Constraints on continental crustal mass loss via INAUGURAL ARTICLE chemical weathering using lithium and its isotopes Xiao-Ming Liu1 and Roberta L. Rudnick1 Department of Geology, University of Maryland, College Park, MD 20742 This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2010. Contributed by Roberta L. Rudnick, November 4, 2011 (sent for review August 2, 2011) Chemical weathering, as well as physical erosion, changes the com- incompatible trace element, whose concentration in crustal rocks position and shapes the surface of the continental crust. However, varies by several orders of magnitude, while Mg, a major element, the amount of continental material that has been lost over Earth’s may vary in concentration by only a factor of two to three. Thus, history due to chemical weathering is poorly constrained. Using a the Li/Mg ratio is sensitive to the more variable Li concentration. mass balance model for lithium inputs and outputs from the con- In addition, Li is a trace element whose partitioning may not al- tinental crust, we find that the mass of continental crust that has ways follow Mg, and, with newer data (see Dataset S1), it is clear been lost due to chemical weathering is at least 15% of the original that the Mg/Li ratio in river waters is quite variable, reflecting the mass of the juvenile continental crust, and may be as high as 60%, influence of factors such as watershed lithology, that influence with a best estimate of approximately 45%. Our results suggest riverine Mg/Li, in addition to continental weathering. that chemical weathering and subsequent subduction of soluble Here, we explore the utility of using a single soluble element, 6 7 elements have major impacts on both the mass and the composi- Li, and its isotopes ( Li and Li), to constrain the mass of con- tional evolution of the continental crust. tinental crust lost to weathering, employing a mass balance ap- proach similar to that of Lee et al. (6). While there are a arc basalts ∣ lithium isotopes ∣ mass balance modeling ∣ number of assumptions that go into this calculation, giving rise crust composition ∣ chemical weathering rate to a family of outcomes, our aim here is not so much to provide the answer regarding the mass of continental crust lost to chemi- EARTH, ATMOSPHERIC, t is well established that the average composition of the conti- cal weathering, but rather, to place limits on this mass. AND PLANETARY SCIENCES Inental crust is intermediate or “andesitic,” if described in terms Lithium Isotopes ¼ 57 ∼ 64 of an igneous rock type (SiO2 wt. %) (1 and references Lithium has two stable isotopes with the following relative abun- therein). However, the magmas that generate the present-day dances: 6Li ∼ 7.5% and 7Li ∼ 92.5%. Because the mass difference continental crust are dominantly basalt (2 and references there- between these two isotopes is relatively large (approximately in). This discrepancy has been referred to as the “crust composi- ” 16%), they show significant mass dependent fractionation in nat- tion paradox (3). Various hypotheses have been proposed to ure (>50‰) (12), expressed in δ7Li notation: δ7Lið‰Þ¼ solve this paradox, including stripping of Mg through chemical ð½7 ∕6 ∕½7 ∕6 − 1Þ × 1000 – Li Li sample Li Li standard , where the standard weathering (4 6), removal of mafic/ultramafic lower crust used is a synthetic Li carbonate, L-SVEC (13). During chemical – through foundering/delamination (7 9), subduction of continen- weathering, secondary minerals, such as clays, take 6Li preferen- “ ” tal crust followed by relamination of buoyant, felsic crust (10), tially into their structure, resulting in heavier Li isotopic compo- or direct addition of tonalites to the crust through slab melting in sition in rivers and lighter isotopic composition in the regolith a hotter Archean Earth (e.g., 2, 3, 11). (14–22). The lithium concentration and isotopic composition During chemical weathering of the continents, soluble ele- of the continental crust, as well as river waters, are well documen- ments (e.g., Na, Ca, Mg, and Li) are dissolved and transported ted by various authors (17, 19–26). In addition, Li concentration to the oceans via rivers and/or groundwater, while insoluble and isotopic composition of potential building blocks of the elements, such as Si and Al, remain in the continental regolith. continental crust, namely, basaltic arc lavas (28–32) and Archean Ultimately, these soluble components may be recycled into the tonalites, trondhjemites, and granodiorites (TTG, 17, 33) are mantle by subduction (e.g., Mg and Ca, 5) or may reenter con- known. Collectively, these studies demonstrate that the δ7Li of tinental crust via arc magmatism (e.g., Na). Therefore, chemical present-day bulk continental crust is 2–3‰ lower than that of weathering may be an important process that controls the mass, its potential building blocks (i.e., mantle-derived basalts), which the composition and the evolution of the continental crust. How- likely reflects the influence of chemical weathering on the bulk ever, only one attempt has previously been made to quantify the crust composition (17, 24). Therefore, Li and its isotopes may influence of chemical weathering on the mass and composition of be useful in quantifying the amount of continental crust lost the continental crust (6). through chemical weathering. Using a mass balance model coupled with the correlation ob- served between lithium and magnesium in modern river waters Mass Balance Model and assumptions regarding the mass lost from the continental The conceptual model of continental crust recycling is illustrated crust due to lower crustal recycling, Lee et al. (6) argued that ap- in Fig. 1, where juvenile arc basalts or felsic slab melts form the proximately 20% of the juvenile continental crustal mass has input to the crust, and there are three outputs: recycled lower been lost from the continental crust due to chemical weathering. crust (lower crust that is lost from the continents by foundering, However, the Li concentration they used for primitive island arc basalts (15 ppm) is about a factor of two higher than average com- Author contributions: X.-M.L. and R.L.R. designed research; X.-M.L. performed research; positions of arc basalts or other oceanic basalts [e.g., mid-ocean X.-M.L. and R.L.R. analyzed data; and X.-M.L. and R.L.R. wrote the paper. ridge basalts (MORB) or ocean island basalts (OIB) see below]. The authors declare no conflict of interest. When using a more accurate concentration (7 ppm), the Mg/Li 1To whom correspondence may be addressed. E-mail: [email protected] or rudnick@ mass balance becomes untenable, as the proportion of bulk con- umd.edu. tinental crust exceeds one, implying mass addition due to chemi- This article contains supporting information online at www.pnas.org/lookup/suppl/ cal weathering. A fundamental problem is that Li is a moderately doi:10.1073/pnas.1115671108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1115671108 PNAS ∣ December 27, 2011 ∣ vol. 108 ∣ no. 52 ∣ 20873–20880 Downloaded by guest on September 26, 2021 where, for an isotope i, the concentration of i in JCC, BCC, RLC, Ci Ci Ci Ci Ci X DIS, and TER are JCC, BCC, RLC, DIS, and TER. And BCC, X X X RLC, DIS, and TER are the mass fractions of BCC, RLC, DIS, and TER relative to JCC, respectively. The fraction of i lost by Ci f i f i ¼ X DIS dissolution is DIS, where DIS DIS Ci . For example, we have JCC 7 6 7 C Li 6 C Li Li DIS Li DIS 7 6 f ¼ X 7 and f ¼ X 6 for Li and Li, respectively. DIS DIS C Li DIS DIS C Li JCC JCC Similar definitions are applied for the fraction of i in the present- day bulk continental crust (BCC), recycled lower crust (RLC), and terrigenous sediments removed via subduction (TER). Thus, Eq. 1 becomes f i þ f i þ f i þ f i ¼ 1 [2] Fig. 1. Cartoon illustrating the mass balance approach used for solving the BCC RLC DIS TER weathering flux from the continents (DIS). Juvenile crust is created via a JCC magmatic flux from the mantle that produces either a basalt (JCC-1) or felsic Rearranging Eqs. 1 and 2, and substituting 7Li and 6Li for i we slab melts (e.g., in a hotter, Archean Earth) (JCC-2). There are three outputs have fluxes from this juvenile crust: recycled lower crust (RLC), soluble components dissolved during weathering (DIS), which wash into the ocean and may be 7Li 7Li 7Li removed by uptake in sea floor sediments and altered basalt, and sediments 7 C þ R C þ R C f Li ¼ 1 − X BCC RLC∕BCC RLC TER∕BCC TER derived from the continents and removed via subduction (TER). The net result 7 DIS BCC C Li of these input and outputs to the continental crust is the present-day bulk JCC continental crust (BCC). 6Li 6Li 6Li 6 C þ R C þ R C f Li ¼ 1 − X BCC RLC∕BCC RLC TER∕BCC TER [3] subduction, or other processes), subducted terrigenous sediments DIS BCC C6Li (net output from the continent to the ocean in solid form, trans- JCC ported as the suspended sediments and bed load of rivers), and R ¼ X ∕X R ¼ X ∕X where RLC∕BCC RLC BCC and TER∕BCC TER BCC. the crust dissolved and removed from the continents by weath- Lithium concentrations and isotopic compositions in different ering. The net effect of these processes is a change of the com- reservoirs, such as those in JCC, BCC, RLC, and TER can be position of the continental crust, leading to the current bulk estimated, along with uncertainties, from published data.