51 research outputs found
Picrite-basalt associated to Ethiopian-Yemeni CFB and their relevance to mantle plume processes
No full textThe Oligocene Northern Ethiopian-Yemeni LIP, represented by a CFB plateau extending ca. 700 km in diameter, is characterized by a well-defined zonal arrangement with increasing plume-related physico-chemical features of erupted magmas, such as thermal regime, incompatible element enrichment and specific Sr-Nd-Pb-He isotopic fingerprint, from the periphery to the central plateau area [1]. Two CFB volcanic piles in the Lalibela district (Northern Ethiopia, ca. 2 km thick) and in the Manakhah section (Northern Yemeni plateau, ca. 1 km thick) which erupted close to the Oligocene Afar plume axis, are similarly characterized by very high-Titransitional basalts and picrites (HT2, [1] [2]) that account for ca. 13% (40,000 km3) of the total Ethiopian-Yemeni CFB lavas. These magmas are characterized, in addition to the extremely high TiO2content (3-6 wt%) by a high MgO content (mostly between 8 and 18 wt%), and show striking compositional analogies with those from the Karoo province and the Siberian meimechites [3] [4]. Petrological modelling based on whole rock FeO-MgO and Ol composition [5] indicates that some of picrites (MgO 16-17 wt%) are near-primary magmas with olivine phenoscrysts up to Fo 90.4. Calculation shows that the primary melts have picrite composition MgO 19.8-20.7 wt% and were generated by polybaric melting in the pressure range 3-4 GPa at a potential temperature of 1570°C. Together with high-MgO lavas from Hawaii and Gorgona, these are the highest temperatures of any OIB and LIP lavas. The available data suggest that HT2 magma sources necessarily require the involvement of specific high-Ti (and Fe) deep-seated sublithospheric components which were entrained and remobilized by the rising plume. References: [1] Beccaluva et al. (2009), J. Petrol. 50, 1377-1403. [2] Beccaluva et al. (2011), GSA Sp. Paper 478, 77-104. [3] Ellam & Cox (1991), Earth Planet. Sci. Lett. 105, 330-342. [4] Heinonen et al.(2014), Earth Planet. Sci. Lett.394, 229-241. [5] Herzberg et al. (2007), Geochem. Geophys. Geosyst. 8, doi:10.1029GC001390
High-MgO lavas associated to CFB as indicators of plume-related thermochemical effects: the case of ultra-titaniferous picrite-basalt from the Northern Ethiopian-Yemeni plateau
No full textA comprehensive petrological and geochemical dataset is reported in order to define the thermo-compositional characteristics of Ti (Fe)-enriched picrite-basalt lavas (HT2, TiO2 3-7 wt%), erupted close to the axial zone of the inferred Afar mantle plume, at the centre of the originally continuous Ethiopian-Yemeni CFB plateau (ca. 30Ma) which is zonally arranged with progressively lower Ti basalts (HT1, TiO2 2-4 wt%; LT, TiO2 1-3 wt%) toward the periphery. Integrated petrogenetic modelling based on major and trace element analyses of bulk rocks, minerals and melt inclusions in olivines, as well as Sr-Nd-Pb-He-O isotope compositional variations enables us to make several conclusions. 1) The phase equilibria constraints indicate that HT2 primary picrites were generated at ca. 1570°C mantle potential temperatures (Tp) in the pressure range 4-5 GPa whereas the HT1 and LT primary melts formed at shallower level (< 2 to 3 GPa, Tp 1530 °C for HT1 and 1430°C for LT). Thus the Afar plume head was a thermally and compositionally zoned melting region with maximum excess temperatures of 300-350°C with respect to the ambient mantle. 2) The HT2 primary melts upwelled nearly adiabatically to the base of the continental crust (ca. 1 GPa) where fractionation of olivine, followed by clinopyroxene, led to variably differentiated picritic and basaltic magmas. 3) Trace element modelling requires that the primary HT2 melts were generated - either by fractional or batch melting (F 9-10%) - from a mixed garnet peridotite source (85%) with 15% eclogite (derived from transitional MORB protoliths included in Panafrican terranes) that has to be considered a specific Ti-Fe and incompatible element enriched component entrained by the Afar plume. 4) The LT, HT1 and HT2 lavas have 143Nd/144Nd = 0.5131-0.5128, whereas Sr-Pb isotopes are positively correlated with TiO2, varying from 87 Sr/86Sr 0.7032 and 206Pb/204Pb 18.2 in LT basalts to 87Sr/86Sr 0.7044 and 206Pb/204Pb 19.4 in HT2 picrite-basalts. High 3He/4He (15-20 RA) ratios are exclusively observed in HT2 lavas, confirming earlier evidence that these magmas require a component of deep mantle in addition to eclogite, while the LT basalts may more effectively reflect the signature of the pre-existing mantle domains. The comparison between high-MgO (13-22%) lavas from several Phanerozoic CFB provinces (Karoo, Paranà-Etendeka, Emeishan, Siberia, Deccan, North Atlantic Province) shows that they share extremely high mantle potential temperatures (Tp 1550-1700°C) supporting the view that hot mantle plumes are favoured candidates for triggering many LIPs. However, the high incompatible element and isotopic variability of these high-MgO lavas (and associated CFB) suggest that plume thermal anomalies are not necessarily accompanied by significant and specific chemical effects, which depend on the nature of mantle materials recycled during the plume rise, as well as by the extent of related mantle enrichments (if any) on the pre-existing lithospheric section
Fluid pressure cycles, variations in permeability, and weakening mechanisms along low-angle normal faults: the Tellaro detachment, Italy
No full textClassical frictional fault reactivation models indicate that slip along misoriented fault planes is not possible under most conditions. Nevertheless, active or exhumed low-angle normal faults have been described in many settings worldwide. This discrepancy is addressed by contrasting models: (1) those proposing that low-angle normal faults result from postkinematic passive rotation of former high-angle extensional faults; and (2) those proposing that specific conditions can promote slip along misoriented fault planes. This paper describes the Tellaro detachment, a mid–late Miocene low-angle normal fault that was responsible for ∼500 m of tectonic vertical thinning in the carbonate-dominated Triassic to Lower Miocene succession of the Northern Apennines, Italy. By integrating structural, petrographic, isotopic, and fluid inclusion data, we show that: (1) the main kinematic activity of the Tellaro detachment occurred between ∼8 and 4 km depths and peak temperature ∼190 °C; (2) dilational breccias, tens of cubic meters in volume, are frequently associated with major low-angle fault segments; (3) slip along misoriented planes was favored by elevated fluid pressures and low differential stress; and (4) the fault system was characterized by transient permeability pulses and overpressure buildups, associated with multiple fracturing and cementation events that caused the downward migration of master slip surfaces.
Results presented in this study show that: (1) in a fluid-active regime, continental crustal thinning can occur for shallow values of fault dip; (2) low-angle normal faults have a great influence on fluid circulation within the upper crust; and (3) episodic permeability enhancement and destruction in detachment faults can promote overpressure buildups, triggering deformation episodes
Evolution of the Earth's mantle-crust-atmosphere system from the trace element and isotope geochemistry of the plume-mantle reservoir
No full textThe 62 million year old lava flows of Baffin Island and West Greenland represent the
earliest phase of magmatism in the North Atlantic Igneous Province (NAIP). These
picritic lavas are characterised by high magnesium contents owing to their high
proportion of olivine crystals. The parental magmas for the picrites are likely to have
accumulated olivine crystals on their transit through the lithosphere and crust. Debate
over the origin of accumulated crystals in the lavas results in uncertainty in the
temperature and composition of the parental magmas for the early NAIP. The
magnesium-rich olivine crystals (up to Fo93) in the picrites of this study are shown
not to have a xenocrystic origin. The samples, therefore, support the inference of
high potential temperatures for the Baffin Island-West Greenland magmas, ~200oC
above ambient mantle.
The picrites of Baffin Island and West Greenland display the highest terrestrial
magmatic 3He/4He (up to 50 Ra, where Ra is the atmospheric value 1.39 x 10-6),
values that are considerably higher than the highest 3He/4He in contemporary ocean
island basalts, which reach a maximum of ~30 Ra. High 3He/4He in Baffin Island and
West Greenland are associated with a wide range of incompatible trace element and
lithophile radiogenic isotopic compositions, not dissimilar to the range of
compositions displayed by lavas at mid-ocean ridges, and overlapping the range
displayed by most northern hemisphere ocean island basalts. Crustal contamination
modelling in which high-grade Proterozoic crustal basement rocks are mixed with
depleted parents cannot account for the compositional trends displayed by the
picrites.
Major and trace element compositions were determined on melt inclusions in high-
3He/4He picrites that span a wide range of whole-rock incompatible trace element
and radiogenic isotopic compositions. The melt inclusions support the findings from
the whole-rock study since melt inclusion compositions reflect the composition of
their associated whole-rock, with no anomalous compositions present. In addition,
there is no evidence for a contribution of a proportion of depleted melts to the source
of the relatively enriched whole-rock samples. Therefore, since all melt inclusions
were contained within high-3He/4He samples, it is shown that high 3He/4He is a
feature of both depleted and relatively enriched melt compositions.
The wide range in whole-rock compositions of the Baffin Island and West Greenland
picrites represents that of the sub-lithospheric mantle source region and is
inconsistent with derivation of the picrites from residues of ancient mantle depletion.
The apparent decoupling of helium from trace elements and radiogenic isotopes is
hard to reconcile with simple mixing of a high-helium concentration, high-3He/4He
reservoir with various depleted and enriched helium-poor mantle reservoirs. It is
possible that primordial helium has diffused into a reservoir with a composition
similar to that of the convecting upper mantle. However, this must have occurred
after the development of existing mantle heterogeneity. The high-3He/4He picrites
require the existence of a deep, primordial helium-rich reservoir. Whether this
reservoir is present in the upper or deep mantle, or even the core, remains uncertain
Radiogenic Isotopes and Mantle Evolution
No full textThe radiogenic decay products of radioactive isotopes have been widely used to constrain the extent of compositional heterogeneity in Earth's mantle and the evolution of the silicate Earth. Several radioactive-radiogenic isotope systems have half-lives that are long compared to the age of the Earth and Solar System and provide a time-integrated history of Earth differentiation. Other systems with much shorter half-lives (e.g., 146Smsingle bond142Nd) elucidate processes that occurred in the early Earth when the parent isotopes were extant. Using meteorites as representative of the initial Solar System it is possible to construct evolutionary models for the bulk Earth. The silicate portion of the Earth differentiated into a continental crust enriched in elements that preferentially enter the melt phase during partial melting and a complementary depleted mantle residuum. Mid ocean ridge and ocean island basalts reveal the extent of isotopic heterogeneity in the sub-lithospheric mantle which can be characterized by a series of end-member components, but which are only speculatively linked to definite sources or geological processes. Isotopic systems that are dominated by radioactive (parent) to radiogenic (daughter) element fractionation due to mantle partial melting e.g., Smsingle bondNd and Lusingle bondHf are relatively coherent whereas systems that are affected by intra-crustal processes e.g., Rbsingle bondSr, Usingle bondPb exhibit more complexity. Continental basalts are more isotopically diverse than their oceanic counterparts suggesting interaction between deep-seated magmas and the lithospheric plates. Both contamination with continental crust and entrainment of sub-continental lithospheric mantle have been implicated
Experimental determination of the equilibrium Fe isotope fractionation between Feaq2+ and FeSm (mackinawite) at 25 and 2°C
No full textWe report the first experimentally-determined metal isotope equilibrium fractionation factors for a metal sulphide at ambient temperatures and pressures. Mackinawite, referred here as FeSm (where the subscript m indicates mackinawite), can be a reactive component in diagenetic pyrite formation and the extent of equilibration between FeS<sub>m</sub> and dissolved Fe(II) has direct implications the δ<sup>56</sup>Fe signatures recorded in diagenetic pyrite. The measured equilibrium Fe isotope fractionation between Fe(II)<sub>aq</sub> and FeS<sub>m</sub> is Δ<sup>56</sup>Fe<sub>Fe(II)–FeS</sub> = −0.52 ± 0.16‰ at 2 °C and Δ<sup>56</sup>Fe<sub>Fe(II)–FeS</sub> = −0.33 ± 0.12‰ at 25 °C and pH 4. At the experimental pH the equilibrium fractionation factor between all dissolved Fe(II) species and FeS<sub>m</sub> (Δ<sup>56</sup>Fe<sub>Fe(II)–FeS</sub>) equates to the fractionation factor between and FeS<sub>m</sub>(Δ<sup>56</sup>Fe<sub>Fe</sub><sup>2+</sup><sub>–FeS</sub>). The measured fractionations are of the same order as other non-redox fractionations measured in low-temperature Fe–C–O systems. We show that at low temperature, the Fe(II)<sub>aq</sub>–FeS<sub>m</sub> system is slowly asymptotic to isotopic equilibrium and consequently, FeS<sub>m</sub> is likely to partially conserve kinetically derived isotopic signatures generated on precipitation. Combined with the range of published kinetic fractionations measured on FeS<sub>m</sub> precipitation, our data suggest that, subject to the degree of isotope exchange during equilibration, FeS<sub>m</sub> can display δ<sup>56</sup>Fe compositions encompassing a range of ∼1.4‰
Bimodality of lavas in the Teide-Pico Viejo succession in Tenerife: the role of crustal melting in the origin of recent phonolites
No full textIn Tenerife, lavas of the recent Teide–Pico Viejo central complex show a marked bimodality in composition from initially mafic lavas (200–30 ka) to highly differentiated phonolites (30–0 ka). After this abrupt change, the bimodality of the lavas continued to manifest itself between the now felsic Teide–Pico Viejo central complex and the adjacent, but exclusively mafic, rift zones. Whole-rock trace element fingerprinting distinguishes three compositional groups (mafic, transitional, felsic). Groundmass Sr–Nd–Pb–O and feldspar δ18O data demonstrate open-system behaviour for the petrogenesis of the Teide–Pico Viejo felsic lavas by high 87Sr/86Sr ratios of up to 0·7049, uniform 206Pb/204Pb (19·75–19·78), variable 207Pb/204Pb (15·53–15·62) and heterogeneous δ18O values (5·43–6·80‰). However, ocean sediment contamination can be excluded because of the low 206Pb/204Pb ratios of North Atlantic sediments. Isotope mixing hyperbolae reproduce the entire Teide–Pico Viejo succession and require an assimilant of predominantly felsic composition. Unsystematic and heterogeneous variation of δ18O in fresh and unaltered feldspars across the Teide–Pico Viejo succession indicates magmatic addition of diverse δ18O assimilants, altered near surface at high and low temperatures. The best fit for these requirements is provided by nepheline syenite that occurs as fresh or altered lithic blocks in voluminous pre-Teide ignimbrite deposits and is similarly heterogeneous in oxygen isotope composition. Nepheline syenite blocks are considered to represent deep remnants of associated earlier eruptions and were thus available for assimilation at depth. Rare earth element modelling indicates that nepheline syenite needs to be melted in bulk to form a suitable end-member composition. Using this assimilant, energy-constrained assimilation fractional crystallization (EC-AFC) modelling reproduces the bulk of the succession, which leads us to suggest that Teide–Pico Viejo petrogenesis is governed by assimilation and fractional crystallization. Both mixing hyperbolae and EC-AFC models indicate that assimilation is more pronounced for the more felsic lavas. The maximum assimilation is evident in the most strongly differentiated (and the most radiogenic in Sr) lava and computes to >97·8% of the assimilant. This most evolved eruption probably represents nepheline syenite bulk melts that formed spatially decoupled from juvenile material. This study therefore recognizes a wider variability of magmatic differentiation processes at Teide–Pico Viejo than previously thought
Silicon effects on biomass carbon and phytolith-occluded carbon in grasslands under high-salinity conditions
No full textChanges in climate and land use are causing grasslands to suffer increasingly from abiotic stresses, including soil salinization. Silicon (Si) amendment has been frequently proposed to improve plant resistance to multiple biotic and abiotic stresses and increase ecosystem productivity while controlling the biogeochemical carbon (C) cycle. However, the effects of Si on plant C distribution and accumulation in salt-suffering grasslands are still unclear. In this study, we investigated how salt ions affected major elemental composition in plants and whether Si enhanced biomass C accumulation in grassland species in situ. In samples from the margins of salt lakes, our results showed that the differing distance away from the shore resulted in distinctive phytocoenosis, including halophytes and moderately salt-tolerant grasses, which are closely related to changing soil properties. Different salinity (Na+/K+, ranging from 0.02 to 11.8) in plants caused negative effects on plant C content that decreased from 53.9 to 29.2% with the increase in salinity. Plant Si storage [0.02–2.29 g Si m–2 dry weight (dw)] and plant Si content (0.53 to 2.58%) were positively correlated with bioavailable Si in soils (ranging from 94.4 to 192 mg kg–1). Although C contents in plants and phytoliths were negatively correlated with plant Si content, biomass C accumulation (1.90–83.5 g C m–2 dw) increased due to the increase of Si storage in plants. Plant phytolith-occluded carbon (PhytOC) increased from 0.07 to 0.28‰ of dry mass with the increase of Si content in moderately salt-tolerant grasses. This study demonstrates the potential of Si in mediating plant salinity and C assimilation, providing a reference for potential manipulation of long-term C sequestration via PhytOC production and biomass C accumulation in Si-accumulator dominated grasslands
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