Summary
Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia-Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. A set of Macquarie Island basaltic glasses has been analysed by electron microprobe for major elements, S, Cl and F; by FTIR for H2O; by LA-ICPMS for trace elements and by SIMS for Sr, Nd and Pb isotopes. An outstanding compositional feature of the data set (47.4-51.1 wt% SiO2, 5.65-8.75 wt% MgO) is the broad range of K2O (0.1-1.8 wt%) and the strong positive covariation of K2O with other incompatible minor and trace elements (e.g. TiO2 0.97-2.1%; Na2O 2.4-4.3%; P2O5 0.08-0.7%; H2O 0.25-1.5%; La 4.3-46.6 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr (87Sr/86Sr = 0.70255-0.70275) and Pb (206Pb/204Pb = 18.951-19.493; 207Pb/204Pb = 15.528-15.589; 208Pb/204Pb = 38.523-38.979), and negatively with Nd (143Nd/144Nd = 0.51310-0.51304). Macquarie Island basaltic glasses are divided into two compositional groups according to their Mg# - K2O relationships. Near-primitive basaltic glasses (Group-I) have the highest Mg# (63-69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primitive olivine (Fo86-89.5). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo90.5).