million years primitive, but not actually primitive because of the higher-than- derive from the oldest terrestrial mantle reservoir. The association chondritic 143Nd/144Nd in the OJP lavas. of these large volcanic events with an ancient primitive mantle However, the discovery of a difference in 142Nd/144Nd between source suggests that its unique geochemical characteristics—it is modern terrestrial rocks and chondrites suggests that the both hotter (it has greater abundances of the radioactive heat- 143Nd/144Nd measured in OJP lavas overlaps with the primitive (albeit producing elements) and more fertile than depleted mantle non-chondritic) terrestrial mantle. Additionally, the 120-Myr-old reservoirs—may strongly affect the generation of flood basalts. early-stage (Kwaimbaita- and Kroenke-type lavas6,7) lavas plot near The discovery of a surviving portion of the early-formed, homo- the 4.43-billion-year (Gyr)-old Pb-isotope geochron, and close to the geneous silicate Earth that existed immediately after formation of the BIWG lavas (which plot closer to the 4.5-Gyr-old geochron), an obser- core—referred to as primitive mantle—would place constraints on the vation that is consistent with these lavas sampling an ancient mantle earliest chemical evolution of the Earth and help to clarify the means source (Fig. 1). Although there are minor differences in the Nd and Pb by which the Earth arrived at its present geochemical state (see, for isotopic composition of the OJP and BIWG lava sources, the overlap- example, refs 8–10). Most models for this primitive mantle are based ping trace-element patterns of the two LIPs suggest an origin from on the assumption that it should have relative abundances of refractory compositionally similar sources whose isotopic compositions are lithophile elements similar to those of carbonaceous chondrites—the within the range expected for an early-formed reservoir. presumed building blocks of the Earth11–13. However, the recent discovery Owing to eruption through oceanic crust, contamination of OJP of small (18 6 5 parts per million, p.p.m.) differences in the 142Nd to lavas by the chemically and isotopically evolved material of the con- 144Nd ratio (146Sm decays to 142Nd with a half-life of 106 million years, tinental crust does not complicate the interpretation of their mantle Myr) between the Earth and chondrites suggests that the Earth’s source to the degree seen in LIPs erupted through continental litho- primitive mantle may not have chondritic relative abundances of sphere (see Methods). In contrast, several of the largest LIPs, including the refractory lithophile elements14–16. the BIWG, were erupted in continental settings where assimilation of Instead, all known modern terrestrial mantle reservoirs may have continental lithosphere can obscure the primary mantle signature of evolved from a primitive precursor with Sm/Nd ratios 4.2–7.3% higher the lavas. Although continental assimilation can drive the Pb-isotopic than that of chondrites, leading to a present-day 143Nd/144Nd of composition of lavas towards either more or less radiogenic values, this 0.51290–0.51309, which translates to a present-day e143Nd of 15.3 mechanism will almost certainly lower magmatic 143Nd/144Nd (ref. 19). to 19.0, relative to the chondritic 143Nd/144Nd ratio of 0.51263 (ref. 17); To test the hypothesis that the least-contaminated LIPs that erupted in the stated uncertainty arises from the range of 142Nd/144Nd found in continental settings contain lavas that, like the OJP and BIWG, have Pb modern terrestrial lavas and chondrites—18 6 5 p.p.m.—that propa- isotopic compositions on the geochron, we examined only the subset of gates into uncertainty in the Sm/Nd ratio and hence into the value of the LIP magmas with high e143Nd(t)(.15.2) that fall closest to the range present-day 143Nd/144Nd of the primitive precursor. If the expectation suggested for a non-chondritic primitive mantle. Such lavas are least of chondritic relative abundances of refractory lithophile elements is likely to have suffered from assimilation of continental crust (see removed, the only remaining signatures of primitive mantle are Pb- Methods). isotopic compositions on the geochron (the line in Pb-isotopic space The ,180-Myr-old Karoo lavas are typical of continental LIPs in defined by samples with constant U/Pb ratios over the Earth’s age) and that they exhibit evidence for continental crust and lithospheric mantle enrichment in the primordial isotope of helium, 3He, relative to the contamination20. However, the high-MgO lavas recently discovered in largely radiogenic isotope, 4He. All three (Nd, Pb and He) of the isotopic the Antarctic portion of the Karoo host high e143Nd(t) ratios 143 characteristics expected for a primitive terrestrial reservoir were iden- (e Nd180 Myr 517.3 to 18.4), and these lavas plot near the Pb geo- tified in 62-Myr-old flood basalts emplaced on Baffin Island and West chron21 (Fig. 1). Greenland (BIWG)2–5. Employing the geochemical insights gained The ,251-Myr-old Siberian Traps generally have too much of a litho- from BIWG, we examine some of the largest large igneous provinces spheric overprint to enable us to discern primary mantle compositions22. 1Department of Earth Sciences, Boston University, 675 Commonwealth Avenue, Boston, Massachusetts 02215, USA. 2Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington DC 20015, USA. 316 | NATURE | VOL 476 | 18 AUGUST 2011 ©2011 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH a 12 10 PREMA DUR-8 10 8 Non-chondritic primitive mantle BI-PI-25 6 ) t 4 1 Nd( 2 143 ε Chondrite 0 OJP (freshest Kroenke) Baffin Island (high 3He/4He) –2 Sample/primitive mantle Depleted MORB mantle –4 Early-depleted reservoir (non-chondritic Earth) 0.1 –6 Rb Ba Th U Nb K La Ce Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Tm Yb Lu –8 Figure 2 | Primitive mantle13 normalized trace-element patterns of OJP b ) 3 4 15.8 lavas compared to high He/ He lavas from BIWG. For the primitive mantle composition to be consistent with past usage of this term, we use the values from ref. 13, because these have served as the model, chondrite-based, primitive 15.7 4.50 Gyr (today) mantle compositions with which the scientific community is most familiar. The NHRL 3 4 4.50 Gyr (at 2504.50 Myr) Gyr (at 120 Myr) 4.43 Gyr (at 2504.43 Myr Gyr (at 4.43120 Myr)Gyr (today) lavas from Baffin Island with highest He/ He, BI-PI-25 and DUR8, represent the normal (N) type and enriched (E) type lavas found in the BIWG LIP, Pb 15.6 respectively. Rb was not reported for BI-PI-25, and Pb concentrations were not 204 5,18 Baffin Island reported for the Baffin Island or the OJP lavas . The variability in Rb and Ba Geochron Pb/ West Greenland (and probably K) in OJP18 and BIWG2 lavas is probably due to alteration, and U 207 OJP (Malaita) 15.5 OJP (Kroenke ODP) has been dramatically modified by alteration in BIWG sample BI-PI-25 (and is OJP (Kwaimbaita ODP) not plotted). Only Kroenke-type OJP lavas are plotted, because they have Kerguelen probably experienced only olivine fractionation, whereas Kwaimbaita-type Deccan 15.4 PREMA Antarctic Karoo lavas are isotopically similar (Fig. 1) but are more evolved. All OJP and BIWG Siberia lavas are corrected for olivine fractionation to be in equilibrium with an olivine Ocean island basalt MORB composition with a forsterite content of 92% (ref. 2). Only the freshest 15.3 Kroenke-type lavas have been plotted, excluding the following samples (using 17 18 19 20 21 22 the same filter as ref. 18): volcaniclastics and lavas with loss on ignition .0.5% 2 206Pb/204Pb and/or K2O/P2O5 . 2. The non-chondritic primitive mantle (or early depleted reservoir; see Supplementary Information) and depleted MORB mantle15 are Figure 1 | Lavas from LIPs that have Nd isotopic compositions within the included.
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