Ca isotope fingerprints of early crust-mantle evolution

Abstract

Ca ratios indistinguishable from our measured modern MORB value (i.e., Ca (3.6) ϳ 0). Greenland samples, however, are very diverse ranging from Ca (3.7) ϭ 0.1 in mafic pillow lavas and felsic sheets from the Isua supracrustal belt, up to very radiogenic signatures ( Ca (3.7) ϭ 2.9) in both mafic rocks of the Akilia association and felsic TTG from the coastal Amîtsoq gneisses.At face value, these results imply the Zimbabwe crust is juvenile whereas most Greenland samples include an earlier crustal component. Yet the west Greenland craton, as with many Archaean localities, has experienced a complex geological history and the interpretation of age-corrected initial isotope values requires great care. Both felsic and mafic samples from Greenland display Ca (3.7) so radiogenic that they are not readily explained by crustal growth scenarios. The presence of such radiogenic 40 Ca/ 44 Ca found in low K/Ca plagioclases requires Ca isotope exchange between plagioclase and whole rock during later metamorphic event(s). In addition the unexpectedly radiogenic Ca isotope ratios in some mafic samples reflect anomalous K/Ca ratios as a result of intense K-metasomatism ϳ3.6 Ga. Thus Ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono-metamorphic processes. Coupled with other isotope data, however, the degree of overprint can be estimated and the 40 Ca/ 44 Ca ratio of a little disturbed sample hints at a small contribution of Hadean protocrust in the coastal part of the Godthåbsfjord area (Southwest Greenland). In the majority of Zimbabwe TTG samples, unradiogenic initial Ca isotope ratios point to very little prior crustal history and minor subsequent disturbance. We thus infer that the modest initial Nd ϳ0.8 of the Zimbabwean samples is representative of the depleted mantle at ϳ3.6 Ga. Furthermore, Ca isotope systematics provide little support for a "steady state" model of crustal growth.

Calcium isotope evidence for early Archaean carbonates and subduction of oceanic crust

“…To allow for easier comparison with published data and to best illustrate complementarity between terrestrial reservoirs, δ 44/42 Ca values are converted to δ 44 Ca by multiplying by 2.05 (the kinetic mass law) and are reported relative to BSE [with a recommended value of +0.95‰ relative to SRM915a 14 , which is +0.25‰ relative to SRM915b in this study]. As 40 Ca is not directly measured, the use of MC-ICP-MS for stable Ca isotope analyses precludes the need for radiogenic 40 Ca corrections, which could otherwise lead to significant variations in Archean-aged samples analyzed by thermal-ionization mass spectrometry 35 , 36 , 40 .…”

Continental growth seen through the sedimentary record

“…little variation in its 40 Ca/ 44 Ca ratio through time, whereas the (high K/Ca) continental crust has developed highly radiogenic 40 Ca/ 44 Ca ratios in the early stages of Earth's evolution (Kreissig and Elliott, 2005). Finally the impact of the 4.1-3.8 Ga late heavy bombardment (LHB) on the destruction and the recycling of the Hadean crust (Marchi et al, 2014;Shibaike et al, 2016), and on the composition of the mantle, needs to be carefully considered when developing more realistic models for the continental growth.…”

Calcium isotope constraints on the uptake and sources of Ca2+ in a base-poor forest: A new concept of combining stable (δ44/42Ca) and radiogenic (εCa) signals

“…These geologically-focused contributions used the K-Ca isotope system to determine the formation age of (i) igneous minerals and rocks DePaolo, 1982, 1989;Shih et al, 1994;Fletcher et al, 1997;Nägler and Villa, 2000), (ii) sedimentary minerals (Baadsgaard, 1987;Gopalan, 2008;Cecil and Ducea, 2010); and more recently also to study (iii) the silicate crust contribution to early Archaean cratons (Kreissig and Elliott, 2005) and the oceanic Ca budget over geological time (Caro et al, 2010). The most recent contribution of Ryu et al (2011), which investigated the Ca isotope systematics during a laboratory-controlled dissolution of Precambrian granite ($1.7 Ga), revealed that the weathering of K-rich minerals can generate large radiogenic 40 Ca enrichments, suggesting that these might be utilized in field-based studies as a unique mineral-specific weathering tracer.…”