Abstract
AbstractHalogens and noble gases within submarine basaltic glasses are critical tracers of interactions between the surface volatile reservoirs and the mantle. However, as the halogens and noble gases are concentrated within seawater, sediments, and the oceanic crust this makes the original volatile signature of submarine basaltic lavas susceptible to geochemical overprinting. This study combines halogen (Cl, Br, and I), noble gas, and K concentrations within a single submarine basaltic quenched margin to quantify the amount of seawater assimilation during eruption, and to further elucidate the mechanisms of overprinting. The outer sections of the glass rim are enriched in Cl compared to the interior of the margin, which maintains mantle‐like Br/Cl, I/Cl, and K/Cl ratios. Low Br/Cl and K/Cl in the outer sections of the basaltic glass margin indicate that the Cl enrichment in the outer glass is derived from the assimilation of a saline brine component with up to 70% of the Cl within the glass being derived from brine assimilation. Atmospheric noble gas contamination is decoupled from halogen contamination with contaminated outer sections maintaining MORB‐like 40Ar/36Ar, suggesting seawater‐derived brine assimilation during eruption is not the dominant source of atmospheric noble gases in submarine basalts. Volatile heterogeneities in submarine basalts introduced during and after eruption, as we have shown in this study, have the potential to expand the range of mantle halogen compositions and only by better understanding these heterogeneities can the Br/Cl and I/Cl variance in mantle derived samples are determined accurately.
Highlights
Volatiles such as the halogens (Cl, Br, and I) and noble gases (He, Ne, Ar, Kr, and Xe) provide powerful tracers for the distribution of primordial and recycled volatile components within the mantle [Graham, 2002; Holland and Ballentine, 2006; Kendrick et al, 2012a]
Halogens and Potassium Halogens (Cl, Br, and I) and K concentrations for each section of the chilled margin are given in Figure 3 and Table 1
Chlorine concentrations of the sections are consistent with previously reported values from N-mid-ocean ridge basalts (MORB) samples (32–630 ppm; Figure 3) and samples originating from slow spreading ridges, but are much lower than some rare, Cl-rich normal MORB (N-MORB) glass shards (4000–7000 ppm) [Jambon et al, 1995; Kendrick et al, 2013; Michael and Cornell, 1998; Portner et al, 2014]
Summary
Volatiles such as the halogens (Cl, Br, and I) and noble gases (He, Ne, Ar, Kr, and Xe) provide powerful tracers for the distribution of primordial and recycled volatile components within the mantle [Graham, 2002; Holland and Ballentine, 2006; Kendrick et al, 2012a]. Submarine basaltic glass is considered to faithfully preserve the original magmatic halogen signature due to the high solubility of halogens in the melt when erupted in seawater at high pressure and depth [Schilling et al, 1980] In spite of their widespread application in mantle volatile studies submarine basaltic glass invariably contains an atmospheric noble gas component [Burnard et al, 2004; Kent et al, 1999; Patterson et al, 1990]. Lowering the K/Cl ratio from the average mantle value is thought to indicate the addition of seawater, hydrothermal brines or hydrothermally altered crust into the melt as seawater-derived components are enriched in Cl and deficient in K compared to silicate melts [Kendrick et al, 2012b; Kent et al, 1999; Michael and Cornell, 1998]. Submarine basalt samples with low K/Cl ratios have been shown to have excess 234U, from the combined assimilation of hydrothermal brines (source of Cl) and hydrothermally altered crust (234U excess) during ascent, indicating assimilation can occur from a variety of sources prior to, during and after eruption [Pietruszka et al, 2013]
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