GEMOC ARC National Key Centre

Natsue ABE¹, Hisatoshi Hirai², Shoji Arai³, and Suzanne Y. O'Reilly¹

1: GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
2: Metallurgical Engineering Section, Kusyu National Industrial Research Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, 807-1 Shuku-machi, Tosu, Saga 841-0052, Japan
3: Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
 

Noyamadake is one of the exceptional localities worldwide that provides samples of the wedge mantle. The locality is a small hill of basanitoid and basanite pyroclastics and lavas of erupted at 6-7.3 Ma (K-Ar ages; Uto, 1995). The occurrence of xenolith types was described in detail by Hirai (1987) and Arai et al (1998). Cumulate mantle xenoliths (dunite, websterite and clinopyroxenite) are almost equal in volume to those of the mantle peridotites in the Noyamadake lava (Hirai, 1987). The Noyamadake spinel peridotite xenoliths are distinctive in recording very high equilibrium temperatures (1100-1250°C).

Peridotite xenoliths from Noyamadake vary from clinopyroxene-rich (23 vol.%) lherzolite to almost clinopyroxene-free (0.1 vol.%) harzburgite. The microstructure of peridotite xenoliths is protogranular to porphyroclastic (Hirai, 1987). The grain sizes of olivine and orthopyroxene are relatively large (2 mm). The grains of pyroxenes have a rounded shape. Olivine grains show remarkable kink-band and undulatory extinction under microscope.

Olivine Fo in peridotites varies from 87.0 to 91.6 and from 83.9 -87.2 in the cumulate xenoliths. Cr# of spinel has a wide range (0.11 to 0.70). Al₂O₃ content in orthopyroxene is very high, up to 6.7 wt%. The Na₂O content in clinopyroxene is higher (up to 1.5 wt%) than that of other localities from Japan arcs.

 Selected clinopyroxenes were analyzed in situ for trace-elements using a laser ablation microprobe (LAM)-ICPMS at GEMOC, Macquarie University. Their chondrite-normalised REE patterns can be classified into three groups; (1) light rare earth element (LREE)-depleted convex-upward pattern, (2) LREE-enriched pattern and (3) HREE-depleted with LREE-enriched pattern.

 The Ti/Zr ratio is has a restricted range (41-245), but the LREE/HREE [Ce/Yb)N (N= chondrite-normalised)] and Ti/Eu ratios vary widely 0.4-9.6 and 2280-6400, respectively.

The clinopyroxenes in the more depleted peridotites (calculated using mineral modes and mineral major-element compositions), is more enriched in LREE and LILE; the so-called "REE paradox (Frey and Green, 1974; McDonough and Frey, 1989; O'Reilly et al, 1991)". These peridotites thus show evidence of "cryptic metasomatism (Dawson, 1984)", as no metasomatic mineral formation accompanies the LREE-enrichment.

It is quite difficult to determine the metasomatic agent for such peridotites. Using the discriminants of (La/Yb)N and Ti/Hf ratio in clinopyroxene as suggested by Coltorti et al. (1999), the involvement of more than one metasomatic agents is indicated as the plotted values span the fields defined to be distinctive for specific metasomatic agents. These fluids are firstly a ubiquitous silicate melt and/or H₂O fluid with silicate component which may be related to the formation of cumulus ultramafic suite such as clinopyroxene, websterite and wehrlite and secondly, a rarer carbonatite melt and/or CO₂ fluid as the plotted values span the distinctive regions for each metasomatic agent type.

References

Arai, S., Abe, N & Hirai, H. 1998. Trends in Mineralogy (India). 2, 39-55.

Arai, S., Hirai, H. & Uto, K. 2000. J. Mineral. Petrol. Sci. 95, 9-23.

Coltorti, M., Bonadiman, C., Hinton, R.W., Siena, F. &Upton, B.G.J. 1999. J. Petrol. 40, 133-165.

Dawson, J.B. 1984. In Kimberlites II; The mantle and crust-mantle relationships. Elsevier, 289-294. Frey, F.A. & Green, D.H. 1974. G.C.A. 38, 1023-1059.

Frey, F.A. & Prinz,  M. 1978. EPSL. 38, 129-176.

Hirai, H. 1987. Doctoral Thesis, University of Tsukuba. p.181.

McDonough, W.F. & Frey, F.A. 1989. Reviews in Mineralogy, 21, Mineral. Soc. Amer. 99-145.

O'Reilly, S.Y., Griffin, W.L. & Ryan, C.G., 1991. Contrib. Miner. Petrol., 109, 98-113.