Collection & Description

The sample suite presented in this chapter is a subset of the dredged products obtained during the SHAARC research voyage (FR04/00) with the RV

Franklin in Papua New Guinea and the Solomon Islands. The SHAARC research

team conducted a multi-disciplinary investigation (volcanology, petrology, geochemistry and economic geology) of submarine volcanic-hydrothermal systems in island arc regions of Papua New Guinea and the Solomon Islands. FR04/00 investigations of the TLTF and Solomon Island convergent margins aimed to generate a better understanding of the fundamental geological processes of submarine arc volcanism (McInnes et al., 2000).

Targets of this research voyage included the active hydrothermal vents on submarine cinder cones off the southern coast of Lihir Island. Dredge and grab sampling programs were successfully implemented on two of these small, explosive seamounts: Edison and Tubaf. The primary purpose of this was to collect xenolith samples of the oceanic lithosphere. These programs were successful, recovering over 250 xenoliths of peridotite, gabbro, basalt and crustal sediments.

On the 7th _{and 8}th _{of May, 2000, at Sites #18 and #19, cobble to pebble sized}

materials were obtained from the summit of Tubaf, comprising phlogopite- amphibole-clinopyroxene-bearing basalt enclosing a variety of xenolithic lithologies. Site #18 (SHDR-08) is located at 3°15.34’S and 152°32.60’E. Site #19 (SHDR-09) is located at 3°15.34’S and 152°32.67’E. In the voyage report (McInnes et al., 2000), the xenolithic lithologies are reported as including harzburgite, gabbro and mudstone, carbonaceous mudstone, and volcaniclastic sedimentary rocks. While the external form of the cobbles and pebbles were rounded, the xenoliths are described as mostly angular and fractured. In addition, many of the gabbros have a tectonic fabric. McInnes et al., (2000) hypothesise that the peridotitic and gabbroic xenoliths are likely to be representative of the deeper

The study that follows is the first to be undertaken on the dredged peridotites from Tubaf that were acquired during the SHAARC research voyage.

4.3.2 Petrography

This study set comprises 10 peridotites with similar modal percentages of olivine, orthopyroxene and clinopyroxene. All samples are classified as harzburgite with the exception of 136309-1A and 136309-1K. Sample 136309-1A exhibits effects of secondary processes, but maintains some of its original texture and is thought to have originally been a dunite. Sample 136309-1K has been extensively re- crystallised. None of the samples are weathered or serpentinised. Full-size scans of the 10 thin sections in plane-polarised and cross-polarised transmitted light are presented in Appendix A.

The harzburgites of this study are all garnet-free. Spinel is ubiquitous but forms a low modal percentage of the xenoliths (~0.5-1.5%) compared with fertile mantle peridotite (Pearson et al., 2003; Ionov, 2007). Unlike the dunites, harzburgites and pyroxenites of Ritter, the host rock of the Tubaf samples contains amphibole and phlogopite. These minerals are only present within the xenoliths as veins, and are a direct product of host magma contamination.

Figure 4.2. Photomicrographs of representative Tubaf host lava. a) & b) 136309-1D in reflected and polarised light. c) & d) 136309-1K in reflected and polarised light.

A range of rocks from Tubaf has been described in the literature. McInnes et al., (2001) outlines three different textural types (Type A – pristine, anhydrous samples displaying no development of fibrous orthopyroxene; Type B – metasomatised samples with ‘clouding’ of pyroxene; and Type C – metasomatised samples with cross-cutting veins and secondary mineral crystallisation). As outlined in Chapter 2, separating source from process is valuable where possible, through petrographical classification. As such, the samples of this study are classified according to the scheme defined in Chapter 2 in association with traditional textural classifications (Table #). The samples range from fine- to medium-grained, with textural classifications of porphyroclastic and granuloblastic (textural definitions from Harte (1977), with reference to Mercier & Nicolas (1975)).

Sample 136309-1B is classified as residual. This sample shows no evidence of veining, or alteration of orthopyroxene, and contains large, clear minerals. 136309-1S also appears to be relatively un-contaminated but does contain phlogopite and amphibole, indicative of host magma infiltration.

Figure 4.3. Residual harzburgite textures. a) 120° grain boundaries in sample 136309-1B. b) Spinel and olivine in sample 136309-1S.

Sample 136309-1K is classified as re-equilibrated due to the presence of localised fine-grained recrystallisation, which is granuloblastic in texture (Fig. 4.4). This is texturally comparable to the re-equilibrated sample suite in Ritter, which has experienced melt percolation in the mantle. This sample does contain phlogopite and amphibole, so is a contaminated sample, but is justifiably not classified as such due to its unique texture and the well-preserved nature of the

been made for the purpose of revealing mineralogical diversity arising from different petrogenetic processes in sections 4.5 and 4.6.

Figure 4.4. Re-equilibrated texture of sample 136309-1K. a) Equigranular olivine. b) Dispersed Cr-spinel grains.

Seven of the Tubaf harzburgites are texturally similar to the contaminated porphyroclastic harzburgites of Ritter. Peridotites from both localities have experienced contamination by fluid/melt invasion resulting in pyroxenite veining. The pyroxenite veins that cross cut the peridotites are orthopyroxene-rich with minor olivine and clinopyroxene. Sample 136309-1A is thought to have been originally dunitic but is now confidently classified as contaminated as it exhibits extensive mineral replacement and acicular orthopyroxene veining.

Figure 4.5. Contamination in sample 136309-1A. a) Acicular orthopyroxene vein. b) Secondary, fibrous orthopyroxene growth.

Olivine ranges in grain-size from >>2 mm to <0.1 mm. The larger grains are predominantly anhedral to subhedral and contain undulose extinction or kink banding (Fig. 4.6). Strain induced elongation is often present in the larger olivine porphyroclasts.

Figure 4.6. Olivine with undulose extinction. a) 136309-1AA. b) 136309-1B.

Coarse orthopyroxene is abundant, often exhibiting fine clinopyroxene lamellae or the crazed texture also seen in the Ritter xenoliths. There is a low modal abundance of clinopyroxene with small grain-size <500 mm.

Figure 4.7. a) Orthopyroxene with exsolution lamellae in sample 136309-1B. b) Orthopyroxene with crazed texture in sample 136309-1A.

Spinel ranges from euhedral to anhedral (Fig. 4.8) and ranges in size up to ~1 mm. The spinel displays characteristic isotropic nature, high relief, no cleavage, and is dark red to black in colour.

4.4

Analytical Methods