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Widespread Assimilation of a Seawater-Derived Component at Loihi Seamount, Hawaii

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Widespread Assimilation of a Seawater-Derived Component at Loihi Seamount, Hawaii Geochimica et Cosmochimica Acta, Vol. 63, No. 18, pp. 2749–2761, 1999 Copyright © 1999 Elsevier Science Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/99 $20.00 ϩ .00 PII S0016-7037(99)00215-X Widespread assimilation of a seawater-derived component at Loihi Seamount, Hawaii 1,2, 3 4 1 2 5 ADAM J. R. KENT, *DAVID A. CLAGUE, MASAHIKO HONDA, EDWARD M. STOLPER, IAN D. HUTCHEON, and MARC D. NORMAN 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA 2Lawrence Livermore National Laboratory, Livermore, CA 94551, USA 3Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-0628, USA 4Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia 5Centre for Ore Deposit Research, School of Earth Sciences, University of Tasmania, Hobart, TAS 7001, Australia (Received February 4, 1999; accepted in revised form June 8, 1999) Abstract—Many tholeiitic and transitional pillow-rim and fragmental glasses from Loihi seamount, Hawaii, have high Cl contents and Cl/K2O ratios (and ratios of Cl to other incompatible components, such as P2O5, H2O, etc.) relative to other Hawaiian subaerial volcanoes (e.g., Mauna Loa, Mauna Kea, and Kilauea). We suggest that this results from widespread contamination of Loihi magmas by a Cl-rich, seawater-derived component. Assimilation of high-Cl phases such as saline brine or Cl-rich minerals (halite or iron–hydroxychlorides) with high Cl/H2O ratios can explain the range and magnitude of Cl contents in Loihi glasses, as well as the variations in the ratios of Cl to other incompatible elements. Brines and Cl-rich minerals are thought to form from seawater within the hydrothermal systems associated with submarine volcanoes, and Loihi magmas could plausibly have assimilated such materials from the hydrothermal envelope adjacent to the magma chamber. Our model can also explain semiquantitatively the observed contamination of Loihi glasses with atmospheric-derived noble gases, provided the assimilant has concentrations of Ne and Ar comparable to or slightly less than seawater. This is more likely for brines than for Cl-rich minerals, leading us to favor brines as the major assimilant. Cl/Br ratios for a limited number of Loihi samples are also seawater-like, and show no indication of the higher values expected to be associated with the assimilation of Cl-rich hydrothermal minerals. Although Cl enrichment is a common feature of lavas from Loihi, submarine glasses from other Hawaiian volcanoes show little (Kilauea) or no (Mauna Loa, Mauna Kea) evidence of this process, suggesting that assimilation of seawater-derived components is more likely to occur in the early stages of growth of oceanic volcanoes. Summit collapse events such as the one observed at Loihi in October 1996 provide a ready mechanism for depositing brine-bearing rocks from the volcanic edifice into the top of a submarine summit magma chamber. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION to have experienced contamination by seawater-derived com- ponents (Michael and Schilling, 1989; Jambon et al., 1995; An important issue in the study of ocean island basalts (OIBs) Michael and Cornell, 1998). There are also several lines of is the degree to which chemical heterogeneity within and evidence from OIB, particularly from Hawaii, that suggest that between volcanoes is influenced by high-level (i.e., lithospheric such components can also be incorporated into magmas in and/or crustal) contamination processes, rather than represen- these environments. Noble gases with atmospheric or near- tative of geochemical variations in mantle source regions (Da- atmospheric isotopic compositions indicate the addition of at- vidson and Bohrson, 1998). Interactions between ascending mospheric components to magmas from Loihi seamount and to magma and the lithospheric mantle, the oceanic crust, and the submarine Kilauea magmas prior to eruption (Patterson et al., volcanic edifice itself can all leave imprints on a magma’s 1990; Honda et al., 1991, 1993). In glasses from a suite of chemical and isotopic composition (e.g., Patterson et al., 1990; submarine Kilauea lavas, Kyser and O’Neil (1984) observed a Martin et al., 1994; Eiler et al., 1996; Garcia et al., 1998a; ␦ correlation between D and H2O, and Chaussidon and Jambon Reiners, 1998). Since the primary goal of many geochemical (1994) observed a correlation between ␦D and ␦11B. Both sets studies of OIB is to investigate the chemical and isotopic of authors suggested that the correlations resulted from assim- compositions of their mantle sources (e.g., Staudacher et al., ilation of seawater or seawater-altered basalt during magma 1986; Zindler and Hart, 1986; Honda et al., 1991; Hoffman, propagation along the East Rift Zone. Clague et al. (1995) 1997), it is important to be able to recognize and, if possible, to noted that Cl concentrations in some submarine glasses from correct for magmatic compositional variations that are due to Kilauea are higher than expected from crystal fractionation and assimilation and contamination. suggested that this could be due to assimilation of seawater- One potential source of contamination for oceanic magmas is altered basalt near the top of the summit magma chamber. sea water, and many mid-ocean ridge basalts (MORB) appear Finally, Kent et al. (1999) suggested that two unusually H2O-, Cl-, and B-rich samples from Loihi represent magmas that assimilated a seawater-derived component; olivine-hosted glass *Address reprint requests to Adam J. R. Kent, L-202, Lawrence Liv- ermore National Laboratory, Livermore, CA 94551 USA (kent5@ inclusions from these samples also show evidence of this llnl.gov). process, and variations in their H2O, Cl, and B contents suggest 2749 2750 A. J. R. Kent et al. variability in the composition of the assimilant. Kent et al. Norman and Garcia (1999). The noble gases in these samples (1999) also speculated that the high Cl contents reported in are of particular interest because their matrix glasses and many other pillow-rim glass samples from Loihi could be another glass inclusions in olivine phenocrysts have high H2O, Cl, and manifestation of contamination by Cl-rich, seawater-derived B concentrations (Kent et al., 1999). As expected (since a components and that assimilation of seawater-derived compo- seawater-derived assimilant would contain atmospheric isoto- nents could be a widespread process at this and other oceanic pic ratios of the noble gases), the 20Ne/22Ne and 40Ar/36Ar volcanoes. In this study, we examine this hypothesis using new ratios of these glasses are close to atmospheric values (Table 1). chemical and isotopic analyses of Loihi glasses and data com- In contrast, the 3He/4He ratios of these samples (as with many piled from the literature. other Loihi samples; e.g., Kurz et al., 1983; Honda et al., 1991, ϳ 1993) are unambiguously of mantle origin ( 27–32 RA for 2. ANALYTICAL TECHNIQUES total fusion and up to 40 RA for He released during individual heating steps; Table 1). Major elements and Cl were analysed by a JEOL 733 electron probe at Lawrence Livermore National Laboratory and at the California Table 2 lists major element, H2O, and Cl analyses of pillow- Institute of Technology usinga5to10nAelectron beam, a 10 ␮m rim glasses from one alkalic and five tholeiitic and transitional beam size, and an accelerating voltage of 15 kV. Data were reduced basalts collected by submersible from 3 to 5 km depth on with the CITZAF correction algorithms using natural and synthetic stan- Loihi’s south rift. Measured Cl and H O contents for these dards (Armstrong, 1995). Analytical precision for electron probe anal- 2 yses, based on counting statistics for the compositions measured, were glasses range between 0.049 and 0.103 and 0.39 and 0.68 wt.%. Ͻ Ͻ 2% for SiO2,Al2O3, CaO, MgO, FeO; 10% for Na2O, K2O, TiO2; Sample locations and noble gas data for pillow-rim glass and Ͻ Ͻ 20% for Cl and P2O5 and 50% for MnO and Cr2O3. Data represent olivine separates from these samples are given in Valbracht et the average of 5 to 10 individual analyses taken from 3 to 5 separate al. (1997). glass chips from each sample. Glass chips free from alteration products were selected using the procedure outlined in Kent et al. (1999), and Table 3 reports on H2O, Cl, B, and Be contents of 15 secondary standards were monitored to ensure the comparability of pillow-rim glasses (from six alkalic lavas and nine tholeiitic data from the two different electron probes used. Cl analyses were and transitional lavas) dredged from ϳ1 to 3 km depth from made with reference to Cl-rich scapolite and sodalite with count times Loihi’s summit and north and south rift zones. The measured Cl of 90 to 120 s and background counts determined during each analysis. The detection limit for Cl was 70 to 80 ppm. All uncertainties stated in and H2O contents of these glasses range between 0.030 and this paper, unless otherwise noted, are Ϯ2␴. 0.122 and 0.38 and 0.97 wt.% and the Be and B contents range H2O, Be, and B were analysed using a modified Cameca 3f ion between 0.7 and 2.0 and 1.2 and 4.9 ppm. Dredge locations for microprobe at Lawrence Livermore National Laboratory following the these samples are given in Moore et al. (1982) and Frey and procedures outlined in Kent et al. (1999). Uncertainties for H2O, Be, and B are based on the reproducibility of 3 to 6 individual analyses on Clague (1983). Noble gas and major element data for these the same glass chips analysed for major elements and are typically glasses were reported by Honda et al. (1993), and Frey and Ϯ Ϯ 15% for H2O and 20% for Be and B.
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