ALBERT

Description: The NeogeneQuaternary alkali-basalthawaiite lavas of the Gharyan volcanic field (NW Libya) contain mantle xenoliths. These mostly consist of protogranular spinel lherzolites with superimposed metasomatic textures represented by reaction patches where primary orthopyroxene (opx), clinopyroxene (cpx) and spinel (sp) are the main reacting phases. The secondary parageneses include clinopyroxene (cpx2), olivine (ol2) and feldspar (feld) as reaction rims around opx, spongy-textured clinopyroxene with recrystallized portions (cpx2{+/-}feldspar), and brown spinel destabilized in a higher Cr/(Cr+Al) black vermicular aggregate (sp2) generally associated with feldspar microlites. Cpx2 are typically depleted in Na2O and Al2O3 relative to cpx; feldspar includes both alkali-feldspar (Or 1751) and plagioclase (An 2364). Bulk rocks have flat heavy rare earth element (HREE) patterns (1.22.3 times chondrite) and are variably enriched in light REE (LREE; LaN/YbN up to 6.6). The constituent clinopyroxenes are characterized by flat HREE distributions (814.5 times chondrite) and variable LREE enrichment with LaN/YbN up to seven, which generally conform to the bulk-rock chemistry. Samples relatively unaffected by metasomatism have clinopyroxene SrNd isotopic composition (87Sr/86Sr down to 0.7023, 143Nd/144Nd up to 0.5139) that approaches the depleted mantle (DM), suggesting that the lithospheric mantle beneath the area underwent a long-term depletion probably by pre-Palaeozoic extraction of basic melts. The remaining samples approach 87Sr/86Sr c. 0.7030, 143Nd/144Nd c. 0.5130, with 206Pb/204Pb up to 19.66. These data imply that the causative agents of metasomatism were Na-alkali silicate melts with a clear HIMU affinity, in accordance with the isotopic signature of the host lavas (87Sr/86Sr=0.7032, 143Nd/144Nd=0.5130, 206Pb/204Pb=19.60). This prevalent HIMU geochemical signature is comparable with that recorded in Cenozoic alkaline basic lavas and associated mantle xenoliths from other occurrences of the northerncentral African lithosphere, suggesting a common regional sub-lithospheric component. The relatively low 3He/4He of the Gharyan xenoliths (5.36.5 Ra) indicates that this component originates within the upper mantle and is unrelated to the deep-seated mantle plume source of the EthiopianYemen plateau basalts. Therefore, the Cenozoic volcanic districts of the Saharan belt could be related to smaller-scale shallow mantle upwellings (also referred to as hot fingers') triggered by intraplate reactivation of regional tectonic lineaments within the Pan-African cratonic basement, as a foreland reaction of the AfricanEurope collisional system.

Description: To evaluate sites for long-term geological storage of CO 2 and optimize techniques for monitoring the fate of injected CO 2 , it is crucial to investigate potential CO 2 migration pathways out of a reservoir and surface leakage magnitudes. For the first time, we calculate CO 2 leakage rates and volumes from ancient fault-related travertines and from an abandoned borehole. U-Th–dated travertine along two faults near Green River, Utah (western United States), shows that leakage has occurred in this area for over 400 k.y. and has switched location repeatedly over kilometer-scale distances. One individual travertine was active for at least 11 k.y. Modern leakage is predominantly through the active Crystal Geyser, which erupts from an abandoned exploration well. Using age data and travertine volume, we calculate magnitudes and rates of CO 2 emission. Fault-focused leakage volume is twice as great as diffuse leakage through unconfined aquifers. The leakage rate from a poorly completed borehole is 13 times greater than the long-term time-averaged fault-focused leakage. Although magnitudes and rates of any leakage from future storage sites will be highly dependent on local geology and pressure regime, our results highlight that leakage from abandoned wells is likely to be more significant than through faults.

Description: Anak Krakatau is a basaltic andesite cone that has grown following the famous caldera-forming 1883 eruption of Krakatau. It breached sea level in 1927 and since the 1950s has been growing at an average rate of ~8 cm a week. We present new major and trace element data combined with whole-rock 18 O, Sr and Nd isotope data for 1883, 1993 and 2002 Krakatau eruptive products and the surrounding crust. Bombs erupted from Anak Krakatau during 2002 contain frothy metasedimentary and plutonic xenoliths that show variable degrees of thermal metamorphism, plastic deformation and partial melting. Contact-metamorphic minerals such as cordierite and tridymite in metasedimentary xenoliths are consistent with high-temperature metamorphism and incorporation at mid- to upper-crustal depth. Energy-constrained assimilation and fractional crystallization modelling of whole-rock data suggests that the Anak Krakatau magmas have a genetic relationship with the 1883 eruption products. The geochemical impact of crustal contaminants on whole-rock compositions is apparently small, and we conclude that low levels of assimilation of a quartzo-feldspathic sediment are recorded in Anak Krakatau magmas. Plagioclase phenocrysts from the 2002 eruption exhibit disequilibrium textures and complex compositional zoning, however, and are also isotopically variable with a total range in 87 Sr/ 86 Sr of 0·7043–0·7048 as determined by in situ laser ablation inductively coupled plasma mass spectrometry. This suggests that although shallow crustal assimilation appears to have had a limited effect on whole-rock chemistry, a complex late-stage differentiation history is recorded within the magma’s cargo of crystals and xenoliths.

Description: New age data from Sr isotope analysis and both planktonic foraminifera and nannofossils are presented and discussed here for the Upper Eocene–Upper Miocene sedimentary rocks of the Degirmenlik (Kythrea) Group. New dating is also given of some Cretaceous and Pliocene sediments. In a revised stratigraphy the Degirmenlik (Kythrea) Group is divided into ten formations. Different Upper Miocene formations are developed to the north and south of a regionally important, E–W-trending syn-sedimentary fault. The samples were dated wherever possible by three independent methods, namely utilizing Sr isotopes, calcareous nannofossils and planktonic foraminifera. Some of the Sr isotopic dates are incompatible with the nannofossil and/or the planktonic foraminiferal dates. This is mainly due to reworking within gravity-deposited or current-affected sediments. When combined, the reliable age data allow an overall biostratigraphy and chronology to be erected. Several of the boundaries of previously defined formations are revised. Sr data that are incompatible with well-constrained biostratigraphical ages are commonly of Early Miocene age. This is attributed to a regional uplift event located to the east of Cyprus, specifically the collision of the Anatolian (Eurasian) and Arabian (African) plates during Early Miocene time. This study, therefore, demonstrates that analytically sound Sr isotopic ages can yield geologically misleading ages, particularly where extensive sediment reworking has occurred. Convincing ages are obtained when isotopic dating is combined with as many forms of biostratigraphical dating as possible, and this may also reveal previously unsuspected geological events (e.g. tectonic uplift or current activity).

Description: In 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 18 O data demonstrate open-system behaviour for the petrogenesis of the Teide–Pico Viejo felsic lavas by high 87 Sr/ 86 Sr ratios of up to 0·7049, uniform 206 Pb/ 204 Pb (19·75–19·78), variable 207 Pb/ 204 Pb (15·53–15·62) and heterogeneous 18 O values (5·43–6·80). However, ocean sediment contamination can be excluded because of the low 206 Pb/ 204 Pb 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 18 O in fresh and unaltered feldspars across the Teide–Pico Viejo succession indicates magmatic addition of diverse 18 O 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.

Description: Regional scale biotite and cordierite-bearing granites (s.l.) in the Variscan of the Central Iberian Zone (CIZ) are spatially closely associated with cordierite-rich nebulites and cordierite-bearing two-mica granites, and with cordierite-rich high grade hornfelses and cordieritites (〉60% cordierite) that are relatively common in the aureoles of these granites. Building on published field evidence, petrological data are presented which, combined with new chemical and isotopic (Sr–Nd) modelling, indicate that the cordierite-bearing granites cannot be derived by simple anatexis of regional sedimentary protoliths; but the data are consistent with a process of reactive assimilation that involves the interaction of biotite granite magma with high-grade host rocks ranging from cordierite nebulites to andalusite-bearing cordieritites. The contribution of the postulated cordierite-rich contaminants to the diversity of cordierite granite compositions is modelled using the compositions of regional Lower Cambrian–Upper Neoproterozoic metasedimentary rocks that are generally chemically mature (CaO very rarely exceeds 1·4%). These rocks include specific horizons in which extreme chemical alteration is attributable to sediment reworking during eustatic falls in sea level. Such compositions may account for the presence of the high concentrations in Al that later produced cordieritites. Fractional crystallisation is also important, particularly in generating the more evolved cordierite granite and cordierite biotite muscovite granite compositions. Although assimilation in situ is normally regarded as a minor contributor volumetrically to evolving plutons, in this instance the emplacement of large volumes of granite magma into a high-T–low-P environment significantly increased the potential for reactive assimilation.