4.2. Trace elements in clinopyroxene
In situ trace element analyses of magmatic clinopyroxene were carried out by LA-ICP-MS in 12 of the thick sections used for EMPA (three clinopyroxenites, two websterites, three troctolites and four gabbronorites). Clinopyroxene analyzed in oxide gabbronorites showed evidence of metamorphic recrystallization and is not discussed here. Representative LA-ICP-MS data are given in Appendix D. Chondrite-normalized (C) trace elements (REE) patterns of clinopyroxene in pyroxenites and gabbroic rocks of the Rio Boba plutonic sequence are displayed in Fig. 8. In a first approximation, incompatible trace elements contents are very low in these rocks, with HREE absolute abundances between 1 and 8 times the chondrite value and sub-chondritic contents of high field strength elements (HFSE), particularly in Nb, Zr, Hf and Ti. The shape of the trace elements patterns are similar, although the ranges of values vary from one group to another (see below). With the exception of clinopyroxenites, no zoning in REE composition has been detected between cores and rims of clinopyroxene grains.
The REE in the clinopyroxene of the clinopyroxenites are highly fractionated [(La/Yb)C=0.16-0.53, average 0.26] and define convex-upward patterns (Fig. 8a, b, c). HREE concentrations are 2-5 times the chondrite values. In general, REE ratios increase from the core towards the rim of the individual grains. They show moderate to strong depletion of Nb, Zr and Hf relative to adjacent Th and LREE, and Ti relative to HREE. They lack a negative Eu anomaly, which could mean that the crystallization of clinopyroxene was not affected by plagioclase fractionation. Websterites present trace elements patterns very similar to clinopyroxenites, although they are distinguished by the extreme fractionation of REE [(La/Yb)C=0.06-0.22, average 0.12] and a more pronounced anomaly in Zr-Hf (Fig. 8d). HREE patterns are flat or shown a slight depletion [(Sm/Yb)C=0.34-1.05, average 0.69]. The REE ratios in clinopyroxene from the Rio Boba pyroxenites are very similar to those in SSZ mantle clinopyroxenites and websterites from the Solomon Islands (Berly et al., 2006), as well as boninite-type mafic-ultramafic cumulates from northern Victoria Land (Tribuzio et al., 2008).