Groundmass

Ilmenite. There are currently no experimentally-determined ilmenite-melt partition coefficients. We must rely instead on phenocryst-groundmass data analyzed by PIXE (Stimac and Hickmott 1994 Ewart and Griffin 1994). Like magnetite, ilmenite is characterised by two relatively small sites, and is therefore unlikely to take up appreciable amounts of lanthanides or large alkaline earths. We will confine ourselves again to U, Th and Pa. 

Figure 7. Two examples of whole-rock isochrons, (a) Isochron obtained on a lava nnit of the Vico volcano (Central Italy) (Villemant and Flehoc 1989). (b) Isochron defined by glass and groundmass analyses of the Laacher See phonolite (Bonidon et al. 1994). In both examples, differentiation occurred shortly before eraption since calculated ages are indistinguishable from eraption ages.

It should be noted that, even when good U-Th mineral isochrons indicate old crystallization ages, Ra- Th disequilibria can be found in minerals, groundmass and whole rocks. It is the case for Mt. St Helens and Mt. Shasta (Volpe et al. 1991, 1992) and Vesuvius (Black et al. 1998) (see Fig. 10c,d) this requires either a selective Ra enrichment in the magma shortly before eruption accompanied by further crystallization... 

The laser 39Ar-40Ar results for a black aphanite clast from 73215 demonstrate that this rock is cogenetic with the aphanite that forms the matrix of the breccia. Ages determined for felsic glass fragments in the two types of aphanite, and for groundmass in... 

Paleozoic limestone (Webb et al. 2004). These kimberlites all have similar groundmass mineralogies consisting mainly of carbonate, spinel, and serpentine with lesser monticellite, mica, apatite, and perovskite (Kong et al. 1999) and they are all of volcaniclastic facies near ground surface. Varying thicknesses of clay and fine marine sediments of the Tyrell Sea ( 4000 - 12000 years BP) and 1 to 4 m of peat overlie kimberlites (Fraser et al. 2005). Bioherms composed of coral and skeletal remains of other marine organisms sometimes outcrop. 

Aii iamprophyres have been ciassified as sannaite iamprophyres, except an uitramafic damtjernite. The sannaite iamprophyres contain kfs, amp cai, cpx, phi, ap, pi, sodaiite, ms, ttn, ep, white mica, chi, zrn, and opaque phases. The coarse-grained damtjernite consists of phenocrystic phiogopite, with groundmass caicite, aikaii feidspar, and apatite. 

The only primary phosphate mineral in the basaltic groundmass is an idiomorphic acicular apatite, which we therefore regard as the most probable primary REE carrier in the residues. However, the REE concentrations of this apatite were below the detection limit of the EDS device of our SEM. 

Nakhlites (Fig. 6.11b) are augite-rich basaltic rocks. They are cumulates formed by concentration of augite and minor olivine. They formed on or close to the surface, as indicated by the fine grain size of their groundmass, which consists of needles of feldspars, delicate spinel crystals, and glass. All the nakhlites are —1.3 billion years old, and compositional similarities indicate that they are related. 

The Basra Petrofabric was identified in an extensive sample of early Islamic ceramics and kiln furniture from Basra. Some of the.kiln furniture examined by Mason formed a part of another INAA study (32). The Basra Petrofabric, as identified in ceramics and kiln furniture, is characterized by a light yellow featureless groundmass containing 2 to 3 percent angular quartz, with a trace amount of amphibole and biotite, less than 0.03 mm. in size (33). Mason also... 

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