Ancient helium and tungsten isotopic signatures preserved in mantle domains least modified by crustal recycling

Abstract

Rare high-³He/⁴He signatures in ocean island basalts (OIB) erupted at volcanic hotspots derive from deep-seated domains preserved in Earth's interior. Only high-³He/⁴He OIB exhibit anomalous ¹⁸²W-an isotopic signature inherited during the earliest history of Earth-supporting an ancient origin of high ³He/⁴He. However, it is not understood why some OIB host anomalous ¹⁸²W while others do not. We provide geochemical data for the highest-³He/⁴He lavas from Iceland (up to 42.9 times atmospheric) with anomalous ¹⁸²W and examine how Sr-Nd-Hf-Pb isotopic variations-useful for tracing subducted, recycled crust-relate to high ³He/⁴He and anomalous ¹⁸²W. These data, together with data on global OIB, show that the highest-³He/⁴He and the largest-magnitude ¹⁸²W anomalies are found only in geochemically depleted mantle domains-with high ¹⁴³Nd/¹⁴⁴Nd and low ²⁰⁶Pb/²⁰⁴Pb-lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low ³He/⁴He and lack anomalous ¹⁸²W. These observations provide important clues regarding the survival of the ancient He and W signatures in Earth's mantle. We show that high-³He/⁴He mantle domains with anomalous ¹⁸²W have low W and ⁴He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low ³He/⁴He and normal (not anomalous) ¹⁸²W characteristic of subducted crust. Thus, high ³He/⁴He and anomalous ¹⁸²W are preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high ³He/⁴He and anomalous ¹⁸²W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth's interior.

Keywords: 182W; 3He/4He; Hadean; hotspot volcanism; mantle geochemistry.

Fig. 1.

Mantle tetrahedron showing that most of the highest ³He/⁴He lavas in Iceland, and global hotspots, do not overlap with the common mantle component [shown as ellipsoid (27)]. Four different high-³He/⁴He (>25 Ra) components in Iceland (Vestfirðir, ERZ, SIVZ, and Baffin Island) are shown. The highest ³He/⁴He (>30 Ra) samples from each global oceanic hotspot with available Sr-Nd-Pb isotopes—Galapagos, Hawaii, and Samoa—are also shown (data sources for these additional lavas are in SI Appendix, Fig. S4). EM1, EM2, and HIMU apices of the tetrahedron are published elsewhere (27), as is MORB (63), but EM2 extends to more extreme compositions (36) (indicated by arrow). Data for the highest ³He/⁴He lavas with available Sr-Nd-Pb isotopes from the different Iceland high-³He/⁴He (>25 Ra) localities are from Dataset S1; the Baffin Island lava is the highest ³He/⁴He (39.9 Ra) lava that is the least crustally contaminated (23). Measured values are shown for all lavas except for Baffin Island, where both age-corrected (c) and measured compositions are shown: The calculated present-day Baffin Island mantle source is indistinguishable from the measured composition in the figure [see (23) and SI Appendix, Fig. S4 legend for details of the age correction and present-day source calculation].

Fig. 2.

Relationships between ³He/⁴He, ⁸⁷Sr/⁸⁶Sr, and ²⁰⁶Pb/²⁰⁴Pb in Iceland hotspot lavas showing four geochemically distinct high-³He/⁴He Iceland components (Vestfirðir, ERZ, SIVZ, and Baffin Island). The highest ³He/⁴He lavas from Hawaii, Galapagos, and Samoa also exhibit heterogeneous Sr-Nd-Pb isotopic compositions. Data sources for these lavas are in SI Appendix, Fig. S4. Only the least crustally contaminated Baffin Island lavas are shown (23). Measured heavy radiogenic isotopic compositions are shown for all samples except for Baffin Island, where present-day mantle source values are plotted (23) [see (23) and SI Appendix, Fig. S4 legend for details of present-day source calculation]. For Iceland hotspot lavas, only high-precision Pb isotopic data (MC-ICP-MS and double-spike TIMS) are shown. (Top) The colored symbols represent lavas with ³He/⁴He >21 Ra sampling all known high-³He/⁴He regions of the Iceland hotspot—ERZ, SIVZ, NRZ (Vaðalda), Vestfirðir, and BIWG—and white symbols represent Iceland lavas (all of which have ³He/⁴He <21 Ra) from the other regions of Iceland. (Bottom) Colored symbols represent lavas with ³He/⁴He >21 Ra sampling all known high-³He/⁴He regions of the Iceland hotspot, and the white symbols represent Iceland lavas with ³He/⁴He ≤21 Ra (or lavas that have not been characterized for ³He/⁴He).

Fig. 3.

Relationships between ³He/⁴He, μ¹⁸²W, ¹⁴³Nd/¹⁴⁴Nd, and ²⁰⁶Pb/²⁰⁴Pb in global OIB showing that only geochemically depleted OIB (i.e., high ¹⁴³Nd/¹⁴⁴Nd and low ²⁰⁶Pb/²⁰⁴Pb) lacking recycled crust signatures host the highest ³He/⁴He and largest-magnitude μ¹⁸²W anomalies. The compilation of He-Sr-Nd-Pb isotopes (from this study and prior studies; Dataset S4) for lavas with available μ¹⁸²W shows that only lavas with low ²⁰⁶Pb/²⁰⁴Pb (Top) or high ¹⁴³Nd/¹⁴⁴Nd (Middle) host the most extreme negative μ¹⁸²W anomalies. Lavas with the most extreme negative μ¹⁸²W anomalies also exhibit high ³He/⁴He (Bottom, where only lavas with available μ¹⁸²W data are shown). In contrast, the mantle endmembers EM1, EM2, and HIMU have low ³He/⁴He and lack anomalous μ¹⁸²W, as does DM. The chondritic reference ¹⁴³Nd/¹⁴⁴Nd is published elsewhere (64). All data and error bars (2σ) are provided in Dataset S4. The ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb for OJP is an average of Kroenke- and Kwaimbaita-type lavas (65); present-day mantle source compositions (23) are show in lieu of measured or age-corrected ages due to the 120-Ma eruption age. The OJP μ¹⁸²W data that are shown lack of anomalous signatures (41, 46).

Fig. 4.

Impact of mixing subducted crust and DM on OIB mantle domains with high-³He/⁴He and anomalous μ¹⁸²W compositions. The mixing models demonstrate how subducted crustal materials, and DM, reduce ³He/⁴He and overprint resolvable μ¹⁸²W anomalies in the mantle, thereby generating the negatively sloping OIB array. Sediment (large gray box), oceanic crust (large gray triangle), and depleted mantle (DM, large black box) are each mixed with four OIB mantle sources with high ³He/⁴He and anomalous μ¹⁸²W. Endmember compositions used are in Dataset S5. Samoa and Hawaii have indistinguishable trends (–16), but Samoan μ¹⁸²W is characterized to higher ³He/⁴He. Intervals are marked at 10% for mixtures with DM; for mixtures with sediment and oceanic crust, intervals are marked at 1% for the first 10% addition of subducted material, thereafter every 20%. Sediment (derived from continental crust) and oceanic crust, when mixed with components that have high-³He/⁴He and anomalous μ¹⁸²W, generate compositions similar to the extreme Samoan EM2 lava (5% subducted sediment), the extreme Pitcairn EM1 lava (3% sediment), and the extreme Mangaia HIMU lavas (25% oceanic crust) (Datasets S4 and S5). OIB data symbols are the same as in Fig. 3. OIB data and error bars (2σ) are provided in Dataset S4.

Fig. 5.

μ¹⁸²W as a function of D^Sr-Nd-Pb showing that OIB sampling significant recycled crust (represented by extreme EM1, EM2, and HIMU lavas), or MORB, lack μ¹⁸²W anomalies. (Left) Samples with the lowest D^Sr-Nd-Pb (defined in the main text) have the smallest contributions from the EM1, EM2, and HIMU mantle endmembers (and the smallest recycled crust contributions). Samples with the highest D^Sr-Nd-Pb plot closest to EM1, EM2, and HIMU in Sr-Nd-Pb isotopic space (and host the greatest recycled crust contributions). Binary mixing lines are shown between a melt with anomalous μ¹⁸²W and melts of EM1 (blue line), EM2 (pink line), HIMU (purple line), and MORB melts of DM (black mixing line) (see Dataset S6 for model compositions); 10% mixing increments are shown. Symbols are the same as Fig. 3 and data (and 2σ errors) are from Dataset S4 (except for OJP data, which are indicated in Fig. 3). (Right) Illustration of calculation of D^Sr-Nd-Pb values and examples of three large D^Sr-Nd-Pb values, one medium D^Sr-Nd-Pb value, and one small D^Sr-Nd-Pb value; all D^Sr-Nd-Pb values are distances in ⁸⁷Sr/⁸⁶Sr-¹⁴³Nd/¹⁴⁴Nd-²⁰⁶Pb/²⁰⁴Pb isotope space calculated between the depleted reference composition (Depl. Ref. Comp.) and OIB samples.