The determination of magnesium in silicates by atomic absorption spectroscopy

The determination of magnesium in silicates by atomic absorption spectroscopy using a hot flame is discussed. Interference by aluminium observed, in the air-acetylene flame is overcome by using the hotter nitrous oxide-acetylene flame. There is some evidence to suggest that the alkali metals interfere in this determination but this is not confirmed in the results obtained on standard rock samples. Values for magnesium are given for some new rock standards recently distributed by the U.S. Geological Survey

The Giles Complex, an example of a deeply eroded volcanic zone

The Giles Plutonic Complex is briefly described. Volcanics occurring in the same tectonic setting are thought to be the extrusive silicic differentiates. Evidence is presented for a differentiation of the parental magma prior to its subsequent intrusion into higher levels where it produced fractionally crystallised rock series, The Volcanics display chemical similarities to other silicic differentiates associated with plutonic basic complexes and the province as a whole has a bimodal character

Crystallisation of an alkali-olivine basalt under controlled PO2, PH2O conditions

An alkali-basalt from Hualalai, Hawaii, previously examined by Yoder and Tilley (1962) has been crystallised under controlled PO2 at 2 kb total pressure. Significant differences, in the order of appearance of the phases result from lowering the PO2 from 10?0.7 to about 10?10 atm. In particular magnetite, present at 1100 °C in the unbuffered runs does not crystallise until below 900 °C in the oxygen controlled runs. There is a drop of up to 50 °C on the liquidus of the system if the oxygen is controlled at the quartz-fayalite-magnetite buffer. Calcium amphiboles crystallised under such conditions are nepheline normative and rich in titanium and aluminium. Decreasing the water content but maintaining total pressure at 2 kb results in an increase in the melting point of the amphibole. There is no data in the runs to support the concept of a primary amphibole gabbro crystallising from a basaltic melt of this composition. Rather, the amphibole would form by reaction of the pyroxene with the liquid, or direct growth at lower temperatures from a basaltic mineralogy

Trace element distributions in the chalcopyrite wall of a black smoker chimney: insights from laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS)

The thin walls of young black smoker chimneys experience steep physico-chemical gradients during active venting of hydrothermal fluid, and these gradients control trace element precipitation within those walls. Here, we utilise a combination of high sensitivity ICPMS and UV laser ablation (resolution of better than 30 ?m) to demonstrate the existence of non-random V, Ag, In, Te, Ba, Au, Pb and U distributions within the chalcopyrite wall of an immature black smoker chimney. The data are the first of their kind to be produced for black smoker chimney walls. Distributions of In and Te are attributed to preferential incorporation into lattices at elevated temperature. Enrichments of U and V derived from seawater are the product of redox immobilisation on sulphide surfaces. The distributions of Au, Ag, Pb and Ba may be related to interactions at the hydrothermal fluid–seawater mixing front; however, comparison of distribution data with reaction-transport models of chimney walls suggest a possible pH control on precipitation. These data illustrate the power of the LA–ICP–MS method, and such spatially resolved data have the potential to constrain models of element precipitation both in chimneys and in associated mounds