A synthesis and meta-analysis of the Fe chemistry of serpentinites and serpentine minerals

Mayhew, L. E.; Ellison, Eric T.

doi:10.1098/rsta.2018.0420

Cited by 28 publications

(29 citation statements)

References 97 publications

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“…Cronstedtite substitution in serpentine describes the partial exchange of Fe 2+ and/or Mg with Fe 3+ , which is accompanied by a Fe 3+ 2 (Fe 2+ Si) -1 exchange to maintain charge balance (e.g., O'Hanley and Dyar, 1993 The presence of cronstedtite by itself is not unusual for marine serpentinites. It can be approximated by analyzing serpentinite Fe(III)/ Fe; generally, between 50 and 60% of the Fe in MOR and passive margin serpentinites are ferric, even in ones containing little magnetite (Mayhew and Ellison, 2020). But X Mg values for serpentine in these rocks rarely fall below 0.9 and the total contents of Fe 2 O 3 in serpentinites are usually below 7-8 wt% (cf.…”

Section: Oxidation and Mass Transfer During Prolonged Fluid-rock Interactionsmentioning

confidence: 99%

“…But X Mg values for serpentine in these rocks rarely fall below 0.9 and the total contents of Fe 2 O 3 in serpentinites are usually below 7-8 wt% (cf. Mayhew and Ellison, 2020).…”

Section: Oxidation and Mass Transfer During Prolonged Fluid-rock Interactionsmentioning

confidence: 99%

“…Frost and Beard, 2007). Instead, more serpentine may form, some of which may hold Fe(III), which itself would increase the rates of H 2 production (see, e.g., reviews by Evans et al, 2013;Mayhew and Ellison, 2020).…”

Section: Introductionmentioning

confidence: 99%

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Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

et al. 2021

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Molecular hydrogen (H2) released during serpentinization of mantle rocks is one of the main fuels for chemosynthetic life. Processes of H2 production at slow-spreading mid-ocean ridges (MORs) have received much attention in the past. Less well understood is serpentinization 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 (<200°C vs. >200°C). To help closing this knowledge gap we investigated drill core samples from the West Iberia margin. Lherzolitic compositions and spinel geochemistry indicate that the exhumed peridotites resemble sub-continental lithospheric mantle. The rocks are strongly serpentinized, mainly consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg + Fe) < 0.8, and shows distinct compositional trends toward a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios are 0.6–0.92 and 0.58–0.8, respectively; our data show that 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the sample’s initial serpentinization produced ∼120 to >300 mmol H2 per kg rock. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation. These results suggest that the H2 generation potential evolves during the transition from continental break-up to ultraslow and, eventually, slow MOR spreading. Metamorphic phase assemblages systematically vary between these settings, which has consequences for H2 yields during serpentinization. At magma-poor rifted margins and ultraslow-spreading MORs, serpentine hosts most Fe(III). Hydrogen yields of 120 to >300 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of <200°C. At slow-spreading MORs, in contrast, serpentinization may produce 200–350 mmol H2, most of which is related to magnetite formation at >200°C. Since, in comparison to slow-spreading MORs, geothermal gradients at magma-poor margins and ultraslow-spreading MORs are lower, larger volumes of low-temperature serpentinite should form in these settings. Serpentinization of lherzolitic rocks at magma-poor margins should produce particularly high amounts of H2 under conditions within the habitable zone. Magma-poor margins may hence be more relevant environments for hydrogenotrophic microbial life than previously thought.

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Section: Oxidation and Mass Transfer During Prolonged Fluid-rock Interactionsmentioning

confidence: 99%

“…But X Mg values for serpentine in these rocks rarely fall below 0.9 and the total contents of Fe 2 O 3 in serpentinites are usually below 7-8 wt% (cf. Mayhew and Ellison, 2020).…”

Section: Oxidation and Mass Transfer During Prolonged Fluid-rock Interactionsmentioning

confidence: 99%

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Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

et al. 2021

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“…2009, Evans et al . 2012, Mayhew and Ellison 2020). Nevertheless, accurate Fe 3+ measurements for samples UWSrp‐4 to UWSrp‐8 and Al06‐44A were previously published for different aliquots of the same samples (Evans et al .…”

Section: Serpentine Reference Materialsmentioning

confidence: 99%

“…Because serpentine can contain variable amount of iron (e.g., Mg# ~ 78–99 mole %; Evans et al . 2009, 2012, Mayhew and Ellison 2020) and of matrix effects that contribute to bias during SIMS analysis related to Mg/Fe variations in some minerals (e.g., olivine, carbonates; e.g., Kita et al . 2010, Śliwiński et al .…”

mentioning

confidence: 99%

In Situ Oxygen Isotope Determination in Serpentine Minerals by SIMS: Addressing Matrix Effects and Providing New Insights on Serpentinisation at Hole BA1B (Samail ophiolite, Oman)

Scicchitano

Spicuzza

Ellison

et al. 2020

Geostandard Geoanalytic Res

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The ability to constrain the petrogenesis of multiple serpentine generations recorded at the microscale is crucial for estimating the extent and conditions of modern versus fossil serpentinisation in ophiolites. To address matrix bias effects during oxygen isotope analysis by SIMS, we present the first investigation analysing antigorite in the compositional range Mg# = 77.5–99.5 mole %, using a CAMECA IMS‐1280 secondary ion mass spectrometer. Spot‐to‐spot homogeneity is ≤ 0.5‰ (2s) for the new antigorite reference materials. The relative bias between antigorite reference materials with different Mg/Fe ratios is described by a second‐order polynomial, and a maximum difference in bias of ~ 1.8‰ was measured for Mg# ~ 78 to 100. We observed a bias up to ~ 1.0‰ between lizardite and antigorite attributed to their different crystal structures. Orientation effects up to ~ 1‰ were observed in chrysotile. The new analytical protocol allowed the identification of oxygen isotope zoning up to ~ 7‰ in serpentine minerals from two serpentinites recovered from an area of active serpentinisation in the Samail ophiolite. Thus, in situ analysis is capable of resolving isotopic heterogeneity that may directly reflect changes in the physical and chemical conditions of multiple serpentinisation events in the Samail ophiolite.

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Early release of H2O during subduction of carbonated ultramafic lithologies

Eberhard

Plümper

Frost

2023

Contrib Mineral Petrol

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To investigate the effect of carbon-bearing phases on the release of fluids in subducted serpentinites, we performed high-pressure multi-anvil experiments on representative ophicarbonate assemblages over a pressure range from 2.5 GPa to 5 GPa and from 450 °C to 900 °C, across the antigorite-out reaction. Parallel experiments were performed on carbonate-free serpentinites. In all experiments, we monitored and/or controlled the oxygen fugacity. The addition of 20 wt. % CaCO3 to a serpentinite assemblage at 2.5 GPa is found to decrease the onset of the serpentine dehydration by over 100 °C, in comparison to carbonate-free assemblages. Similarly, the final disappearance of serpentine is also affected by the presence of CaCO3. For a bulk CaCO3 content of 20 wt. %, this causes a decrease in maximum stability of antigorite by 50 °C. For a bulk CaCO3 content exceeding 25 wt. %, this difference can be as high as 100 °C in warm and 150 °C in cold subduction zones, causing antigorite to be completely dehydrated at 500 °C. This results from the reaction of CaCO3 with serpentine to form clinopyroxene and Mg-rich carbonates. This reaction, however, causes no discernible decrease in the proportion of carbonate, indicating that the amount of released carbon is insignificant. Whilst CaCO3, therefore, influences serpentine stability, there is no significant effect of hydrous phases on the carbonate stability. On the other hand, a MgCO3-bearing system shows no significant effects on the serpentinite stability field. Further experiments and oxygen fugacity calculations indicate that graphite is not stable in typical magnetite-bearing serpentinites. The reduction of carbonates to graphite would require oxygen fugacities that are 1–2 log units below those of magnetite-bearing serpentinites. This confirms earlier studies and indicates that reduction of carbonates can only occur through the infiltration of external H2-rich fluids.

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A synthesis and meta-analysis of the Fe chemistry of serpentinites and serpentine minerals

Cited by 28 publications

References 97 publications

Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

In Situ Oxygen Isotope Determination in Serpentine Minerals by SIMS: Addressing Matrix Effects and Providing New Insights on Serpentinisation at Hole BA1B (Samail ophiolite, Oman)

Early release of H2O during subduction of carbonated ultramafic lithologies

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