Geochemistry of clinopyroxene in peridotites from the Nukabira complex, Kamuikotan zone, Hokkaido, Japan : a LA-ICP-MS study

Trace-element compositions of clinopyroxene in peridotite from the Nukabira complex were examined to mainly discuss partial melting processes involved in their formation. The Nukabira complex in the Kamuikotan zone, Hokkaido, Japan, is mainly composed of mantle-derived peridotites with wide compositional variations. The Iherzolite (e. g. , C# of spinel=0. 18-0. 34) and depleted harzburgite (e. g. , Cr#=0. 4-0. 7) are predominant, and dunite is often surrounded by them. Samples of 8 Iherzolites, 3 harzburgites and 4 dunites from the complex were selected, and their clinopyroxene geochemical characteristics were determined by using LA-ICP-MS. Chondrite-normalized trace-element patterns of clinopyroxene in the Iherzolites except one sample are characterized by strong depletion of light-REE (LREE) and middle-REE (MREE) relative to high heavy-REE (HREE) (e. g. , (Nd/Yb) _<0. 004, YbN=2. 7-6. 1). One Iherzolite sample, on the other hand, has high REE abundances of clinopyroxene (e. g. CeN≈0. 05, YbN≈5. 3, (Ce/Yb) N=0. 02-0. 06). The trace-element patterns of clinopyroxene in harzburgites are "spoon-shaped" being high in MREE and LREE and low in HREE abundance (e. g. , (Nd/Yb) N<0. 09, YbN=1. 0-1. 7). The abundances of MREE and LREE are variable in each harzburgite sample (e. g. , CeN=0. 008-0. 03, NdN=0. 02-0. 08). The dunites tend to be similar in clinopyroxene compositions to surrounding peridotites. The variations of Iherzolite can be ascribed not only to the difference of melting degree but also to conditions on melting, such as with or without residual garnet prior to spinel peridotite stability field melting. Most of the Iherzolites probably experienced the melting in the garnet peridotite stability field. The further melting of Iherzolites should generate the harzburgites. The clinopyroxene geochemistry, however, indicates that the harzburgite was produced by different melting processes from the Iherzolite formation. The melting assisted by LREE-rich hydrous fluid is more preferable to explain the harzburgite features and origin.