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

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. Ca-44/40 of these N-MORB samples display a small variation ranging from 0.750.05 to 0.860.03 (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 1.050.04 parts per thousand and the Bulk Silicate Earth (BSE) value of 0.94 +/- 0.05 parts per thousand. This phenomenon cannot be explained by fractional crystallization, because olivine and orthopyroxene fractional crystallization has limited influence on Ca-44/40 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 Ca-44/40 of N-MORB samples compared to their mantle source is most likely caused by partial melting. The offset in Ca-44/40 between N-MORB and BSE indicates that at least 0.1-0.2 parts per thousand 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 Ca-44/40 of melt-depleted peridotites are higher than fertile peridotites in literature. Therefore, partial melting is an important process that can decrease Ca-44/40 in basalts and induce Ca isotopic heterogeneity in Earth's mantle.