Halogen variations through the quenched margin of a MORB lava: Evidence for direct assimilation of seawater during eruption

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 18; no. 7; pp. 2413 - 2428
• Main Authors: Broadley, Michael W., Burgess, Ray, Kumagai, Hidenori, Curran, Natalie M., Ballentine, Chris J.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.07.2017; AGU and the Geochemical Society
• Subjects: Basalt; Brines; Chemical analysis; Earth Sciences; Gases; Geochemistry; Geophysics; Glass; Halogens; Interactions; Lava; Mantle; Marine environment; noble gases; Oceanic crust; Rare gases; Ratios; Sciences of the Universe; Seawater; seawater assimilation; Tracers; Volcanic glass; Water analysis; halogens; mantle; seawater assimilation; noble gases
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2016GC006711

Halogens 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. Key Points Halogens within submarine basaltic glass are susceptible to being overprinted from the surrounding marine environment Up to 70% of the Cl in the quenched basaltic glass margin was found to be derived from the assimilation of a Cl‐rich brine during eruption Alteration of basaltic glass in direct contact with seawater can introduce an I‐rich component and overprint the original MORB I/Cl ratio