Marine Carbonates in the Mantle Source of Oceanic Basalts: Pb Isotopic Constraints

Abstract

For almost fifty years, geochemists have been interpreting the clues from Pb isotopic ratios concerning mantle composition and evolution separately. The Pb isotopes of ocean island basalts (OIB) indicate that their mantle source is heterogeneous, most likely due to the presence of end-components derived from recycled crust and sediment. Some OIB have unusually high 206Pb/204Pb coming from one of the end-components with a long time-integrated high 238U/204Pb or μ (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high 206Pb/204Pb, indicating a HIMU-like source. Moreover, measured 232Th/238U (κ) for most MORB are lower than those deduced from their 208Pb/204Pb and 206Pb/204Pb. Such high μ and low κ features of oceanic basalts are inconsistent with the known geochemical behavior of U, Pb and Th and temporal evolution of the mantle; these have been respectively termed the 1st and 2nd Pb paradox. Here we show that subducted marine carbonates can be a source for HIMU and a solution to the Pb paradoxes. The results are consistent with the predictions of the marine carbonate recycling hypothesis that posits the Pb isotopes of oceanic basalts indicate a common origin and/or magma generation process.

Isotopic Evidence for Multiple Recycled Sulfur Reservoirs in the Mangaia Mantle Plume

“…Cabral et al (2014) argue for a carbonated source at HIMU Mangaia on the basis of inferred (predegassing) high primary CO 2 in melt inclusions that are required to explain their elevated CaO/Al 2 O 3 and low SiO 2 , as well as the presence of carbonatite observed in Mangaia melt inclusions by Saal et al (1998) and Yurimoto et al (2004). Castillo et al (2018) suggest a component of recycled carbonate in the source of HIMU, and such may be the source of carbonation for the HIMU mantle source. If the carbonated component originates from marine carbonates, or carbonated shales, as suggested by Castillo et al (2018), they could have influenced the isotopic composition of sulfides.…”

“…The HIMU mantle reservoir is generally thought to reflect mantle mixed with subducted oceanic crust that experienced sulfide loss, causing depletion in Pb (Chauvel et al, 1992; Kelley et al, 2005; Lassiter et al, 2003); however, some proposals also call for a mantellic origin (Thirlwall, 1997), formation by recycled carbonate (Castillo, 2015; Castillo, 2016; Castillo et al, 2018), recycled serpentinites (Kendrick et al, 2017), carbonatitic metasomatism of the subcontinental lithospheric mantle that in turn delaminated and sank to the core‐mantle boundary (Weiss et al, 2016), or an origin in metasomatic veins hosted in ancient subducted mantle lithosphere (Pilet et al, 2008). A wide range of ages have been proposed for the U/Pb fractionation that generated HIMU (~550 Ma to 3.0 Ga; Hauri and Hart, 1993; Mazza et al, 2019; Nebel et al, 2013; Thirlwall, 1997).…”

“…Cabral et al (2014) argue for a carbonated source at HIMU Mangaia on the basis of inferred (predegassing) high primary CO 2 in melt inclusions that are required to explain their elevated CaO/Al 2 O 3 and low SiO 2 , as well as the presence of carbonatite observed in Mangaia melt inclusions by Saal et al (1998) and Yurimoto et al (2004). Castillo et al (2018) suggest a component of…”

Fingerprinting subducted oceanic crust and Hainan Plume in the melt sources of Cenozoic Basalts from the South China Sea Region

“…The mantle component featuring low Th/U ratios that started to influence the melt evolution of Cenozoic basalts in the SCS region around 25 Ma (Figure 3d) points to the contribution of recycled marine carbonates (Castillo, 2015; Castillo et al., 2018). Mounting geochemical evidence demonstrates that recycled marine carbonates have been abundant in the mantle source of Cenozoic basalts in eastern Eurasia, e.g.…”

“…Simultaneously, FOZO basalts exhibit lower 207 Pb/ 204 Pb and 208 Pb/ 204 Pb than OIBs with strong EM1 and EM2 affinities for given 206 Pb/ 204 Pb ratios (Figure 3a,b). Therefore, it is posited that marine carbonates which have an inherently decoupled high U/Pb (and U/Th) and low Rb/Sr (~0.005) ratios were involved in the mantle source of typical FOZO basalts (Castillo, 2015; Castillo, MacIsaac, Perry, & Veizer, 2018).…”