Halmyrolysis

Halmyrolysis, also known as submarine weathering, is the alteration of sediments and rocks on the seafloor through reaction with seawater. ^[1] It is the oceanic counterpart of subaerial weathering and, like subaerial weathering, produces new minerals and releases ions to the water column. What distinguishes halmyrolysis from terrestrial weathering is not just the setting but the chemistry: the reactions involve seawater with its distinctive ion budget, and at high-temperature sites along mid-ocean ridges, the fluid-rock exchange operates at temperatures and fluid fluxes that have a measurable effect on the overall chemical composition of the oceans.

Low-Temperature Alteration Processes

Halmyrolysis includes several distinct types of alteration that take place at low temperatures. These include the conversion of clay minerals from one type to another; the formation of glauconite from feldspars and micas; the formation of phillipsite (a zeolite mineral) and palagonite (altered volcanic glass) from volcanic ash; and the dissolution of the siliceous and calcareous tests of organisms. ^[1]

Low-temperature alteration takes place as seawater percolates through fractures and voids in the upper part of the oceanic crust, perhaps extending to depths of 2-5 km. ^[1] In basalts, olivine and interstitial glass are replaced by smectite clay minerals, and further alteration may lead to the formation of zeolite minerals and chlorite. ^[1] As a result of these changes, chemical elements are exchanged between rock and water, and large volumes of seawater become fixed in the oceanic crust as hydrous clay minerals and zeolites. ^[1] This fixation of water in the crust is geologically significant: it represents a major sink for seawater and contributes to the budget of elements in the ocean. It also has consequences for the crust itself, because hydrated minerals are weaker and denser than their dry equivalents, influencing how oceanic crust behaves when it eventually subducts.

High-Temperature Hydrothermal Systems

Large-scale hydrothermal activity takes place in the ocean at sites where seawater enters the ocean crust along fractures or other voids and comes into contact with hot volcanic rock. The heated water then flows back out through vents on the ocean floor and mixes with the overlying seawater, rising as hydrothermal plumes 100-300 m above the vent field. Exceptional plumes rising to heights of 1000 m have also been reported. ^[1]

Hydrothermal systems occur along mid-ocean ridges in both the Pacific and Atlantic oceans, as well as along convergent plate margins, in backarc basins, and even on midplate volcanoes in the Hawaiian chain. ^[1] Alteration of oceanic rocks occurs across a wide temperature range - both at low temperatures below 20°C and at higher temperatures ranging to approximately 350°C. ^[1]

Chemical Exchange Between Rock and Seawater

Reactions between hot basalt and seawater play a role in regulating the chemical composition of seawater. During this exchange, magnesium, sulfate, and sodium ions are removed from seawater, while many other elements are enriched in the seawater: calcium, iron, manganese, silicon, potassium, lithium, and strontium. ^[1] This exchange is not a small-scale effect. The entire ocean apparently circulates through ocean-floor hydrothermal systems on a timescale of 10⁶-10⁷ years, which has a significant impact on the budget of several elements, including silica. ^[1]

Both seafloor hydrothermal reactions and continental weathering processes supply ions to the ocean that may eventually be extracted to form chemically deposited rocks such as limestones, iron-rich sedimentary rocks, and cherts. ^[1] Halmyrolysis is therefore not just an agent of rock destruction - it is also a source of the raw materials for chemical sedimentary rocks.

Effect of Spreading Rate on Ocean Chemistry

Changes in spreading rates along mid-ocean ridges, where hydrothermal activity takes place, have exerted major control on the calcium and magnesium content of seawater throughout geologic time. High spreading rates result in significant adsorption and loss of magnesium with a concomitant increase in calcium, causing a decrease in the ratio of magnesium to calcium (Mg/Ca). Low spreading rates have the opposite effect of increasing the Mg/Ca ratio. ^[1] These changes in seawater Mg/Ca have important implications for the kinds of calcium-carbonate minerals deposited in the ocean. ^[1] Specifically, high Mg/Ca seawater favours the precipitation of aragonite and high-magnesium calcite, while low Mg/Ca seawater favours low-magnesium calcite - a distinction that influences the character of limestones deposited at different times in Earth history.

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