Isotopic Constraints on Earth System Processes. Группа авторов

number of Ca‐, Mg‐, and Fe‐rich carbonatites spanning from ~3 Ga to ~1 Ma. Their findings are important because they may provide timing constraints for the onset of plate tectonics and subduction of marine sediments. Furthermore, their conclusions imply that since at least 3 Ga there has been relatively similar levels of marine biomineralization, seawater conditions, and continental weathering to produce isotopically light marine carbonate. This does not appear to be true, however, as over approximately the same timespan (~3.0 to 0.7 Ga) there is little evidence for isotopically light marine carbonate in the geologic record (Blatter & Higgins, 2017). In fact, the extensive Precambrian carbonate record reported by Blatter and Higgins (2017) is dominated by BSE calcium isotope compositions. This record poses a serious problem for the interpretations of Amsellem et al. (2020), which necessitate that ancient carbonatites form from recycled light marine carbonates.

      The Laacher See carbonatite is notable, in part, because it is the youngest intrusive carbonatite example known on Earth. It may also be important that it is an intrusive rather than an extruded sample. Another significant characteristic of the Laacher See carbonatite is that its oxygen isotopic composition is similar to the relatively narrow range (δ¹⁸O_SMOW = 5–8‰) exhibited by mantle‐derived mafic rocks (Schmitt et al. 2010; Taylor et al., 1967). In general, more silicic igneous rocks and most metamorphic and sedimentary rocks exhibit higher δ¹⁸O values. This similarity has been used to suggest a possible genetic relationship between the Laacher See carbonatites and rocks that are commonly believed to originate in the lower crust or upper mantle. Because oxygen is a major component of the studied materials, their oxygen isotopic composition has been used to argue that it is difficult for the Laacher See carbonatites to have formed by addition of any significant sedimentary carbonate component, which generally have much higher oxygen isotope compositions (δ¹⁸O_SMOW = 15–25‰; Taylor et al., 1967). It follows, therefore, that its CO₂, and perhaps calcium, is likely also primary and not due to marine carbonate recycling. Taylor et al. (1967) do point out, however, that the Laacher See carbonatites could form from assimilation of carbonate if these materials had undergone recrystallization and oxygen exchange with primary igneous rocks. It should also be noted that despite the fact that a large number of carbonatite complexes around the world do exhibit mantle‐like C and O isotopic signatures, some exhibit much heavier oxygen isotope compositions than the Laacher See carbonatites. These heavy oxygen isotope values are consistent with what might be produced by assimilation of sedimentary carbonate country rock (e.g., Santos and Clayton, 1995; Wei et al., 2020). Consequently, the relatively constant δ⁴⁴Ca ~ –0.4 values seen in carbonatites, over the past 3 Ga, probably reflect a more universal mass‐fractionation process rather than the incorporation of a relatively constant amount (6–7%) of recycled marine carbonate sediment into their mantle sources (cf. Amsellem et al., 2020).

      Laacher See carbonatite has had a protracted sub‐solidus history as an intrusive rock and has a calcium isotopic composition that is similar in magnitude and sign to the mass‐fractionation seen for carbonatite metasomatism (Ionov et al., 2019). Therefore, the most likely explanation for its calcium isotopic composition is that its formation included fluid alteration and/or a partially dehydrated source rock from which isotopically heavy Ca‐bearing fluids have been lost, analogous to that reported by John et al. (2012) for metasomatism in subduction zones. Given that a majority of carbonatites have a similar light calcium isotopic composition, and yet marine carbonates may not have had isotopically light calcium isotope compositions in the past, there is no simple scenario in which marine carbonate recycling is the dominant explanation for their calcium isotopic composition.

3.5. CONCLUSIONS

Studied Central American arc magmas have no calcium isotope evidence for carbonate recycling. This is observed even for magmas with strong trace element and radiogenic isotope signatures indicating sediment subduction. The decoupled signatures are important because they likely reflect different sources and processes, and demonstrate that sediment subduction is not a bulk mixing process. Fresh Oldoinyo Lengai carbonatites also have δ⁴⁴Ca values that are similar to unmelted peridotites, komatiites, and most basalts, which again implies a primitive mantle‐derived calcium source. The possible exception comes from the intrusive Laacher See carbonatite. Its calcium isotopic signature could be related to carbonate sediment recycling in a long deep convective cycle. Alternatively, the relatively light calcium isotopic composition (δ⁴⁴Ca = –0.39) measured in the Laacher See carbonatite, and recently found in other carbonatites, may be associated with a process in which an isotopically heavy Ca‐bearing fluid was extracted during dehydration of its lithospheric mantle source, mass‐fractionating its original BSE calcium isotopic composition.

ACKNOWLEDGMENTS

      The author would like to thank Don DePaolo for his friendship, support, and tutelage. Being a part of the Earth and Planetary Science Department at University of California at Berkeley and in particular a member of the Center for Isotope Geochemistry was a highlight of my career. I have learned a great deal from discussions and collaborations with past and present members of the CIG group. The editorial handling by Ken Sims and helpful constructive reviews by Michael Antonelli and two anonymous reviewers are much appreciated. Michael Carr, Tobias Fisher, and Axel Schmitt are thanked for generously providing sample material. This work was supported by NSF Petrology and Geochemistry and NASA Astrobiology programs (that partly supported my postdoctoral research with Don DePaolo while at the University California at Berkeley) and NASA Planetary Science funding.

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