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).