Missing Pb, High 3He/4He, Ancient Sulfides and Continent Formation

Shichun Huang¹, Cin-Ty A. Lee² & Qing-Zhu Yin³

¹Department of Geoscience, University of Nevada, Las Vegas, shichun.huang@unlv.edu

²Department of Earth Science, Rice University, ctlee@rice.edu

³Department of Earth and Planetary Sciences, University of California, Davis, qyin@ucdavis.edu

This webpage is a summary of: Huang, Shichun, Cin-Ty A. Lee & Qing-Zhu Yin, Missing Lead and High ³He/⁴He in Ancient Sulfides Associated with Continental Crust Formation, Scientific Reports, 4, DOI: 10.1038/srep05314

Introduction

Continent formation is a major process that fractionates the Earth. It generates complementary reservoirs that may be sampled by ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) (e.g., Hofmann, 1988; Jacobsen, 1988). For example, Parman (2007) found that the pulses of continent formation, represented by age distribution of continental zircons, are couple with ³He/⁴He distribution peaks in OIBs and MORBs. Lee et al. (2012) found that continental crust is depleted in Cu, and they attributed this Cu depletion to fractionation of sulfide-rich mafic cumulates under arcs during continent formation. Since Cu is highly compatible in sulfides, sulfide-rich mafic cumulates formed during arc magma evolution are enriched in Cu. These sulfide-rich mafic cumulates are then removed from continental crust because of their higher density, a process called delamination, which in turn, results in a felsic and Cu-poor composition for continental crust. Such sulfide-rich mafic cumulates are delaminated into the mantle or stay at the bottom of the continental lithosphere, and may be sampled by OIBs or continental flood basalts (CFBs) via mantle plumes, small-scale convective instabilities, or melting during lithospheric heating and extension.

What are the distinctive geochemical signatures of such sulfide-rich mafic cumulates?

Sulfides are enriched in Pb and depleted in U and Th (e.g., Hart & Gaetani, 2006). Based on published noble gas solubility studies, which show that the solubility of a noble gas in a liquid is a function of the macroscopic surface tension of the liquid (e.g., Uhlig, 1937; Blander et al., 1959; Jambon et al., 1986; Lux, 1987), Huang et al. (2014) showed that He solubility in sulfide melt is close to that in basaltic melt. Consequently, they argue that the sulfide-rich mafic cumulates are also enriched in He. That is, sulfide-rich mafic cumulates are characterized by low (U+Th)/Pb and (U+Th)/He, and they would “freeze” in the ³He/⁴He, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb*/²⁰⁶Pb* ratios of the ambient mantle at the time of their formation. (²⁰⁷Pb*/²⁰⁶Pb* measures the ratio of radiogenic ingrowth of ²⁰⁷Pb and ²⁰⁶Pb since the formation of the Earth.) If formed early in Earth’s history, they would be characterized by high ³He/⁴He and ²⁰⁷Pb*/²⁰⁶Pb*, and low ²⁰⁶Pb/²⁰⁴Pb, i.e., unradiogenic He and Pb isotopic signatures.

Are such distinctive geochemical signature observed in OIBs and CFBs?

Jackson et al. (2010) argued that at Baffin Island, high ³He/⁴He is associated with an ancient Pb isotopic signature, i.e., low ²⁰⁶Pb/²⁰⁴Pb and high ²⁰⁷Pb*/²⁰⁶Pb*. Recently, Huang et al. (2014) showed that ³He/⁴He is negatively correlated with ²⁰⁶Pb/²⁰⁴Pb and positively correlated with ²⁰⁷Pb*/²⁰⁶Pb* in OIBs (Figure 1).

Figure 1: a: ²⁰⁶Pb/²⁰⁴Pb vs. ³He/⁴He for sulfides formed at different ages and ocean island basalts (OIBs) and continental flood basalts (CFBs). Lines are cross plots of ²⁰⁶Pb/²⁰⁴Pb vs. ³He/⁴He for sulfides formed at different ages. Since sulfides have very low U/Pb and (U+Th)/He ratios, they preserve the mantle ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb*/²⁰⁶Pb* and ³He/⁴He signatures at the time they formed. In OIBs, high-³He/⁴He is associated with low-²⁰⁶Pb/²⁰⁴Pb. b, c: ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb*/²⁰⁶Pb* vs. ³He/⁴He for the average of each OIB-CFB suite. No age-corrections were applied to the ³He/⁴He of Baffin Island and West Greenland. Consequently the plotted values are their minima. d, e: average ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb*/²⁰⁶Pb* vs. maximum ³He/⁴He of each OIB-CFB suite.

Of particular interest, Huang et al. (2014) also observed a similar correlation in Yellowstone-Snake River Plain basalts (Figure 2), one of the few regions of continental intraplate volcanism that has been extensively studied for Pb and He isotopes. This volcanic track has been postulated to be formed by the Yellowstone plume (e.g., Hanan et al., 2008). Along this track, ³He/⁴He and ²⁰⁷Pb*/²⁰⁶Pb* increase and ²⁰⁶Pb/²⁰⁴Pb decreases from west to east. Lavas erupted on the older Precambrian North American Craton have higher ³He/⁴He and ²⁰⁷Pb*/²⁰⁶Pb*, and lower ²⁰⁶Pb/²⁰⁴Pb while lavas emplaced through the younger Mesozoic accreted terrains have lower ³He/⁴He and ²⁰⁷Pb*/²⁰⁶Pb*, and higher ²⁰⁶Pb/²⁰⁴Pb. Such correlations may reflect sampling of sulfide-rich cumulates with different ages, i.e. Precambrian vs. Mesozoic.

Figure 2: ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb*/²⁰⁶Pb* and ³He/⁴He vs. longitude for basalts from the Northwestern United States. High-³He/⁴He is associated with low-²⁰⁶Pb/²⁰⁴Pb. The ⁸⁷Sr/⁸⁶Sr = 0.706 line indicates the boundary of the North American Craton.

The sulfide-rich mafic cumulate model also provides alternative interpretations for two long-lasting questions in the field of mantle geochemistry–the origin of high-³He/⁴He, and the classic “Pb isotope paradox”.

Many OIBs have ³He/⁴He higher than that in MORBs, which sample the Earth’s upper mantle. The origin of the high ³He/⁴He mantle reservoir is the center of many debates. Since ⁴He is mostly produced by U-Th-Pb decay, a high ³He/⁴He signature requires an ancient reservoir with low (U+Th)/He. In most cases, He escapes to the atmosphere. Therefore, in the traditional view, any mantle process would increase the (U+Th)/He ratio. Consequently, the high ³He/⁴He reservoir has been postulated to be the primitive mantle (e.g., Kurz et al., 1983; Jackson et al., 2010), partially degassed, or less processed primitive mantle (e.g., Class & Goldstein, 2005; Porcelli & Elliott, 2008; Gonnermann & Mukhopadhyay, 2009). According to another view, a low (U+Th)/He reservoir may form by dissolving or diffusing He into a depleted mantle reservoir with low U+Th abundances (e.g., Parman et al., 2005; Albarède, 2008; Hart et al., 2008; Jackson et al., 2013). That is, He added to a depleted mantle reservoir forms a low (U+Th)/He reservoir.

Although one of the best studied elements, the Pb budget of the Earth is not fully understood. Major crustal and mantle reservoirs have Pb isotopes more radiogenic than bulk silicate Earth. Consequently, there must be a reservoir, whose location is unknown, with unradiogenic Pb (Allègre, 1968). This is the classic “Pb isotope paradox”. The missing unradiogenic Pb has been argued to be in the lower continental crust (e.g., O’Nions et al., 1979) or dissolved in the core early in Earth’s history (e.g., Allègre et al., 1995; Wood & Halliday, 2010). However, granulite terranes are not sufficiently unradiogenic (e.g., Zartmen & Haines, 1988; Rudnick & Goldstein, 1990; Rudnick & Gao, 2003) and the timing of core formation implied by Pb sequestration into the core is too young compared to that determined by ¹⁸²Hf-¹⁸²W systematics (Kleine et al., 2002; Yin et al., 2002).

In the model of Huang et al. (2014), ancient sulfide-rich mafic cumulates are the host of both the “missing Pb” and the high-³He/⁴He signature. Thus, the high-³He/⁴He, high-²⁰⁷Pb*/²⁰⁶Pb* and low-²⁰⁶Pb/²⁰⁴Pb reservoirs in the OIB-CFB sources may correspond to the complements of continent formation rather than primordial parcels of mantle that have remained unprocessed through Earth’s entire history.

References