Importancia y prevalencia

Arc-derived peridotites and mantle exposures in arc localities are extremely rare even in well-explored subaerial arcs. The peridotite and pyroxenite xenoliths from Ritter are a unique occurrence in the West Bismarck Arc and are among the rare occurrences from arcs globally. Previous studies of mantle xenoliths have been conducted in arc localities including the Aleutian Arc (Delong et al., 1975; Conrad et al., 1983; Conrad & Kay, 1984; Debari et al., 1987), Andean Arc (Gorring & Kay, 2000; Laurora et al., 2001; Bjerg et al., 2005; Schilling et al., 2005; 2008; Aliani et al., 2009; Rivalenti et al., 2004; Conceição et al., 2005), Cascades Arc (Brandon & Draper, 1996; Ertan & Leeman, 1996), Honshu Arc (Kushiro & Kuno, 1963; Sakuyama, 1984; Goto & Arai, 1987; Umino & Yoshizawa, 1996; Arai et al., 2000; Senda et al., 2007; Tasaka et al., 2008), Izu-Bonin-Mariana Arc (Ishii, 1992; Parkinson & Pearce, 1998; Ohara et al., 2002; Dare et al., 2009; Michibayashi et al., 2009), Kamchatka (Bryant et al., 2007; Ishimaru et al., 2007; Halama et al., 2009; Ionov, 2010; Soustelle et al., 2010), Kurile Arc (Volynets et al., 1990), Lesser Antilles (Parkinson et al., 2003; Vannucci et al., 2007), Philippines (Maury et al., 1992; Métrich et al., 1999; Arai et al., 2004; Grégoire et al., 2008), Mexican Volcanic Belt (Heinrich & Besch, 1992; Luhr & Aranda-Gómez, 1997; Blatter & Carmichael, 1998; Mukasa et al., 2007; Housh et al., 2010), Scotia Arc (Pearce et al., 2000), Solomon Islands (Ishikawa et al., 2007), Talkeetna (Debari & Coleman, 1989), Tabar-Lihir-Tanga-Feni (TLTF) Arc (Grégoire et al., 2001; McInnes et al., 2001; Franz et al., 2002; Kamenov et al., 2008), Tonga Arc (Bloomer & Fisher,

1987) and Vanuatu (Barsdell & Smith, 1989).This may seem like a good coverage

of samples but if you consider that there are ~40,000 km of active arc zones, and that it is possible to count the number of ultramafic xenolith-bearing localities very quickly, these samples become more important. Additional to being the only samples of this nature from the New Britain – West Bismarck Arc, this one locality delivers a suite of dunites, harzburgites and pyroxenites, all of which display evidence of complex histories. The nature of the subarc mantle, and that of arc cumulate sequences, is of considerable scientific interest as it provides constraints on many of the unresolved questions in geochemistry and igneous petrology, such as fundamental mass balance considerations of the differentiated earth; why arcs are more oxidised than Mid Ocean Ridge Basalts (MORB); how depleted/re- fertilised the subarc mantle is; the nature of the melts forming the continental

crust; the composition of the primary magma being generated in these environments; and if this subarc mantle is rich in the metals which concentrate in arc settings. Without the study of samples such as those presented in this chapter and the following, researchers are reduced to inferences about the subarc environment, typically drawn from primitive arc rocks (or the components thereof) that are thought to be derived from these ultramafic lithologies. Arc xenoliths remove the process based obscurities and necessary assumptions created by this method, and allow for direct, first order observations of the nature of ‘what lies beneath’ or perhaps more correctly what lies ‘between’ in terms of a magma and its next pulse. Before some of the above geochemical unknowns may be addressed, a detailed petrographical study of these xenoliths is vital. It is important understand what exactly it is that these rocks represent.

This chapter consists of a detailed petrographical study of a unique suite of arc-related samples from the West Bismarck Arc in Papua New Guinea. In documenting, investigating and analysing the textures, mineral assemblages and melt features of this suite, the aim is to determine the history and genesis of these samples and thus deepen the understanding of subarc processes and provide the baseline for further geochemical study. The companion piece (Chapter 3) presents the major and trace element chemistry of the xenolith mineral assemblage and whole-rock major and trace element geochemistry of select samples from this xenolith suite.

2.2 Samples

Small submerged satellite cones and rifts are common around the major subaerial volcanoes of the Bismarck volcanic arc front. During the 2007 research

voyage of the Marine National Facility (RV Southern Surveyor) the SS06-2007

West Bismarck Vents Expedition (WeBIVE) explored these promising submerged portions of the Bismarck Arc. Dredging was used to recover rock samples and a multibeam swath mapper used to explore the bathymetry and sea floor properties of the Arc. A large collection of picrites, peridotite-cored bombs and less commonly, pyroxenite-cored bombs, was dredged from 4 of the 5 submarine satellite cones to the west of Ritter Island. These are emergent from the debris field of the 1888 tsunamigenic collapse of the main Ritter cone (see Chapter 1, Section 1.3.1).

On Friday 10 August 2007, West Bismarck Dredge #62 (WBD-62) was deployed at 1520 and returned to deck at 1625. The dredge recovered a haul of fresh scoriaceous olivine-clinopyroxene-plagioclase-phyric basalt, some of which had xenolithic cores of peridotite. The dredge recorded no tension bites indicating that it sampled unconsolidated material and that Cone #1 may be a cinder cone.

RV Southern Surveyor then moved ~2½ nautical miles northwest to dredge a

second acoustically reflective cone (Cone #2). WBD-63 left deck at 1807 and was recovered at 1904. The results of the operation mirrored those of WBD-62. The dredge showed no tension bites, and recovered a haul of fresh scoriaceous olivine- clinopyroxene-plagioclase-phyric basalt, some with xenolithic cores of peridotite, and muddy volcaniclastic sediment. With excitement levels high amongst the igneous petrologists aboard, a decision was made to re-dredge Cones #1 and #2, in order to recover additional xenolithic material. WBD-64, at Cone #2, was deployed at 1928 and recovered at 2015, and WBD-65, at Cone #1, commenced at 2107 and concluded at 2210. Both dredges gave similar results to WBD-62 and WBD-63.

Shortly after midnight, the vessel moved ~1½ nautical miles to the northwest of

Cone #2 to dredge Cone #3, a feature on a ridge that runs northeast from Umboi Island. WBD-66 was deployed at 0032 and recovered at 0130, with a load of Fe- oxide stained vesicular olivine-phyric basalt and muddy volcaniclastic sediment. As

the science watch changed, RVSouthern Surveyor moved 1½ nautical miles east to

dredge another volcanic cone (Cone #4). WBD-67 left deck at 0204 and was completed at 0305. The dredge recovered at least 30 kg of peridotite xenoliths wrapped in fresh, glassy basalt, as well as one eroded piece of more massive weathered basalt, and a galatheid crab (Arculus, 2007).

The sample set selected for study and presented in this chapter comprises 36 fresh xenoliths recovered from the dredging of Cones #1, #2 and #4.