Isotopic Constraints on Earth System Processes. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

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δ44Ca that is indistinguishable from BSE calcium isotope composition.

      3.4.1. Calcium Isotopic Record of Marine Carbonates

Image described by caption.

      3.4.2. Calcium Isotopic Record of Mantle‐Derived Rocks

      3.4.3. Calcium Isotopes Exhibit no Evidence for Carbonate Sediment Recycling at Subduction Zones

      In the studied Central American arc magmas, I found no evidence for calcium isotopic heterogeneity and thus no evidence for carbonate recycling or any isotopic fractionation related to subduction. This is the case despite the fact that I selected rocks that have both little to no geochemical evidence for sediment subduction, i.e., YO1 has MORB‐like trace element signatures and depleted mantle (DM) radiogenic isotope compositions, and rocks with strong trace element and radiogenic isotope signatures for carbonate sediment subduction (Fig. 3.2).

      To date, resolvable radiogenic calcium isotopic signatures have not been observed in any oceanic or arc basalts (Huang et al., 2011; Marshall & DePaolo, 1989; Simon et al., 2009). This might not be surprising given the work of Caro et al. (2010) who, despite finding well‐defined excesses of 40Ca in some river waters draining into the ocean, report that no discernable effects of 40K decay, to within their reported analytical precision (~0.4 epsilon units, 2σ), exist in marine carbonate samples ranging in age from Archean to recent.

      There have been recent studies of mantle‐derived rocks that find little evidence that recycling of carbonates affects the calcium isotope values of the mantle on a global or regional scale (Antonelli et al. 2019a; Ionov et al., 2019). However, other calcium isotope studies of primitive igneous rocks report evidence for recycling, e.g., Banergee and Chakrabarti (2019), Chen et al. (2018), Huang et al. (2011), Kang et al. (2016, 2017), and Liu et al. (2017). My results are significant since the trace element and radiogenic isotope signatures (e.g., high Ba/La, Ba/Th, 87Sr/86Sr, 206Pb/204Pb; see Figs. 3.2 and 3.4) of Central American lavas suggest a significant contribution from subducted carbonates (Patino et al., 2000; Sadofsky et al., 2008). The geochemical decoupling reported herein contrasts with the signatures reported for the ocean island basalts studied by Huang et al. (2011). In the Huang et al. (2011) study, stable mass‐dependent calcium isotope signatures vary and correlate with other geochemical parameters (i.e., Sr/Nb and 87Sr/86Sr) used to support the interpretation that Hawaiian lavas represent recycling of ancient calcium bearing surface materials.

      All samples from the volcanic front (VF) are interpreted to be elevated in their Pb and Sr radiogenic isotopes above values for back‐arc lavas (BA) by the addition of a sedimentary subducted component (see Fig. 3.4; Carr et al., 1990; Feigenson & Carr, 1986; Feigenson et al., 2004; Patino et al. 1997, 2000). Back‐arc lavas including YO1 remain within the mantle field, reflecting mixtures of MORB‐like depleted mantle (DM) and enriched mantle (HIMU). The potential sedimentary contribution