A petrological study of peridotite and pyroxenite xenoliths from the West Bismarck Arc and the Tabar-Lihir-Tanga-Feni Arc, Papua New Guinea
Creators: name: McAlpine, Sarlae Ruth Buffett
Publisher: name: The Australian National University
Year: 2015
Subjects: sub-arc mantle
peridotite
mantle xenolith
lithosphere
redox state
petrology
metasomatism
picrite
arc magmatism

description: Some of the most refractory peridotite samples described in the literature comprise clasts up to 15 cm in size, hosted in satellite cones of Ritter Volcano in the West Bismarck Arc, Papua New Guinea. Host lavas are MgO-rich (13.9-16.6 wt%), mostly non-accumulative picritic tholeiites, representing the most primitive magma types in the region. The lava can be divided into two distinct geochemical groups: a low-Ti series (TiO2 0.25-0.3 wt%) and a high-TiO2 series (TiO2 0.4-0.45 wt%). This thesis documents the chemical composition and mineralogy of the picritic hosts and peridotite suite of Ritter, and compares the latter with a peridotite suite from the Tubaf Seamount in the Tabar-Lihir-Tanga-Feni Arc of Papua New Guinea. The Ritter and Tubaf peridotite suites have experienced minimal alteration through serpentinisation or chloritisation. Petrologic study reveals however, that they have experienced various degrees of melt depletion, host magma infiltration, metasomatism, dissolution/re-precipitation and replacement. The sample suite can be divided into three broad groups: residues from partial melting, re-equilibrated samples and a third category comprising samples from both the ‘residual’ and ‘re-equilibrated’ categories that have been ‘contaminated’ by secondary melt infiltration processes. Olivine-spinel exchange geothermometric calculations give temperatures of ~670 to 1140 oC for Ritter, and 755 to 840 oC for Tubaf, consistent with entrainment in host lavas from the sub-arc lithosphere. However, the bulk compositions, crystalline phase major element compositions coupled with trace element geochemical characteristics of these suites reflects a complex petrogenetic history, likely established in regions of magma generation in a supra-subduction zone, mantle wedge setting. Olivine is highly forsteritic (Fo# 86.8-95.7 for Ritter, and Fo# 87-91 for Tubaf), spinel is extremely Cr-rich (Cr# 40.4-89.3 for Ritter, and Cr# 45.0-69.1 for Tubaf), CaO in olivine, and Al2O3 in orthopyroxene are consistently very low (&lt;0.05 wt% and &lt;2 wt% respectively), and primary clinopyroxene is absent. The trace element abundance patterns of primary orthopyroxene and secondary clinopyroxene display depletions relative to rare earth elements in high field strength elements, consistent with equilibration with arc-type magmas. Olivine-spinel oxygen barometry shows a range from reduced to oxidised conditions relative to the fayalite-magnetite-quartz buffer for both Ritter (-1.43 to +1.84 log10 units fO2) and Tubaf (-1.26 to +0.86). Evidence from Zn/Fe, V/Sc and Mn/Fe systematics suggests that independent of tectono-magmatic setting, the source of arc magmas, evidenced by these peridotites, may be indistinguishable in terms of oxidation state to that of mid ocean ridge basalts. This study gives a rare insight into the nature of the sub-arc mantle and the generation of arc magmas.