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Melt inclusion records of volatile and metal degassing at Brothers submarine volcano, Kermadec arc

Shane M. Rooyakkers 1, Ery C. Hughes1, Cornel E. J. de Ronde1, Frank J. Tepley III2, Michael C. Rowe3, Penny E. Wieser4, Ariadni Georgatou5, Philip A. Brandl6, ­ Christian Timm6, Isabelle Chambefort7

  • Affiliations: 1 GNS Science, Lower Hutt, New Zealand; 2Oregon State University, Corvalis, Oregon, USA; 3University of Auckland, Auckland, New Zealand; 4 University of California Berkeley, California, USA; 5National and Kapodistrian University of Athens, Athens, Greece; 6GEOMAR, Kiel, Germany; 7GNS Science, Wairakei, New Zealand 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 30

  • Programme No: 4.1.21

  • Theme 4 > Session 1

Abstract

In recent years, studies have suggested that meals in seafloor massive sulphides at Brothers volcano (Kermadec arc) are degassed from underlying magma. To test this hypothesis, we measured the volatile contents (H2O, CO2, S, Cl: by Raman, SIMS and EPMA), sulphur speciation (by EPMA) and select trace metal contents (by SIMS) of glassy plagioclase-hosted dacitic Brothers melt inclusions (MIs). Measurable CO2­ was not found in MI-hosted vapour bubbles. The MIs are oxidised (S6+/Stotal ~0.8, fO2 ~FMQ+2) and have variable CO2 (3--264 ppm) and H2O contents (2.3--5.1 wt%), recording degassing between ~50--200 MPa. Sulphur contents are low ([<]{.underline}350 ppm), reflecting partitioning into vapour and sulfides. Moderate metal contents (e.g., up to 53 ppm Cu) are retained by the dacitic melts due to oxidising conditions that minimise losses to sulfides. Chlorine contents (max = 6270 ppm) are among the highest known for any subalkaline arc magmas, but only minimal Cl has degassed to vapour. Hypersaline brine inclusions found previously at Brothers imply degassing of more saline fluids at later stages of crystallization than recorded by the MIs. Very high magmatic Cl/H2O ratios and incompatible behaviour of Cl may eventually drive mushy parts of the system to brine saturation, releasing saline fluids that scavenge metals from the residual melt. Our work supports other evidence for magmatic contributions to massive sulfide deposits at Brothers, but suggests that most metals degas later than in large porphyry copper deposits.