Peridotite mantle nodule in basalt

--- from Mount Leura, western Victoria, Australia

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Figure 1. A splendid example of a mantle xenolith, a somewhat rounded nodule composed largely of the mineral olivine, enclosed within a much younger host rock, fine-grained black basalt. The nodule lithology is dunite, or more generally speaking, peridotite, an olivine-dominated ultramafic rock --- given sufficient orthopyroxene and clinopyroxene, the specific term would be lherzolite. The darker green grains in the rock may be chrome diopside, a form of clinopyroxene. On the right is a view of the poorly consolidated volcaniclastic host rock of the nodules. Sample and the 3 photographs of the locality courtesy of Prof. Peter Hollings of Lakehead University in Ontario, Canada.

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Figure 2. Images of the Mount Leura locality, by Pete Hollings. Left: the scoria cone. Right: nodules occur with basaltic "rind" in a poorly consolidated scoria deposit.

"Rock of the Month #155, posted for May 2014" ---

A mantle xenolith

from a volcanic field in western Victoria state. Mount Leura, Victoria is a volcanic centre on the east side of the town of Camperdown in western Victoria. Much of the land is private and there are two operating tuff pits on the northern rim of a tuff ring. Mount Sugarloaf is a National Trust property. The site is a nested tuff ring maar, with a scoria cone surrounding a dry crater 100 m deep. The shallow maar crater measures 2.5x1.7 km, surrounded by a low tuff ring. Inside the cone are secondary eruption points, including Mount Leura and several mounds and cones of scoria. Mount Sugarloaf is a conical mound on the southwest flank of Mount Leura, representing the final phase of volcanism. The scoria at Mount Leura is notable for abundant lower crust and mantle xenoliths (`peridotite'). The site is one of the largest such structures in the Newer Volcanic province (Rosengren, 1994).

Across the huge region of scattered mantle rock exposures of eastern Australia, the chrome-diopside spinel lherzolites are 80-90% of all mantle xenoliths (O'Reilly and Griffin, 1987). The mineralogy is simple, largely free of volatile-bearing minerals, containing only olivine, orthopyroxene, clinopyroxene and spinel plus, in four localities, garnet. Chrome diopside spinel lherzolite xenoliths show mantle metasomatism in which amphibole, mica and apatite replace primary pyroxene and spinel. The alteration is ascribed to CO2-rich fluids, found in fluid inclusions (O'Reilly and Griffin, 1988). The Al-augite suite of xenoliths include wehrlites and pyroxenites with Al augite (as opposed to Cr diopside) clinopyroxenes. These Al-augite rocks occur as veins or dykes in Cr-diopside spinel lherzolite of the upper mantle (Griffin et al., 1988; Molin, 1989).

By coincidence, "primitive" mantle rocks are recorded from TWO Australian sites of the same name, the other Mount Leura being in the Eocene to Miocene Hoy basalt province of Queensland. The Queensland locality contains an unusual anorthoclase- corundum rock and has long been associated with sapphire mining (O'Reilly and Griffin, 1987; Robertson and Sutherland, 1992).

And another mantle xenolith

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Figure 3. Images of a metapyroxenite nodule from Mount Leura, western Victoria. Left: a scanned image of the whole thin section, showing the equant grains in a recrystallized texture. Area of sample is about 5.6 cm2. Centre and right: two photomicrographs showing fine-scale clinopyroxene exsolution in coarse orthopyroxene grains, with adjacent grains of olivine and clinopyroxene. Nominal magnification 50X, long-axis field of view 1.7 mm, in crossed-polarized transmitted light.

The thin section in Figure 3 samples another lithology from Mount Leura, a spinel-bearing metapyroxenite of the Al-augite suite. The rock is composed largely of orthopyroxene plus lesser clinopyroxene and olivine, and accessory spinel grains up to 1 mm in diameter. The latter may be green and translucent, suggesting a high content of the Fe-Al spinel, hercynite. Approximately 5% of the section appears to be the basaltic matrix of the nodule, invading the mantle assemblage along fractures. The basalt consists of plagioclase feldspar microlites, pyroxene, dark brown glass and small (0.1-0.3 mm) vesicles (gas bubbles).

References

Griffin,WL, O'Reilly,SY and Stabel,A (1988) Mantle metasomatism beneath western Victoria, Australia: II. Isotopic geochemistry of Cr-diopside lherzolites and Al-augite pyroxenites. Geochim.Cosmochim.Acta 52, 449-459.

Molin,GM (1989) Crystal-chemical study of cation disordering in Al-rich and Al-poor orthopyroxenes from spinel lherzolite xenoliths. Amer.Mineral. 74, 593-598.

O'Reilly,SY and Griffin,WL (1987) Eastern Australia - 4000 kilometres of mantle samples. In `Mantle Xenoliths' (Nixon,PH editor), Wiley-Interscience, John Wiley and Sons Ltd, 844pp., 267-280.

O'Reilly,SY and Griffin,WL (1988) Mantle metasomatism beneath western Victoria, Australia: I. Metasomatic processes in Cr-diopside lherzolites. Geochim.Cosmochim.Acta 52, 433-447.

Robertson,ADC and Sutherland,FL (1992) Possible origins and ages for sapphire and diamond from the central Queensland gem fields. In `R.O. Chalmers, Commemorative Papers (Mineralogy, Meteoritics, Geology)' (Sutherland,FL editor), Records of the Australian Museum, Supplement 15, 136pp., 45-54.

Rosengren,NJ (1994) Eruption Points of the Newer Volcanic Province of Victoria: an inventory and evaluation of scientific significance. National Trust of Australia (Victoria) and Geol.Soc.Australia (Victoria), 387pp.

Graham Wilson, 25-28 February 2014, 18 April 2014.

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