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International audience; Rifting of the Red Sea resulted in the formation of one of the highest escarpments on our planet: the Great Escarpment of southwestern Saudi Arabia. Published low-temperature geochronology ages are around the Paleogene-Neogene transition, but geomorphic features like the height, the steepness and the preservation of the escarpment may indicate a younger aspect to the exhumation history. Here we use apatite fission track (AFT) and (U-Th-Sm)/He (AHe) cooling ages in combination with a geomorphic analysis of the region from Jeddah to Jizan to test this idea. Pooled AFT ages range from to Ma (1σ). The base of the AFT partial annealing zone prior to rock uplift is at ∼200 m elevation in most of the studied area, but track length data show that the amount of exhumation is insufficient to affect all coastal plain samples. Consistent with earlier estimates in the literature, the total amount of rock uplift is estimated to be ∼4 km. Single grain AHe ages range from to Ma (1σ) and do not have a clear trend with distance from the escarpment. Overall, the distribution of data appears to indicate widespread downwearing of the elevated topography around the rift flank. In contrast, geomorphic analysis that indicates escarpment retreat including a flexural uplift response. We suggest that the conflict between geomorphological and geochronological data can be resolved by invoking a two-stage erosion model. The first stage involved dome-shaped rift flank uplift since initiation of the Afar plume, downwearing of this topography and cooling recorded by the AFT ages. The second stage involved the massive erosion that excavated the present day Saudi escarpment and reset the AHe ages. We suggest that this second stage is related to the formation of oceanic lithosphere since 13 Ma in the Red Sea, associated downwarping of the margin and flexural updoming of the eastern rift flank. The consequential orographic precipitation initiated asymmetric erosion and formation of a retreating escarpment. This model is consistent with the large age difference between Miocene AFT and Pliocene AHe ages and the geomorphic metrics.

Mantle plumes have been proposed to exert a first-order control on the morphology of Earth's surface. However, there is little consensus on the lifespan of the convectively supported topography. Here, we focus on the Cenozoic uplift and exhumation history of the British Isles. While uplift in the absence of major regional tectonic activity has long been documented, the causative mechanism is highly controversial, and direct exhumation estimates are hindered by the near-complete absence of onshore post-Cretaceous sediments (outside Northern Ireland) and the truncated stratigraphic record of many offshore basins. Two main hypotheses have been developed by previous studies: epeirogenic exhumation driven by the proto-Iceland plume, or multiple phases of Cenozoic compression driven by far-field stresses. Here, we present a new thermochronological dataset comprising 43 apatite fission track (AFT) and 102 (U–Th–Sm)/He (AHe) dates from the onshore British Isles. Inverse modelling of vertical sample profiles allows us to define well-constrained regional cooling histories. Crucially, during the Paleocene, the thermal history models show that a rapid exhumation pulse (1–2.5 km) occurred, focused on the Irish Sea. Exhumation is greatest in the north of the Irish Sea region, and decreases in intensity to the south and west. The spatial pattern of Paleocene exhumation is in agreement with the extent of magmatic underplating inferred from geophysical studies, and the timing of uplift and exhumation is synchronous with emplacement of the plume-related British and Irish Paleogene Igneous Province (BIPIP). Prior to the Paleocene exhumation pulse, the Mesozoic onshore exhumation pulse is mainly linked to the uplift and erosion of the hinterland during the complex and long-lived rifting history of the neighbouring offshore basins. The extent of Neogene exhumation is difficult to constrain due to the poor sensitivity of the AHe and AFT systems at low temperatures. We conclude that the Cenozoic topographic evolution of the British Isles is the result of plume-driven uplift and exhumation, with inversion under compressive stress playing a secondary role.

The isotopic composition of the noble gases, in particular Ar, in samples of ancient atmosphere trapped in rocks and minerals provides the strongest constraints on the timing and rate of Earth atmosphere formation by degassing of the Earth's interior. We have re-measured the isotopic composition of argon in the Rhynie chert from northeast Scotland using a high precision mass spectrometer in an effort to provide constraints on the composition of Devonian atmosphere. Irradiated chert samples yield 40Ar/36Ar ratios that are often below the modern atmosphere value. The data define a 40Ar/36Ar value of 289.5 ± 0.4 at K/36Ar = 0. Similarly low 40Ar/36Ar are measured in un-irradiated chert samples. The simplest explanation for the low 40Ar/36Ar is the preservation of Devonian atmosphere-derived Ar in the chert, with the intercept value in 40Ar–39Ar–36Ar space representing an upper limit. In this case the Earth's atmosphere has accumulated only 3% ( 5.1 ± 0.4 × 10 16 mol ) of the total 40Ar inventory since the Devonian. The average accumulation rate of 1.27 ± 0.09 × 10 8 mol 40Ar/yr overlaps the rate over the last 800 kyr. This implies that there has been no resolvable temporal change in the outgassing rate of the Earth since the mid-Palaeozoic despite the likely episodicity of Ar degassing from the continental crust. Incorporating the new Devonian atmosphere 40Ar/36Ar into the Earth degassing model of Pujol et al. (2013) provides the most precise constraints on atmosphere formation so far. The atmosphere formed in the first ∼100 Ma after initial accretion during a catastrophic degassing episode. A significant volume of 40Ar did not start to accumulate in the atmosphere until after 4 Ga which implies that stable K-rich continental crust did not develop until this time.

An ankaramite lava from Fogo, Cape Verdes, has been used to determine the long-term cosmogenic 3He (3Hecos) production rate at low latitude (14.9°N). 40Ar/39Ar incremental heating yield an eruption age of 125.8±7 ka. Cosmogenic 3He abundances yield local production rates of 75.6±7.0 atoms/g/year in olivine (1σ, n=10) and 70.1±2.9 atoms/g/yr in pyroxene (1σ, n=11). The lower 3Hecos production rate in pyroxene is consistent with compositional control on production. When scaled to sea level high latitude using available algorithms, the olivine 3Hecos production rate ranges from 86–109 atoms/g/yr. These values overlap the long-term production rate determined at the Canary Islands (99 atoms/g/yr). They are, however, lower than previously reported long-term production rates from Hawaii and southern South America (129–152 atoms/g/yr), and the range of

Andesite magmatism at convergent margins is essential for the differentiation of silicate Earth, but no consensus exists as to andesite petrogenesis. Models proposing origin of primary andesite melts from mantle and/or slab materials remain in deadlock with the seemingly irrefutable petrographic and chemical evidence for andesite formation through mixing of basaltic mantle melts with silicic components from the overlying crust. Here we use 3He/4He ratios of high-Ni olivines to demonstrate the mantle origin of basaltic to andesitic arc magmas in the central Mexican Volcanic Belt (MVB) that is constructed on ~ 50 km thick continental crust. We propose that the central MVB arc magmas are hybrids of high-Mg# > 70 basaltic and dacitic initial mantle melts which were produced by melting of a peridotite subarc mantle interspersed with silica-deficient and silica-excess pyroxenite veins. These veins formed by infiltration of reactive silicic components from the subducting slab. Partial melts from pyroxenites, and minor component melts from peridotite, mix in variable proportions to produce high-Mg# basaltic, andesitic and dacitic magmas. Moderate fractional crystallization and recharge melt mixing in the overlying crust produces then the lower-Mg# magmas erupted. Our model accounts for the contrast between the arc-typical SiO2 variability at a given Mg# and the strong correlation between major element oxides SiO2, MgO and FeO which is not reproduced by mantle–crust mixing models. Our data further indicate that viscous high-silica mantle magmas may preferentially be emplaced as intrusive silicic plutonic rocks in the crust rather than erupt. Ultimately, our results imply a stronger turnover of slab and mantle materials in subduction zones with a negligible, or lesser dilution, by materials from the overlying crust.

A detailed study of the geochemistry of a new suite of early Iceland plume picrites shows that extremely high 3He/4He ratios (up to 50 Ra) are found in picrites from Baffin Island and West Greenland. High 3He/4He picrites display a wide range in 87Sr/86Sr (0.70288–0.70403), 143Nd/144Nd (0.51288–0.51308) and incompatible trace element ratios (e.g. La/Smn = 0.5–1.6). These overlap the complete range of compositions of mid-ocean ridge basalts and most northern hemisphere ocean island basalts, including Iceland. Crustal contamination modelling in which high-grade Proterozoic crustal basement rocks for the region are mixed with a depleted parent cannot account for the trend displayed by the Baffin Island and West Greenland picrites. This rules out the possibility that the incompatible trace element, Sr and Nd isotope range of the high 3He/4He picrites is due to crustal contamination. The compositional range at high 3He/4He is also inconsistent with derivation from a primordial-He-rich reservoir that is a residue of ancient mantle depletion. This implies that the composition of the high 3He/4He mantle cannot be determined simply by extrapolating ocean island basalt He–Sr–Nd–Pb–Os isotope data. The apparent decoupling of He from trace element and lithophile radiogenic isotope tracers is difficult to attain by simple mixing of a high-[He], high 3He/4He reservoir with various depleted and enriched He-poor mantle reservoirs. The possibility that primordial He has diffused into a reservoir with a composition typical of convecting upper mantle cannot be ruled out. If so, the process must have occurred after the development of existing mantle heterogeneity, and requires the existence of a deep, primordial He-rich reservoir.

Apatite (U–Th)/He and fission track data from profiles in western Scotland constrain the timing and magnitude of denudation during the early Cenozoic when the north Atlantic region was the site of intense magmatic activity related to the proto-Icelandic plume. Apatite helium ages vary from 77 ± 8 to 265 ± 27 Ma (± 2σ) at Sgorr Dhonuill, Ballachulish, and from 104 ± 10 Ma to 166 ± 17 Ma at Clisham, Outer Hebrides. At both locations apatite fission track (AFT) ages are older than the corresponding He ages; at Clisham they vary from 189 ± 28 Ma to 242 ± 26 Ma, and from 186 ± 6 Ma to 257 ± 12 Ma at Sgorr Dhonuill. Apatite He ages increase linearly with elevation suggesting that the cooling rate remained constant in the late Mesozoic. However, the apatite He age profile requires a period of rapid cooling after ∼ 100 Ma. Apatite He ages predicted from the AFT-derived thermal histories are indistinguishable from measured He ages for a rapid cooling event of 1 to 10 Myr duration between 61 and 47 Ma at Sgorr Dhonuill and 65 to 49 Ma at Clisham. The combined apatite FT- and He-derived thermal histories constrain the early Cenozoic geothermal gradient at 39 ± 9 °C/km at Sgorr Dhonuill and 19 ± 6 °C/km at Clisham. Amounts of denudation related to the rapid cooling event vary from 1330 ± 230 m at Sgorr Dhonuill to 2250 ± 750 m at Clisham, in agreement with models that predict greater amounts of denudation where magmatic underplating is thicker. However, the direct correlation between underplating-driven surface uplift and denudation may only be apparent and a more complex link between spatial variation of surface uplift and denudation is suggested. The integration of results from multiple low-temperature thermochronometers, combined with inverse and forward modelling, provides a convincing and quantitative method to deduce onshore erosional histories, and provides critical information about the spatial distribution of erosion that cannot be derived from the offshore record.

The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr–Nd–Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra

Accurate determination of cosmogenic 3He concentrations in minerals has to account for nucleogenic 3He produced from the reaction of thermalized neutrons with 6Li. Thermal neutrons from the cosmic ray cascade are typically not considered. However, in exposed basalts and granites the flux of cosmogenic thermal neutrons (CTN) is generally 10–200 times higher than the thermal neutron flux produced by (α,n) reactions in the rock, with the absolute values dependent on composition and geographic location. The reaction of CTN with Li can account for 5–50% of the cosmogenic 3He in minerals in typical granitiod rocks, depending on the Li content and distribution, and petrography of the rock, and may be considerably higher in special cases. In basalts this contribution is usually lower (1.5–6%) but may still warrant consideration. The strong depth-dependence of CTN-produced 3He in rock may be used to assess erosion of landforms, similar to CTN-produced 36Cl; however, as 3He is stable it has the potential to extend the useful time range to older surfaces.

Apatite (U–Th)/He and fission-track thermochronometers are combined to constrain models of denudation and escarpment development of the high elevation Eritrean margin along the southern Red Sea. (U–Th)/He ages have been determined for apatites from two coast-perpendicular transects that were previously used for apatite fission-track analysis [E. Abbate, M.L. Balestrieri, G. Bigazzi, Morphostructural development of the Eritrean rift flank (southern Red Sea) inferred from apatite fission track analysis, J. Geophys. Res., 107, B11, doi: 10.1029/2001JB001009, (2002)]. In central Eritrea near Asmara, He ages increase from c. 7 Ma at the coast to 152 Ma on the plateau along a transect. Further south, He ages from the margin border to the base of the escarpment span a narrower range (7 to 12 Ma). One sample from the top of a marginal high (2950 m) yields older ages (22–43 Ma). Forward modeling suggests that the He age distribution across the margin should allow scarp retreat and downwearing mechanisms to be distinguished in the case of a margin formed in the last twenty million years. The distribution of He ages suggests that the escarpment evolved by downwearing and that post-break up erosion of the escarpment was facilitated by in situ excavation rather than parallel retreat. This implies the existence of an inland drainage divide located seaward of the present day escarpment rim. We envisage that the inland divide formed during or immediately after continent break up, and that the eastern marginal belt represents a remnant of the apex structure. The short distance between the eastern marginal belt and the rim (c. 10 km) represents the total retreat of the escarpment. Comparison of the measured He ages from the coastal plain with those predicted from forward modelling indicates that the main phase of post-break up erosion started at about 15 Ma, closer to the initiation of seafloor spreading in the Red Sea than was thought previously.

In situ produced cosmogenic 3 He analyses provide independent support for the model of a stable, hyper-arid polar climate persisting in East Antarctica since the mid-Miocene and provide quantitative constraints on long-term rates of erosion within the Dry Valleys. In the Coombs Hills area, a series of cobble-size boulders form mega-ripples with wavelengths of approximately 50 m. Their topographic position and association with features characteristic of scabland, such as stripped, corrugated bedrock surfaces, indicate the boulders were deposited by subglacial floodwaters. Such outburst flooding could only have occurred during overriding of the northern Dry Valleys by a greatly expanded East Antarctic ice sheet. Timing of the overriding episode has been previously assigned to 14.8 to 13.6 Ma by correlation with volcanic ash deposits dated by 40 Ar/ 39 Ar in the Asgard Range of the Dry Valleys. Cosmogenic 3 He concentrations in clinopyroxene from Ferrar dolerite boulders are consistent with 8.6 to 10.4 Ma exposure, calculated using scaling factors appropriate for Antarctica and assuming zero erosion. These are among the oldest surface exposure dates yet measured on Earth, but are not however consistent with the 40 Ar/ 39 Ar chronology used to define the age of the landscape due to unconstrained levels of erosion. Erosion rates of 0.03–0.06 m Ma −1 are necessary to have produced the measured boulder exposure age if they were deposited at 14.8 Ma. These are less than half the steady-state erosion rate derived from cosmogenic 3 He in the nearby bedrock surfaces (0.17 m Ma −1 ) and testify to the extreme stability of the landscape.

European Social Fund - Scottish Universities - Carnegie Trust for the Universities of Scotland.

The southeast Australian margin, like other high elevation passive margins, is characterised by a steep escarpment that separates a dissected coastal plain from a low relief inland plateau. Quantitative constraints on the generation of escarpments can be provided by apatite (U–Th)/He ages. Here we use a coast-perpendicular traverse across the coastal lowlands, escarpment and plateau to test the three prevailing models of SE Australian escarpment formation, namely retreat into a downwarped rift shoulder, escarpment retreat and down-wearing on high elevation rift shoulder with flexural rebound. Apatites from the coastal plain have He ages of between 87 and 112 Ma, suggesting that the coastal lowlands developed very rapidly after rifting and continental break-up at 85–100 Ma. The He age data are inconsistent with the erosion of a downwarped rift margin, and cannot be explained by a constant post-break-up rate of lateral escarpment retreat across the coastal plain or by constant down-wearing. The data require either rapid escarpment retreat or rapid in-place excavation of the escarpment soon after break-up, in response to rifting and the lowering of base levels on the margin of the new continent at break-up, followed by a period of landscape stability and low erosion. Combined with the existing apatite fission track record, the He data are consistent with erosion of 3–4 km within a maximum of 28 Myr of break-up, at a minimum vertical erosion rate of 130 m Myr−1 along the coast. The rapid denudation period across the coastal plain in this region took less than 48 Myr (from the coast to the escarpment base), which corresponds to an average vertical erosion rate of 45 m Myr−1. This is equivalent to a mean escarpment retreat rate of 5–10 km Myr−1. Apatite He ages from the plateau (183–247 Ma) indicate that the highlands remained stable throughout continental break-up, experiencing average erosion rates of less than 10 m Myr−1 since the late Palaeozoic/early Mesozoic.

Helium isotopes have been measured in olivines from igneous rocks spanning the history of the Palaeocene volcanic centre at the Isle of Skye, northwest of Scotland. 3He/4He in olivine phenocrysts from basalts from the Skye Lava Field are up to 22 times the atmospheric ratio which is consistent with their origin from the proto-Iceland mantle plume despite being erupted 1300 km from the plume axis. The 3He-anomaly of the Palaeocene starting plume in the North Atlantic appears to have been up to three times greater, for the same distance from the plume axis, than the modern Iceland plume. The Pb isotope and trace element (ΔNb) composition of the least crustally contaminated Skye magmas resemble North Atlantic MORB rather than Iceland basalts. The proto-Iceland plume head appears to have been dominated by upper mantle material with a small deep mantle He contribution rather than undiluted deep mantle material as predicted in the classic plume model.

Combined laser extraction and manometric-quadrupole-mass spectroscopic measurements permit the simultaneous analysis of CO2, He and Ar in selected fluid inclusion generations trapped in a dunite nodule from Reunion Island. Variations in 40Ar36Ar in primary fluid inclusions (1000–8500) implies that atmospheric Ar is present in all the trapped fluids, possibly introduced by percolation of seawater into the magma from which the olivine crystallised. Later inclusion generations have lower 40Ar36Ar (500–3500). CO23He ratios ((1–9.6) × 109) are at the lower range of reported values from high 3He4He xenoliths ((1–20) × 109) but show no systematic variation with inclusion generation. 4He40Ar∗ values are constant (2.4 ± 0.1) for all inclusion generations and over wide ranges in CO2He ((0.2–8.2) × 105). This is not consistent with the relative solubilities of CO2, He and Ar in basaltic melts but implies that ƒCO2 is determined by some other process, possibly the partitioning of CO2 into an immiscible phase. Mixing between high 3He4He, high He abundance mantle (primitive He mantle) with a degassed and incompatible element enriched component can generate the coupled high 3He4He-radiogenic Sr and Pb isotopic compositions of Reunion lavas. The range in C3He likely for the primitive He mantle ((1–20) × 109) spans the estimated C3He of chondrites (≈ 2 × 109). The primitive He component cannot represent undegassed primitive mantle as the C36Ar values of Reunion (> 1 × 109) are higher than those of chondrites (8 × 107).