The role of C-O-H-F-Cl fluids in the making of Earth's continental roots

Abstract

The cratonic 'roots' of Earth's major continents extend to depths of over 160 km and have remained stable for more than 2.5 billion years due to buoyant, refractory harzburgites formed by Archean mantle melting. However, mantle harzburgites from some global cratons (e.g., Kaapvaal, Siberia, Slave, Rae and Tanzania) show unusual orthopyroxene and silica enrichment, alongside titanium depletion, which cannot be explained by simple melting processes. The origins of the orthopyroxene-rich harzburgites are debated and include high-pressure melting residues, with komatiite melt interaction, or subduction-related silicic melts and fluids. To further investigate this we analysed volatile (H₂O, F, Cl) contents in Kaapvaal craton peridotites. Orthopyroxene-rich harzburgites, including a diamond-bearing sample, show elevated volatile contents, suggesting infiltration by supercritical C-O-H fluids-rich in silica, fluorine and chlorine and depleted in Ti-fluxed from subducted oceanic lithosphere (carbonated pelites, eclogites and serpentinites). These findings highlight the role of C-O-H-F-Cl bearing fluids in shaping cratonic lithosphere and offer a new framework for understanding craton evolution, mantle metasomatism and diamond genesis in early Earth.

Fig. 1. Spatial distribution of orthopyroxene-rich garnet harzburgites in global Archaean cratons.

Not all cratons are made equal. Mantle peridotites with greater than 25% orthopyroxene cannot be explained as single-stage melt residues and require the addition of silica (data from this work, refs. ^,,,,). Contours for lithospheric thickness are shown at 10 km intervals for regions where this exceeds 160 km, and are taken from seismic tomography (modified from ref. , data from ref. ). Outlines of shaded regions are selected on the basis of lithosphere >170 km so as to distinguish specific cratons from the more laterally extensive continental cores that are frequently composed of amalgamated cratonic blocks. For example, the Slave and Rae cratons amalgamated with the Superior and Wyoming cratons to form the North American craton and are located close to the thickest part of the continental core. The Kaapvaal craton forms the southern part of the larger Kalahari craton of southern Africa. Some of the deepest roots of Eurasia are associated with the Siberia craton. Mid-ocean ridges are illustrated by pale blue lines.

Fig. 2. Variation of modal amounts of olivine and orthopyroxene in the Kaapvaal craton.

Panel a shows that deviations from the main linear trend are primarily due to addition of garnet and or clinopyroxene to the melt residue as a consequence of metasomatism. The dashed line marks the threshold above which peridotites are described as having excess orthopyroxene compared to those in melt residues. Panel b shows that the modal amount of olivine in mantle peridotites primarily varies linearly with the amount of orthopyroxene and bulk MgO/SiO₂ content. The excess amount of orthopyroxene (>25%) primarily reflects the reaction of silica in a melt or fluid with olivine and can be described by the following equation: MgFe₂SiO₄ + SiO₂ → MgFe₂SiO₆. Mantle peridotites from the Kaapvaal and Tanzania cratons studied in this work are shown by large symbols. Orthopyroxene-rich garnet harzburgites are shown by circles and garnet harzburgites and lherzolites are shown by triangles. Small symbols are for mantle peridotites from the Kaapvaal craton published by refs. ^,.

Fig. 3. Variation of modal abundance of olivine (volume %) with Fo content.

The Fo contents (Mg/Mg+Fe) of olivines in mantle peridotites are thought to be the most reliable tracer of mantle melting. a Numerical models of mantle melting derived from experimental studies indicate that typical cratonic peridotites have experienced 20% or more melt extraction. Co-variations with the amount of modal olivine show that many of those from the cratonic mantle deviate away from the oceanic trend. The Kaapvaal craton shows some of the greatest deviations and contrasts with the East Greenland craton, where there is a close approximation to the trend predicted for melts of peridotite at 3 GPa (red curve). While the deviation of cratonic mantle peridotites away from the oceanic and hypothetical 3 GPa melt residue trends may be explained by high-pressure melting (6 GPa, black curve), the amounts of fusion are implausibly high. As a consequence, Tomlinson and Kamber proposed that the anomalous enrichments in silica can be explained by the melting of a hybrid komatiite-peridotite source (blue curve), which requires lower amounts of melting. Isobaric melt models are after Tomlinson and Kamber. Data are from this work (Supplementary Data 1), refs. ^,,,,. b Modal olivine versus Fo content plot together with the melt extraction trend for Phanerozoic mantle (purple arrow) coloured for bulk H₂O + F contents of cratonic mantle. Data for abyssal peridotites and continental off-craton mantle (this work and ref. ) are shown for comparison.

Fig. 4. Bulk rare-earth element (REE) concentrations of cratonic peridotites.

a Orthopyroxene-rich garnet harzburgites and b other types of peridotites in mantle xenolith suites from the Kaapvaal craton of southern Africa. Elements increase in incompatibility during mantle melting from right to left. The orthopyroxene-rich garnet harzburgites exhibit sinusoidal REE patterns when normalised to the MORB source mantle. These patterns vary with the amounts of orthopyroxene. As the amount of orthopyroxene increases, the contents of light and heavy REEs decrease and middle REEs increase (as illustrated by the black arrows). The most extreme example of this in our sample suite is the diamondiferous orthopyroxene-rich harzburgite BD2125 from Lesotho. This xenolith equilibrated just below the graphite-diamond boundary (1060 °C and 4.4 GPa). Data are from this work (Supplementary Data 1) unless stated otherwise in the legend. A MORB-source normalisation is used because cratonic mantle is widely believed to represent aggregated residues of large amounts of melting.

Fig. 5. Bulk xenolith concentrations of incompatible trace and volatile elements (H₂O and F) in orthopyroxene-rich garnet harzburgites from the Kaapvaal craton of southern Africa normalised to MORB source upper mantle.

Elements increase in incompatibility during mantle melting from right to left. Incompatible trace element concentrations in our samples of orthopyroxene-rich garnet harzburgites resemble those in the dataset of ref. . All of the orthopyroxene-rich garnet harzburgites are depleted in Ti and the heavy rare-earth elements relative to other types of peridotites (lherzolites, harzburgites and dunites). Diamondiferous orthopyroxene-rich garnet harzburgite (BD2125) exhibits the greatest relative enrichments in H₂O, F, Pb, Th and U and depletion in Rb. Data are from this work (Supplementary Data 1) and ref. .

Fig. 6. Box and whisker plots showing variations in reconstructed bulk contents of volatiles in different types of mantle peridotites from the Kaapvaal craton.

Panel a shows the variations in H₂O, b F and c H₂O + F. The boxes define the edge of the 1st and 3rd quartile ranges, the grey line inside the box is the median value and the whiskers extend to 1.5× the interquartile range and represent the range of the data. Data are from this work and Peslier et al.. Statistical analyses are available in Supplementary Data.

Fig. 7. Modal abundance of orthopyroxene (volume %) versus mean F content of mineral phases in Kaapvaal peridotites.

a F content of orthopyroxene and clinopyroxene in garnet lherzolites and orthopyroxene-rich harzburgites from the Kaapvaal craton. Vectors show the effects of melt- and fluid-rock reactions on mantle residues, which are depleted in F (e.g., depleted MORB mantle has 12 µg/g F). The volume of fluid controls the magnitude of the increase in excess modal orthopyroxene (garnet and phlogopite) and fluorine concentration. b Same as for (a) except the maximum value of the y-axis scale is increased to also include olivine, and the minimum value of the x-axis scale is increased so that only orthopyroxene-rich garnet harzburgites are shown. Data are from this work (Supplementary Data 1).

Fig. 8. Bulk MgO/SiO₂ versus Ti in cratonic mantle peridotites and melting residues predicted from thermodynamic models.

Curved lines show models for isobaric melting of mantle peridotite (KR4003) at 3 and 6 GPa; and reaction of komatiite melts with peridotite (KR4003) at 6 GPa. The 2 and 3 GPa melting trends are similar. The amount of melting associated with the residues is given as % values. All symbols are coloured according to the modal % of orthopyroxene. Only melting at 6 GPa generates the high Mg# of olivine and orthopyroxene observed in the orthopyroxene-rich harzburgites, but the Kaapvaal orthopyroxene-rich harzburgites have much lower bulk Ti contents than the residues predicted from 6 GPa melt models. An alternative mechanism that causes depletion in Ti and enrichment in Si is therefore required. Bulk MgO/SiO₂ data for peridotite xenoliths are reconstructed from EPMA analyses of minerals based on this work (Supplementary Data 1); Gibson et al. and whole-rock XRF analyses (Simon et al.). Bulk xenolith Ti (µg/g) contents are also reconstructed, but from LA-ICP-MS analyses, which are at a higher precision than those from EPMA. The highest MgO/SiO₂ (1.18) in our sample suite from the Kaapvaal craton occurs in a metasomatised dunite (BD2153).

Fig. 9. Comparison of MORB-source normalised incompatible trace and volatile element patterns in orthopyroxene-rich garnet peridotites from the Kaapvaal craton (this work and ref. ) with potential sources of excess silica.

The compositions of the orthopyroxene-rich garnet harzburgites are compared with: a mean composition of Al-depleted and Al-enriched komatiites^,,; b subduction-related supercritical fluids generated in high-pressure experiments; and c fluids with a similar composition to those found in diamond inclusions. The calculated concentrations of a 50%:50% mix of depleted melt residue and Al-depleted komatiite are shown as a green dashed line on (a). (b) and (c) also show the calculated composition of a metasomatic agent in equilibrium with garnet in orthopyroxene-rich garnet harzburgite BD2125. This assumes that 30% of the incompatible trace element content of BD2125 garnets is derived from the metasomatic agent, consistent with mass balance calculations of the amount of excess silica (see text for discussion). Elements increase in incompatibility during mantle melting from right to left. Data are from ref. unless stated otherwise in the legend.

Fig. 10. Schematic illustration of the formation of excess orthopyroxene in Archaean cratons.

Hashed region illustrates where orthopyroxene-rich garnet harzburgites are most likely to form in relation to subduction zones. Devolatilisation of carbonated pelite, altered oceanic crust or serpentinite at subduction zones—via aqueous fluids, supercritical fluids or carbonated-silicate melts—provides the primary flux of C, H and halogens (F, Cl) into the convecting mantle^,–. Fluxing of the carbonated pelites by fluids from underlying basaltic crust (eclogites) and serpentinites may lead to the mobilisation, ascent and reactive infiltration of Si, Mg, C, F, H₂O and Cl in the overlying lithospheric mantle to form excess orthopyroxene and volatile enrichment. Since the Archaean, some of this signature may have been overprinted by infiltrating small-fraction volatile-rich melts from the convecting mantle, especially at the base of the lithosphere. The diamond-graphite boundary is from ref. .