How much energy for life (H2) is generated by serpentinization at passive continental margins?
Authors: Albers Elmar , Bach Wolfgang , Pérez-Gussinyé Marta , McCammon Catherine , Frederichs Thomas 
Published: March 2021

&lt;p&gt;Molecular hydrogen (H&lt;sub&gt;2&lt;/sub&gt;) released during serpentinization of oceanic mantle is one of the main fuels for chemosynthetic-based deep life. Hydrogen is produced during the oxidation of ferrous to ferric iron, and the amount of H&lt;sub&gt;2&lt;/sub&gt; generated strongly depends on rock type, fluid composition, alteration temperature, and water-to-rock ratio.&lt;/p&gt;&lt;p&gt;Progress has been made in understanding serpentinization and related H&lt;sub&gt;2&lt;/sub&gt; production at slow-spreading mid-ocean ridges (MORs). Less attention has been paid to the hydration of mantle rocks at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (&lt;200&amp;#176;C vs. &gt;200&amp;#176;C). To close this knowledge gap we investigated serpentinization and H&lt;sub&gt;2&lt;/sub&gt; production using drill core samples from the West Iberia margin (Ocean Drilling Program Leg 103, Hole 637A).&lt;/p&gt;&lt;p&gt;Lherzolitic compositions indicate that the exhumed peridotites represent sub-continental lithospheric mantle. The rocks are strongly serpentinized and mainly consist of serpentine with little magnetite and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with &lt;em&gt;X&lt;/em&gt;&lt;sub&gt;Mg&lt;/sub&gt; = Mg/(Mg+Fe) &lt; 0.8, and shows distinct compositional trends towards a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/&amp;#8721;Fe ratios range from 0.6&amp;#8211;0.92 and 0.58&amp;#8211;0.8, respectively. Our data show that more than 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the initial serpentinization of the samples at temperatures of &lt;200&amp;#176;C likely produced about 100&amp;#8211;250&amp;#160;mmol H&lt;sub&gt;2&lt;/sub&gt; per kg rock, which is 2&amp;#8211;3 times more than previously estimated.&lt;/p&gt;&lt;p&gt;These results lead us to suggest that the generation potential of H&lt;sub&gt;2&lt;/sub&gt; evolves from continental break-up to ultraslow and eventually slow MOR spreading. The observed metamorphic phase assemblages systematically vary between these different settings, which has consequences for H&lt;sub&gt;2&lt;/sub&gt; yields during serpentinization. At passive margins and ultraslow-spreading MORs, the main phase hosting Fe(III) appears to be serpentine, and H&lt;sub&gt;2&lt;/sub&gt; yields of 100&amp;#8211;250 mmol and 50&amp;#8211;150 mmol H&lt;sub&gt;2&lt;/sub&gt; per kg rock, respectively, may be expected at temperatures of &lt;200&amp;#176;C. At slow-spreading MORs, in contrast, serpentinization of harzburgite may produce 200&amp;#8211;350&amp;#160;mmol H&lt;sub&gt;2&lt;/sub&gt; per kg most of which is related to the formation of magnetite at &gt;200&amp;#176;C. Within the same (low) temperature range, larger volumes of serpentinite should form at passive margins than at slow-spreading MORs, owing to lower geothermal gradients. Relative to both slow- and ultraslow-spreading MORs, serpentinization at passive margins likely produces more H&lt;sub&gt;2&lt;/sub&gt; and under conditions closer to/within the habitable zone. These sites may hence be suitable environments for hydrogenotrophic microbial life.&lt;/p&gt;