Evidence for carbonate platform failure during rapid sea‐level rise; ca 14 000 year old bioclastic flow deposits in the Lesser Antilles

Bioclastic flow deposits offshore from the Soufrière Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south‐west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse‐grained and either ungraded or poorly graded, and were deposited by non‐cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub‐units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub‐units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi‐stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea‐level change.     

Origin of Phenocrysts and Compositional Diversity in Pre-Mazama Rhyodacite Lavas, Crater Lake, Oregon

Phenocrysts in porphyritic volcanic rocks may originate in avariety of ways in addition to nucleation and growth in thematrix in which they are found. Porphyritic rhyodacite lavasthat underlie the eastern half of Mount Mazama, the High Cascadeandesite/dacite volcano that contains Crater Lake caldera, containevidence that bears on the general problem of phenocryst origin.Phenocrysts in these lavas apparently formed by crystallizationnear the margins of a magma chamber and were admixed into convectingmagma before eruption. About 20 km³ of pre-Mazama rhyodacite magma erupted during arelatively short period between400 and 500 ka; exposed pre-Mazamadacites are older and less voluminous. The rhyodacites formedas many as 40 lava domes and flows that can be assigned to threeeruptive groups on the basis of composition and phenocryst content.Phenocryst abundance decreases (from 32 to 8 vol.%) and SiO₂content increases (from 68 to 73 wt.%) in the apparent orderof eruption. Phenocrysts (plagioclase, orthopyroxene, augite,and Fe-Ti oxides) are commonly fragmental or form polycrystallineaggregates with interstitial glass. Discrete phenocrysts withcomplete euhedral outlines are rare except for small elongatedcrystals. The abundance of discrete phenocrysts increases withthat of aggregates. The grain-size of minerals in the aggregatescovers the range of discrete phenocrysts (0.2–4.2 mm).Rim compositions of phenocrysts and the range of chemical zoningare almost uniform among the three rhyodacite groups, regardlessof whether crystals are discrete or in aggregates. However,a small fraction of phenocrysts, especially small elongatedcrystals, have different compositions: plagioclase with Fe-richcores and augite with Wo-poor cores, both of which are characteristicof crystals in undercooled andesite enclaves in the rhyodacites.The majority of phenocrysts were derived by disintegration ofpolycrystalline aggregates; rare, small phenocrysts crystallizedin andesitic magma similar to that represented by the andesiteenclaves. The modal and chemical compositions of the rhyodacites can beexplained by different degrees of admixing of crystals, representedby the aggregates, into magma having 4 vol.% ‘true’phenocrysts, mainly plagioclase. The aggregates may be partsof the rind formed by in situ crystallization near the walland roof of the magma chamber. The rind was disrupted duringor just before eruption, and pieces were variably disaggregatedand incorporated into erupting magma. The amount of rind incorporateddeclined during the sequence of eruptions. Owing to vesiculationof interstitial liquid and shearing during flow, crystals inthe aggregates were separated and became phenocrysts. Pre-Mazamarhyodacite was erupted dominantly as lava, as opposed to thecompositionally similar rhyodacite pumice of the Holocene caldera-formingeruption of Mount Mazama, apparently because its source chamberwas crystallizing inward rather than actively growing.     

Petrology of Lower Crustal and Upper Mantle Xenoliths from the Cima Volcanic Field, California

Basaltic rocks of the Cima volcanic field in the southern Basinand Range province contain abundant gabbro, pyroxenite, andperidotite xenoliths. Composite xenoliths containing two ormore rock types show that upper-mantle spinel peridotite wasenriched by multiple dike intrusions in at least three episodes;the mantle was further enriched by intergranular and shear-zonemelt infiltration in at least two episodes. The oldest dikes,now metamorphosed, are Cr-diopside websterite. Dikes of intermediateage are most abundant at Cima and consist of igneous-texturedwebsterite and two-pyroxene gabbro and microgabbro of tholeiiticor calcalkalic parentage. The youngest dikes are igneous-texturedclinopyroxenite, gabbro, and olivine microgabbro of alkalicparentage. The dikes in peridotite are interpreted as partsof a system of conduits through which tholeiitic (or calcalkalic)and alkalic magmas fed lower-crustal intrusions, which are representedby abundant xenoliths of the same igneous rock types as observedin the dikes. Mineral assemblages of dikes in peridotite indicatethat an enriched uppermost mantle zone no thicker than 15 kmcould have been sampled. Because of their high densities, thegabbros and pyroxenites can occupy the zone immediately abovethe present Moho (modeled on seismic data as 10-13 km thick,with V_p 6.8 km/s) only if their seismic velocities are reducedby the joints, partial melts, and fluid inclusions that occurin them. Alternatively, these xenoliths may have been derivedentirely from beneath the Moho, in which case the Moho is notthe local crust-mantle boundary.     

Carbonate mineral cycles generated by foraminiferal and pteropod response to Pleistocene climate: west Florida ramp slope

Carbonate ramps are gently sloping depositional surfaces where shallow-water, coarse-grained facies pass basinward into fine-grained, deep-water sediments, with no abrupt change in slope. The objectives of this study are: (i) to integrate the depositional processes recorded in the Pleistocene stratigraphy of the west Florida outer ramp into a palaeoclimatic and palaeoceanographic framework for the eastern Gulf of Mexico; and (ii) to examine the origin of mineralogical and sedimentary cycles in the light of pteropod and planktonic foraminiferal populations corresponding to climatic oscillations. Aragonitic, pteropod-rich sediments with large amounts of insoluble residue occur in sediments deposited during glacial intervals; sandy calcitic sediments with abundant planktonic foraminifera accumulate during interglacials. These cycles reflect variations in biological productivity of pelagic pteropods and planktonic foraminifera, rather than preferential dissolution of either aragonitic or calcitic fractions. Species assemblages suggest that the productivity cycles are linked to changes in upwelling intensity at the margins of the Loop Current and variations in water mass salinities, as well as terrigenous dilution from the Mississippi Delta. These cycles are the response to Pleistocene glacial-interglacial oscillations, controlled by Milankovitch orbital parameters. Although the organisms contributing to deep-water carbonate environments have changed through geological time, facies patterns, as well as sedimentary textures and structures, identified in the west Florida sediments provide criteria for recognition of ancient ramps. An understanding of the processes on a modern ramp slope, such as west Florida, may prove valuable in palaeoclimatic and palaeoenvironmental analysis of epicontinental carbonate sequences and ramps in the rock record.     

Petrology of Mantle Xenoliths from Harrat al Kishb: The Mantle beneath Western Saudi Arabia

Tertiary to Recent continental rifting and sea floor spreadingformed the Red Sea. Mantle xenoliths from the Saudi ArabianRed Sea margin provide an opportunity to study the mantle beneaththe flanks of this young ocean basin. The Harrat al Kishb mantlexenolith suite consists of Cr-diopside group spinel harzburgiteand lherzolite mantle wall rock, and a variety of pyroxenitesproduced by crystallization from mafic magmas within the mantle.The pyroxenites include two texturally distinct varieties ofCr-diopside group spinel websterites, and Al-augite group spinelpyroxenite, garnet-spinel websterite, and garnet-bearing spinelclinopyroxenite. All Harrat al Kishb xenoliths are deformedto some degree and many are recrystallized. Mineral exsolutionand zoning textures indicate reequilibration to decreasing temperatureconditions. Several xenoliths provide evidence for metasomaticprocesses in the mantle beneath western Saudi Arabia. Estimates of peridotite temperatures are 900–980?C withpressure bracketed between 13 and 19 kb. Al-augite spinel pyroxenitesyield temperatures of 1050–1070?C. Garnet-spinel websteritesyield temperatures and pressures in the range 1000–1030?C,13.8–16.5 kb. These P-T estimates show that mantle temperatures are elevatedwell above those predicted by low surface heat flow measurements.Mantle heating associated with rifting is young enough thatsurface heat flow has not yet equilibrated. The xenolith dataare consistent with a model of asthenosphere upwelling beneaththe Red Sea rift. Comparison of xenolith data with existingseismic refraction data reveals a coherent picture of the compositionof the western Saudi Arabian lithosphere.     

Cosmogenic radionuclide dating indicates ice‐sheet configuration during MIS 2 on Nordaustlandet,Svalbard

Hormes, A., Akçar, N. & Kubik, P. W. 2011: Cosmogenic radionuclide dating indicates ice‐sheet configuration during MIS 2 on Nordaustlandet, Svalbard. Boreas, 10.1111/j.1502‐3885.2011.00215.x. ISSN 0300‐9483.0300‐9843 Glacial geological field surveys, aerial image interpretation and cosmogenic radionuclide (CRN) dating allowed us to reconstruct the ice‐sheet configuration on Nordaustlandet, the northernmost island of the European sector on the margin of the Arctic Ocean. The timing of deglaciation was investigated by determining the ²⁶Al and ¹⁰Be ages of glacially scoured bedrock, weathered periglacial blockfields and glacial erratic boulders. Only ¹⁰Be ages were useful for our interpretations, because of unresolved analytical problems with ²⁶Al. Fjords and lowlands on Nordaustlandet yielded Late Weichselian ¹⁰Be ages, indicating that actively erosive ice streams scoured the coastal fjord bathymetry during marine isotope stage (MIS) 2. In Murchisonfjorden, ground‐truthed air‐photograph interpretation and ¹⁰Be ages of boulders indicated a cold‐based glacier ice cover during MIS 2 on higher plateaus. ¹⁰Be ages and lithological studies of erratic boulders on higher and interior plateaus of Prins Oscars Land (>200–230 m a.s.l.) suggest that the Mid‐Weichselian glaciation (MIS 4) might have been more extensive than that during MIS 2.