Carbon Dioxide Emissions from Subaerial Volcanic Regions: Two Decades in Review

TLDR: In this article, the authors quantify the current volcanic flux of carbon to the atmosphere and understand the factors that control this flux, which is of fundamental importance for stabilization of atmospheric CO2 and for long-term climate.

Abstract: Volcanism and metamorphism are the principal geologic processes that drive carbon transfer from the interior of Earth to the surface reservoir. Input of carbon to the surface reservoir through volcanic degassing is balanced by removal through silicate weathering and the subduction of carbon-bearing marine deposits over million-year timescales. The magnitude of the volcanic carbon flux is thus of fundamental importance for stabilization of atmospheric CO2 and for long-term climate. It is likely that the “deep” carbon reservoir far exceeds the size of the surface reservoir in terms of mass; more than 99% of Earth’s carbonmay reside in the core, mantle, and crust. The relatively high flux of volcanic carbon to the surface reservoir, combined with the reservoir’s small size, results in a short residence time for carbon in the ocean–atmosphere–biosphere system (~200 ka). The implication is that changes in the flux of volcanic carbon can affect the climate and ultimately the habitability of the planet on geologic timescales. In order to understand this delicate balance, we must first quantify the current volcanic flux of carbon to the atmosphere and understand the factors that control this flux. The three most abundant magmatic volatiles are water (H2O), carbon dioxide (CO2), and sulfur (S), with CO2 being the least soluble in silicate melts. 8 For this reason, it is not only Earth’s active volcanoes that are a source of magmatic CO2, but also numerous inactive volcanoes with magma bodies present at depth in the crust that contribute to the carbon emissions (Figure 8.1). Emissions from active volcanoes are released through crater fumaroles and open vents to form visible volcanic plumes, but diffuse degassing and degassing through springs on the volcano flanks also contribute to the total flux of carbon from a volcano. Plume gas emissions typically dominate over flank gas emissions and are highest during periods of eruptive activity. Due to the hazard associated with eruptions and the value of volcanic gas monitoring to aid in eruption forecasting, much of our knowledge about the degassing of volcanic systems comes from active volcanoes, and typically during periods of unrest. At less active and dormant (i.e. inactive) volcanoes, magmatic emissions of CO2 are less obvious. CO2 emissions are typically highest in thermal areas where gases are emitted through small fumaroles, soils, and fractures as diffuse degassing and through hot and cold

Figures

Figure 5.3 (a and b) SiO2 and Cl versus MgO content of HDF microinclusions in 89 fibrous diamonds from different lithospheric provinces (in wt.% on a water- and carbonate-free basis). The high-Mg carbonatitic compositions are close to experimental near-solidus melts of carbonateperidotite, while the low-Mg carbonatitic to silicic HDFs form an array that is close in composition to experimentally produced fluids/melts in the eclogite + carbonate water system.62,63,65–67 The saline HDF end-members have been related to fluids derived from seawater-altered subducted slabs.68 Data: DeBeers-Pool, Koingnaas, and Kankan fromWeiss et al.;62,69 Koffiefontein from Izraeli et al.;70 Brazil from Shiryaev et al.;71 Diavik and Siberia from Klein Ben-David et al.;72,73 Jwaneng from Schrauder and Navon;74 Panda from Tomlinson et al.;75 Wawa from Smith et al.76

Figure 5.10 (a) Metallic inclusions in a 9.56 carat CLIPPIR diamond with an enlargement of one of the inclusions. These metallic inclusions sometimes have a needle-like tail and typically have large, graphitic decompression cracks around them. (b) Depth constraints place the origin of these diamonds within 360–750‑km depths in the mantle, where they are associated with subducted lithologies. The metallic inclusions are evidence for reduced, metal-bearing regions of the deep mantle below a depth of approximately 250 km.

Figure 5.2 (a) Cathodoluminescence (CL) image of Marange diamond MAR06b,40 showing core-torim secondary ion mass spectrometry analytical spots. (b) Raman map showing distribution of graphite and CH4 micro-inclusions in a homogeneously grown cuboid zone. (c) Outwardly decreasing nitrogen content (atomic ppm) with increasing δ13C (‰) in this same cuboid growth zone (red) and other cuboid growth zones (gray). The modeled trend (red dashed line) is for a CH4: CO2 ratio of 1:1 and assumes an initial δ 13C for the fluid of –7.4‰. For an assumed water content of 98 mol.%, the observed variation corresponds to 0.7% crystallization of the entire fluid (and 35% of the carbon in the fluid). For details on modelling, see Stachel et al.51

Figure 5.8 Lherzolitic diamond formation through time: ca. 2.1 to 1.8 Ga, diamonds from Premier (Kaapvaal craton) and 23rd Party Congress/Udachnaya (Siberian craton); 1.4 Ga, diamonds from Ellendale (Western Australia);161 1.1 to 1.0 Ga, diamonds from 23rd Party Congress/Mir (Siberian craton)162 and Venetia (Zimbabwe craton);163 and 0.72 Ga, diamonds from Attawapiskat (Superior craton).164 Numbers in parentheses give host kimberlite eruption ages (in giga-years) to illustrate the delay between lherzolitization and kimberlite magmatism. Shown for comparison is the age distribution of kimberlites from Tappe et al.,160 Os model ages of mantle sulfides from Griffin et al.,165 which predominantly reflect the time of Archaean craton formation and the creation of strongly refractory and reducing mantle lithosphere, and for xenoliths from the Siberian craton, which show a major Paleoproterozoic lithospheric mantle formation event.166

Figure 5.4 Δ33S (‰) versus δ34S in mid-ocean ridge basalt (MORB; blue rectangle87,89,90), sulfide inclusions in diamonds (yellow hexagons83,86), and sulfides from high-µ mantle reservoir (HIMU) ocean island basalt (OIB; Mangaïa,91 light green squares) and enriched mantle reservoir 1 (EM1) OIB (Pitcairn,92 dark green squares). While MORB are homogeneous and devoid of MIF, sulfides from SCLM and from some OIB contain the relict of Archean surficial sulfur. Sulfide inclusion compositions are best explained by a combination of atmospheric and biotic effect and resemble what has been previously observed in Archean chemical sediments. Sulfide in OIB carry negative Δ33S together with negative δ34S, as previously observed in sulfides from altered oceanic crust.

Figure 5.6 Photograph of Juína diamond JuC-29 and a magnified view of the ringwoodite inclusion (right panel, center of image) showing the characteristic indigo–blue color of ringwoodite.

Full text

5
Diamonds and the Mantle Geodynamics of Carbon
Deep Mantle Carbon Evolution from the Diamond Record
steven b. shirey, karen v. smit, d. graham pearson,
michael j. walter, sonja aulbach, frank e. brenker, he
´
le
`
ne bureau,
antony d. burnham, pierre cartigny, thomas chacko,
daniel j . frost, erik h. hauri, dorrit e . jacob, st even d. jacobsen,
simon c. ko hn, robert w. luth, sami mikh ail, oded navon,
fabrizio nestola, paolo nimi s,medericpalot,evanm.smith,
thomas stachel, vincenzo stagno, andrew steele, richard a. ste rn,
emilie thomassot, andrew r. thomson, an d yaakov weiss
5.1 Introduction
The importance of diamond to carbon in Earth is due to the fact that diamond is the only
mineral and especially the only carbon mineral to crystallize throughout the silicate Earth –
from the crust to the lower mantle. To study diamond is to study deep carbon directly
throughout Earth, allowing us to see the inaccessible part of the deep carbon cycle. By
using the properties of diamond, including its ability to preserve included minerals,
important questions relating to carbon and its role in planetary-scale geology can be
addressed:
•
What is the mineralogy of phases from Earth’s mantle transition zone and lower mantle?
•
What are the pressures and temperatures of diamond growth?
•
What is the chemical speciation of recycled and deep carbon?
•
What are the reactions that produce reduced carbon?
•
What are the sources of carbon and its associated volatiles (H
2
O, CH
4
,CO
2
,N
2
,NH
3
,
and S)?
•
How do these findings vary with global-scale geological processes?
•
How have these processes changed over billions of years of geologic history?
Diamonds for scientific study are difficult to obtain and the nature of diamond presents
special research challenges. Diamonds, whether they are lithospheric or sublithospheric
(see the paragraph after next below), are xenocrysts in kimberlitic magma that travel a long
path (as much as 150 to >400 km!) during eruption to Earth’s surface. On strict petrologic
grounds, by the time a diamond reaches Earth’s surface, there is little direct evidence that it
is related to any neighboring diamond. However, the suites of diamonds that occur in close
spatial association at Earth’s surface in a mine may have similar physical characteristics
and may also record similar pressure–temperature conditions and ages. If so, these features
would suggest that the host kimberlite delivered a diamond suite to the surface from a
spatially restricted mantle source hundreds of kilometers distant. Kimberlite magmas can
89
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transport some diamonds in mantle blocks that apparently disaggregate near the end of
their upward journey. Nonetheless, each diamond and its inclusions is a case study unto
itself until its association with other diamonds can be confirmed.
The Diamonds and Mantle Geodynamics of Carbon (DMGC) consortium was con-
ceived early in the existence of the Deep Carbon Observatory (DCO) for the specific
purpose of breaking down the traditional barriers to research on single diamonds and
directing research toward global carbon questions. From the outset, the DMGC consortium
focused on making cross-disciplinary research tools available, sharing samples so that
more definitive results could be obtained, and enhancing intellectual stimulation across
research groups so that new ideas would develop. The purpose of this chapter is
to showcase results from the major collaborative resear ch areas that have emerged
within the DMGC consortium: (1) geothermobarometry to allow the depth of diamond
crystallization and constraints on diamond exhumation to be determined (Section 5.2);
(2) diamond-forming reactions, C and N isotopic compositions, and d iamond-forming
fluids to understand how diamonds form in the mantle (Section 5.3); (3) the sources of
carbon either from the surface or within the mantle to provide information on the way
carbon and other volatiles are recycled by global processes (Section 5.4); and (4) the
mineralogy, trace element, and isot opic composition of mineral inclusions and their host
diamonds to relate diamond formation to geologic conditions in the lithospheric and deeper
convecting mantle (Section 5.5).
A general review and summary of diamond research can be obtained by consulting
previous works.
1–16
Much of this literature focuses on diamonds and the mineral inclusions
that have been encapsulated when these diamonds crystallized in the subcontinental
lithospheric mantle (SCLM). These so-called lithospheric diamonds can be classified as
eclogitic or peridotitic (harzbugitic, lherzolitic, or websteritic) based on the composition of
silicate or sulfide inclusions.
10,16
Lithospheric diamonds crystallize at depths of around
100–200 km and temperatures of around 1160  100

C.
16
Peridotitic diamonds typically
have restricted, mantle-like C isotopic compositions , whereas eclogitic diamonds have
more variable and sometimes d istinctly lighter C isotopic compositions.
8
Lithospheric
diamonds are likely to contain appreciable nitrogen (mostly Type I; 0–3830 at. ppm,
median = 91 at. ppm
16
). Their ages range from Cretaceous to Mesoarchean, but most are
Proterozoic to Neoarchean.
1,7,17
The study of lithospheric diamonds has led to advances in
understanding of the stabilization of the continents and their mantle keels , the onset of plate
tectonics, and the nature of continental margin subduction, especially in the ancient past.
In the last two decades, attention has turned to the study of diamonds whose inclusion
mineralogies and estimated pressures of origin put them at mantle depths well below
the lithospheric mantle beneath continents. These “sublithospheric” or so-called super-
deep diamonds can occur at any depth down to and including the top of the lower
mantle (660–690 km), but a great many crystallize in the mantle transition zone (e.g.
410–660 km)
9,12,18
at higher temperatures (between 100 and 400

C higher)
19
than litho-
spheric diamonds. Unlike lithospheric diamonds, super-deep diamonds are not as easily
classified as eclogitic or peridotitic. However, super-deep diamonds do carry mineral
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phases that are the high-pressure derivatives of basaltic and ultramafic precursors such as
majorite or bridgmanite, respectively,
12
so a petrologic parallel exists in super-deep
diamonds with the eclogitic and peridotitic lithospheric diamond designation. Super-deep
diamonds are typically low nitrogen (e.g. Type IIa or IIb) and display quite variable
C isotopic compositions, even extending to quite light compositions (Section 5.4.2 and
Ref. 20). Age determinations on super-deep diamonds are rare, but the few that exist
21
support thei r being much younger than lithospheric diamonds – an expected result given
the known antiquity of the continents and their attached mantle keels relative to the
convecting mantle. The study of super-deep diamonds has led to advances in understand-
ing of the deep recycling of elements from the surface (e.g. H
2
O, B, C, and S), the redox
structure of the mantle, and the highly heterogeneous nature of the mantle transition zone.
Much of the research described in this chapter focuses on super-deep diamonds since
the study of super-deep diamonds has the greatest relevance to the deep carbon cycle.
5.2 Physical Conditions of Diamond Formation
5.2.1 Measuring the Depth of Diamond Formation
An essential question is the depth at which a diamond forms. Geobarometry of diamonds
based on the stability of their included minerals has provided important constraints on the
deep carbon cycle. Application of these methods has yielded the whole range of depths
from 110 to 150 km, corresponding to the graphite–diamond boundary in the lithosphere,
to over 660 km, lying within the lower mantle.
14,22–25
Thus, these studies have provided
direct evidence for the recycling of surficial carbon to lower-mantle depths. Traditional
geobarometric methods, however, have several limitations: they can only be applied to rare
types of mineral inclusions; touching inclusions may re-equilibrate after diamond growth;
non-touching inclusions may be incorporated under different conditions and may not be in
equilibrium; and protogenetic inclusions
26,27
may not re-equilibrate completely during
diamond growth.
In order to avoid some of these drawbacks, alternative approaches that are independent
of chemical equilibria are increasingly being explored. Elastic geobarometry is based on
the determination of the residual pressure on the inclusion, P
inc
, which builds up on an
inclusion when the diamond is exhumed to the surface as a result of the difference in the
elastic properties of the inclusion and host. If these properties are known and the entrap-
ment temperature is derived independently or its effect is demonstrably negligible, then the
entrapment pressure can be calculated back from the P
inc
determined at room conditions.
The idea of using P
inc
to calculate entrapment conditions is not new,
28
but practical
methods have recently been developed that allow more robust estimates of minerals with
known elastic properties.
29,30
In principle, elastic geobarometry can be applied to any
inclusion in a diamond if: (1) the inclusion–diamond interaction is purely elastic, otherwise
only minimum estimates can generally be o btained; (2) the geometry of the inclusion–host
system is properly considered; and (3) mineral-specific calibrations are available to
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calculate P
inc
from X-ray diffraction or micro-Raman spectroscopy data. Contrary to
common pract ice, calibrations should take into account the effect of deviatoric stresses,
which typically develop in inclusion–diamond systems upon exhumation. For example,
Anzolini et al.
31
showed that only an accurat e choice of Raman peaks could provide
reliable estimates for a CaSiO
3
–walstromite inclusion in diamond, yielding a minimum
formation depth of 260 km and supporting CaSiO
3
–walstromite as a potential indicator of
sublithospheric origin. The effect of the presence of fluid films around the inclusions,
which has recently been docum ented in some lithospheric diamonds,
32
still demands
proper evalua tion. In addition, the ability of diamond to deform plastically, especially
under sublithospheric conditions, is well known, but methods to quantify any effects on
elastic geobarometry are not available. Therefore, in many cases, only minimum estimates
can be obtained. Nonetheless, we are now able to provide depth or minimum depth
estimates for a number of new single-phase assemblages that would not be possible with
more traditional methods. Future geobarometry of larger sets of diamonds, using both
elastic and traditional approaches, will allow more comprehensive data to be gathered on
the conditions for diamond-forming reactions and on the deep carbon cycle.
5.2.2 Thermal Modeling of Diamond in the Mantle from Fourier-Transform
Infrared Spectroscopy Maps
The defects trapped in diamonds can be used to constrain estimates of the temperature that
prevailed during the residence of a diamond in the mantle and can help constrain estimates
of the return path of carbon to the surface. Pressure and temperature covary with depth in
Earth, and the ability of the diamond lattice to record temperature history in its defect
structure provides an additional independent constraint on estimates of mantle location.
These measurements are there fore complementary to those on inclusions that can be used
to determine the pressure and temperature conditions during the trapp ing of inclusions
during diamond growth. The general concepts and calibration of a thermochronometer
based on nitrogen defect aggregation are well established.
33
The technique is based on the
kinetics of aggregation of pairs of nitrogen atoms (called A centers) into groups of four
nitrogen atoms around a vacancy (called B centers) and measurement of these defect
concentrations using Fourier-transform infrared (FTIR) spectroscopy. The FTIR spectros-
copy method has long been used as one of the standard characterization techniques for
diamonds, mostly for whole stones, but also as FTIR spectroscopy maps showing the
distribution of defect concentrations across diamond plates.
34,35
Only recently has the full
potential of FTIR spectroscopy for determining the thermal history of a diamond been
recognized. The major recent developments have been: (1) improvements in the methods
for acquiring and processing FTIR spectroscopy maps
34
; (2) a better understanding of the
temperature history that is available from zoned diamonds
36
; and (3) unlocking the
abundant information that is provided by the FTIR spectroscopy signal of platelets – planar
defects created with B centers during nitrogen aggrega tion.
37,38
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Figure 5.1a shows a map of “model temperatures” made up by automated fitting of
several thousand FTIR spectra in a map of a diamond from Murowa, Zimbabwe. The
higher temperatures in the core and lower temperatures in the rim reflect a growth and
annealing history with at least two stages. The key idea is that the N aggregation in the rim
only occurs during the second stage of annealing, but that N aggregation in the core occurs
throughout the residence period of the diamond in the mantle (i.e. during both stages of
annealing). Even if the date of rim growth is not known, there is an interplay between the
temperatures of the two stages and the time of rim formation (Figure 5.1b). While these
data provide a combination of time and temperature, if the dates of each stage of diamond
formation are accurately known (by dating of inclusions) and the date of kimberlite
eruption is known, the temperatures during the two stages can be determined. The model
in Figure 5.1b assumes core growth at 3.2 Ga followed by a period of annealing, then rim
growth and finally a second period of annealing. If a constant temperature prevailed
throughout the history of the diamond’s residence in the lithosphere, the ages of the two
periods of growth are 3.2 and 1.1 Ga. If the earlier history of the diamond was hotter, the
overgrowth must be older. Using this method, the mean temperature variation over very
long (billion-year) timescales at a specific location in the lithosphere can be determined.
An alternati ve way to learn about the history of a diamond is to study the production and
degradation of platelets. By comparing transmission electron microscopy and FTIR
Figure 5.1 (a) Example of a map of “model temperatures” made up by automated fitting of several
thousand FTIR spectra in a map of a diamond from Murowa, Zimbabwe. Model temperatures are
calculated using a single assumed mantle residence time. The higher model temperatures in the core
and lower model temperatures in the rim reflect a growth and annealing history with at least two
stages. (b) Modeling the possible combinations of temperature and time that could explain the FTIR
spectroscopy characteristics of a zoned diamond from Murowa.
36
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Frequently asked questions

Q1. What are the contributions in "Diamonds and the mantle geodynamics of carbon deep mantle carbon evolution from the diamond record" ?

The diamond and mantle geodynamics of carbon ( DMGC ) consortium this paper was formed to study the deep carbon cycle. 

Q2. What are the future works in "Diamonds and the mantle geodynamics of carbon deep mantle carbon evolution from the diamond record" ?

Future directions in diamond research in relation to the bigger picture of carbon in the deep Earth are best summed up with a list of questions: • Taken in total, the studies made by the DMGC consortium will provide new insights into how carbon behaves and resides in both the lithosphere and the deeper convecting mantle. Moreover, through diamond ’ s remarkable attributes, diamond studies will allow us to go beyond the study of carbon alone, to make fundamental discoveries on the nature of the deep Earth that is inaccessible in any other way, and to understand the spectrum of geological processes that govern how carbon gets into the mantle and the form in which it resides. 

Q3. What is the role of trace elements in the origin of super-deep diamonds?

The involvement of recycled crustal components in the origin of super-deep diamonds is supported by observations that the diamond hosts show a wide range of carbon isotope compositions extending to very light values (e.g. ~0% to –25%)20,21 and that majorite garnet and other silicate inclusions have isotopically heavy oxygen isotope compositions. 

Q4. What is the alternative to diamond growth from cooling or decompressing?

The alternative to diamond growth from cooling or decompressing C–H–O-rich liquids is precipitation due to redox reactions with iron or potentially sulfide species in surrounding minerals or melts. 

Q5. What is the bulk composition of the inclusions in super-deep diamonds?

The bulk composition and trace element distributions in mineral inclusions in super-deep diamonds provide information about the conversion of carbonate to diamond in the deep upper mantle, transition zone, and lower mantle. 

Q6. How can the authors study the uplift and exhumation of diamond?

advances in the correlation of spectral features with newly understood defect types may allow diamond to emerge as a prime mineral for studying the uplift and exhumation in the global tectonic cycle (e.g. Ref. 39). 

Q7. What is the role of metal in the evolution of mantle?

Further research into the influence of metallic iron on carbon in the mantle will explore the evolution of storage and cycling, from core formation to the onset of modern-style plate tectonics. 

Q8. What is the preferred mechanism to form these diamonds?

The preferred mechanism to form these diamonds is by dissolution and re-precipitation (Figure 5.5c), where subducted metastable graphite would be converted into an oxidized or reduced species during fluidaided dissolution, before being re-precipitated as diamond. 

Q9. What is the carbon isotopic signature of transition zone diamonds?

The carbon and nitrogen isotopic signatures of transition zone diamonds worldwide indicate that they likely crystallized from fluids derived from subducted material, illustrating the deep cycling of surficial carbon and nitrogen into and through the transition zone. 

Q10. What is the role of polycrystalline diamonds in the deep carbon cycle?

Obtaining accurate information on the age and depth of polycrystalline diamond formation is the next step to addressing their role in the deep carbon cycle, since they may represent the shallowest form of diamond-forming fluid. 

Q11. How is the study of diamonds done?

As diamond is thought to crystallize from these species by different mechanisms, its study becomes a key way to understand these carbon-bearing fluids. 

Q12. What is the way to determine the origin of carbonate inclusions in diamonds?

The identification of carbonate inclusions in ultra-deep diamonds indicates that carbonate may be efficiently transported deep into the mantle. 

Q13. What is the future of the study of sulfur pools?

In the future, studying covariations of Δ33S and Δ36S may help to provide a more complete assessment of the recycled sulfur pools and ultimately add new constraints upon crust/mantle dynamics. 

Q14. What is the way to determine the speciation of the diamond-forming fluid?

In fluid inclusion-free diamonds, core-to-rim trends in δ13C and N content have been (and probably should not have been) used to infer the speciation of the diamond-forming fluid. 

Q15. What is the key zone for the storage and return of recycled volatiles in Earth?

This region of Earth’s mantle is therefore a key zone for the storage and ultimate return of recycled volatiles, including carbon, in Earth. 

Q16. What is the way to track the fate of specific sedimentary pools?

multiple S isotopic systematics is a robust tracer of the Archean surficial sulfur,83 but can also be used to track the fate of specific sedimentary pools.