Understanding magma genesis through analysis of melt inclusions: application of innovative micro-sampling techniques

Abstract

Melt entrapped as inclusions in early-formed phenocrysts provide geochemists with an exceptional opportunity to study sample material from the earliest stages in the formation of a suite of lavas. With a foucus on olivine-hosted melt inclusions, this Ph.D. thesis has explored the potentials for obtaining Sr isotope ratios on individual olivine-hosted melt inclusions, and examined the potentials for Sr isotope studies on melt inclusions to reveal new information on the origin of CFB and OIB. A novel technique is introduced that facilitate precise and accurate Sr isotope and trace element analysis of individual melt inclusions at sub-nanogram levels - thus applicable to typical melt inclusion suites from OIB and CFB, and in general to 'problems' where precise and accurate Sr isotope and trace element information is required on sub-nanogram Sr samples. The technique developed combines off-line sampling by micro-milling, micro Sr column chemistry, Sr isotope determination by TIMS, and trace element analysis by ICPMS. Olivine-hosted melt inclusions from two suites of high (^3)He/(^4)He lavas of the North Atlantic Igneous Province are studied. These reveal that Sr isotope and trace element measurements on individual melt inclusions provide a higher resolution picture of the pre-aggregated melt compositions and the different mantle and crustal components involved in the magma genesis, which otherwise were obscured within the whole-rock data. The Sr isotope and elemental variability recorded by the olivine-hosted melt inclusions contrast the more subtle variations of the host lava suites and raises the question of whether the (^3)He/(^4)He measured in melt inclusions in olivine phenocrysts should be related to the chemistry of melt inclusions rather than the bulk lava chemistry. The study further provides strong evidence that the extreme, high (^3)He/(^4)He signature observed in magmas from the North Atlantic Igneous Province is derived from a depleted component in their source, and hence such He isotopic signature should no longer be regarded as canonical evidence for primitive, lower mantle source.