Melt Enrichment of Shallow Depleted Mantle: a Detailed Petrological, Trace Element and Isotopic Study of Mantle-Derived Xenoliths and Megacrysts from the Cameroon Line

Major element, trace element and Sr–Nd–Pb isotopiccompositions of ultramafic xenoliths and megacrysts from thecontinental Cameroon line provide evidence for metasomatismof the upper most lithospheric mantle by enriched melts duringthe Mesozoic The megacrysts probably crystallized within thelower continental crust from melts similar to the host magmas.All the xenoliths originated as depleted residues after theextraction of basaltic melts, but some indicate evidence ofinteraction with enriched partial melts before entrainment.The U–Pb isotopic data on garnet are consistent with coolingthrough >900C at >300 Ma. The Sm–Nd isotope systematicsin constituent phases appear to have been in equilibrium ona xenolith scale at the time of entrainment, indicating derivationfrom mantle that remained at temperatures >600C until eruption.Spinel therzolies that show simple light rare earth element(LREE) depletions are characterized by isotopic compositionsthat are comparable with, but slightly more depleted than AtlanticN-MORB, suggesting that the unmetasomatized sub-continentallithosphere of the Cameroon line may be isotopically similarto that of sub-oceanic lithosphere. The Nd-depleted mantle modelages of these xenoliths are consistent with late Proterozoicdepletion, similar in age to much of the overlying continentalcrust. In contrast, samples that have LREE-enriched clinopyr-oxenes(La/Yb =4.7–9.4) contain trace amounts of amphibole, areenriched in U and have more radiogenic Pb and Sr. These xenolithsyield U–Pb and Sm–Nd model ages consistent withMesozoic enrichment, in agreement with the age of enrichmentof the source regions of the basalts, as deduced from Pb isotopiccompositions. Clinopyroxenes record three orders of magnitudeenrichment in U and LREE accompanied by progressive K depletionassociated with the growth of trace amphibole, with K/U ratiosthat range from 12000 to 1. The ratios of the trace elementsthought to have similar bulk D in mantle melting, Ce/Pb, Ba/Rband Nd/Sr ratios, display regional variations related to thetime integrated history of enrichments indicated by Nd isotopiccompositions. Mass balance calculations suggest that the meltsresponsible for the most recent enrichment of the lithospherehad higher La/Yb and U/Pb than Cameroon line host magmas, andwere probably the product of small degrees of partial meltingassociated with the earliest stages of the breakup of Pangea. KEY WORDS: Cameroon line; mantle xenoliths; megacrysts; REE; isotopic composition; trace element     

FACTOR ANALYSIS OF A SIMULATED DATA MATRIX INVOLVING AQUEOUS COMPLEX EQUILIBRIA

Abstract factor analyses were performed on databases consisting of simulated samples from aqueousequilbria.The program COMPLEX was used to generate equilibrium species in a system of three reactantmetals and five reactant bases.Reactant concentrations and pH were drawn from random-normaldistributions so that sample data vectors comprised a multivariate log-normal distribution of equilibriumconcentrations.In addition,sample groups were created containing different distributions for pH andreactant concentrations.Equilibrium species were shown to contain variance contributed by change in pH among samples aswell as change in reactant concentrations.Factor modelling revealed the qualitative relationships amongthe species and how the relationships change with pH.Factors also revealed those reactants containingvariance in the data matrix.In some cases,reactant variance obscured relationships between pH and theequilibrium species.Since factor modelling of a simulated data matrix revealed the expected chemical equilibriuminteractions,a potentially powerful tool exists for investigating the effects of outliers and error.     

RANK-ORDER ANALYSIS FOR ROBUST MULTIRESPONSE,MULTIBLOCK COMPARISONS:EVALUATION OF HERBICIDE INTERACTIONS

Studies of interactions among bioactive compounds are often difficult to interpret unambiguously.A priori assumptions about the nature of such interactions can seriously distort analysis of the data.Byapplying a rank order analysis appropriate to the naturally ordinal scale of response to xenobiotic insult,several co-herbicides were successfully identified from among numerous candidates in an experimentinvolving multiple blocks,rates and species.Moreover,underlying herbicide interactions weresubstantiated and identified which were not apparent by more traditional parametric analysis.     

Acanthodian scales and worm tubes from the Kapp Kjeldsen Division of the Lower Devonian Wood Bay Formation, Spitsbergen

Results of a thin section study of bonebeds from the Kapp Kjeldsen Division are presented and discussed. The material for this study was collected in the talus on the south-eastern side of Bockfjorden, northern Spitsbergen. The stratigraphical position of the Kapp Kjeldsen Division within the Devonian strata of Spitsbergen is shown in Table 1.
The thin sections for this study were not made to standard thickness, but are slightly thicker for better resolution of the vertebrate material. Each thin section was individually polished. The photos are made with crossed nichols.     

Insect traces from nonmarine ephemeral puddles

Metz, Robert 1987 06 01: Insect traces from nonmarine ephemeral puddles. Boreas , Vol. 16, pp. 189–195. Oslo. ISSN 0300–9483.
Field observations of nonmarine clay-lined ephemeral puddles have revealed numerous exogenic traces formed by a variety of insects. Most of these traces are described here for the first time. A number of different insects produced similar appearing traces. Relatively rapid drying of the shallow puddles often traps the insect, so that the trace and its maker are initially well preserved. This dessication process, in contrast to many lakes, streams and ponds having deeper water conditions, offers high potential for the exogenic trace to be preserved as a trace fossil. Trace fossil forms similar to those documented for ephemeral puddles include Gordia, Cochlichnus, Lithographus, Helminthopis and Isopodichnus . Ephemeral puddles should be considered a prime candidate for the origin of many of the trace fossils found in ancient nonmarine sedimentary rocks.     

Isotopic and Chemical Evidence Concerning the Genesis and Contamination of Basaltic and Rhyolitic Magma Beneath the Yellowstone Plateau Volcanic Field

Since 2.2 Ma, the Yellowstone Plateau volcanic field has produced6000 km³ of rhyolite tuffs and lavas in >60 separate eruptions,as well as 100 km³ of tholeiitic basalt from >50 vents peripheralto the silicic focus. Intermediate eruptive products are absent.Large calderas collapsed at 2?0, 1?3, and 0?6 Ma on eruptionof ash-flow sheets representing at least 2500, 280, and 1000km³ of zoned magma. Early postcollapse rhyolites show largeshifts in Nd, Sr, Pb, and O isotopic compositions caused byassimilation of roof rocks and hydrothermal brines during collapseand resurgence. Younger intracaldera rhyolite lavas record partialisotopic recovery toward precaldera ration. Thirteen extracalderarhyolites show none of these effects and have sources independentof the subcaldera magma system. Contributions from the Archaeancrust have extreme values and wide ranges of Nd-, Sr-, and Pb-isotoperatios, but Yellowstone rhyolites have moderate values and limitedranges. This requires their deep-crustal sources to have beenpervasively hybridized (and the Archaean components diluted)by distributed intrusion of Cenozoic basalt, most of which wasprobably contemporaneous with the Pliocene and Qualernary volcanism.In hybrid sources yielding magmas parental to the subcalderarhyolites, half or more of the Nd and Sr may have been contributedby such young basalt. Parents for the extracaldera rhyolites,generated beyond the leading edge of the northeast-propagatingfocus of basaltic intrusion and deep-crustal mobilization, containedsmaller fractions of mantle-derived components. Most Yellowstone basalts had undergone cryptic clinopyroxenefractionation in the lower crust or crust-mantle transitionzone and, having also ascended through or adjacent to crustalzones of silicic-magma generation, most underwent some crustalcontamination. A high fraction of the Pb in most basalts isof crustal derivation. Anomalously low seismic velocities toa depth of 250 km and a high flux of ³He at Yellowstone suggestsublithospheric magma contributions. Elevated baseline Nd- andSr-isotopc ratios suggest additional contributions from oldlithospheric mantle, but this is hard to quantify because ofthe crustal overprint. Foundering of crustally contaminatedmain-stage cumulates into the low-viscosity upper mantle beneaththe principal focus of basaltic intrusion may influence theisotopic compositions of low-K tholeiites and Snake River olivinetholeiites subsequently generated along the Snake River Plainaxis in the wake of the main migrating melting anomaly.     

An Experimental Study of the Sulfur Content in Basaltic Melts Saturated with Immiscible Sulfide or Sulfate Liquids at 1300{degrees}C and 1{middle dot}0 GPa

The sulfur content in basaltic melts coexisting with eithersulfide or sulfate melts was determined experimentally. Theexperimental conditions were in the range of 1300–1355°Cand 1·0–1·6 GPa, conditions appropriatefor the melting of the upper mantle above subduction zones.Under these conditions, both sulfide and sulfate were presentas immiscible liquids, as inferred from the round geometriesof the quenched sulfide and sulfate phases. The measured S contentin basaltic melts saturated with sulfate liquids ([S] = 1·5± 0·2 wt %) was 10 times higher than the S contentin basaltic melts saturated with sulfide liquids ([S] = 0·14± 0·02 wt %). In our experiments, sulfate liquidswere stable at fO₂ as low as FMQ = +1·85 [FMQ = log (fO₂)_sample– log (fO₂)_FMQ, where FMQ is the fayalite–magnetite–quartzoxygen buffer], and evidence from other sources indicates thatsulfates will be stable at lower fO₂ in melts with lower activitiesof silica. Because chalcophile and highly siderophile elements,such as Cu, Ni, Au, and Pd, are partitioned preferentially intosulfide phases, melting of sufficiently oxidized sources, inwhich sulfides are not stable, would favor incorporation ofthese elements into the silicate melt produced. Such melts wouldhave a higher potential to generate ore deposits. This studyshows that the high sulfur contents of such oxidized basaltsalso means that relatively small amounts of such magmas canprovide significant amounts of sulfur to exsolving volatilephases and account for the bulk of the sulfur expelled in somevolcanic eruptions, such the 1991 eruption of Mount Pinatubo. KEY WORDS: basalt; mantle; oxidation state; sulfate; sulfur