Melt generation models for the Auckland volcanic field, New Zealand: constraints from U[sbnd]Th isotopes

Abstract

New major and trace element, Sr, Nd, Pb, and UTh isotope results are presented for Quaternary volcanic rocks from the Auckland volcanic field, North Island, New Zealand. The rocks are predominantly small volume basanites and alkali basalts, with just 4.1 km3 having been erupted from 49 centres in the last 150 ka. They have many of the characteristics of HIMU Ocean Island Basalts (OIB), including high Ce/Pb, Nb/Ba, Nb/Ce, U/Pb and low Nb/U, and Sr and Nd isotope ratios (0.70287–0.70327 and 0.51315-0.51289) that plot on the low side of the mantle array. Their initial Pb isotope ratios (206Pb/204Pb= ∼ 19.2) are unradiogenic relative to those in HIMU OIB, suggesting that the HIMU signature in the source of the Auckland rocks is young (< 0.2 Ga). The Auckland rocks are unusually primitive (MgO > 11%; mg* = 0.65–0.70), and they exhibit some of the most marked (230Th/238U) disequilibrium (28–48%) known from intraplate basalts. Olivine fractionation-corrected trace element arrays are used to assess the degrees of partial melting (∼ 1.5–3.5%), the relative partition coefficients, and the source trace element ratios. The initial (230Th/232Th) values are much higher than those inferred for their source regions, and so some form of dynamic melting model is required, and the matrix peridotite must have been in the melt zone for relatively long periods of time, typically 0.5–1.0 Ma. Using dynamic and melt percolation models (after [1,2]) the calculated melt rates are low (2 × 10−5–2 × 10−4 kg m−3 yr−1), consistent with the low eruption rate of 1–2 km3 in the last 50,000 yr. It is inferred that these reflect very low upwelling rates beneath the Auckland volcanic field, and that the source characteristics (HIMU) of the Auckland volcanics are from relatively shallow levels in the upper mantle (80–140 km), rather than a deep-seated mantle plume.