Читать книгу Isotopic Constraints on Earth System Processes - Группа авторов - Страница 74

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 to the arc magmas is believed to be uniform from Guatemala through northern Costa Rica and the sedimentary sequence has been well‐documented by the Deep Sea Drilling Program (DSDP); see Patino et al. (2000). The lower section of the sedimentary sequence consists of middle‐lower Miocene chalky carbonate ooze and manganiferous chalk and chert that are on average ~50 wt. % CaO (von Huene et al., 1982).

Subducted carbonate sediments along the Central American trough have compositions (Ba/La_avg = 244, Sr/Nb_avg = 3418, ⁸⁷Sr/⁸⁶Sr_avg = 0.7086) that can produce a distinct signature in the arc basalts (Patino et al., 2000; Sadofsky et al., 2008). The calcium isotopic composition of this sediment has not been measured, but modern carbonate ooze (DSDP 590B) has a δ⁴⁴Ca = –0.36 ± 0.15‰ (2σ) (Fantle & DePaolo, 2005), similar to the modern riverine inputs to the oceans (DePaolo, 2004); see Fig. 3.3. The calcium isotope homogeneity among the studied arc magmas is particularly notable when one considers the fact that they exhibit a large range in their sediment contribution signatures. They exhibit compositions that range from non‐existent levels typical of MORB up to those near BSE in these sediment signatures (e.g., Ba/La varies from ~4 to 117, Fig. 3.2; Sr/Nb varies from 16–328; and ⁸⁷Sr/⁸⁶Sr varies from 0.7029–0.7041, Fig. 3.4) compared to the more limited range recorded by the Hawaiian tholeiites (Sr/Nb = 25–55, ⁸⁷Sr/⁸⁶Sr = 0.7035–0.7042), considered by Huang et al. (2011) to reflect ancient carbonate recycling. These observations imply that some traditional geochemical signatures for carbonate sediment subduction in arc magmas are at odds with their calcium isotopic signatures.

There are several ways to explain the decoupling of the calcium isotopes and the more traditional geochemical carbonate sediment signatures. First, the calcium isotope compositions of the subducted carbonates could have had little mass‐fractionated calcium (i.e., Blattler & Higgins, 2017; Farkas et al., 2007) or their original light calcium isotopic signatures could have been modified, perhaps during diagenesis, prior to subduction and therefore have had limited effect on the calcium isotope composition of the studied arc magmas. I rule this out because it is unlikely that modern (≤35 Ma) subducted carbonate sediments had BSE‐like calcium isotope compositions, as shown by the work of Fantle and Tipper (2014; references therein). Likewise, even while the enhanced reaction in young carbonates increases the diagenetic effects, and therefore their δ⁴⁴Ca, the maximum effect (<0.15‰) of diagenesis is not large enough to erase the ≥0.3‰ light isotope effects typical of the carbonates; see Fantle and DePaolo (2007).

Second, it is possible that the flux of subducted sediment to the arc, and in particular marine carbonate, is overestimated. Despite the large volumes (100s m thick) of carbonate inferred from the DSDP drill cores from the subducting sediment sequence along the volcanic front, it is unknown how much sediment‐derived calcium is hybridized within the source reservoir(s) of the arc magmas. On average CaO is 4–5× more enriched in the carbonate sediment than the arc magmas. MORB‐like magmas have CaO contents that are the same or slightly higher than comparable primitive arc basalts (Patino et al. 1997; Presnall & Hoover, 1987). So, unlike Pb, Sr, and Nd that are found in relatively low abundances in the mantle, and therefore must come from a subducted component, perhaps sediments, to explain the elevated abundances in the arc basalts, all of the CaO needed in the arc basalts can be potentially provided by the mantle. For the most part these trace elements also behave incompatibly and will preferentially contribute, along with H₂O and other fluid mobile LILE, to the arc flux. In contrast, calcium acts compatibly. Therefore, any process that creates new Ca‐bearing phases, such as the “reaction pyroxenite” described by Straub et al. (2008; 2011), would effectively trap sediment‐derived Ca. As this material sinks into the mantle, this may help explain why some or most subducted carbonate calcium is added to long, deep mantle convection cycles contributing to the formation of ocean island basalts as suggested by Huang et al. (2011) (and possibly the carbonatites discussed in the following section), rather than the arc.

Third, the calcium isotope signatures may reflect a scenario in which the light calcium isotopic signatures of the subducted sediment are diluted by mixture with relatively heavy calcium reservoir(s), i.e., seawater and/or crustal rocks with BSE calcium isotopic compositions. Clear evidence of sediment subduction in the Central American arc is complicated by the fact that the radiogenic isotope compositions of the subducted sediments are relatively unradiogenic for marine sediments (cf. Feigenson et al., 2004). The exception may be Guatemalan lava AT‐50, but notably its Nd‐Sr isotopes could also reflect addition of crust and not necessarily subducted sediment (Fig. 3.4). Some of the trace element variability used as evidence for subducted sediment (Ba/Th) could also be explained by the mobilization of Ba over Th in fluids derived from subducted altered oceanic crust (Fig. 3.4) as seen in oceanic island arc basalts (Hawkesworth et al., 1997; Turner et al., 1996). Moreover, calcium mobilized in fluids extracted via deserpentinization reactions, as subducted rocks rise in temperature and pressure, might buffer the light carbonate signatures. It has been reported that subducted fluids could evolve isotopically during transport and fluid‐rock interaction, becoming enriched in heavy isotopes as they rise through the slab into the subvolcanic arc (John et al., 2012). It follows that mixing with isotopically heavy altered oceanic crust could offset the effects of the calcium from carbonate sediment, which is isotopically light, and produce BSE calcium isotope compositions for the arc lavas.