Mantle plumes, such as the one responsible for generating the Hawaiian Archipelago, represent a rare opportunity to indirectly sample the Earth's mantle. Geochemical studies of ocean island basalts (OIB) show that the mantle is chemically and isotopically heterogeneous at scales ranging from millimeters to thousands of kilometers (Hofmann, 2003). Volcanoes in the Hawaiian Islands are distributed along two subparallel geographical trends that are geochemically distinct (

Hydrothermal vent deposits form on the seafloor as a result of cooling and mixing of hot hydrothermal fluids with cold seawater. Amongst the major sulfide and sulfate minerals that are preserved at vent sites, barite (BaSO 4 ) is unique because it requires the direct mixing of Ba-rich hydrothermal fluid with sulfate-rich seawater in order for precipitation to occur. Because of its extremely low solubility, barite crystals preserve geochemical fingerprints associated with conditions of formation. Here, we present data from petrographic and geochemical analyses of hydrothermal barite from the Endeavour Segment of the Juan de Fuca Ridge, northeast Pacific Ocean, in order to determine the physical and chemical conditions under which barite precipitates within seafloor hydrothermal vent systems. Petrographic analyses of 22 barite-rich samples show a range of barite crystal morphologies: dendritic and acicular barite forms near the exterior vent walls, whereas larger bladed and tabular crystals occur within the interior of chimneys. A two component mixing model based on Sr concentrations and 87 Sr/ 86 Sr of both seawater and hydrothermal fluid, combined with 87 Sr/ 86 Sr data from whole rock and laser-ablation ICP-MS analyses of barite crystals indicate that barite precipitates from mixtures containing as low as 17% and as high as 88% hydrothermal fluid component, relative to seawater. Geochemical modelling of the relationship between aqueous species concentrations and degree of fluid mixing indicates that Ba 2+ availability is the dominant control on mineral saturation. Observations combined with model results support that dendritic barite forms from fluids of less than 40% hydrothermal component and with a saturation index greater than $0.6, whereas more euhedral crystals form at lower levels of supersaturation associated with greater contributions of hydrothermal fluid. This document is a U.S. government work and is not subject to copyright in the United States.from cyclical oscillations in hydrothermal fluid flux. Barite chemistry and morphology can be used as a reliable indicator for past conditions of mineralization within both extinct seafloor hydrothermal deposits and ancient land-based volcanogenic massive sulfide deposits.

For~82 million years, the Hawaiian-Emperor chain volcanoes have sampled the Pacific mantle via the Hawaiian mantle plume, providing evidence that its composition varies on a range of temporal and spatial scales. Hawaiian volcanoes from 2 to 0 Ma are divided into southwestern (Loa) and northeastern (Kea) geographic and geochemical trends that are interpreted to reflect the bilateral chemical structure of the underlying plume and its corresponding deep mantle sources. Hawaiian volcanoes that formed between 8 and 3 Ma record a geochemical transition between the Kea-dominated Northwest Hawaiian Ridge (49 to 8 Ma) and the bilateral trends of the younger Hawaiian Islands. High-precision Pb isotopic analyses conducted on 55 new shield-stage samples from two of these key volcanoes, Kaua'i and Wai'anae, show that Loa-like Pb isotopic ratios (e.g., elevated 208 Pb*/ 206 Pb*) gradually increase with decreasing age among the northern Hawaiian volcanoes and dominate for over 2 million years prior to the onset of the bilateral Loa and Kea geochemical trends. Distinct isotopic groups are observed across Kaua'i and the distribution of Loa and Kea isotopic compositions is rotated relative to that observed on the younger Hawaiian Islands. Protracted Loa compositions and the atypical Loa-Kea trend on Kaua'i are accounted for by (1) the arrival of a voluminous, Loa mantle heterogeneity possibly associated with anchoring of the Hawaiian plume to the Pacific Large Low Shear Velocity Province and (2) a different orientation of the Pacific plate relative to the Loa-Kea compositional boundary prior to 2 Ma.

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