62,888 research outputs found
Constraints on mantle source and interactions from He-Sr isotope variation in Italian Plio-Quaternary volcanism
No full textHelium isotope ratios of olivine and pyroxene phenocrysts from Plio-Quaternary volcanic rocks from southern Italy (seven Aeolian Islands, Mt. Vulture, Etna, Ustica, and Pantelleria) range from 2.3 to 7.1 R <sub>a</sub>. Importantly, the phenocryst <sup>3</sup>He/<sup>4</sup>He correlate well with whole rock Sr isotopic composition (0.70309–0.70711), reflecting the mixing of two sources. A significant contribution of He from crustal contamination is recorded only occasionally (e.g., pyroxenes from Vulcano). When merged with data from the Roman Comagmatic Province, a remarkably strong near-linear He-Sr isotope correlation is apparent. The general northward decrease in <sup>3</sup>He/<sup>4</sup>He corresponds to an increase in <sup>87</sup>Sr/<sup>86</sup>Sr (and a decrease in <sup>143</sup>Nd/<sup>144</sup>Nd and <sup>206</sup>206Pb/<sup>204</sup>Pb) that is due to increasing metasomatic enrichment of the mantle wedge via subduction of the Ionian-Adriatic plate. Calculations based on the ingrowth of <sup>4</sup>He in the wedge and on the <sup>4</sup>He content of the subducting crust show that mechanisms of enrichment in radiogenic He are effective only if the wedge is strongly depleted in He relative to best estimates of the depleted mantle. This can be accommodated if the process of metasomatism by the subduction fluids depletes the mantle wedge. The <sup>3</sup>He/<sup>4</sup>He of Pantelleria, Etna, Iblei, Ustica, Alicudi, and Filicudi basalts (7.0 ± 0.6 R a) define the mantle composition least affected by subduction-related metasomatism. Although these volcanoes are from a variety of tectonic regimes (subduction-related, intraplate, rifting), their similarities suggest a common origin of geochemical features. Their characteristics are consistent with a HIMU-type mantle that either is younger than the Cook-Austral island end-member or has a lower <sup>238</sup>U/<sup>204</sup>Pb
Helium-strontium isotope constraints on mantle evolution beneath the Roman Comagmatic Province, Italy
No full textSpinel-lherzolite xenoliths from the Hoggar swell: evidence for intracratonic asthenosphere upwelling and lithosphere rejuvenation.
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The role of HIMU metasomatic components in the North African lithospheric mantle: Petrological evidence from the Gharyan lherzolite xenoliths, NW Libya
No full textThe Neogene-Quaternary alkali-basalt-hawaiite 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 Na 2O and Al 2O 3 relative to cpx; feldspar includes both alkali-feldspar (Or 17-51) and plagioclase (An 23-64). Bulk rocks have flat heavy rare earth element (HREE) patterns (1.2-2.3 times chondrite) and are variably enriched in light REE (LREE; La N/Yb N up to 6.6). The constituent clinopyroxenes are characterized by flat HREE distributions (8-14.5 times chondrite) and variable LREE enrichment with La N/Yb N uP to seven, which generally conform to the bulk-rock chemistry. Samples relatively unaffected by metasomatism have clinopyroxene Sr-Nd isotopic composition ( 87Sr/ 86Sr down to 0.7023, 143Nd/ 144Nd up to 0.5139) that approaches the depleted mantle (DM), suggesting that the lifliospheric 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 northern-central African lithosphere, suggesting a common regional sub-lithospheric component. The relatively low 3He/ 4He of the Gharyan xenoliths (5.3-6.5 R a) indicates that this component originates within the upper mantle and is unrelated to the deep-seated mantle plume source of the Ethiopian - Yemen 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 African-Europe collisional system
He isotopes in fluid inclusions of mafic phenocrysts from the Plio-Quaternary volcanism in Italy.
No full textThe graphical presentation of lead isotope data for environmental source apportionment
No full textLead isotope ratios are widely used to identify original sources of Pb in the environment. Such source apportionment depends on the ability to distinguish potential sources on the basis of their isotopic composition. However, almost all terrestrial Pb is co-linear in some of the plots i.e. <sup>206</sup>Pb/<sup>208</sup>Pb versus <sup>206</sup>Pb/<sup>207</sup>Pb and <sup>206</sup>Pb/<sup>204</sup>Pb versus <sup>206</sup>Pb/<sup>207</sup>Pb commonly presented in the literature. These diagrams are unable to distinguish more than two sources of environmental Pb. Linear trends in such plots are an inevitable consequence of the co-linearity of terrestrial leads and should not be taken necessarily to indicate simple binary mixing of sources. A more reliable test for multiple source mixing can be obtained from plots involving <sup>206</sup>Pb/<sup>204</sup>Pb, <sup>207</sup>Pb/<sup>204</sup>Pb and <sup>208</sup>Pb/<sup>204</sup>Pb and therefore requires measurements of the minor <sup>204</sup>Pb isotope
Metasomatic events in the lithospheric mantle beneath NW Libya: petrological evidence from the Gharyan peridotite xenoliths
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Petrology of mantle xenoliths from the Northern Ethiopian plateau: Clues of plume related metasomatism?
No full textLithosphere/asthenosphere interaction in a plume region: Evidence from Ethiopian mantle xenoliths.
No full textStratigraphy of Cretaceous to Lower Pliocene sediments in the northern part of Cyprus based on comparative 87Sr/86Sr isotopic, nannofossil and planktonic foraminiferal dating
No full textNew 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 Deg ̆irmenlik (Kythrea) Group. New dating is also given of some Cretaceous and Pliocene sediments. In a revised stratigraphy the Deg ̆irmenlik (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)
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