In the past Raphaël Pik has collaborated on articles with Luc Bastian and Pierre-Henri Blard. One of their most recent publications is Helium isotopic signature of modern and fossil fluids associated with the Corinth rift fault zone (Greece): Implication for fault connectivity in the lower crust. Which was published in journal Chemical Geology.

More information about Raphaël Pik research including statistics on their citations can be found on their Copernicus Academic profile page.

Raphaël Pik's Articles: (6)

Helium isotopic signature of modern and fossil fluids associated with the Corinth rift fault zone (Greece): Implication for fault connectivity in the lower crust

AbstractHelium isotopic ratios of modern groundwaters and of fossil fluids trapped in secondary calcites from fault planes have been measured along the southern Corinth rift in order to investigate the nature of fluids circulating in these fault zones and to set constraints on tectonic processes responsible for this extension. The observed isotopic ratios of modern fluids range from the air composition to moderately radiogenic compositions (0.15–0.4 Ra, with Ra being the atmospheric 3He/4He ratio), whereas fossil fluids display more radiogenic compositions (0.04 to 0.15 Ra), suggesting that they have been trapped deeper in the fault zone in the absence of significant atmospheric contamination. These compositions result from mixing of a deep radiogenic and hot fluid with cold and shallow air-saturated aquifer waters.The absence of a mantle-He signal in the Corinth rift fluids is remarkable and evidences the lack of active volcanic fluids in the basin. It further suggests that the fault system is not connected at depth with zones where mantle-He might have been trapped. At the scale of the Corinth rift, this observation suggests that faults are rooted in the upper crust. If extension results from displacement of the crust above a north-dipping shear zone, then this shear zone should be relayed at depth through the lower crust by impermeable ductile deformation.At a larger scale, it appears that basins in the western side of the Aegean host crustal helium whereas basins in the Anatolian side present systematically mantle He. This He isotope distribution is difficult to explain by symmetrical tectonic evolution of the Aegean domain, and could instead be related to the recent propagation of the North Anatolian Fault (NAF). Propagation of the NAF may have recently disturbed hydrothermal circulations established after Miocene extension, by progressively reactivating the western basins with a different style of tectonic deformation, re-localising most of the fault controlled flow-pathway in shallower basins and as a consequence displacing the fluids mixture toward a crustal origin.

Chemical erosion rates in the upper Blue Nile Basin and related atmospheric CO2 consumption

AbstractSilicate weathering of basaltic rocks constitutes a non-negligible sink of atmospheric CO2 but the role it plays in the regulation of past and future global climate is still matter of debate. In this study, silicate weathering rates for various sub-basins of the Ethiopian Traps, emplaced 30 million years ago, and the corresponding atmospheric CO2 consumption rates are evaluated. For this, major and trace elements were measured in the dissolved phases and in the sedimentary particles carried and deposited by the main rivers flowing through this steep region. Lithium isotopes and major elements were also measured in the extracted clay fractions in order to infer complementary information on weathering processes in this region. Clay δ7Li values correlate positively with Mg/Ti ratios, and are best explained by varying ratios of leaching versus clay formation rate.Although located in a region annually submitted to monsoon, average silicate weathering rate (16.1 tons/km2/year) and CO2 consumption rate (0.65 × 1012 mol/year) are estimated to be low when compared to other basaltic regions such as the Deccan Traps, and volcanically active islands of the tropical zone. This is surprising since the concentrations of Total Dissolved Solids of the Ethiopian rivers are among the highest ones. With a 2D rainfall model that takes into account the detailed topography of the region, annual occurrence of the Monsoon, and monitoring station data, we show that runoff intensity is a key parameter that explains this difference. We determine that, at present, the weathering of the Ethiopian Traps plays a negligible role in the carbon cycle. However, simple calculations, which integrate recent knowledge on African climate variations and on weathering controls, illustrate that during the African Humid Period (14–8 kyr), a significant increase of Monsoon precipitation may have resulted in much higher weathering rates and related CO2 consumption (0.91–1.5 × 1012 mol/year). This study therefore evidences the potential importance of this region in the past, and the need to quantify more precisely the variations of the monsoon intensity and its impact on tropical watersheds for reconstructing past CO2 levels.

Fossil cosmogenic 3He record from K–Ar dated basaltic flows of Mount Etna volcano (Sicily, 38°N): Evaluation of a new paleoaltimeter

AbstractThis study investigates the reliability of a paleoaltimeter based on the elevation dependency of the cosmogenic nuclides production rate. The proposed method relies on “fossil” cosmogenic record. Indeed, measuring the cosmogenic nuclide concentration in an exposed and then buried lava flow combined with an independent estimate (by K–Ar or 40Ar / 39Ar dating) of the past exposure duration theoretically allows recording “fossil” cosmogenic production rates, which can thus be converted in paleoelevations. This approach was tested by measuring cosmogenic 3He (3Hec) in olivines and clinopyroxenes of Quaternary K–Ar dated basaltic flows of Mount Etna volcano (Sicily, 38°N). The agreement within uncertainties (1σ ≤ 500 m) between current and cosmogenic-derived elevations demonstrates the paleoaltimeter reliability. However, dating imprecision, Earth's paleomagnetic fluctuations and erosion contemporary to the flow exposure are parameters that may affect the accuracy and the precision of the method on older geological scales. Their respective influences were thus carefully evaluated and discussed to conclude that the paleoaltimeter can potentially reach resolutions better than ∼1000 m, even for Cenozoic records.

Production of 3He in crustal rocks by cosmogenic thermal neutrons

AbstractAccurate 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.

Original ArticlesIsotopic and trace element signatures of Ethiopian flood basalts: evidence for plume–lithosphere interactions

AbstractTrace element and radiogenic isotope data have been measured on Oligocene flood basalts from the northwestern Ethiopian plateau. Our aim was to investigate and identify the nature of mantle and crustal sources involved in the genesis of this huge volume of pre-rift basalts to constrain the interaction between the Afar mantle plume and the lithosphere at the onset of continental break-up. The three magma types previously identified on this plateau display contrasting geochemical signatures. The Low-Ti magma type (LT) basalts display a strong and variably developed lithospheric signature characterized by relative depletions in Nb, Ta, Th, and Rb and peaks at Ba and Pb compared to oceanic basalts. The High-Ti magma type basalts (HT2) display much more homogeneous compositions and have ocean island basalt-like trace element signatures, whereas HT1 basalts exhibit intermediate compositions between those of the two other groups. In contrast to the wide range of trace element compositions, Sr, Nd, and Pb isotope ratios display limited variations (87Sr/86Sr = 0.70304–0.70429; 143Nd/144Nd = 0.51271–0.51298; 206Pb/204Pb = 18.00–18.86). Correlations among isotopic and trace element ratios provide evidence for the involvement of various mantle and crustal components in the petrogenesis of these flood basalts. Two distinct mantle components are involved in the genesis of the LT and HT2 extreme magma types. The HT2 basalts were derived from an ocean island basalt-like mantle component (87Sr/86Sr ∼ 0.704; 143Nd/144Nd ∼ 0.51295; 206Pb/204Pb ∼ 18.8) that corresponds to the initial material of the Afar mantle plume. By contrast, the LT basalts result from the melting of a more depleted mantle component (87Sr/86Sr ∼ 0.7033; 143Nd/144Nd ∼ 0.5130; 206Pb/204Pb ∼ 18.6), either intrinsic to the plume itself or entrained in the Afar plume head during its ascent. Correlations of incompatible trace element and isotopic ratios with differentiation indices indicate that the more or less pronounced lithospheric signature of the Ethiopian flood basalts was acquired by crustal contamination of the magmas during their variable residence time in the lower and upper crust. The effects of crustal contamination are much more evident in the LT basalts because of their much less enriched initial characteristics.

The northwestern Ethiopian Plateau flood basalts: Classification and spatial distribution of magma types

AbstractThe extensive, complex, continental flood basalt (CFB) province which occurs in Ethiopia and Yemen consists of Oligocene prerift volcanism related to the Africa–Arabia continental break-up. Basalts from the northwestern Ethiopian Plateau exhibit a particularly large range of compositions and, for the first time in the Afro-Arabian CFB province, low-Ti basalts have been encountered. Major and some trace element data have been used to identify distinct geochemical groups and evaluate the role of differentiation processes. Three magma types have been distinguished: two high-Ti groups (HT1 and HT2) and one low-Ti group (LT). The transitional to tholeiitic LT suite exhibits low TiO2 (1–2.6%), Fe2O3* (10.5–14.8%), CaO/Al2O3 (0.4–0.75), Nb/La (0.55–0.85) and high SiO2 (47–51%). In contrast, the HT2 suite exhibits high TiO2 (2.6–5%), Fe2O3* (13.1–14.7%), CaO/Al2O3 (0.9–1.43), Nb/La (1.1–1.4) and low SiO2 (44–48.3%). The HT1 series is intermediate between the LT and HT2 groups. These three groups of lavas originated from different parental magmas. They display distinct differentiation trends, either controlled by the removal of a shallow level gabbroic (Pl+Ol+Cpx) assemblage (LT and HT1 suites) or by deeper Ol+Cpx fractionation (HT2 suite). Most of this thick continental flood lava pile was emplaced over a short time interval (about 1–2 Ma). The three contrasted magma types do not reflect a temporal evolution of their sources but rather a strong spatial control. Indeed, the northwestern Plateau may be subdivided into two different subprovinces as all the low-Ti basalts are located in the northern part of the plateau, and the high-Ti basalts are exposed in the eastern and southern parts. The LT and HT1 basalts display compositional ranges similar to those of the low- and high-Ti groups from other main CFB provinces (e.g. Parana, Deccan, Karoo, Siberia, …). However, the HT2 group exhibits extreme OIB-like compositions. This unusual geochemical signature suggests the involvement of deep mantle in the genesis of the HT2 magmas. The LT compositions rather reflect the participation of the continental lithosphere, through mantle derived melts and/or crustal contamination.

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