Calcium isotope compositions of mantle pyroxenites

Variations of the stable Ca isotopic compositions (noted as delta Ca-44/40 relative to the SRM915a standard) of basalts are interpreted as effects of mantle sources. Mantle pyroxenites are a minor but integral part of the mantle and, as fusible components, they are important source rocks to understand chemical and isotopic heterogeneity in mantle-derived magmas. However, the effect of pyroxenites on the Ca isotopic composition of the mantle has been poorly constrained. To which extent mantle pyroxenites and their formation processes such as melt-peridotite reaction, particularly involving recycled crustal materials, lead to heterogeneity in delta Ca-44/40 of the mantle is unknown. Here, we report delta Ca-44/40 of different types of pyroxenites (spinel pyroxenites, garnet pyroxenites, and phlogopite-bearing spinel clinopyroxenites) and minerals separates, along with surrounding peridotites from Hannuoba xenoliths, North China Craton to address the issue. Initial Sr-87/Sr-86 ratio indicates that recycled crustal materials were incorporated into parental magmas of garnet pyroxenites (0.70391-0.70715) and phlogopite-bearing clinopyroxenites (0.7142-0.7149), consistent with previous conclusions from Sr-Nd isotopes. Overall, the delta Ca-44/40 of garnet pyroxenites ranges from 0.86 parts per thousand to 0.98 parts per thousand (average 0.90 +/- 0.05 parts per thousand, n = 10) and the host peridotites affected by the infiltrating melts from 0.87 parts per thousand to 0.93 parts per thousand (0.89 +/- 0.04 parts per thousand, n = 8). Each pair (n = 8) of garnet pyroxenite and host peridotite displays no measurable difference in delta Ca-44/40 The spinel pyroxenites and phlogopite-bearing spinel clinopyroxenites also show similar delta Ca-44/40 (0.94 +/- 0.06 parts per thousand and 0.98 +/- 0.05 parts per thousand, respectively). The indistinguishable delta Ca-44/40 among these different types of pyroxenites and surrounding peridotites suggest no obvious Ca isotope variations during silicate meltperidotite interaction and fractional crystallization, even if recycled silicate materials were involved. These results indicate that the mantle source with variable proportions of pyroxenites in equilibrium conditions overall would show uniform delta Ca-44/40. It implies that the basic magmas derived from such peridotite-pyroxenite source would display limited variations in delta Ca-44/40, even if their radiogenic isotopes show strong heterogeneity. The conclusion is consistent with no systematic variations in the available delta Ca-44/40 of different basalt types such as DMM, EM1 and HIMU. However, garnets in mantle rocks of recent work and this study generally display heavier . delta Ca-44/40 than co-existing clinopyroxenes, implying that melts generated by partial melting of mantle sources with abundant residual garnets may show lighter delta Ca-44/40 values than MORBs. (C) 2019 Elsevier Ltd. All rights reserved.