minerals

Article
Contrasted Effect of Spinel and Pyroxene on Molecular
Hydrogen (H2) Production during Serpentinization of Olivine
Ruifang Huang 1, * , Xing Ding 2 , Weidong Sun 3,4 and Xiuqi Shang 3,4,5

1 SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology,
Shenzhen 518055, China
2 State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy
of Sciences, Guangzhou 510640, China; xding@gig.ac.cn
3 Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;
weidongsun@gig.ac.cn (W.S.); shangxq@qdio.ac.cn (X.S.)
4 Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology,
Qingdao 266237, China
5 Institute of Oceanology, University of Chinese Academy of Sciences, Beijing 100049, China
* Correspondence: huangrf@sustech.edu.cn

Abstract: Serpentinization produces molecular hydrogen (H2 ) and hydrocarbons that can feed the
colonies of microbes in hydrothermal vent fields, and therefore serpentinization may be important for
the origins of life. However, the mechanisms that control molecular hydrogen (H2 ) production during
serpentinization remain poorly understood. Here the effect of pyroxene minerals and spinel on
molecular hydrogen (H2 ) generation during serpentinization is experimentally studied at 311–500 ◦ C
and 3.0 kbar, where olivine, individually and in combinations with pyroxene and/or spinel, is reacted



with saline solutions (0.5 M NaCl). The results show a contrasting influence of spinel and pyroxeneon


molecular hydrogen (H2 ) production. At 311 ◦ C and 3.0 kbar, spinel promotes H2 generation by
Citation: Huang, R.; Ding, X.; Sun, around two times, and pyroxene minerals decrease molecular hydrogen (H2 ) production by around
W.; Shang, X. Contrasted Effect of one order of magnitude. Spinel leaches aluminum (Al) and chromium (Cr) during hydrothermal
Spinel and Pyroxene on Molecular
alteration, and Al and Cr enhance molecular hydrogen (H2 ) production. This is confirmed by
Hydrogen (H2 ) Production during
performing experiments on the serpentinization of olivine with the addition of Al2 O3 or Cr2 O3
Serpentinization of Olivine. Minerals
powders, and an increase in molecular hydrogen (H2 ) production was observed. Pyroxene minerals,
2021, 11, 794. https://doi.org/
however, not only leach Al and Cr, but they also release silica (SiO2 ) during serpentinization. The
10.3390/min11080794
sharp decline in molecular hydrogen (H2 ) production in experiments with a combination of olivine
Academic Editor: Shoji Arai and pyroxene minerals may be attributed to releases of silica from pyroxene minerals. With increasing
temperatures (e.g., 400–500 ◦ C), the effect of spinel and pyroxene minerals on molecular hydrogen
Received: 3 July 2021 (H2 ) production is much less significant, which is possibly related tothe sluggish kinetics of olivine
Accepted: 19 July 2021 serpentinization under these T-P conditions. In natural geological settings, olivine is commonly
Published: 22 July 2021 associated with spinel and pyroxene, and molecular hydrogen (H2 ) during serpentinization can be
greatly affected.
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in Keywords: molecular hydrogen; serpentinization; spinel; pyroxene minerals
published maps and institutional affil-
iations.

1. Introduction
Serpentinization is a hydrothermal alteration of ultramafic rocks at temperatures of
Copyright: © 2021 by the authors. ≤500 ◦ C, and the reaction of olivine and pyroxene in ultramafic rocks with aqueous fluids
Licensee MDPI, Basel, Switzerland.
results in the production of serpentine, (±) brucite, (±) talc and (±) magnetite. Mantle
This article is an open access article
peridotites are abundantly exposed at mid-ocean ridges, and hydrothermal activity results
distributed under the terms and
in serpentinization of mantle peridotites. The presence of a serpentinite layer near the
conditions of the Creative Commons
base of the mantle wedge has been indicated by geological and geophysical evidence [1–3].
Attribution (CC BY) license (https://
Fluids associated with serpentinizing peridotites commonly contain abundant molecular
creativecommons.org/licenses/by/
4.0/).
hydrogen (H2 ) and hydrocarbons [4–7]. Molecular hydrogen (H2 ) and methane (CH4 )

Minerals 2021, 11, 794. https://doi.org/10.3390/min11080794 https://www.mdpi.com/journal/minerals

Minerals 2021, 11, 794 2 of 15

can feed the colonies of microbes from many geological settings, such as hydrothermal
vents, alkaline springs, and the deep subsurface [8–12], indicating that serpentinization
may be crucial for the genesis of life [13–17]. Occurrences of H2 - and CH4 -rich fluid
inclusions derived from a harzburgite in subduction zones may be closely associated with
serpentinization of peridotites in the mantle wedge [18], which may greatly influence
oxygen fugacity of subduction zones.
Thermodynamic models and experiments have been performed to study molecular
hydrogen (H2 ) production during serpentinization, mostly regarding the serpentiniza-
tion of olivine [19–32]. Molecular hydrogen (H2 ) production was found to depend on
many factors, including temperature, water/rock ratios, chemical compositions and pH of
starting fluids [23,29,30,33,34]. The influence of other factors (e.g., pyroxene and spinel)
on molecular hydrogen (H2 ) production, however, remains unclear. Our experimental
studies suggest that the serpentinization of peridotite proceeds at faster rates compared to
olivine hydrothermal alteration, resulting from the effect of pyroxene and spinel leaching
aluminum and chromium [35,36]. The rates of reaction are directly linked to molecular
hydrogen (H2 ) production during serpentinization [24], and therefore pyroxene and spinel
may affect molecular hydrogen (H2 ) production.
In this study, we carried out serpentinization experiments at 311–500 ◦ C and 3.0 kbar,
and a combination of olivine, (±) spinel and (±) pyroxene minerals was reacted with
saline solutions. The aims of this study are to (1) quantify the effect of spinel and pyroxene
on molecular hydrogen (H2 ) production during olivine serpentinization, (2) investigate
temperature dependence for the effect of spinel and pyroxene on molecular hydrogen
(H2 ) formation, and (3) study the factors that control molecular hydrogen (H2 ) production
during the serpentinization of olivine.

2. Materials and Methods

Olivine and spinel were chosen from a peridotite sample that occurs as xenoliths in
basaltsat Panshishan, Jiangsu Province, China [37–39]. Peridotite has been used in previous
experiments [29,35,36]. Olivine and spinel were ground in an agate mortar and sieved
into starting grain sizes of <30 µm, and they were cleaned in an ultrasonic water bath to
remove fine particles. A combination of olivine and spinel/pyroxene was made. Saline
solutions (0.5 M NaCl), the starting fluids of this study, were prepared with analytical
reagent sodium chloride and deionized water.
The experiments were carried out in cold-sealed hydrothermal vessels [29,35,36].
Gold capsules with solid reactants and starting solutions (mass ratios of ~1.0) were welded
at two sides and placed into hydrothermal vessels (Table 1). After experiments, the
amounts of molecular hydrogen (H2 ) in the gold capsules were measured by using an
Agilent 7890A gas chromatographat the State Key Laboratory of Organic Geochemistry,
Guangzhou Institute of Geochemistry, Guangzhou, China, with a detailed description of
analytical procedures presented by several studies [40,41]. The solid run products were
completely recovered, and the XRD patterns were collected utilizing a Rigaku Smartlab
X-ray diffractometer in a high-pressure laboratory at the Southern University of Science
and Technology„ Guangzhou, China, with 2θ ranging from 10◦ to 90◦ at a step size of
0.02◦ and a counting time of 10 s per step. The morphology of the solid run products was
observed using a Zeiss Ultra 55 field emission gun scanning electron microscope at 5 kVat
Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China. The
products were placed on a carbon tape, and they were subsequently coated with a thin
platinum film. FTIR measurements (in transmission mode) were performed with a Bruker
Vector−33 FTIR spectrometerat Analytical and Testing Center of South China University of
Technology. Potassium bromide (KBr) pellets were prepared, and around 1 mg of sample
was homogenized with 200 mg of KBr. The spectra were collected from 400 to 4000 cm−1
at a resolution of 4 cm−1 , with 32 scans for each spectrum.
Minerals 2021, 11, 794 3 of 15

Table 1. Experimental conditions.

T P Olivine Solution Spinel, Al2 O3 , W/R Time H2 Reaction

Run no.
(◦ C) (kbar) (mg) (mg) Cr2 O3 (mg) Ratios (Days) (mmol/kg) Extent (%) c
Olivine Only
HR-16 311 3.0 50.7 50.9 — 1.0 13 100 69 (1.3)
HR-76 311 3.0 49.2 51.2 — 0.96 27 80 76 (1.7)
HR-106 311 3.0 49.3 50.2 — 1.0 10 95 62 (0.5)
Fe-57 a 400 3.5 35.9 46.7 — 1.3 19 0.83 0.5 (0.5)
Fe-46 a 535 3.6 65.7 101.7 — 1.5 18 2.2 0.0 (0.0)
Olivine + Al2 O3
HR-92 311 3.0 39.4 37.5 6.5 0.82 29 203 90 (0.7)
HR-94 311 3.0 50.0 50.9 12.7 0.95 27 132 —
HR-108 311 3.0 51.3 51.1 6.5 0.88 19 233 54 (3.8)
HR-72 505 3.0 36.2 37.9 11.8 0.79 20 3.7 0.5 (0.2)
Olivine + Cr2 O3
HR-95 311 3.0 49.4 50.9 6.3 0.91 27 166 77 (3.2)
HR-99 311 3.0 51.7 57.8 13.2 0.90 27 271 84 (2.8)
HR-109 311 3.0 52.0 50.4 6.7 0.86 18 268 54 (3.8)
Olivine + Spinel
HR-84 200 3.1 32.7 28.7 7.1 0.72 13 1.5 0.4 (0.5)
HR-93 311 3.0 49.3 47.5 1.9 0.93 28 259 99 (0.9)
HR-75 311 3.0 49.7 51.4 6.6 0.92 27 260 100 (1.0)
HR-110 311 3.0 52.0 51.6 6.3 0.88 15 130 64 (0.6)
HR-73 505 3.0 36.4 36.7 12.8 0.75 20 5.7 0.5 (0.6)
Olivine + Pyroxene
HR-88 311 3.0 30.7 27.8 — 0.90 27 15 100 (1.0)
HR-68 405 3.0 51.4 36.5 — 0.71 20 2.3 53 (3.5)
HR-70 505 3.0 20.1 19.6 — 0.97 19 6.3 5 (0.1)
HR-83 b 505 3.0 50.3 51.5 — 1.0 36 1.7 5 (0.3)
Spinel-Bearing Peridotite
HR86 311 3.0 59.2 51.1 — 0.86 27 119 99 (1.7)
HR-69 405 3.0 54.2 41.2 — 0.76 20 16 99 (2.2)
HR-71 505 3.0 20.1 19.5 — 0.97 20 14 6 (0.1)
W/R ratios: ratios between the mass of a saline solution and solid reactants loaded into gold capsules. a Data from [27], olivine with
starting grain sizes of 100–177 µm was used. b Peridotite with starting grain sizes of 100–177 µm was used. c The numbers in brackets are
standard deviation of at least three times analyses.

3. Results and Discussion

3.1. Characterization of Experimental Products
The conventional methods (X-ray diffraction, SEM and FTIR) revealed that the major
hydrous phase in experimental products at 311 ◦ C and 3.0 kbar was fibrous chrysotile
except those using mechanical mixtures of olivine and ~12.0 mg Al2 O3 powders, where
chlorite was produced (Figures 1 and 2). Serpentine formation is evidenced by infrared
spectra with stretching modes at 954 cm−1 and 3686 cm−1 [42,43]. The band at 954 cm−1 is
attributed to the stretching mode of the Si-O group in serpentine, and the band at 3686 cm−1
represents the stretching vibration of the –OH group in serpentine [42,43]. At 400 ◦ C and
3.0 kbar, the dominant secondary hydrous phase was lizardite (Figure 2). Lizardite and
talc were produced in experiments at 500 ◦ C and 3.0 kbar, and talc formation is attested by
its stretching modes at 671 cm−1 and 3677 cm−1 [44].
Minerals 2021, 11, 794 4 of 15
Minerals 2021, 11, x 4 of 15

.
Figure 1. XRD patterns of typical experimental products of this study. (a) HR-16, 311 ◦ C/3.0 kbar.
Figure 1.XRD patterns of typical experimental products of this study. (a) HR-16, 311 °C/3.0 kbar.
Olivine (Ol) was taken as the starting reactant; (b) HR-93, 311 ◦ C/3.0 kbar. A combination of olivine
Olivine (Ol) was taken as the starting reactant; (b) HR-93, 311 °C/3.0 kbar. A combination of oli-
(Ol) and spinel (Spl) was used; (c) HR-86, 311 ◦ C/3.0 kbar. Mechanical mixtures of olivine (Ol)
vine (Ol) and spinel (Spl) was used; (c) HR-86, 311 °C/3.0 kbar. Mechanical mixtures of olivine (Ol)
and
and pyroxene
pyroxene minerals
minerals (Pyr)
(Pyr) were
wereused.
used. Mgt:
Mgt: magnetite,
magnetite, Bru:
Bru: brucite,
brucite, Cpx:
Cpx: clinopyroxene,
clinopyroxene, Srp:
Srp:
serpentine.
serpentine.

Figure 2.
Figure 2. Identification
Identification of
of secondary
secondary hydrous
hydrous minerals
minerals with
with scanning
scanning electron
electronmicroscope
microscopeimaging
imaging
(a,b)and
(a,b) andFTIR
FTIRanalyses
analyses(c).
(c).
Minerals 2021, 11, 794 5 of 15

In spite of the presence of brucite in many natural serpentinites [45,46], it was absent in
all experiments at 400–500 ◦ C and 3.0 kbar and in most experiments at 311 ◦ C and 3.0 kbar,
except those at 311 ◦ C and 3.0 kbar with a combination of olivine and spinel (Figure 1).
Experimental and thermodynamic simulations suggest that brucite formation during
serpentinization can be affected by temperature, silica activity and chemical compositions
of starting fluids [23,47]. First, thermodynamic models show that brucite production
decreases significantly at temperatures of ≥350 ◦ C, indicating that the stability of brucite
may be reduced at higher temperatures [23]. Consistently, experimental studies show that
brucite was formed during serpentinization at temperatures of ≤300 ◦ C, and it was not
found in experiments at higher temperatures (e.g., 350–500 ◦ C) [21,22,30,31]. Moreover,
brucite is not stable at high silica activity, under which conditions the reaction of brucite
with silica results in the production of serpentine minerals [24,34,47].

Mg2 SiO4 + 4Mg(OH)2 + 3SiO2 = 2Mg3 Si2 O5 (OH)4

(1)
olivine brucite silica serpentine

As revealed by thermodynamic models, silica activity of peridotite serpentinization is

around 1–2 orders of magnitude higher than that of olivine hydrothermal alteration [21].
As a consequence, brucite may not be stable in experiments with the presence of pyroxene.
The absence of brucite during peridotite serpentinization was also reported by several
previous experimental studies at temperatures of 300–500 ◦ C and 3.0 kbar [24,35,48]. At
relatively low temperatures (200–230 ◦ C), brucite appeared only very late in the reaction
(after 100 days)) [31]. Additionally, analyses of natural serpentinites suggest that brucite
tends to dissolve in seawater [49], and therefore brucite was not formed in experiments of
this study with saline solutions (0.5 M NaCl).

3.2. Influence of Spinel and Pyroxene Minerals on Molecular Hydrogen (H2 ) Production
Blank experiments were carried out at 311–500 ◦ C and 3.0 kbar, and solid reactants
were loaded into gold capsules without any starting fluids. After 27 days, the concentra-
tions of molecular hydrogen (H2 ) and hydrocarbons were below the limit of detection of
gas chromatograph, indicating that olivine, pyroxene and spinel used in this study may
not have any molecular hydrogen (H2 ) and hydrocarbons. Otherwise, elevated amounts
of H2 and hydrocarbons (methane, ethane and propane) can be produced due to the de-
composition of long-chain hydrocarbons [50]. This suggests that molecular hydrogen (H2 )
detected in the serpentinization experiments was produced during hydrothermal alteration
of olivine.
The influence of spinel on molecular hydrogen (H2 ) during serpentinization was ex-
amined, and olivine, individually and in combinations with spinel, was reacted with saline
solutions. In olivine experiments at 311 ◦ C and 3.0 kbar, 80 mmol/kg H2 was produced
after 27 days, which is around three times higher with the addition of spinel (Figure 3,
Table 1). This suggests that spinel accelerates molecular hydrogen (H2 ) production during
serpentinization.
Consistently, previous experiments showed a positive relationship between molecular
hydrogen (H2 ) production and the amounts of spinel in starting reactants (R2 = 0.97), with
higher H2 for experiments with more abundant spinel [25]. The influence of spinel has been
proposed to result from the transfer of electrons to water adsorbed to the spinel surfaces,
producing magnetite rinds around spinel [25]. Spinel in natural serpentinites is typically
hydrothermally altered, leading to the formation of magnetite rinds [51–54]. The formation
of Cr- and Al-depleted magnetite rinds indicates releases of aluminum (Al) and chromium
(Cr) during hydrothermal alteration [35,51–54]. Aluminum and chromium accelerate the
serpentinization of olivine [35,55], and they may impede the formation of iron oxide [48].
Therefore, Al and Cr may influence the production of H2 during hydrothermal alteration
of olivine. In order to test such hypothesis, we carried out serpentinization experiments at
311 ◦ C and 3.0 kbar with a combination of olivine and Al2 O3 or Cr2 O3 powders (Table 1),
and molecular hydrogen (H2 ) production during the serpentinization of olivine becomes
Minerals 2021, 11, 794 6 of 15

around 2–3 times higher (Figure 3), suggesting that Al and Cr enhance molecular hydrogen
Minerals 2021, 11, x
(H2 ) production during olivine serpentinization. Therefore, the effect of spinel on molecular
6 of 15
hydrogen (H2 ) production may result from releases of Al and Cr during hydrothermal
alteration.

Figure 3. The
Theconcentrations
concentrationsofofHH2 2ininaqueous
aqueousfluids
fluids(mmol/kg)
(mmol/kg)forfor
experiments at 311
experiments ◦ C and
°C and
at 311 3.0
kbar as a function of experimental durations (in day), showing that spinel, aluminum
3.0 kbar as a function of experimental durations (in day), showing that spinel, aluminum and and chro-
mium accelerate
chromium the production
accelerate of H2ofduring
the production serpentinization.
H during serpentinization.
2

Consistently,
Aluminum and previous
chromium experiments
promoteshowed a positive
the production of relationship
H2 during the between molec-
serpentiniza-
ular hydrogen (H ) production and the amounts of spinel in starting
tion of olivine, which is closely associated with a dramatic increase in reaction rates. As
2 reactants (R 2 = 0.97),

with higherbyHprevious
suggested 2 for experiments
experimental with studies,
more abundant
aluminum spinel
and [25]. The influence
chromium speed upofthe spinel
ser-
has been proposed
pentinization of olivine to result from the
[35,55,56]. Thetransfer
progressofofelectrons
reaction to in water adsorbed
experiments with toserpentine
the spinel
surfaces, producing
as the dominant magnetite
hydrous phase rinds
wasaround
calibratedspinel
based[25].
onSpinel
standardin natural
curvesserpentinites
described in is a
typically hydrothermally
previous study altered, leading
[35]. For experiments to the formation
with serpentine and brucite of magnetite
as the dominantrinds hydrous
[51–54].
The formation
phases, the extentof Cr- and Al-depleted
of reaction magnetite
was calibrated rinds indicates
according releases
to a calibration of aluminum
curve based on
(Al) andspectra
infrared chromium (Cr) duringofhydrothermal
of a combination alteration
olivine, serpentine, [35,51–54].
and brucite with the Aluminum
percentage and
of
olivine ranging
chromium from 10%
accelerate to 48%. The proportions
the serpentinization of olivine of [35,55],
serpentine andhave
theyamay positive
impedecorrela-
the
tion with log(A
formation of iron441 /Aoxide (R2 = 0.98) (Figure
503 )[48].Therefore, Al and4a). Cr
A441 mayis the integrated
influence the infrared
production intensity
of H2
of the vibration modealteration
at 441 cmof −1 , and A
during hydrothermal In is
olivine. 503 the to
order integrated
test suchinfrared
hypothesis,intensity of the
we carried
bending
out mode of Si-Oexperiments
serpentinization band in olivine at 503
at 311 °C andcm−3.0
1 . Molecular hydrogen (H ) production
kbar with a combination 2 of olivine
has a positive correlation with the extent of reaction, with
and Al2O3 or Cr2O3 powders (Table 1), and molecular hydrogen (H2) production duringa higher extent of reaction for
the production of more
the serpentinization of olivine H 2 (Figure 4b). Chromium slightly increases
becomes around 2–3 times higher (Figure 3), suggestingthe rates of olivine
hydrothermal
that Al and Cralteration, and it has hydrogen
enhance molecular a pronounced (H2) effect on molecular
production during hydrogen (H2 ) pro-
olivine serpentini-
ductionTherefore,
zation. (Figure 4b). the In contrast,
effect spinel
of spinel not only enhances
on molecular hydrogenthe (Hhydrothermal
2) production may alteration
result
of olivine,
from butofitAl
releases also anddramatically increases molecular
Cr during hydrothermal alteration.hydrogen (H2 ) production. This
also indicates
Aluminum that and thechromium
dramatic increase
promoteinthe molecular
production hydrogen
of H2 (H 2 ) production
during with the
the serpentiniza-
addition of spinel is closely associated with aluminum and chromium
tion of olivine, which is closely associated with a dramatic increase in reaction rates. rather than solely
As
aluminum.
suggested by previous experimental studies, aluminum and chromium speed up the
serpentinization of olivine [35,55,56]. The progress of reaction in experiments with ser-
pentine as the dominant hydrous phase was calibrated based on standard curves de-
scribed in a previous study [35]. For experiments with serpentine and brucite as the
dominant hydrous phases, the extent of reaction was calibrated according to a calibration
curve based on infrared spectra of a combination of olivine, serpentine, and brucite with
the percentage of olivine ranging from 10% to 48%. The proportions of serpentine have a
positive correlation with log(A441/A503) (R2 = 0.98) (Figure 4a). A441 is the integrated infra-
red intensity of the vibration mode at 441 cm−1, and A503 is the integrated infrared inten-
sity of the bending mode of Si-O band in olivine at 503 cm−1. Molecular hydrogen (H2)
production has a positive correlation with the extent of reaction, with a higher extent of
reaction for the production of more H2 (Figure 4b). Chromium slightly increases the rates
 gen (H2) production (Figure 4b). In contrast, spinel not only enhances the hydrothermal
alteration of olivine, but it also dramatically increases molecular hydrogen (H2) produc-
Minerals 2021, 11, 794 tion. This also indicates that the dramatic increase in molecular hydrogen (H2) produc-
7 of 15
tion with the addition of spinel is closely associated with aluminum and chromium ra-
ther than solely aluminum.

Figure 4. (a) Standard curves established to calibrate the proportions of serpentine in experiments of
Figure 4. (a) Standard curves established to calibrate the proportions of serpentine in
this study at 311 ◦ C and 3.0 kbar, with serpentine and brucite as major hydrous minerals. (b) Positive
experiments of this study at 311 °C and 3.0 kbar, with serpentine and brucite as major
correlation between the production of H2 and the progress of reaction for experiments at 311 ◦ C and
hydrous minerals. (b) Positive correlation between the production of H2 and the progress
3.0 kbar, and the data correspond to experiments with a duration of 27–29 days.
of reaction for experiments at 311 °C and 3.0 kbar, and the data correspond to experi-
mentsAnalyses
with a duration of serpentinites
of natural 27–29 days. show that their Al2 O3 is inversely correlated with
SiO2 contents [57], suggesting that Al3+ substitutes for Si4+ in tetrahedral sites of serpentine.
Analyses of natural serpentinites show that their Al2O3 is inversely correlated with
The substitution of Al3+ for Si in serpentine minerals is also observed for experiments of
SiO2 contents [57], suggesting that Al3+substitutes for Si4+ in tetrahedral sites of serpen-
this study, which is indicated by the broadening of the Si-O infrared band at 956 cm−1
tine. The substitution of Al3+ for Si in serpentine minerals is also observed for experi-
(Figure 2). This suggests that aluminum is mobile during hydrothermal alteration of
ments of this study, which is indicated by the broadening of the Si-O infrared band at 956
peridotite, and the mobility of Al is also indicated by chlorite formation in experiments
cm−1 (Figure 2). This suggests that aluminum is mobile during hydrothermal alteration
with a combination of olivine and 13 wt% spinel or Al2 O3 (Figure 2). The incorporation of
of peridotite,
Al3+ and the
into serpentine mobility
results of Al is
in a charge also indicated
deficit, which maybyleadchlorite
to theformation
distributionin of
experi-
more
ments
3+ with a combination of olivine and 13 wt% spinel or Al 2O3 (Figure 2).The incorpo-
Fe in octahedral sites of serpentine for compensation, and consequently the production
ration of Al3+ intoSuch
of H2 increases. serpentine results
hypothesis in a charge
is supported bydeficit, which
a decrease in may lead to the of
the production distribution
magnetite
of more Fe3+ in octahedral sites of serpentine for compensation, and consequently the
with the presence of Al2 O3 powders and the incorporation of more Fe into serpentine
minerals [17,55]. The influence of Cr is possibly associated with the oxidation of Fe2+
derived from olivine and pyroxene minerals by Cr6+ as proposed previously [35].
Orthopyroxene taken in our experiments contains 4.14 wt% Al2 O3 and 0.40 wt% Cr2 O3 ,
and clinopyroxene has 6.04 wt% Al2 O3 and 1.04 wt% Cr2 O3 . Chemical compositions of
natural serpentinized peridotites and experimental products after serpentinization suggest
that pyroxene-derived serpentine has much less Al2 O3 than the Al2 O3 contents of pyroxene,
Minerals 2021, 11, 794 8 of 15

indicating that pyroxene leached some Al during serpentinization [35,36,58]. It has been
estimated that ~50% of Al could be released from pyroxene [58]. The mobility of Cr is
indicated by largely scattered Cr2 O3 contents of orthopyroxene-derived serpentine [35,58].
These observations indicate that pyroxene minerals may affect molecular hydrogen (H2 )
production during serpentinization.
The effect of pyroxene minerals on molecular hydrogen (H2 ) production was investi-
gated by reacting a combination of olivine and pyroxene with saline solutions at 311 ◦ C
and 3.0 kbar. With the presence of pyroxene minerals, a sharp decline in molecular hydro-
gen (H2 ) production was observed, e.g., the concentrations of H2 were 15 mmol/kg after
27 days, which are lower by more than one order of magnitude compared to H2 produced in
experiments with olivine. Therefore, pyroxene minerals impede molecular hydrogen (H2 )
production during serpentinization, in great contrast with spinel that promotes molecular
hydrogen (H2 ) production.
Compared to olivine, pyroxene minerals have more abundant SiO2 . Analyses of
natural serpentinites and experimental products suggest that serpentine derived from
pyroxene hydrothermal alteration contains smaller amounts of SiO2 compared to the SiO2
contents of pyroxene, which indicates releases of SiO2 from pyroxene during hydrothermal
alteration. Silica leached from pyroxene minerals participates in the reaction of olivine
with H2 O [36]. As indicated by thermodynamic models, silica significantly decreases
H2 production during olivine serpentinization [47,59]. The possibility that the formation
of Si-rich surface layers inhibits the dissolution of olivine and pyroxene [25,43,60] and
decreases the production of H2 , however, is excluded. The absence of Si-rich surface layers
is indicated by analyses of the experimental products with scanning electron microscopy
and infrared spectroscopy.

3.3. Temperature Dependence of the Influence of Spinel and Pyroxene Minerals

Figure 5 shows that the effect of spinel and pyroxene on molecular hydrogen (H2 )
production during serpentinization is temperature dependent. The effect of pyroxene
and spinel on molecular hydrogen (H2 ) production is most pronounced at 311 ◦ C and
3.0 kbar, and it becomes much less significant at lower (200 ◦ C/3.0 kbar) and higher temper-
atures (400–500 ◦ C/3.0 kbar). Consistently, previous experiments showed that molecular
hydrogen (H2 ) produced in olivine-experiments has no difference from H2 formed in
experiments with the presence of spinel [61]. At 400 ◦ C and 3.0 kbar, a sharp decline in
molecular hydrogen (H2 ) production was observed, e.g., H2 in olivine-experiments de-
creased to 0.83 mmol/kg after 19 days (Table 1), which has the same trend as that observed
for experiments with a combination of olivine and spinel/pyroxene. For experiments at
400–500 ◦ C and 3.0 kbar with spinel-bearing peridotite (~60–65% olivine, 30–35% pyroxene
and 1–2% spinel), molecular hydrogen (H2 ) production was slightly higher than H2 formed
in olivine-experiments (Figure 5), indicating that spinel combined with pyroxene minerals
promotes the production of H2 after olivine serpentinization.
Previous experiments suggest that molecular hydrogen (H2 ) production during serpen-
tinization is directly linked to reaction rates [21,24,30]. The kinetics of olivine serpentiniza-
tion increase greatly at higher temperatures, with a maximum value at ~300 ◦ C [30,62,63].
With increasing temperatures (e.g., ≥350 ◦ C), the kinetics of serpentinization are signifi-
cantly slowed down [21,30,62]. Consistently, infrared spectra of the experimental products
in this study at 300 ◦ C and 3.0 kbar show a strong –OH band of serpentine and a weak
Si-O band of olivine located at 505 cm−1 ; for experiments at 400–500 ◦ C and 3.0 kbar, a
strong Si-O band of olivine located at 505 cm−1 and a weak -OH band of serpentine were
observed (Figure 2), suggesting sluggish rates of reaction at 400–500 ◦ C and 3.0 kbar. As a
consequence, molecular hydrogen (H2 ) production decreases greatly at 400–500 ◦ C and
3.0 kbar. Hydrothermal experiments suggest that pyroxene and spinel slightly enhance the
serpentinization of olivine at 400–500 ◦ C and 3.0 kbar, which is attributed to a decrease in
Gibbs energy of olivine serpentinization with the presence of silica released from pyroxene
Minerals 2021, 11, 794 9 of 15

Minerals 2021, 11, x 9 of 15

minerals [36]. This may consequently increase molecular hydrogen (H2 ) production during
serpentinization.

Figure
Figure 5.
5. (a)
(a)Temperature
Temperature dependence
dependence of of the
the influence
influence of
of spinel
spinel and
and pyroxene
pyroxene minerals
minerals onon H22
production during serpentinization. (b) An enlargement of the yellow rectangle in (a). Ol:
production during serpentinization. (b) An enlargement of the yellow rectangle in (a). Ol: olivine, olivine,
Pyr:
Pyr:pyroxene,
pyroxene,Spl:
Spl:spinel,
spinel,Prt:
Prt:peridotite.
peridotite.

Previous
Temperature experiments
dependence suggest
of H2 that molecular
production afterhydrogen (H2) production
olivine serpentinization mayduring
reflect
serpentinization
distinct mineralogicalis directly linked to
assemblages of reaction rates [21,24,30].
the experimental products The kinetics
at lower of olivine
(e.g., 200–300ser-◦C
pentinization
and 3.0 kbar) increase
and higher greatly at higher (e.g.,
temperatures temperatures,
400–500 with◦ C anda 3.0maximum value at electron
kbar). Scanning ~300 °C
[30,62,63].
microscopeWith increasing
imaging and FTIR temperatures
analyses have (e.g., ≥350 °C),
revealed thatthe kinetics
fibrous of serpentinization
chrysotile was the main
are significantly
secondary hydrous slowed down
mineral for [21,30,62].
most experimentsConsistently, ◦ C and 3.0
at 311 infrared spectra
kbar. of
Thethe experi-
dominant
mental
hydrous products
phase ininexperiments
this study atat300 400°C◦ Cand
and3.0 3.0kbar
kbarshow a strong and
was lizardite, –OHlizardite
band of andserpen-
talc
tine
wereand a weak at
generated Si-O ◦
500band of olivine
C and located
3.0 kbar. at 505 cm indicates
Talc formation −1 ; for experiments
that silicaatactivity
400–500may °C
be higher
and than
3.0 kbar, that of Si-O
a strong experiments
band of with
olivine serpentine
located at[21].
505 Thecm−1increase
and a weakin silica
‒OH activity
band of at
higher temperatures
serpentine were observed may lead
(Figureto a2),
decline in molecular
suggesting sluggish hydrogen (H2 ) production
rates of reaction at 400–500 [59].°C
and 3.0 kbar. As a consequence, molecular hydrogen (H2) production decreases greatly
3.4.
at Comparison
400–500 °C and of H
3.02 Production in This Study
kbar. Hydrothermal with That from
experiments suggestPrevious
that Studies
pyroxene and spinel
Chromite, an accessory mineral of ultramafic rocks,
slightly enhance the serpentinization of olivine at 400–500 °C and 3.0 kbar, is commonly present in serpen-
which is at-
tinization experiments [24,25,35,36,61,64]. Only a few studies
tributed to a decrease in Gibbs energy of olivine serpentinization with the presence have reported that chromite of
greatly
silica accelerates
released frommolecular
pyroxenehydrogen
minerals (H 2 ) production
[36]. during serpentinization
This may consequently [25], and
increase molecular
most studies
hydrogen (H2)have proposed
production duringa negligible influence of chromite [24,61,64]. Experiments
serpentinization.
of Mayhew et al. [25] were conducted at relatively low temperatures, ◦ C, with an
55–100 may
Temperature dependence of H2 production after olivine serpentinization reflect
experimental duration up to 100 days, and they showed that chromite
distinct mineralogical assemblages of the experimental products at lower (e.g., 200–300 enhanced molecular
hydrogen
°C (H2 ) production
and 3.0 kbar) and higher by around three
temperatures times.
(e.g., In contrast,
400–500 °C and 3.0 thekbar).
influence of chromi-
Scanning elec-
tron microscope imaging and FTIR analyses have revealed that fibrous chrysotile was the
main secondary hydrous mineral for most experiments at 311 °C and 3.0 kbar. The
dominant hydrous phase in experiments at 400 °C and 3.0 kbar was lizardite, and lizard-
Minerals 2021, 11, 794 10 of 15

teon H2 production was found to be negligible, based on short-period experiments (40

days) at 200 ◦ C and 200 bar with and without the addition of 1 wt% chromite [61]. The
discrepancy may be attributed to the production of lower molecular hydrogen (H2 ) due to
relatively shorter periods [61], which may obscure the influence of chromite. Consistently,
for experiments of this study at 200 ◦ C and 3.0 kbar with a combination of olivine and 22%
spinel, molecular hydrogen (H2 ) produced after 13 days was notably similar to H2 formed
in olivine-only experiments (Table 1). Experiments of Lazar et al. [64] were performed
at 300 ◦ C and 350 bar, and they showed that the serpentinization of komatiite with the
presence of chromite produced similar amounts of H2 as that formed during komatiite
hydrothermal alteration (Table 2). The contrast between experimental results of Lazar
et al. [64] from this study may result from the presence of glass, Cu-Zn alloys and sulfides
in komatiite, which may influence the formation of H2 .

Table 2. The production of H2 in previous experimental studies.

Solid Starting H2
T (◦ C) P (bar) W/R Ratios Time (Days) References
Reactant Fluids (mmol/kg)
300 500 Ol NaCl 2.25 69 158 [19]
200 500 Prt seawater 1.1 328 77 [22]
seawater +
200 300 Ol 2.5 33 2 [33]
NaHCO3
300 300 Prt H2 O 0.67 70 76 [24]
200 200 Ol + Chr a H2 O 2.5 21 4 [61]
200 200 Ol H2 O 2.5 21 4 [61]
300 350 Koma H2 O 3.7 63 74 [64]
300 350 Koma + Chr H2 O 4.7 63 55 [64]
300 500 Ol NaCl 2.1 111 11 [30]
200 500 Ol NaCl 1.8 138 0.09 [30]
400 500 Ol NaCl + MgCl2 4.0 64 1.2 [21]
400 500 Prt b NaCl + MgCl2 4.0 60 6.8 [21]
Ol: olivine, Prt: peridotite, Chr: chromite, Koma: komatiite. a The starting material is olivine and 1 wt% chromite. b The starting material is 76% olivine,
17% orthopyroxene, and 7% clinopyroxene.

Hydrothermal experiments of Shibuya et al. [17] were conducted at 300 ◦ C and

500 bar using two types of komatiites that have essentially the same chemical compositions
except Al2 O3 contents, 5% and 10%, respectively. It was revealed that H2 production in
the experiment with 5% Al2 O3 is higher than that with 10% Al2 O3 , which suggests that
aluminum impedes the production of H2 during hydrothermal alteration of komatiite [17].
The discrepancy from experimental findings of this study may result from different starting
reactants. Komatiite is composed of olivine, glass, and minor chromite, and it contains
more abundant SiO2 compared to olivine [17]. Hydrothermal alteration of komatiite could
release more SiO2 into aqueous fluids compared to that after hydrothermal alteration of
olivine [19,65,66], which suggests that the serpentinization of olivine is distinct from the
hydrothermal alteration of komatiite.
Peridotites or mixtures of olivine and orthopyroxene have been used as starting
reactants in serpentinization experiments [21,22,24,31]. At relatively low temperatures
(230 ◦ C), orthopyroxene was found to have a negligible influence on molecular hydrogen
(H2 ) production during the serpentinization of olivine, and it enhanced molecular hydro-
gen (H2 ) production after very long periods (e.g., 386 days), which is associated with a
dramatic increase in pH [31]. The discrepancy from experimental results of this study may
be attributed to the different experimental conditions, which include relatively low temper-
atures (230 ◦ C), the production of brucite and a dramatic increase in pH [31]. The kinetics
of orthopyroxene serpentinization under these conditions are 2–3 times faster than the
rates of olivine hydrothermal alteration [31], which is distinct from the serpentinization of
peridotite at 311 ◦ C and 3.0 kbar with slower rates of pyroxene serpentinization at reaction
extent of ≤70% [35]. With increasing temperatures (400 ◦ C), the serpentinization of olivine
Minerals 2021, 11, 794 11 of 15

produced the same amounts of molecular hydrogen (H2 ) as H2 formed in experiments with
a combination of olivine and pyroxene minerals [21]. This indicates a negligible influence
of pyroxene minerals on molecular hydrogen (H2 ) production at higher temperatures,
which agrees well with experimental results of this study.

3.5. Geological Implications

This study shows that spinel and pyroxene strongly influence the production of H2
during hydrothermal alteration of olivine, which is connected with the mobility of Al, Cr
and silica. Analyses of natural serpentinite ssuggest that they contain abundant Al2 O3 ,
ranging from <1 wt% to >20 wt% [46,58]. Although the solubility of Al2 O3 in aqueous fluids
is very low, <100 ppm [67], the mobility of Al during serpentinization is supported by the
following geological evidence: (1) occurrences of aluminum hydroxide in serpentinites [68],
(2) the formation of pyroxene-derived serpentine with less Al2 O3 compared to that of
primary pyroxene [46,57,58], and (3) the production of ferrichromite and magnetite rinds
around primary spinel [54,58,69]. Olivine in natural geological settings is intimately
associated with minerals with abundant Al2 O3 , such as pyroxene, plagioclase, and spinel,
which could release some of their Al during hydrothermal alteration of ultramafic rocks,
and consequently the production of H2 can be enhanced.
Fluids circulating serpentinizing peridotites commonly contain abundant SiO2 [5].
Analyses of natural serpentinites show that serpentine minerals derived from the hy-
drothermal alteration of pyroxene typically have smaller amounts of SiO2 compared to the
SiO2 contents of pyroxene [35,48,57,58], indicating that pyroxene minerals leached some
of their SiO2 during hydrothermal alteration. The mobility of silica during hydrothermal
alteration of ultramafic rocks is also indicated by the replacement of plagioclase by various
Si-poor silicates, such as grossular, wollastonite and zoisite [68]. Experimental and ther-
modynamic studies have revealed that silica impedes the oxidation of Fe2+ derived from
olivine and pyroxene into Fe3+ [47], which may consequently reduce molecular hydrogen
(H2 ) production during serpentinization.
The experimental results of this study may have important implications for H2 pro-
duction during serpentinization. Molecular hydrogen can feed communities of microorgan-
isms in serpentinite-hosted settings such as deep-sea hydrothermal vents, alkaline springs,
and the deep subsurface [8–12]. The microorganisms can endure high temperature (e.g.,
~100 ◦ C), high pressure, and extremely acidic or alkaline conditions. This may be an analog
for the genesis of life in the Hadean ocean. Geological studies suggest that life has been
present on Earth for at least 3.5 Gyr, and it probably began before 3.8 Gyr [70]. Biological
studies indicate that the ancestor of life is hyperthermophile, i.e., the organism that lives
at temperatures of 80–110 ◦ C or more [71]. Therefore, serpentinization may therefore be
favorable for the origin and evolution of life [72,73]. There should be some critical steps
for the early Earth to evolve from a magma ocean to a habitable planet in the Hadean eon.
Ultramafic rocks may be widely exposed on the surface of the early Earth, and the atmo-
sphere is mainly composed of H2 O and CO2 [71,74]. Interactions between ultramafic rocks
and H2 O in early atmosphere may significantly change the physicochemical condition of
the Earth’s surface and initiate the prebiotic chemical conditions for life genesis.

4. Conclusions
The effect of pyroxene and spinel on molecular hydrogen (H2 ) generation during the
serpentinization of olivine was experimentally studied at 311–500 ◦ C and 3.0 kbar, where
olivine, individually or in combined with pyroxene and spinel, was reacted with saline
solutions (0.5 M NaCl). At 311 ◦ C and 3.0 kbar, spinel was found to accelerate H2 generation
by around 2–3 times, and the increase in H2 generation is directly linked to releases of
aluminum and chromium during hydrothermal alteration. Aluminum and chromium
significantly enhance molecular hydrogen (H2 ) generation during olivine serpentinization,
which was confirmed by serpentinization experiments conducted at 311 ◦ C and 3.0 kbar
with the presence of Al2 O3 and Cr2 O3 powders. In spite of releases of aluminum and
Minerals 2021, 11, 794 12 of 15

chromium from pyroxene minerals, pyroxene minerals greatly inhibit H2 generation during
serpentinization, which may result from releases of silica from pyroxene minerals. The
influence of pyroxene minerals and spinel on H2 generation is temperature dependent, and
it becomes negligible at lower temperatures (200 ◦ C and 3.0 kbar). At higher temperatures
(400–500 ◦ C and 3.0 kbar), the addition of the combination of pyroxene and spinel slightly
increases H2 generation during olivine serpentinization, and the presence of individual
pyroxene or spinel has a negligible effect. Olivine in natural geological settings is commonly
associated with aluminum-rich minerals (e.g., pyroxene, spinel and plagioclase), and
therefore H2 generation can be greatly affected.

Author Contributions: R.H. conceived of the primary data, conducted all the experiments, analyzed
most samples and wrote the manuscript. W.S. and X.S. co-write the manuscript. X.D., X.S. analyzed
some samples. All authors have read and agreed to the published version of the manuscript.
Funding: This work was financially supported by the Natural Science Foundation of China (41873069),
the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22050103), and the
National Key R&D Program of China (2016YFC0600408).
Data Availability Statement: The data presented in this study are available within the article.
Acknowledgments: We thank J. H. Zhu from the Second Institute of Oceanography, State Oceanic
Administration of China for performing scanning electron microscope imaging. We would like to
express our gratitude to S. Jiang from South China University of Technology for the assistance during
FTIR analyses.
Conflicts of Interest: The authors declare that there are no conflict of interest regarding the publica-
tion of this paper.

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