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Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts

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Science  13 May 2016:
Vol. 352, Issue 6287, pp. 809-812
DOI: 10.1126/science.aad8563

Isotopes isolated after impact

Details about how Earth formed are gleaned from the daughter products of certain short-lived radioactive isotopes found in rocks. Rizo et al. describe subtle tungsten isotope variations in rocks from the very deep mantle in Baffin Island and the Ontong Java Plateau (see the Perspective by Dahl). The results suggest that portions of Earth have remained unmixed since it formed. The unmixed deep mantle rocks also imply that Earth's core formed from several large impact events.

Science, this issue p. 809; see also p. 768

Abstract

How much of Earth's compositional variation dates to processes that occurred during planet formation remains an unanswered question. High-precision tungsten isotopic data from rocks from two large igneous provinces, the North Atlantic Igneous Province and the Ontong Java Plateau, reveal preservation to the Phanerozoic of tungsten isotopic heterogeneities in the mantle. These heterogeneities, caused by the decay of hafnium-182 in mantle domains with high hafnium/tungsten ratios, were created during the first ~50 million years of solar system history, indicating that portions of the mantle that formed during Earth’s primary accretionary period have survived to the present.

Four and a half billion years of geologic activity have overprinted much of the evidence for the processes involved in Earth’s formation and initial chemical differentiation. High-precision isotopic measurements of the decay products of short-lived radionuclides that were present when Earth formed can provide a view of events that occurred during the first tens to hundreds of million years of Earth history. Data from both the 146Sm-142Nd (half-life, t1/2 = 103 million years) and 129I-129Xe (t1/2 = 15.7 million years) systems show the importance of early mantle differentiation and outgassing events but provide conflicting evidence about the preservation of early-formed mantle reservoirs to the present day (14). Of the short-lived systems, the 182Hf-182W (t1/2 = 8.9 million years) system is distinctively sensitive to metal-silicate separation and has been used effectively to trace the timing and processes of core formation (5), which is arguably the most important chemical differentiation event to occur on a rocky planet. Only recently, however, have measurement techniques improved to the point of resolving 182W/184W variability in ancient (>2.7 billion years old) terrestrial rocks; such variability reflects the preservation of compositionally distinct domains in Earth’s interior that were probably created during Earth’s formation (610). Young mantle-derived rocks examined to date have shown neither 142Nd nor 182W isotopic heterogeneity, suggesting that the early-formed compositional domains in Earth’s interior were largely destroyed by mantle-mixing processes during the first half of Earth history (14, 610). Here we report 182W/184W ratios in Phanerozoic flood basalts from Baffin Bay and the Ontong Java Plateau, some of which are among the highest ever measured in terrestrial rocks. These results document the preservation of regions within Earth’s interior whose compositions were established by events that occurred within the first ~50 million years of solar system history. This study, consequently, provides new insights into the processes at work during planet formation, the chemical structure of Earth’s interior, and the interior dynamics that allowed the preservation of chemical heterogeneities for 4.5 billion years.

Flood basalts are the largest volcanic eruptions identified in the geological record. These types of eruptions created both the North Atlantic Igneous Province, which hosts the Baffin Bay locale (11), and the Ontong Java Plateau in the western Pacific Ocean (12). We studied pillow lavas with high-MgO picritic compositions (table S1) from Padloping Island, Baffin Bay (samples Pi-23 and Pd-2). We targeted these rocks because some Baffin Bay lavas contain the highest 3He/4He ratios ever measured (13), along with Pb isotopic compositions (14) and D/H ratios (15) that indicate that their mantle source was relatively primitive and undegassed, consistent with it being isolated since shortly after Earth’s formation. Ontong Java is Earth’s largest known volcanic province and shares chemical and isotopic similarities with the Baffin Bay lavas, consistent with a primitive mantle source (16). The Ontong Java sample (192-1187A-009R-04R) is a basalt (table S1) collected from the plateau’s eastern flank by Ocean Drilling Project Leg 192.

We present data from the short-lived 182Hf-182W and 146Sm-142Nd systems, because these two systems are variably sensitive to the core formation and mantle differentiation processes that occurred early in Earth history. We compare these data with data from the long-lived U-Th-He, 147Sm-143Nd, and 187Re-187Os isotope systems, together with W and highly siderophile element (HSE; Re, Os, Ir, Ru, Pt, and Pd) concentrations, to better distinguish early differentiation events from those occurring over all of Earth history.

Glassy rim and core pieces of sample Pi-23 (Pi-23a and Pi-23b, respectively), a bulk sample of Pd-2, and a bulk sample of 192-1187A-009R-04R are characterized by high 182W/184W ratios that are well resolved from standards, with μ182W values ranging from +10 to +48 {where μ182W = [(182W/184W)sample/(182W/184W)standard – 1] × 106} (Fig. 1 and Table 1) (17). The μ182W values for samples Pi-23a and Pi-23b are in good agreement. This rules out the influence of stable W isotope fractionation through interaction of seawater with the pillow rim in creating the measured 182W values (17). Samples 192-1187A-009R-04R and Pd-2 had the lowest W concentrations and the highest μ182W values (Table 1). The W concentrations of these two samples [23 and 26 parts per billion (ppb), respectively] are broadly consistent with those of magmas derived by 15-to-20% partial melting of a mantle source with ~5 ppb W, indicating a primitive source free of W-rich recycled crust (fig. S2) (17). The geological reference materials VE-32 (mid-ocean ridge glass) and BHVO-1 (Hawaiian basalt) were measured at the same time as the Baffin Bay and Ontong Java samples and yielded μ182W values of –0.8 ± 4.5 and –2.3 ± 7.7, respectively (Table 1). These μ182W values are indistinguishable from the terrestrial Alfa Aesar W standard (μ182W = 0) and other modern rocks (610). The 3He/4He ratios measured in olivines from the Baffin Bay samples (table S3) (17) yielded values up to 48.4 times higher than the 3He/4He ratio of the atmosphere (1.39 × 10−6) in agreement with previous findings (13), indicating that the source of these lavas is relatively undegassed and possibly has been isolated since Earth’s formation. The HSE abundances and initial 187Os/188Os ratios (which provide a record of the long-term Re/Os ratio) from the Baffin Bay and Ontong Java Plateau samples are indistinguishable from other modern mantle-derived lavas with similar MgO abundances that do not show elevated μ182W (Fig. 2 and table S4) (18).

Fig. 1 μ182W values measured for the Baffin Bay and Ontong Java Plateau samples, the geological reference materials VE-32 and BHVO-1, and the Alfa Aesar W standard.

The values are expressed as deviations, in parts per million (ppm), from the average value measured for the W standard. The gray shaded area represents 2σ for the average W standard value. Errors for each data point are 2σ.

Table 1 Tungsten concentrations and isotopic compositions.

Included are data from the Baffin Bay samples Pi-23a, Pi-23b, and Pd-2; the Ontong Java Plateau sample 192-1187A-009R-04R; the mid-ocean ridge glass sample VE-32; and the BHVO-1 basalt standard. Uncertainties are ±2σ. More details are given in (17) and table S2.

View this table:
Fig. 2 HSE abundances for the Baffin Bay and Ontong Java Plateau (OJP) samples.

Abundances are normalized to the HSEs of carbonaceous chondrites (CI group) from (25). The gray shaded area shows the range of HSE abundances for type-2 Hawaiian picrites (26).

Variability in 182W/184W ratios reflects Hf/W fractionation while 182Hf was extant. Hf/W fractionation has been observed in early solar system materials (5), so variable W isotopic compositions in terrestrial samples can reflect the imperfect mixing of late additions of such materials (6, 9). The μ182W value of +48 for Baffin Bay sample Pd-2, however, is larger than can be accounted for by this process, and so this possibility is discounted (supplementary text). Hf/W fractionation can also occur as the result of endogenous Earth differentiation processes, such as magma ocean crystallization (7) and core formation (9). However, silicate fractionation processes cannot be responsible for the generation of the anomalous 182W in the sources of the Baffin Bay and Ontong Java lavas. If the high μ182W was due to silicate fractionation in a magma ocean while 182Hf was extant, then μ182W should positively correlate with μ142Nd, the decay product of the short-lived 146Sm (t1/2 = 103 million years) isotope system. Instead, the μ142Nd values of the samples are indistinguishable from all other modern basalts measured so far (fig. S3 and table S5) (17).

This leaves Hf/W fractionation resulting from metal-silicate segregation accompanying core formation as the probable cause of the observed anomalies in the Phanerozoic samples. Metal-silicate segregation is the process that can fractionate Hf/W most effectively, because Hf is a strongly lithophile trace element, whereas W is moderately siderophile. The low W concentrations estimated for the mantle source of the flood basalts described here are consistent with mantle domains that experienced metal-silicate segregation (Table 1 and supplementary text). Repeated metal-silicate segregation events during planet formation (19) could create one or more mantle domains with distinct μ182W, without affecting the Sm-Nd system. Such events would result in variable μ182W, due to metal-silicate segregation events occurring at different times (Fig. 3), or, alternatively, different Hf/W ratios in the resulting mantle reservoirs that reflect different oxidation states and, hence, different partitioning of W into metal (supplementary text).

Fig. 3 Model for the creation of mantle reservoirs with distinct W isotopic compositions.

(A) Early core formation leaves the proto-Earth’s mantle with a high Hf/W ratio that, with time, evolves to a high μ182W value (i). (B) The impact of a large body affects the Hf/W ratio and W isotopic composition of a portion of the proto-Earth’s mantle. (C) Evolution of the portion of the mantle (ii) affected by the impact of a large body, involving some degree of isotopic equilibration between the impactor materials and the mantle. The core of the impactor subsequently merges with the core of the proto-Earth. (D) Possible scenario after isostatic adjustment (27) and creation of a mantle with heterogeneous μ182W through impacts of large bodies. Mantle domains affected by impacts that occur after the extinction of 182Hf no longer generate radiogenic 182W, so their 182W/184W ratios can change only by mixing with other terrestrial reservoirs or with late-accreted chondritic material. (E) Late accretion, representing ~0.5% of Earth’s mass, decreases the 182W/184W ratio of all the earlier-formed reservoirs by ~15 ppm. This last accretion is responsible for endowing the modern mantle with chondritic relative abundances of the HSEs.

The key observation to consider, which is supported by the results reported here, is that terrestrial samples with 182W excesses do not seem to derive from sources depleted in HSEs (Fig. 2) (610). HSEs have partition coefficients between metal and silicate of >104 (20); thus, their concentrations in metal-depleted mantle domains are expected to be very low. Evolving oxidation states during Earth accretion might explain the decoupling of 182W and HSEs, because although W becomes less siderophile under oxidizing conditions, the HSEs, even at high oxidation states, are not soluble in silicates (20). This model, however, would require subsequent late-accreted HSEs to have been mixed into different mantle domains without the mixing away of W isotopic heterogeneity. Alternatively, the observed decoupling could be explained if some metal from the core of the Moon-forming giant impactor was retained in the mantle, followed by a minor amount of late accretion (9). This model would require a substantial mass of high-density metal to have been retained in Earth's mantle after the impact, when the mantle was partially or wholly molten, and this retained metal would have to have contained chondritic relative abundances of the HSEs. These models are summarized in the supplementary text, along with a few additional potential explanations for the apparent decoupling of W isotopic compositions and HSE abundance variations.

Regardless of the origin of the 182W variability, what is arguably more unexpected than the fact that Earth experienced such early differentiation events is that reservoirs formed by these early processes remain in the mantle today. This conclusion is now supported by isotopic variability in both W and Xe (1, 21), but not in 142Nd, which suggests that the observed heterogeneity in 142Nd/144Nd ratios was reduced to an unobservable level by the end of the Archean, probably through the mixing caused by mantle convection. Perhaps the key to reconciling these observations is that the 129I-129Xe system primarily reflects mantle outgassing, and the 182Hf-182W system reflects metal-silicate separation, whereas the 146Sm-142Nd system is controlled by internal mantle differentiation. For both W and Xe, one component of the complementary chemical differentiation (the core for W and the atmosphere for Xe) may not be available for effective recycling and mixing in the mantle (22). In contrast, for Nd, the main reservoirs created during early Earth differentiation may have been in a portion of the mantle that has since been effectively mixed by mantle circulation. Estimates for how much of the mantle can remain unmixed depend on the rheological properties assigned to the various materials involved. Some models (23) show that as much as 20% of the mantle may remain isolated as distributed masses. An important aspect of the results presented here is that both 182W anomalies and elevated 3He/4He ratios (table S3) appear in at least two major flood basalts. Flood basalt eruptions require the melting of large volumes of mantle during unusual thermal events in the history of mantle circulation. The large size and sporadic nature of flood basalt eruptions are perhaps indicative of a layer in the mantle whose density or rheological properties keep it from effectively mixing with the rest of the mantle. The large low–shear velocity provinces (LLSVPs) that have been imaged at the base of the mantle (24) may constitute such a reservoir. These regions appear to be warmer and compositionally different from the surrounding mantle. Estimates of their volume range to as high as 7% of the mantle, or on the order of 6 × 1010 km3. If the LLSVPs are remnants of early differentiation events on Earth, they must have a delicately balanced density contrast relative to the surrounding mantle to allow their survival through 4.5 billion years of dynamic Earth history.

Supplementary Materials

www.sciencemag.org/content/352/6287/809/suppl/DC1

Materials and Methods

Figs. S1 to S6

Tables S1 to S5

References (2856)

References and Notes

  1. Materials and methods are available as supplementary materials on Science Online.
Acknowledgments: We thank T. Mock for assistance with mass spectrometry. This work was improved by helpful discussions with J. O’Neil, S. Shirey, and B. Wood. We thank S. Shirey, who provided sample VE-32. We also appreciate the helpful comments and suggestions from three anonymous reviewers. We thank M. Garçon, who developed the four-step 142Nd acquisition method. This work was supported by NSF grant EAR-1265169 (Cooperative Studies of the Earth’s Deep Interior program) to R.J.W. and grant EAR-1250419 to S.M. All data are available in the main manuscript and supplementary materials.
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