Pb and Other Trace Elements in Melt Inclusions From Modern Seafloor Tectonic Settings

Evidence for the contribution of magmatic metal-bearing fluids to modern seafloor hydrothermal systems has been presented for key ore metals such as Cu, Zn, Au and Ag but not for Pb. The source of Pb in these systems can be attributed to leaching of host rocks by hydrothermal fluids and/or potentially by direct degassing of magma. We present data from 103 melt inclusions mainly in plagioclase crystals hosted in basaltic to rhyolitic lavas from 9 modern seafloor tectonic settings: oceanic back-arcs (Manus Basin including an ODP Leg 193 drill hole, Bransfield Straits), continental back-arc (Okinawa Trough), mid-ocean ridges (Explorer, MAR), seamounts (Foundation, Axial, Pito), and a transform (Garrett). Most sites host active hydrothermal systems that are producing polymetallic sulfides. Inclusions were analyzed for Pb and other trace elements by LA- ICP-MS at ETH Zurich. Although inclusion trace element compositions vary between tectonic settings, for a given setting variability between different volcanic sequences is low and follows a general mafic to felsic fractionation trend. N-MORB compositions dominate most inclusion populations with exceptions seen at Explorer (plume-ridge interaction), Garrett (near-ultramafic source) and in some Foundation samples (seamounts affected by ridge interaction). Results show high Pb concentrations in inclusions from oceanic back arc and continental back-arc settings coinciding with evolved volcanic chemistries and related in the latter to contamination from sediments or continental crust. Sulfide deposits in these back-arcs are also Pb-rich. Inclusions from primitive basalt and near-ultramafic hosted samples from MOR, intraplate seamounts and transforms have low Pb abundances. In the most Pb abundant settings, melt inclusions have Pb concentrations 1-3 times higher than that of the bulk rock or volcanic glass indicating possible Pb removal from the magma prior to eruption. This loss may be explained by direct degassing of the magma into the hydrothermal circulation. Exceptions to the behavior of Pb in the studied settings include: elevated Pb in inclusions hosted in primitive basalts TAG (MAR) and some samples from the Okinawa Trough showing low Pb with respect to bulk rock.