Abstract

High-resolution trace element mapping (2 µm beam) was performed by synchrotron radiation X-ray fluorescence microscopy (SXFM) on Cu- and Zn-rich chimneys from Brothers volcano, Kermadec arc. The maps cover 84- to 136-mm2 cross sections of the inner chimney walls and document the distribution of Fe, Cu, Zn, As, Se, Sr, Pb ± Ga, Au, Bi, and U. Comparative element maps were generated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at a lower resolution (47 µm beam), which additionally measured Co, Ni, Mo, Ag, Cd, In, Sn, Sb, Ba, Au, Tl ± V, Te, and Bi. In two Zn-rich chimneys, Cu distribution varies from Cu-bearing sphalerite with chalcopyrite disease to distinct chalcopyrite-lined channels, implying a progression of chalcopyrite replacement of sphalerite. Conversely, the two Cu-rich chimneys have different styles of massive chalcopyrite lining their conduits. The first displays elongate chalcopyrite grains that radiate into and infill the conduit; these radial grains may be related to decreased fluid flow during sealing of the chimney at both its top and bottom. The second style involves multiple, concentric chalcopyrite laminations (0.25–1 mm) inside the conduit, which progressively narrowed the orifice as they were deposited. Fine (15–40 µm) trace element rings are revealed within and between laminations that exhibit variable contents and distributions of Co, Ni, Zn, As, Se, Mo, Ag, Cd, Sn, Te, Au, Tl, Pb, Bi, and U. The presence of U in the rings specifically indicates seawater ingress into the chimney interior despite it vigorously discharging 274°C fluids at the time of sampling. During these periodic seawater incursions, rapidly changing chemical gradients within the chimney wall induce the instantaneous precipitation of metals from the vent fluid. Thus, the trace element rings are a proxy for the secular evolution of vent fluid compositions. We compared enrichment factors of trace element rings to those of fumarole condensates studied at subaerial arc and rift volcanoes and molten S pooled atop a submarine back-arc volcano. Our enrichment factors show remarkable consistency with the other volcanoes and indicate Au, Te, Bi, Cu, Ag, and Se in Brothers’ chimneys were magmatically derived.

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