Lamproitic Rocks Research Articles

Appraisal of the new occurrence of götzenite ss, khibinskite and apophyllite in kalsilite-bearing lavas from San Venanzo and Cupaello (Umbria), Italy

Minerals corresponding in chemical composition to probable götzenite solid solutions, götzenite ss, ideally (Ca, Na) 3 (Ti, Zr) (Si 2O 7) (F, O, OH) 2, khibinskite (K 4Zr 2Si 4O 14) and apophyllite (KCa 4Si 8O 20(OH, F)) were first reported in an aphyric, glass-bearing kalsilitite from Cupaello (Umbria), Italy, by Cundari and Ferguson (1991). To the authors knowledge, this occurrence of khibinskite is only the second recorded. Götzenite ss was also found in a leucite-, melilite-rich pegmatoid in an olivine-melilitite from San Venanzo, S. Venanzo, in the same region. The S. Venanzo phase, with significant Zr (0.18 atoms per formula unit) is transitional to rosenbuschite, which is the Zr-rich end-member of the isostructural series götzenite-rosenbuschite, while that from Cupaello, virtually Zr free, is close to the ideal götzenite end-member. The S. Venanzo phase crystallized as the preferred Zr phase from the relatively low-Zr (Zr=335 ppm) S. Venanzo lava. This probably inhibited khibinskite crystallization in the S. Venanzo lava, whereas khibinskite is the preferred Zr phase in the Cupaello lava (Zr=760 ppm), coexisting with low-Zr götzenite ss. The structural formula of the Cupaello khibinskite, based on 10 cations, yielded: K 3.89[Zr 1.99Mg 0.01Fe 0.04 2+Ti 0.02] 2.06Si 4.05O 14.1 which is very close to the composition first reported by Khomiakov et al. (1974). Apophyllite, also occurring in the Cupaello assemblages and in cavities in the lava, yielded a composition close to the ideal formula and supports the important role of alkali-rich fluids in the crystallization history of this lava. The rare records of these minerals reflect, at least in part, the difficulty in identifying them. Their role as potential acceptors of both large ion lithophile and high field strength elements in peralkaline rock compositions is important in understanding the geochemistry of lamproitic rocks and in detecting possible genetic links with related rocks.

Post-magmatic apatite + hematite + carbonate assemblage in the Jumilla lamproites. A fluid inclusion and isotope study

The lamproitic rocks of Jumilla (southeastern Spain) consist of magnesian olivine, phlogopite, clinopyroxene, F-apatite, sanidine and analcime in variable amounts, and of Cr-rich spinel (mostly included in olivine and clinopyroxene), K-rich amphibole, Ti-magnetite, ilmenite and late calcite. In a small area, a post-magmatic apatite + hematite + carbonate assemblage occurs in the form of thin veins. F-apatite, hematite and calcite are the dominant phases. Apatite is rich in several generations of fluid (sensu lato) inclusions. The prevalent primary inclusions are filled in part with a solidified saline melt and in part with a very low density gas; the solid/gas volume ratio in the inclusions is not constant suggesting that, at the time of entrapment in apatite, a vapour phase was coexisting with the saline melt. The final melting temperature of the solids contained in the inclusions is mostly in the range 630–700°C, which represents the minimum temperature of entrapment. Other inclusions are concentrated along healed fractures. Some of them are similar to the solid salt inclusions described above, but most of them contain liquid, vapour and, sometimes, abundant daughter minerals. The temperature of initial melting of frozen liquid-bearing inclusions is variable (ca. −54°C, −35/−39°C, ca. −11°C, ca. −2/−3°C) suggesting that aqueous fluids carrying different components were entrapped at different times. The strontium isotope ratios and the high content of fluorine and REE suggest that apatite crystallized from fluids prevalently segregated from the lamproitic magma. Strontium as well as oxygen and carbon isotopes of the carbonates associated with apatite suggest that the parent fluids were poligenic (magmatic and sedimentary components) or of prevalent sedimentary provenance.

LAMPROITES OF NORTHERN ALGERIA

The mineralogy and chemistry of lamproitic rocks from two bodies in northern Algeria, near the city of Constantine, have been studied in detail. On the basis of the ultra-alkaline, highly potassic and highly mag-nesian composition and the presence of magnesian olivine, low-alumina and low-soda diopside and other minerals, these rocks are classified as fortunites. There appears to be a lamproite belt in the western Mediterranean region extending through Spain, Morocco, Algeria and Tunisia.

Trace-element analysis of argyle diamonds using instrumental neutron activation analysis

Thirty four elements were determined by instrumental neutron activation analysis in colourless, brown, and pink diamonds, with and without inclusions. These were compared with data obtained for similar elements in the host lamproite rock. The natural radioactivity of these samples was measured by instrumental techniques, and found to be negligible.

Diamond exploration in Western Australia

Lamproitic rocks of Western Australia have produced significant amounts of diamond and have given rise to the world's largest diamond mine. This has led to a reappraisal of the relationship between diamond and its host rock as well as new exploration techniques in the search for diamond. Procedures for success follow a modelling system using the most modern methods available in the Earth‐related sciences.