High CaO Concentrations Research Articles

‘Terra Sigillata imitations’ from Nyon (SW Switzerland): one of Fronto’s workshops?

The production of Terra Sigillata imitations (TSI) from Nyon (south-west Switzerland) (15–30 A.D.) was studied from the chemical-mineralogical point of view, in order to define a new reference group and to prove if one piece stamped by the potter Fronto (ZA165) and found at the same place, was produced in the workshop of Nyon. Three Fronto stamped wasters found at Augst were compared with the local Nyon TSI in order to verify the possibility of the existence of two different workshops of this potter during the Gallo-Roman period. The analysed Nyon wasters may be divided into two groups, differing in CaO content. Most of the samples are characterised by high CaO concentrations and the Fronto stamped piece (ZA 165) is similar to this group. Nine samples form a small cluster characterised by lower CaO and MgO contents. Stamped wasters from Augst show a very different chemical pattern compared with the TSI of Nyon. One new reference group is proposed, consisting of CaO-rich samples from Nyon. Samples from Augst were certainly produced in a different workshop. The presence of two Fronto workshops is therefore presumed.

Late Pleistocene basaltic ash and volcanic eruptions in the Bonneville basin, Utah

Microprobe analyses of glass and radiocarbon dating of samples associated with three basaltic volcanic ashes in the deposits of Lake Bonneville make the ashes useful stratigraphic markers for correlation within the Bonneville basin. Two of the ashes were derived from tuff cones (Pavant Butte and Tabernacle Hill) that erupted into Lake Bonneville in what is now the Black Rock Desert of south-central Utah. Near Kanosh, Utah, the Pavant Butte ash is interbedded with barrier-beach lagoon marl that is slightly less than 16,000 yr old. Five radiocarbon dates on carbonate materials stratigraphically associated with the ash elsewhere in the Sevier Desert average 15,300 yr B.P. Lake Bonneville was in its transgressive phase, about 15 m below the Bonneville Shoreline, when the Pavant Butte eruption began. At the Tabernacle Hill tuff cone, a basalt flow that has pillows, wave-rounded cobbles, and tufa on its outer margins was erupted into Lake Bonneville shortly after the tuff cone at or near the Provo Shoreline. The Tabernacle Hill tuff cone and basalt flow are older than a radio-carbon date of about 14,300 yr B.P. on tufa collected from the outer margin of the basalt flow, but younger than the Bonneville Flood at about 14,500 yr B.P. The third ash, informally termed the Thiokol ash, has been found interbedded with Lake Bonneville deposits in two exposures in northern Utah and in sediment cores from the Great Salt Lake. It is about 25,000 yr old, and may have been erupted from a volcanic field on the northwest shore of Great Salt Lake or from the Snake River Plain. Microprobe analyses of glass from samples of the Black Rock Desert ashes show them to have similar chemical compositions, but glass from Tabernacle Hill can be distinguished from Pavant Butte glass by its higher concentrations of CaO and P 2 O 5 . Systematic differences in chemical composition between samples of Pavant Butte glass can be explained by comagmatic processes. Thiokol glass has less SiO 2 , AI 2 O 3 , MgO, and Na 2 O but greater total iron and P 2 O 5 than do the Black Rock Desert glasses.

A geochemical study of rocks from the Walvis Ridge, South Atlantic

Basalts from the Walvis Ridge, an aseismic ridge in the eastern South Atlantic Ocean, have been analyzed to investigate the origin and evolution of this feature, and its relation to Tristan da Cunha. The principal minerals in the basalts are plagioclase, clinopyroxene and Fe-oxides. The alkalic nature of the samples is indicated by the high content of the orthoclase end-member in the feldspars, and the aluminous titanaugites, which contain up to 3.2 wt.% TiO 2. Chemically, the basalts can be divided into two related major groups. Group I is slightly more evolved and is characterized by high TiO 2, FeO∗, V and Zr concentrations, while group II contains relatively low TiO 2 and FeO∗ contents, but has high concentrations of CaO, Na 2O and Al 2O 3. The differences between them are explained by the relative importance of ilmenite precipitation during crystallization in group I and plagioclase precipitation in group II. All of the basalts from the Walvis Ridge are geochemically distinct from Mid-Ocean Ridge basalts originating at spreading centers including those at equivalent latitudes in the South Atlantic. A change in the Zr/Nb ratio of the basalts is observed along the Walvis Ridge — from a value of 10 at the eastern end, 6 in the central section, to ∼4 for Tristan da Cunha. This suggests that the composition of the mantle source was different for the various provinces along the ridge and may have changed progressively with time if the Walvis Ridge represents the trace of the Tristan da Cunha hot spot.

Compositions of magnesite articles and the analysis of phase equilibria in the CaO-MgO-SiO2 system

The norms in the State Standards for magnesite products have been confirmed from the point of view of phase equilibria. The deviations from the norms towards higher concentrations of CaO and SiO2 are accompanied by an increase in the amount of melt and sometimes by a lowering of the temperature at which the melt appears.