Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)

Abstract

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 Fourier transform infrared spectroscopy for H2O; by laser ablation-inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry 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 rations of Sr (Sr-87/Sr-86 = 0.70255-0.70275) and Pb (Pb-206/Pb-204 = 18.951-19.493; Pb-207/Pb-204 = 15.528-15.589; Pb-208/Pb-204 = 38.523-38.979), and negatively with Nd (Nd-143/Nd-144 = 0.51310-0.51304). Macquarie Island basaltic glasses are divided into two compositional groups according to their mg-number-K2O relationships. Near-primitive basaltic glasses (Group I) have the highest mg-number (63-69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primitive olivine (Fo(86-89.5)). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo(90.5)). Fractionated, Group II, basaltic glasses are saturated with olivine + plagioclase +/- clinopyroxene, and have lower mg-number (57-67), and relatively low Al2O3 and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that represent a new primitive enriched MORB end-member. Compared with N-MORB, this new end-member is characterized by relatively low contents of MgO, FeO, SiO2 and CaO, coupled with high contents of Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements, and has the most radiogenic Sr and Pb regional isotope composition. These unusual melt compositions could have been generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island region must have been enriched and heterogeneous on a very fine scale.We suggest that the mantle enrichment implicated in this study is more likely to be a regional signature that is shared by the Balleny Islands magmatism than directly related to the hypothetical Balleny plume itself.