Wehrlites from continental mantle monitor the passage and degassing of carbonated melts

Continental rifting has been linked to the thinning and destruction of cratonic lithosphere and to the release of enough CO2 to impact the global climate. This fundamental plate tectonic process facilitates the infiltration and mobilisation of smallvolume carbonated melts, which may interact with mantle peridotite to form wehrlite through the reaction: enstatite thorn dolomite (melt) = forsterite thorn diopside thorn CO2 (vapour). Application to mantle xenolith suites from various rifts and basins shows that 2.9 to 10.2 kg CO2 are released per 100 kg of wehrlite formed. For the Eastern Rift (Africa), this results in estimated CO2 fluxes of 6.5 +/- 4.1 Mt yr(-1), similar to estimates of mantle contributions based on surficial CO2 surveys. Thus, wehrlite-bearing xenolith suites can be used to monitor present and past CO2 mobility through the continental lithosphere, ultimately with diffuse degassing to the atmosphere. They may also reveal the CO2 flux in lithospheric provinces where carbonated melts or continent-scale rifts are not observed at the surface.This work and collaborationwere stimulated by an invitation to SA and GMY to present at the Deep Carbon Observatory’s Deep Carbon 2019: Launching the Next Decade of Deep Carbon Science meeting in Washington DC (USA), and by an Alexander von Humboldt Fellowship to GMY, which we gratefully acknowledge. It was written while SA was funded through German Research Foundation fellowship AU356/11

The discovery of kimberlites in antarctica extends the vast gondwanan cretaceous province

Kimberlites are a volumetrically minor component of the Earth's volcanic record, but are very important as the major commercial source of diamonds and as the deepest samples of the Earth's mantle. They were predominantly emplaced from ≈2,100 Ma to ≈1

Evidence of sub-arc mantle oxidation by sulphur and carbon

The oxygen fugacity (ƒO2) of the Earth’s mantle at subduction zones exerts a primary control on the genesis of mineral deposits in the overlying magmatic arcs and on speciation of volcanic gases emitted into the atmosphere. However, the processes governing mantle ƒO2 such as the introduction of oxidised material by subduction are still unresolved. Here, we present evidence for the reduction of oxidised fluid-borne sulphur and carbon during alteration of depleted mantle by slab fluids at ultra-high pressure in the Bardane peridotite (Western Gneiss Region, Norway). Elevated ferric iron in metasomatic garnet, determined using synchrotron X-ray absorption near edge structure (XANES) spectroscopy, indicates that this process drove oxidation of the silicate assemblage. Our finding indicates that subduction oxidises the Earth’s mantle by cycling of sulphur and carbon.This research was funded by a Monash Research Accelerator Grant to A. Tomkins and by a Society of Economic Geologists (SEG) student research grant to A. Rielli. O. Nebel was supported through an ARC DECRA fellowship (DE120100513)

Revealing Repton: bringing landscape to life at Sheringham Park

The year 2012 marked 200 years since Humphry Repton (1752–1818) produced his design for Sheringham Park in north Norfolk, bound as one of his Red Books. On paper, Repton is England’s best-known and most influential landscape gardener. On the ground, his work is much harder to identify, focused as it was on light touches that equated more to landscape makeover than the landscape making of his predecessor Lancelot “Capability” Brown. This paper documents and evaluates a project that celebrated this bicentenary through a temporary exhibition within the visitor centre of Sheringham Park, whilst also making reference to the commemoration of his work in other places and on paper. In attempting to reveal Repton at Sheringham, we explore the context of the 1812 commission and the longer landscape history of the site, as well as the different methods of representing Repton on site that are open to site owners and managers

Detrital apatite geochemistry and its application in provenance studies

Single-grain, laser-ablation inductively coupled plasma-mass spectrometry analyses of detrital apatites from Pliocene sandstones in the South Caspian Basin (Azerbaijan) and Devonian-Carboniferous sandstones from Clair oil field, west of Shetland (UK), demonstrate that apatite geochemistry has significant potential in provenance analysis. Apatites in Pliocene sandstones deposited by the paleo-Kura River system, which drained the Lesser Caucasus region, were derived largely from mafic to intermediate and alkaline rocks. Apatite populations in Pliocene sediments transported by the paleo-Volga River system, which drained the Russian Platform, show greater compositional diversity and indicate supply from granitoids or other acidic rocks together with subordinate mafic to intermediate and alkaline rocks. Apatites in the Devonian-Carboniferous succession west of Britain were derived predominantly from acidic rocks, either directly from Archean gneisses or indirectly from metasedimentary rocks. In the two case studies, the most useful discriminators of apatite provenance proved to be La/Nd and La + Ce/ΣREE. Since apatite is stable during burial in sedimentary basins, apatite geochemistry can be used to determine provenance of sandstones from the full range of diagenetic environments, although the instability of apatite during weathering means that the method will be difficult to apply to sandstones with prolonged weathering history. At present, identification of provenance using apatite geochemistry is limited by the lack of a comprehensive database on apatite compositions in some of the potential source rocks, particularly those of metamorphic origin. The role played by sediment recycling is another factor that requires consideration when reconstructing source areas on the basis of apatite compositions

High-pressure partial melting of gabbro and its role in the Hawaiian magma source

We have conducted high-pressure experiments on a natural oceanic gabbro composition (Gb108). Our aim was to test recent proposals that Sr-enrichment in rare primitive melt inclusions from Mauna Loa, Hawaii, may have resulted from melting of garnet pyroxenite formed in the magma source regions by reaction of peridotite with siliceous, Sr-enriched partial melts of eclogite of gabbroic composition. Gb108 is a natural, Sr-enriched olivine gabbro, which has a strong positive Sr anomaly superimposed on an overall depleted incompatible trace element pattern, reflecting its origin as a plagioclase-rich cumulate. At high pressures it crystallises as a coesite eclogite assemblage, with the solidus between 1,300 and 1,350°C at 3.5 GPa and 1,450 and 1,500°C at 4.5 GPa. Clinopyroxenes contain 4-9% Ca-eskolaite component, which varies systematically with pressure and temperature. Garnets are almandine and grossular-rich. Low degree partial melts are highly siliceous in composition, resembling dacites. Coesite is eliminated between 50 and 100°C above the solidus. The whole-rock Sr-enrichment is primarily hosted by clinopyroxene. This phase dominates the mode (>75 wt%) at all investigated PT conditions, and is the major contributor to partial melts of this eclogite composition. Hence the partial melts have trace element patterns sub-parallel to those of clinopyroxene with ≈10× greater overall abundances and with strong positive Sr anomalies. Recent studies of primitive Hawaiian volcanics have suggested the incorporation into their source regions of eclogite, formerly gabbroic material recycled through the mantle at subduction zones. The models suggest that formerly gabbroic material, present as eclogite in the Hawaiian plume, partially melted earlier than surrounding peridotite (i.e. at higher pressure) because of the lower solidus temperature of eclogite compared with peridotite. This produced highly siliceous melts which reacted with surrounding peridotite producing hybrid pyroxene + garnet lithologies. The Sr-enriched nature of the formerly plagioclase-rich gabbro was present in the siliceous partial melts, as demonstrated by these experiments, and was transferred to the reactive pyroxenite. These in turn partially melted, producing Sr-enriched picritic liquids which mixed with normal picritic partial melts of peridotite before eruption. On rare occasions these mixed, relatively Sr-rich melts were trapped as melt inclusions in primitive olivine phenocrysts

Phase relations of carbonate-bearing eclogite assemblages from 2.5 to 5.5 GPa: implications for petrogenesis of carbonatites

We have experimentally investigated the phase and melting relations of garnet + clinopyroxene + carbonate assemblages at 2.5-5.5 GPa, to assess the feasibility of carbonated eclogite as a source for some crustally emplaced carbonatites. The solidus of our composition was at ≈1,125 °C at 2.5 GPa, ≈1,225 °C at 3.5 GPa and ≈1,310 °C at 5.0 GPa. Melts were sodic calcio-dolomitic carbonatites, and were markedly more calcic than the dolomitic melts produced by partial melting of carbonated peridotite. Na contents of the experimental carbonatites decreased with increasing pressure when compared at similar degrees of melting, and SiO2 contents increased with degree of melting. Experiments on a second composition with enhanced Na2O demonstrated its strong effect in lowering melting temperatures in carbonate eclogite. Natural carbonated eclogite bodies in the peridotitic upper mantle will have a range of solidus temperatures. In many cases, carbonate will be molten in the upper ≥250 km. Carbonate melt would segregate from its source eclogite at very low melt fractions and infiltrate surrounding peridotitic wall rock. This would result in metasomatic enrichment of the peridotitic wall rock, but its exact nature will depend on the relative P-T positions of the eclogite + CO2 and peridotite + CO2 solidii. As a result of these inevitable metasomatic interactions, it is considered unlikely that carbonatite melts derived from carbonated eclogite in the upper mantle could be emplaced into the crust unmodified. However, they may have a role in metasomatically enriching and carbonating parts of the upper mantle, producing sources suitable for subsequent production of silica undersaturated silicate liquids and carbonatites ultimately emplaced in the crust

In situ origin for glass in mantle xenoliths from southeastern Australia: insights from trace element compositions of glasses and metasomatic phases

Siliceous, aluminous and alkali-rich glasses, commonly found in patches and veins in spinel peridotite xenoliths, have been attributed to a number of different origins. These include low-degree primary melts of the mantle, exotic metasomatic melts influxing into the lithosphere, or breakdown of amphibole, and other phases during high-temperature transport of the xenoliths to the surface in their host magmas. We present new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of trace element abundances in glasses, and in metasomatically introduced phases (clinopyroxene, amphibole, phlogopite, apatite) from a suite of spinel wehrlite, Iherzolite and harzburgite xenoliths from southeastern Australia. The majority of glass compositions are best explained by melting of amphibole (usually complete, as amphibole is now absent from most samples) with varying but significant contributions from partial melting of clinopyroxene. However, some glasses require additional components derived from partial or complete modal melting of phlogopite, or apatite. The data confirm our earlier model, that the glass present in patches in these samples derives from high-temperature, transport-related breakdown of a metasomatic phase assemblage (amphibole + clinopyroxene ± phlogopite ± apatite) present in the xenoliths prior to their entrainment in the host magmas

The role of detrital zircons in Hadean crustal research

Meso-Archean sedimentary sequences at Mt. Narryer and the Jack Hills of the Narryer Terrane in Western Australia's Yilgarn Craton contain detrital zircon grains with ages as old as 4.37. Ga, the oldest preserved terrestrial matter. These grains are rare remnants of Hadean (4.5-4.0. Ga) terrestrial crust and their survival stems from the crystallographic properties of zircon during crustal reworking: they are resistant to physical and chemical weathering. Zircons are further suitable for single grain, precise age determinations making them a unique archive of the crustal past. Only a small proportion of all detrital zircons from the Narryer Terrane show Hadean age spectra and younger overgrowth rims on all 'Hadean' grains indicate multiple recycling events. Numerous studies that applied a spectacular range of analytical tools and proxies have been undertaken to decipher the geochemical nature of these zircons' host rocks, in order to place constraints on Hadean geodynamics and the processes responsible for creating the earliest terrestrial crust. Their elemental and isotope budget and mineral inclusions have helped to develop an emerging picture of a water-rich, evolved Hadean crust. However, subsequent studies have challenged this view and it seems that each piece of new evidence indicative of an early, evolved continental crust has non-unique interpretations also permissive of mafic to ultra-mafic crust. In this review we examine these disparate interpretations and their possible implications and conclude that at least parts of the earliest terrestrial crust were hydrated. However, to date there is no conclusive evidence for preserved granitic, continental crust. The protoliths of the Hadean detrital zircons were likely acidic in nature, yet the composition of the greater terrane from which these melts were derived was probably mafic. It remains unclear if the zircons formed in a geodynamic environment that includes Hadean subduction. We suspect that the Hadean crust was an initially homogeneous, thin, mafic layer. It was spiked with minor, low-degree, anatectic melts of granitoid composition formed from material that formerly resided at the surface and was subsequently buried. The process responsible for this was likely sag-subduction triggered by repeated volcanic resurfacing, possibly fed by early mantle plumes. Regional scale granitoid plutonism of the tonalite-trondhjemite-granodiorite suite (TTG) predominates granitoid-generating processes in the Eo-Archean, along with the first appearance of low-Ca (s-type) granites at around 3.9. Ga, evidenced by the first occurrence of detrital monazite in the Narryer Terrane. This coincides with the first addition of juvenile crust as documented by the global detrital zircon record and temperature signatures of the late heavy bombardment in Narryer Terrane zircons. This age probably marks the onset of Archean-style tectonics, likely associated with subduction activity, which lasted until ~. 3. Ga, when modern style plate tectonics emerged