THE ABUNDANCE AND DISTRIBUTION OF THE RARE-EARTH ELEMENTS LA, CE, AND ND IN THE GEORGE ASHLEY BLOCK PEGMATITE BODY

In the George Ashley Block pegmatite body four rare-earth elements are present in sufficient quantities to be measured (in ppm); these are La, Ce, Nd, and Yb. The elements Eu and Ho were looked for, but did not occur above the level of detection (2 and 4 ppm). The distribution of minerals in this pegmatite is not homogeneous. In particular, garnet (volume%) varies by a factor of at least 20 in different samples, and ≈80% occurs in the lower part (46%) of the pegmatite body. The abundances of three of the rare-earth elements (La, Ce, Nd) varies as does the abundance of garnet within the pegmatite body. The abundance of these elements also varies as does Mn, and garnet is the dominant Mn-bearing mineral. This data indicates that the La, Ce, and Nd are contained within the garnet in amounts of a few ppm and that their variation in rock samples is controlled by the variation in the abundance of garnet.

This data also indicates there is a tendency for these three elements to be, specifically, included in the garnet at a level of a few ppm. Note that in studies of thin sections and with the microprobe, there were no apparent blebs of melt or other inclusions in the garnet (grain size average ≈0.05 cm). Other minerals nucleated at the same time as garnet, and they show no tendencies to contain these rare-earth elements. This, then, indicates that these atoms (≈1.85Å, metallic radii) are present in a very few, scattered, large structural defects in the garnet--one such defect per 10¹⁰ structural sites should be sufficient to contain a few ppm of these rare-earth elements. The other possibility is that there were inclusions that favored garnet, and they are too small to observe. There is a vague indication that Yb behaves in a similar way, but with respect to muscovite.