There were three negative seawater strontium-isotope excursions (shifts to lower 87 Sr/ 86 Sr values) during the Jurassic and Cretaceous that were of relatively short duration (5-13 my) and showed a relatively quick recovery to pre-excursion 87 Sr/ 86 Sr ratios. These excursions occurred in the Pliensbachian-Toarcian (Early Jurassic), Aptian-Albian, and Cenomanian-Santonian (Early and Late Cretaceous respec- tively). Each excursion coincided closely in time with an Oceanic Anoxic Event (OAE) marked by sediments unusually rich in organic carbon. The Jurassic OAE occurred at the end of the strontium-isotope excursion, whereas the two Cretaceous OAEs oc- curred at the onset of the accompanying strontium-isotope excursions. The possible causes of these excursions were evaluated by successively examining the changes in the riverine strontium fluxes, riverine 87 Sr/ 86 Sr ratios, or hydrothermal strontium fluxes required to produce each excursion. A range of seawater strontium budgets was used to encompass the uncertainties in modern and ancient cycles. To produce the excursions, we calculate that the riverine strontium fluxes would have had to decrease by 6 to 15 percent or the fluvial 87 Sr/ 86 Sr ratios by 0.00019 to 0.00046. The uncertainties largely stem from the assumed magnitude of the hydrothermal strontium flux at the onset of each excursion. Alternatively, increases in sea-floor hydrothermal activity of 7 to 104 percent could also have produced the strontium-isotope excursions. This large range is due mostly to uncertainties in the relative flux of strontium from axial high-temperature hydrothermal systems and low-temperature off-axis systems. Only a small portion of this range stems from uncertainties in the riverine strontium terms. The possible causes of the excursions were further evaluated by examining several geologic factors that could have affected riverine strontium, including climate change, sealevel, and the eruption of flood basalts. We conclude that neither variations in riverine strontium fluxes nor in 87 Sr/ 86 Sr ratios is the likely cause of the strontium- isotope excursions. The most probable explanation is increased rates of hydrothermal activity related to increased ocean-crust production at the mid-ocean ridges. The close correlation in time between the strontium-isotope excursions and the major Oceanic Anoxic Events (OAEs) is compatible with a causal linkage. We propose that increased ocean-crust production led to enhanced CO2 outgassing and global warming, which in turn led to several processes that acted to make surface ocean waters more productive. However, because OAEs did not occur throughout the proposed periods of enhanced hydrothermal activity, it appears that these processes only preconditioned the oceans for the OAEs: sealevel rise may have been the final trigger. This model explains why all three OAEs did not occur at the same time relative to the onset of excess hydrothermal activity and why OAEs are not associated with every sealevel rise documented in the stratigraphic record.

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Seawater Strontium Isotopes, Oceanic Anoxic Events, and Seafloor Hydrothermal Activity in the Jurassic and Cretaceous

Dramatic coastline changes demonstrate rapid Quaternary uplift of Calabria in southern Italy. Because most of the west (Tyrrhenian Sea) coast is normal fault bounded, previous work has asserted that its uplift is local footwall uplift related to extension. However, the east (Ionian Sea) coast is also uplifting but is not normal fault bounded. This reanalysis, based on original fieldwork and reinterpretation of the literature, reaches the following conclusions. First, although radiometric dating of only one marine terrace sequence in Calabria is available, on the east coast at Crotone, terraces occur at similar elevations, and can thus be correlated, throughout this coast and the west coast of northern and central Calabria. Uplift rate at both coasts and across the region in between is the same, 1.0 ± 0.1 mm yr−1, provided my correlations are correct. Second, the oldest raised shorelines date from the 0.9 Ma marine highstand and have uplifted ∼700 m since ∼0.7 Ma. Regional uplift rate earlier was minimal, possibly zero. Third, localities on this west coast show marginally (up to ∼5%) higher uplift rates, indicating some local footwall uplift (at ∼0.05 mm yr−) associated with slow extension (at ∼0.1 mm yr−1). Fourth, the 12-m Holocene marine terrace formed around 7 ka. Allowing for a 3-m range of wave and tidal action, it is interpreted as the result of 7 m of tectonic uplift and 2 m of eustatic sea level fall during ∼6–4 ka. Finally, the west coast of southern Calabria shows significant elevation changes caused by active normal faulting, as well as regional uplift. Footwall localities have uplifted up to 1300 m since 0.9 Ma. The estimated maximum present-day uplift rate of 1.67 mm yr−1 is interpreted as 1 mm yr of regional uplift plus 0.67 mm yr of local footwall uplift, indicating much faster extension than farther north. It is suggested that the Tyrrhenian Benioff zone detached from beneath Calabria shortly before 0.7 Ma. The regional uplift of the overlying landmass is thus explained as its transient isostatic response to removal of this load. Order-of-magnitude calculations suggest that 1 mm yr−1 uplift rate is reasonable, with ∼2 km total uplift expected, indicating that uplift will continue for > ∼1 Myr into the future. Extension of Calabria appears to have begun at ∼11 Ma, at the same time as formation of the Tyrrhenian Sea and the related subduction at its Benioff zone. Extension rate in southern Calabria abruptly increased from ∼0.1 to ∼1 mm yr−1 around 0.9–0.7 Ma., when the slab detached and the regional uplift began; extension in northern and central Calabria continues at the same ∼0.1 mm yr−1 rate as before.

Quaternary uplift of southern Italy

Two distinct phases of continental magmatism are evident in western Arabia The first, from about 30 to 20 Ma, produced tholeiitic-to-transitional lavas emplaced along NW trends The second, from about 12 Ma to Recent, produced transitional-to-strongly-alkalic lavas emplaced along N-S trends The older phase is attributed to passive-mantle upwelling during extension of the Red Sea Basin, whereas the younger phase is attributed to active-mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision The younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the West Arabian Swell after about 14 Ma A variety of evidence suggests that the West Arabian Swell is thermally supported by hot, upwelling asthenosphere In contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the Red Sea Basin, these processes were symmetric across a N-S line marking the central axis of the West Arabian Swell This axis coincides with two fundamental features: the Ha'il-Rutbah Arch in the north, and the Makkah-Madinah-Nafud (MMN) volcanic line in the south The symmetry of magmatism is demonstrated by petrochemical evidence that the MMN harrats were derived by greater degrees of partial melting, at shallower depths, than those harrats lying to the west and east of the MMN line The potential temperature of the asthenosphere is estimated to be about 1436°C beneath the MMN line and about 1354°C beneath the flanking, more undersaturated harrats The source of upwelling is either a mantle plume centrally located beneath the West Arabian Swell or an elongated and extended lobe of hot asthenosphere emanating from the Ethiopian mantle plume Convective flow may have been channelled along a preexisting, regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the AfroArabian Dome This crest of mantle upwelling underlies the MMN line in the north and the Danakil Depression and Ethiopian rift system in the south It also passes through the Red Sea Basin at the midpoint of an unusual, doubly propagating rift system where axial seafloor spreading began 4–5 my ago The NW trend of the Red Sea Basin was well established by crustal attenuation during the older magmatic phase The Pliocene invasion of this basin by a N-S zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions

Upwelling asthenosphere beneath western Arabia and its regional implications

The Lesser Antilles volcanic chain : a study in arc magmatism.

We report on the rare earth and Nd-Sr-Pb isotopic composition of basalts dredged along the Sheba Ridge axis in the Gulf of Aden and its extension into the Gulf of Tadjoura and subaerial basalts from the Ardoukoba Rift in east Afar. The sampling profile provides a means to study the evolutionary nature of the mantle sources involved in the melting process associated with the interaction of the head of a starting mantle plume with continental lithosphere and an ocean basin at a nascent stage of formation. An 800-km-long Nd-Sr-Pb isotopic and La/Sm gradient, sinusoidally modulated, is apparent from the Afar eastward. The first enrichment peak occurs in the Gulf of Tadjoura, where diffuse extension of the Danakil-Aisha continental lithospheric block and westward rift propagation is currently progressing. The second enrichment peak at 46°E is associated with a mantle buoyancy anomaly and related constructional volcanism. East of 48°E, the MORBs are typically light rare earth element depleted, whereas 206Pb/204Pb and 87Sr/86Sr slightly increase, suggesting recent decoupling. In Nd-Sr-Pb isotope ratio space, three distinct vector trends are observed within a plane. The mixing vectors point toward three mantle source end-members which can be interpreted as Pan-African continental lithosphere along the Gulf of Tadjoura (a hybrid EM-l-EM-2), a mantle plume (relatively young HIMU-like) which dominates the 46°E anomaly, and the depleted asthenosphere east of 48°E (DUPAL-like). Combined data from the Gulf of Aden-Red Sea-Afar-Ethiopian rifted zones suggest a radial pattern of geochemical and isotopic variation about the Afar. A working dynamical-thermal model is presented for the past 30–40 m.y. history of the Horn of Africa. It invokes both passive rifting/seafloor spreading in the Red Sea/Gulf of Aden and the flattening and interaction of the starting head of a toruslike thermal mantle plume with the Pan-African continental lithosphere which is slowly moving northeastward with the plume head attached at its base. The plume flattened into a pancakelike form, twice the diameter of the original head which is estimated to be of the order of 700 km in diameter. The thinning of the lithosphere by stretching and thermal erosion by the mantle plume has not yet been completed. A working ternary mixing model constrained by the isotope data indicates that within the 800–1000 km radius of influence of the Afar mantle plume, melting of the lithosphere mantle and the depleted asthenosphere apparently entrained by the ascending mantle plume dominates initially. Only along the three rifting zones intersecting the flattened plume ring, 450±150 km in radius, composed of original HIMU-like plume material does the original plume component play a more dominant role. Judging from the spatial isotopic composition variation of the basalts, the plume torus may be apparent along (1) the 46°E Gulf of Aden anomaly where seafloor spreading is now well established; (2) the 13°–16°N southern Red Sea segment, which represents a rift zone at a transient stage of either development or abandonment (overlapping with the Afar NW neovolcanic zone), where ocean island alkali volcanism dominates and diffuse lithosphere extension may still operate; (3) the high alkaline field of the Aden Volcanic Series; and (4) the Ethiopian Rift around 8°N in a purely continental setting. The NW Afar neovolcanic zone, which is essentially at a nascent stage of seafloor spreading and is overlapping the ring and the center of the pancakelike flattened mantle plume, is dominated by tholeiites derived from depleted asthenospheric material entrained by the plume during its original ascent. Plate reconstructions further suggest that the original center of the flattened mantle plume head has moved with the lithosphere some 900 km northeastward. The stem feeder of the plume has now been drawn or tilted toward the Afar as a result of the migration of the Gulf of Aden/Red Sea spreading centers which act as sinks of asthenospheric material and the likelihood that the feeder of the mantle plume is encountering with time an African lithosphere increasing in age, thickness, and rigidity.

Nd‐Sr‐Pb isotopic variations along the Gulf of Aden: Evidence for Afar Mantle Plume‐Continental Lithosphere Interaction

The paper presents major and trace element data and mineral compositions for a series of foiditic-tephritic to phonolitic rocks coming from Monte Vulture, Southern Italy, and investigates their origin, evolution and relationship with the other centres of the Roman province. Major and trace element variation in the foiditic to tephritic suite agrees with a hypothesis of evolution by simple crystal/liquid fractionation, whereas the early erupted phonolitic trachytes and phonolites have geochemical characteristics which do not support their derivation from tephritic magma by crystal fractionation. Foiditic and phonolitic rocks have mineral compositions which are interpreted as indicating magma mixing. However geochemical evidence shows that this process did not play an important role during the magma evolution. The Vulture rocks have compositional peculiarities such as high abundance of Na2O, CaO, Cl and S, when compared with other Roman volcanics. Instead, the distribution of incompatible elements is similar to those of Roman rocks, except for a lower content of Rb and K, higher P and lower Th/Ta and Th/Nb ratios which are still close to the values of arc volcanics. The high contents of Na, Ca and of volatile components are tentatively attributed to the interaction of magma with aqueous solutions, rich in calcium sulphate and sodium chloride, related to the Miocene or Triassic evaporites occurring within the sedimentary sequence underlying the volcano. The distribution pattern of the incompatible elements is interpreted as indicative of magma-forming in a subduction modified upper mantle and of the peculiar location of M. Vulture.

Petrogenesis of Monte Vulture volcano (Italy): inferences from mineral chemistry, major and trace element data

The Yemen trap series: Genesis and evolution of a continental flood basalt province

The Vulture complex is made up of foiditic, tephritic, phonolitic-trachytic and phonolitic products. New rock analyses have been performed in order to ascertain whether the various rock types derive from a unique parental magma and, if so, to define its nature. The data presented support that the Vulture suite originated from a foiditic melt which had differentiated at low pressures. The main process determining the foidite → → tephrite → phonolitic trachyte evolution seems to be the crystal fractionation of mainly clinopyroxenes, and opaques, with the contribution of plagioclases and hauyne too in the tephrite → trachyte evolution. Additionary role must have been played by a mixing of melts at different evolution stages occurred in a shallow seated magma chamber.

Magma evolution at mount vulture (Southern Italy)

The volcanic units outcropping in southeastern central Afar include a sequence of thick, stratoid lava flows with interbedded acidic rocks, ranging in age from 25 to 1 m.y. This sequence is overlain by a number of volcanic ranges, some of them in axial position (Manda-Inakir) and others trans-versally oriented in respect to the axis of the depression (Assab, Edd). The age of all these volcanic ranges seem to be younger than 0.6 m.y. The flood basalts sequence is not well defined from a petrological and geochemical standpoint: its products range from mildly alkaline to alkali-basaltic terms. On the contrary, each volcanic range is well characterized in terms of the nature of its products; Manda-Inakir products form a series starting from mildly alkaline basalts and the transversal volcanic ranges consist of typical alkaline series. The geochemical characters of these rocks seem to be roughly consistent with the general evolution pattern inferrable for a spreading area.

Structural Meaning of East-Central Afar Volcanism (Ethiopia, T.F.A.I.)

Two distinct periods of volcanic activity are distinguishable at Monte Vulture volcanic complex: the first activity started with tephritic and foiditic volcanics, documented by xenoliths in the phonotrachytic (Ph-T) ignimbrites, lava blocks in a basal explosion breccia deposit beneath the Ph-T ignimbrites, sandy lenses rich in volcanic components in fluviatile conglomerates, and ended with emplacement of phonotrachytic deposits (Ph-T ignimbrites and lava domes); whereas subsequent activity produced deposits of foiditic and tephritic composition which built up the central volcano. The volcanics of the two periods do not fit a simple evolutionary model on the basis of geochemical signatures.

M. De Fino

Papers

Petrogenesis of Monte Vulture volcano (Italy): inferences from mineral chemistry, major and trace element data

The Yemen trap series: Genesis and evolution of a continental flood basalt province

Magma evolution at mount vulture (Southern Italy)

Structural Meaning of East-Central Afar Volcanism (Ethiopia, T.F.A.I.)

Mineral Chemistry of Monte Vulture Volcanics: Petrological Implications