Hf Isotope Evidence for a Miocene Change in the Kerguelen Mantle Plume Composition

We report high-precision multiple collector inductively coupledplasma mass spectrometry (MC-ICP-MS) analyses of Hf isotopiccompositions for 39 Kerguelen Archipelago volcanic rocks thatadd a new perspective to the temporal evolution of the Kerguelenmantle plume. Samples cover the entire range of chemical andSr, Nd and Pb isotopic variations reported for the archipelago,and vary in age from 29 to 0·1 Ma. Their Hf isotopiccompositions show the largest variation for an individual oceanicisland and     

The Tatahouine diogenite: Mineralogical and chemical effects of sixty-three years of terrestrial residence

Abstract— Comparison between clasts of the Tatahouine diogenite, collected the day of the fall in 1931 and 63 years later in 1994, allows the evaluation of mineralogical and chemical effects of terrestrial residence on meteorites. Secondary minerals are found in the 1994 samples: iron stains and carbonate rosettes. Major and trace element abundances have been determined on fallen and found clasts. No significant differences have been observed for most elements with the exception of Rb, Sr, and (in a single case) the light rare earth elements (LREE). In this case, the REE pattern of a 1994 clast displays a weak positive Ce anomaly, probably linked to the presence of iron hydroxide. The contents of Rb and Sr are significantly higher in the 1994 samples than in the 1931 clasts and reflect the formation of calcite inside some of the clast fractures. These results demonstrate that weathering processes may change the chemistry of meteorites in a very short time.     

Geochemical Constraints on the Origin of the Late Archean Skjoldungen Alkaline Igneous Province, SE Greenland

The present work reports the first broad geochemical investigationof the recently discovered late Archean (2700 Ma) Skjoldungenalkaline igneous province (SAP) in southeast Greenland. Therocks studied range in composition from ultramafic to felsicand comprise pyroxenites, hornblendites, hornblende noritesand diorites, monzonites, syenites, and nephelinitic rocks andcarbonatites. Various lithologic units from the host Archeangneissic basement are also investigated. The magmatic rocksshow remarkably coherent major element, trace element, rareearth element (REE), and Sr and Nd isotope systematics, suggestinga petrogenetic relationship. The most important geochemicalfeatures are high normative proportions of nepheline, forsteriteand albite, low TiO₂ (<15 wt %) and moderate FeO (total)(<12 wt %) contents, enrichments in large ion lithophileelements (LILE) and light rare earth elements both absoluteand relative to high field strength elements (HFSE) that displaylarge negative anomalies, and generally low to moderate abundancesof compatible elements. Field relations and REE and compatibleelement systematics among Skjoldungen rocks suggest that maficand ultramafic hornblende-rich samples may represent cumulatelithologies of the regional parental magma. On the basis ofmineral data, this is deduced to have had mg-number of 064,shoshonitic affinities (K₂O15 wt %), been close to silica saturationand volatile rich. Major element, trace element and REE systematicsfurther suggest that felsic intrusions are related to the maficregional parental magma through extensive olivine, hyperstheneand hornblende fractionation. Lack of correlation between La/Yband other critical trace and REE ratios indicates that apatite,zircon and titaniferous minerals were not important cumulusphases at advanced stages of evolution. The measured Sm–Ndwhole-rock isochron age is 2716 23 Ma (2 error) [mean squareof weighted deviates (MSWD) = 14], whereas linear regressionof the Sr isotope data yields an age of 26047 Ma (2 error)(MSWD = 22•2). The age obtained by Nd isotopes is corroboratedby U–Pb zircon results (2698 7 Ma), suggesting thatthe Sm–Nd system remained closed since crystallization.By contrast, the 100 Ma younger age obtained by Sr isotopessuggests that the Rb–Sr system has been disturbed. Initial¹⁴³Nd/¹⁴⁴ Nd ratios span a narrow range corresponding to _Nd(27Ga) =+074 to –109, whereas initial _Sr values at 27Ga cover a comparatively larger interval from –10 to +20.Neither initial _Nd nor initial _Sr values conform to previouslysuggested mantle depletion curves and no meaningful correlationexists between Nd and Sr isotopes for the Skjoldungen magmaticrocks as a whole. Although compositionally heterogeneous, theanalyzed suite of samples from the host agmatitic basement isextremely homogeneous with respect to age, with T^CHUR crustalresidence times around 2700–2800 Ma confirming limitedavailable isotopic evidence. Large-scale assimilation of Archeancrust or recycling of sediments derived from the local basementinto the mantle source fails to explain adequately negativeNb anomalies and low ^Nd signatures characteristic of the Skjoldungenintrusions. Rather, the nearchondritic isotopic compositionof Nd in the Skjoldungen samples together with the decoupledLILE and HFSE enrichment and slightly positive _Sr values areconsidered to reflect characteristics of the mantle source ina subduction zone environment. The geodynamic site hosting theSkjoldungen province thus may be an early manifestation of modern-styleplate tectonics. KEY WORDS: Skjoldungen province; Greenland; Archean; alkaline igneous rocks; geochronology; geochemistry ^*Corresponding author. Present address: Ecole Normale Suprieure de Lyon, 46 AlLe d'Italie, 69364 Lyon Cedex 07, France     

The Prinsen af Wales Bjerge Formation Lavas, East Greenland: the Transition from Tholeiitic to Alkalic Magmatism during Palaeogene Continental Break-up

We present elemental and isotopic (Sr–Nd–Pb–Hf–Os–He)data on primitive alkalic lavas from the Prinsen af Wales Bjerge,East Greenland. Stratigraphical, compositional and ⁴⁰Ar–³⁹Ardata indicate that this inland alkalic activity was contemporaneouswith the upper parts of the main tholeiitic plateau basaltsand also post-dated them. The alkalic rocks show a marked crustalinfluence, indicating establishment of new magmatic plumbingsystems distinct from the long-lived coastal systems that fedthe relatively uncontaminated plateau basalts. The least contaminatedlavas have high ³He/⁴He isotope ratios (R/R_A 12·4–18·5),sub-chondritic ¹⁸⁷Os/¹⁸⁸Os_i (0·120–0·126),low     

Timing of high-pressure metamorphism and exhumation of the eclogite type-locality (Kupplerbrunn–Prickler Halt, Saualpe, south-eastern Austria): constraints from correlations of the Sm–Nd, Lu–Hf, U–Pb and Rb–Sr isotopic systems

Sm–Nd, Lu–Hf, Rb–Sr and SIMS U–Pb data are presented for meta‐gabbroic eclogites from the eclogite type‐locality ( Haüy, 1822 ) Kupplerbrunn–Prickler Halt and other areas of the Saualpe (SE Austria) and Pohorje Mountains (Slovenia). Mg‐rich eclogites derived from early gabbroic cumulates are kyanite‐ and zoisite rich, whereas eclogites with lower Mg contents contain clinozoisite ± kyanite. Calculated P–T conditions at the final stages of high‐pressure metamorphism are 2.2 ± 0.2 GPa at 630–740 °C. Kyanite‐rich eclogites did not yield geologically meaningful Sm–Nd ages due to incomplete Nd isotope equilibration, whereas Sm–Nd multifraction garnet–omphacite regression for a low‐Mg eclogite from Kupplerbrunn yields an age of 91.1 ± 1.3 Ma. The Sm–Nd age of 94.1 ± 0.8 Ma obtained from the Fe‐rich core fraction of this garnet dates the initial stages of garnet growth. Zircon that also crystallized at eclogite facies conditions gives a weighted mean U–Pb SIMS age of 88.4 ± 8.1 Ma. Lu–Hf isotope analysis of a kyanite–eclogite from Kupplerbrunn yields 88.4 ± 4.7 Ma for the garnet–omphacite pair. Two low‐Mg eclogites from the Gertrusk locality of the Saualpe yield a multimineral Sm–Nd age of 90.6 ± 1.0 Ma. A low‐Mg eclogite from the Pohorje Mountains (70 km to the SE) gives a garnet–whole‐rock Lu–Hf age of 93.3 ± 2.8 Ma. These new age data and published Sm–Nd ages of metasedimentary host rocks constrain the final stages of the eo‐Alpine high‐pressure event in the Saualpe–Pohorje part of the south‐easternmost Austroalpine nappe system suggesting that garnet growth in the high‐pressure assemblages started at c. 95–94 Ma and ceased at c. 90–88 Ma, probably at the final pressure peak. Zircon and amphibole crystallization was still possible during incipient isothermal decompression. Rapid exhumation of the high‐pressure rocks was induced by collision of the northern Apulian plate with parts of the Austroalpine microplate, following Jurassic closure of the Permo‐Triassic Meliata back‐arc basin.     

Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu-Hf Isotopic Systematics

Hafnium isotopic compositions have been determined on a suiteof calc-alkaline and high-alumina-olivine tholeiitic lavas fromthe Lassen region of California and are used, in conjunctionwith previously published mineralogical, geochemical, and isotopicdata, to constrain their petrogenesis. Positive correlationbetween     

Geochemical Constraints on the Role of Oceanic Lithosphere in Intra-Volcano Heterogeneity at West Maui, Hawaii

Stratigraphically well-constrained sequences of late shield-buildingstage lavas from West Maui volcano, Hawaii, show age-dependentcompositional variability distinct from that seen in shield-stagelavas from any other Hawaiian volcano. These distinctions aredefined by ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb variation as well as⁸⁷Sr/⁸⁶Sr correlation with ²⁰⁶Pb/²⁰⁴Pb and trace element compositions.The West Maui lavas from stratigraphically higher in the sequencehave major and trace element and Sr–Pb–Hf–Ndisotopic compositions similar to Kea-type lavas sampled at theyounger Mauna Kea and Kilauea volcanoes, indicating that theKea compositional end-member of Hawaiian lavas has remainedhomogeneous over