Compositional variations and heterogeneity in fertile lithospheric mantle: peridotite xenoliths in basalts from Tariat,
- PDF / 1,126,342 Bytes
- 23 Pages / 595.276 x 790.866 pts Page_size
- 61 Downloads / 165 Views
ORIGINAL PAPER
Compositional variations and heterogeneity in fertile lithospheric mantle: peridotite xenoliths in basalts from Tariat, Mongolia Dmitri A. Ionov
Received: 12 December 2006 / Accepted: 3 April 2007 / Published online: 26 April 2007 Springer-Verlag 2007
Abstract Clinopyroxene-rich, poorly metasomatised spinel lherzolites are rare worldwide but predominate among xenoliths in five Quaternary basaltic eruption centres in Tariat, central Mongolia. High-precision analyses of the most fertile Tariat lherzolites are used to evaluate estimates of primitive mantle compositions; they indicate Mg#PM = 0.890 while lower Mg# in the mantle are likely related to metasomatic enrichments in iron. Within a 10 · 20 km area, and between ~45 and ‡60 km depth, the sampled xenoliths suggest that the Tariat mantle does not show km-scale chemical heterogeneities and mainly consists of residues after low-degree melt extraction at 1–3 GPa. However, accessory (3–4 cm in size, which appeared least altered, were collected; their numbers ranged from about 30 for Tsagan and Shute to some 70 at Zala. One sample was taken in a side vent (Bosko) of the Haer eruption centre.
Sample processing and analytical techniques The samples were sliced with a rock saw and inspected under binocular microscope. Over 70 peridotite xenoliths were thin-sectioned. Of the >70 samples, 30 were selected for whole-rock studies based on their size (sufficient to obtain >100 g of clean crushed material), low degrees of alteration and the absence of basaltic veins. Their rims were removed and the interiors crushed in a bench-top jaw crusher, carefully cleaned after each sample to avoid crosscontamination. Splits of the crushed whole-rock samples were ground in agate mortars to fine powder. Minerals were hand-picked from 0.5–1.0 mm size fractions of another split to produce polished grain mounts for electron probe micro-beam analyses (EPMA).
457
Two sets of analytical data were obtained. The first one is based on conventional EPMA and X-ray fluorescence (XRF) methods. Mineral compositions in over 70 xenoliths (listed in Table 1) were determined from polished thin sections by wavelength-dispersive EPMA at Macquarie University (MU) with a Cameca SX50 instrument at an accelerating voltage of 15 kV and sample current of 20 nA. Counting times were 10 s for peaks and 5 s for background on each side of the peak. Standards were natural and synthetic minerals and matrix corrections were by the PAP method. Attention was given to establishing heterogeneities and zoning patterns of pyroxenes (core-rim profiles were run in selected samples) and their relationships to textural position and grain size. A small number of samples were analysed on a Cameca SX-100 at the Service Microsonde Sud, Universite´ Montpellier II. Major elements in bulk rocks were determined in 1996–1997 at MU by XRF using high-dilution (1:10) glass fusion discs as described by O’Reilly and Griffin (1988). The second set of data was obtained using high-precision methods designed for peridotite analys
Data Loading...
