Calcium Isotopic Compositions of Normal Mid‐Ocean Ridge Basalts From the Southern Juan de Fuca Ridge

Abstract

Mantle peridotites show that Ca is isotopically heterogeneous in Earth's mantle, but the mechanism for such heterogeneity remains obscure. To investigate the effect of partial melting on Ca isotopic fractionation and the mechanism for Ca isotopic heterogeneity in the mantle, we report high‐precision Ca isotopic compositions of the normal Mid‐Ocean Ridge Basalts (N‐MORB) from the southern Juan de Fuca Ridge. δ44/40Ca of these N‐MORB samples display a small variation ranging from 0.75 ± 0.05 to 0.86 ± 0.03‰ (relative to NIST SRM 915a, a standard reference material produced by the National Institute of Standards and Technology), which are slightly lower than the estimated Upper Mantle value of 1.05 ± 0.04‰ and the Bulk Silicate Earth (BSE) value of 0.94 ± 0.05‰. This phenomenon cannot be explained by fractional crystallization, because olivine and orthopyroxene fractional crystallization has limited influence on δ44/40Ca of N‐MORB due to their low CaO contents, while plagioclase fractional crystallization cannot lead to light Ca isotopic compositions of the residue magma. Instead, the lower δ44/40Ca of N‐MORB samples compared to their mantle source is most likely caused by partial melting. The offset in δ44/40Ca between N‐MORB and BSE indicates that at least 0.1–0.2‰ fractionation would occur during partial melting and light Ca isotopes are preferred to be enriched in magma melt, which is in accordance with the fact that δ44/40Ca of melt‐depleted peridotites are higher than fertile peridotites in literature. Therefore, partial melting is an important process that can decrease δ44/40Ca in basalts and induce Ca isotopic heterogeneity in Earth's mantle.

The effects of high-temperature fractional crystallization on calcium isotopic composition

“…Calcium isotopic values of the olivine-amphibolite group range from 1.45 to 1.61‰, representing the isotopically heaviest samples of the GAC. While the olivine-amphiboles are heavier than both terrestrial basalts (Valdes et al, 2014) Kang et al, 2017), the rest of the samples are significantly lighter than most previously measured terrestrial rocks, including basalts, peridotites, granites, rhyolites, carbonatites, and komatiites, (Richter et al, 2003;DePaolo, 2004;Amini et al, 2009;Wombacher et al, 2009;Huang et al, 2010Huang et al, , 2011Simon and DePaolo, 2010;Ryu et al, 2011;Chen et al, 2014;Valdes et al, 2014;He et al, 2017;Kang et al, 2017;Liu et al, 2017;Zhao et al, 2017;Zhu et al, 2018). The spinel-amphibolite group has Ca isotopic values ranging from −0.39 to 0.13‰.…”

“…Shasta plagioclase and Mauna Koa tholeiites (Δ 44/40 plag-thol~− 0.10‰; Skulan et al, 1997). While partial melting has been shown to fractionate Ca isotopes (Amini et al, 2009;Chen et al, 2014;He et al, 2017;Kang et al, 2017;Liu et al, 2017;Zhu et al, 2018), fractional crystallization has been disregarded as a source of inter-phase Ca isotope fractionation (He et al, 2017;Zhu et al, 2018). Before an assessment can be made on the effect of fractionation crystallization on variations in δ 44/40 Ca between different GAC layers, it is important to evaluate whether the GAC represents a magmatic suite formed from a single parental magma and whether it has been significantly affected on the sample scale by secondary processes.…”

“…Likewise, co-genetic mafic samples from the Canary Islands have similar δ 44/40 Ca values despite substantial variability in CaO content, suggesting that fractional crystallization of the parent magma had minimal effect on Ca isotopes (Chen et al, 2014). Most recently, Zhu et al (2018) measured normal mid-ocean ridge basalts (N-MORB) from the southern Juan de Fuca ridge to be δ 44/ 40 Ca = 0.75‰ ± 0.05-0.86‰ ± 0.03, lighter than estimations for the upper mantle (1.05‰ ± 0.04; Huang et al, 2010). The N-MORBs are thought to have formed from partial melts of the upper mantle that have evolved through fractional crystallization (e.g.…”

“…Smith et al, 1994). Based on mass balance calculations, Zhu et al (2018) suggested that fractional crystallization is unable to generate significant Ca isotope fractionation of N-MORB from the upper mantle. Nevertheless, our results stand in contrast to previous studies and suggest that fractional crystallization should not be eliminated as a source of Ca isotope variability among co-genetic samples.…”

“…This study aims to better understand the effect of fractional crystallization on inter-phase Ca isotope fractionation. Previous studies that have evaluated high temperature Ca isotope fractionation have largely aimed to understand partial melting of the mantle as a source of fractionation (Amini et al, 2009;Chen et al, 2014;Kang et al, 2017;Liu et al, 2017;Zhu et al, 2018). If fractional crystallization, which inherently modifies the elemental composition of the residual liquid, causes inter-phase Ca isotope fractionation, then co-crystallized mineralogical assemblages should bear Ca isotopic signatures that reflect the compositional evolution of the liquid.…”