Geochemical fingerprint of hyaloclasts in glassy fragmental rocks of Macquarie Island (Southern Ocean): implications for volcanogenic sedimentary processes at a waning mid-ocean ridge

Abstract

Two main types of glassy fragmental rocks formed along the Proto-Macquarie Spreading Ridge: (i) hyaloclastite breccia; and (ii) pillow-fragment breccia. Examples now exposed on Macquarie Island, Southern Ocean, were largely sourced from proximal pillow lavas. In each of seven samples examined, hyaloclasts (basaltic glass grains) have a narrow major-and trace-element geochemical range, consistent with derivation of each sample from a single volcanic eruption event. Moreover, every sample analysed within the one stratigraphic section (at three sites) displays distinctive major-and trace-element geochemistry compared with the other two sections. This suggests that hyaloclasts at each site represent discrete magma batches. A single source for these glassy fragmental rocks contrasts with the dominant fault-scarp-derived polymict sedimentary rocks on Macquarie Island. We suggest that the hyaloclasts analysed in this study were deposited in small basins between the slopes of growing pillow cones along the mid-ocean ridge. The geochemical analyses presented here encompass (weakly) fractionated (e.g. Bauer Bay) to near-primitive (e.g. Pyramid Peak) compositions. All samples presented here lie within the range of the enriched-to normal-MORB suites previously reported for the island that include the least fractionated MORB melts known globally. The interpretation of geochemically distinct magma batches over the small area of the island suggests very limited magma mixing consistent with an immature or waning magmatic system. We relate these geochemical characteristics to: (i) volcanism near a very long offset transform; and (ii) genesis of magmas during the waning stages of slow seafloor spreading within a very narrow (550 km-wide) spreading corridor.

The Volcano‐Tectonic Evolution of the Macquarie Ridge Complex, Australia‐Pacific Plate Boundary South of New Zealand

“…These features constrain a model for the formation of Macquarie breccias: destabilisation of a growing pillow cone resulted in a voluminous eruption phase responsible for the fragmentation, transportation and emplacement of glass and pillow lava clasts down-slope of the cone. The narrow major and trace element compositional range defined by numerous glass grains in each Macquarie Island breccia sample is consistent with derivation from a single magmatic source (Daczko et al, 2009). 4.1).…”

“…Glass shards from individual volcaniclastic breccia samples define tight major element compositional clusters that are less than the analytical uncertainty for each oxide (Figs. 4.3,4.4; Chapter 2, Table 2.3), indicating that shards for each sample originate from a single magmatic source (Daczko et al, 2009). Analytical precision of EPMA analyses is presented in Chapter 2 (see Table 2.3).…”

“…Investigation of glass shards in volcaniclastic breccias in several studies on Macquarie Island has provided insight into the mechanisms of volcaniclastic rock formation at mid-ocean ridges. Three key characteristics are indentified: a tight compositional domain defined by samples of the same unit (Portner et al, 2009); preservation of distinct geochemical features between units (Daczko et al, 2009); textural characteristics indicating deposition while grains were hot (Dickinson et al, 2009). These features constrain a model for the formation of Macquarie breccias: destabilisation of a growing pillow cone resulted in a voluminous eruption phase responsible for the fragmentation, transportation and emplacement of glass and pillow lava clasts down-slope of the cone.…”

“…Griffin & Varne, 1980;Duncan & Varne, 1988). Recent work on the island has included structural studies (Goscombe & Everard, 2001;Daczko et al, 2003;Wertz et al, 2003;Dijkstra & Cawood, 2004;Mosher & Massell-Symons, 2008) and petrologic and geochemical examination of volcanic (Kamenetsky et al, 2000;Kamenetsky & Maas, 2002;Wertz, 2003;Daczko et al, 2009;Portner et al, 2009) and plutonic (Bazylev & Kamenetsky, 1998;Dijkstra et al, 2009) rocks. Macquarie Island crust has been dated at between 12 and 8 Ma with 40 Ar-39 Ar dating of basaltic samples (Duncan & Varne, 1988) and biostratigraphic ages from microfossils in sediment between pillow lavas (Quilty et al, 2008).…”

“…The result indicates that volcanism driven by low degrees of partial melting was widespread during oblique divergence and/or the oceanic crust that comprises MRC seamounts was generated within narrow corridors of seafloor spreading, similar to Macquarie Island. The preservation of distinct magmatic batches over small areas at individual MRC seamounts and on Macquarie Island is the result of limited mixing of melts at the plate boundary during the waning stages of seafloor spreading (Daczko et al, 2009;Wertz, 2003).…”