U432164.pdf (153.32 MB)
Inclusion studies in ijolitic and carbonatitic rocks, with particular reference to the identification of their solid components.
thesisposted on 2015-11-19, 09:17 authored by John Arthur. Aspden
A variety of primary inclusions are present within apatite crystals extracted from East African and Swedish ijolites and carbonatites. They provide unique, albeit rather minute, geochemical samples of the crystallising fluids which were present during the growth of apatite. These inclusions contain varying amounts of solid material and by using the E.M. P., together with a combination of optical and microchemical tests the major chemistry of the various solid components has been established. In the ijolitic rocks from the Usaki complex of Western Kenya multisolid, carbonate-rich melt inclusions, which consist of alkali-rich carbonates, sulphates, and halides are present. The composition of the Ca-Na-K Carbonates within these inclusions is extremely similar to that recorded for the major carbonate phases from the natrocarbonatite lavas of Oldoinyo Lengai. Silicate glass inclusions are also present in some samples and these consist of two chemically contrasting types, one of which is Na-rich and undersaturated while the other is K-rich and oversaturated. These two glasses appear to show an immiscible relationship. Alkali-rich aqueous inclusions containing nahcolite (NaHCO3) together with solid inclusions composed of single crystals of calcite are also present within the ijolitic apatites of the Usaki complex. Apatite crystals which occur within late-stage patches of calcite in Alno ijolite pegmatites from Sweden do not contain melt inclusions but are dominated by concentrated aqueous saline types. Individual inclusions often contain up to 6o% of solids which include nahcolite, kalicine (KHCO3), alkali chlorides, alkali sulphates and opaque specks. These inclusions formed at lower temperatures (=450°C-) than the melt inclusions of the Usaki ijolites (= 680 - 950°C). Solid inclusions which contain calcite, pyrrhotite biotite and magnetite have also been recognised. Aqueous-rich inclusions are dominant in the apatites of the Tororo carbonatite of Eastern Uganda but a variety of solid inclusions are also present. The following types have been recognized: calcite, shortite, pyrrhotite and magnetite. It is concluded that these apatites were formed directly from an aqueous-rich alkaline solution at temperatures in the region of 400°C. The majority of inclusions studied contain minor amounts of opaque material and in both ijolitic and carbonatitic apatites pyrrhotite and magnetite have been recognised.
Date of award1977-01-01
Author affiliationPhysics and Astronomy
Awarding institutionUniversity of Leicester