Island is an exposure above sea-level of part of the crest 38•979), and negatively with Nd ( 143 Nd/ 144 Nd = 0•51310of the Macquarie Ridge. The ridge marks the Australia-Pacific 0•51304). Macquarie Island basaltic glasses are divided into plate boundary south of New Zealand, where the plate boundary two compositional groups according to their mg-number-K 2 O has evolved progressively since Eocene times from an oceanic spreading relationships. Near-primitive basaltic glasses (Group I) have the system into a system of long transform faults linked by short highest mg-number (63-69), and high Al 2 O 3 and CaO contents spreading segments, and currently into a right-lateral strike-slip at a given K 2 O content, and carry microphenocrysts of primitive plate boundary. The rocks of Macquarie Island were formed during olivine (Fo 86-89•5 ). Their bulk compositions are used to calculate spreading at this plate boundary in Miocene times, and include primary melt compositions in equilibrium with the most magnesian intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted Macquarie Island olivines (Fo 90•5 ). Fractionated, Group II, basaltic dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow glasses are saturated with olivine + plagioclase ± clinopyroxene, lavas, picrites, breccias, hyaloclastites), and associated sediments. and have lower mg-number (57-67), and relatively low Al 2 O 3 A set of Macquarie Island basaltic glasses has been analysed by and CaO contents. Group I glasses define a seriate variation within electron microprobe for major elements, S, Cl and F; by Fourier the compositional spectrum of MORB, and extend the compositional transform infrared spectroscopy for H 2 O; by laser ablationrange from N-MORB compositions to enriched compositions that inductively coupled plasma mass spectrometry for trace elements; represent a new primitive enriched MORB end-member. Compared and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. with N-MORB, this new end-member is characterized by relatively An outstanding compositional feature of the data set (47•4-51•1 low contents of MgO, FeO, SiO 2 and CaO, coupled with high wt % SiO 2 , 5•65-8•75 wt % MgO) is the broad range of K 2 O contents of Al 2 O 3 , TiO 2 , Na 2 O, P 2 O 5 , K 2 O and incompatible trace (0•1-1•8 wt %) and the strong positive covariation of K 2 O with elements, and has the most radiogenic Sr and Pb regional isotope other incompatible minor and trace elements (e.g. TiO 2 0•97-2•1%; composition. These unusual melt compositions could have been Na 2 O 2•4-4•3%; P 2 O 5 0•08-0•7%; H 2 O 0•25-1•5%; La generated by low-degree partial melting of an enriched mantle 4•3-46•6 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr ( 87 Sr/ 86 Sr = peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island 0•70255-0•70275) and Pb ( 206 Pb/ 204 Pb = 18•951-19•493;
