The salinity of Archaean oceans impacted both the climate and the evolution of early life. In the modern oceans, the halogen content (here we focus on Cl, Br and I specifically) is mainly regulated by riverine input of continental weathering products, subduction, and sequestration of iodine by burial of I-rich organic matter like algae. In the oceans of the Archaean eon (4.0–2.5 Ga) were life probably first emerged, continents were largely submerged by oceans that lacked Br-I-fixing organisms. Hence, Archaean seawater salinity has been assumed to be derived from vigorous submarine volcanism. However, available halogen data of Archaean seawater samples (it's mindbending that such samples exist and have been preserved as small inclusions of fluid in chert / quartz that precipitated from Archaean oceans!) are at apparent odds with a conventionally assumed mantle origin of sea salt. This mismatch highlights that we currently don’t understand how the Archaean oceans became salty.

A candidate mechanism for how mantle halogens enter the oceans is degassing from ocean floor volcanism, but we don't know how the degassing process affects the halogens in nature, nor whether magmatic fluids inherit the halogen signature (e.g., Br/Cl and I/Cl ratios) of their parental melts. Magmatic fluids are challenging to sample and their primary signatures easily overprinted by external fluids.

To resolve the problem, I analysed the compositions of magmatic fluid inclusions (including those of Cl-Br-I) trapped within quartz, using laser ablation inductively coupled mass spectrometry (LA-ICP-MS). The samples come from miarolitic cavities within an exposed gabbro intrusion belonging to the ~4 Ma Vesturhorn intrusive complex in East Iceland. The fluids appear to represent magmatic fluids soon after separation from an evolved residual melt, based on high estimated temperatures of >740°C—hotter than any fluids sampled from active hydrothermal systems. 

Highlights of the study:

• First Cl-Br-I data of magmatic fluid inclusions in oceanic-type crust.

• A demonstration that magmatic degassing fractionates the halogens during ocean floor magmatism.

• Magmatic fluids have a unique Br/Cl and I/Cl signature, which is useful for tracing fluid origins in magmatic-hydrothermal environments.

• Overlap of Archaean seawater and magmatic fluid Br/Cl and I/Cl suggests that magma degassing indeed appears to be the main source of salt in early oceans.