Fractures, convection and underpressure: hydrogeology on the southern margin of the Piceance basin, west-central Colorado, USA

Grand Mesa is an erosional remnant on the southern margin of the Piceance basin (Colorado, USA) that appears to host topographically driven groundwater flow in low permeability strata via a pervasive network of vertical extensional fractures. The vertical fractures cut more than 1 km of clay-rich lithology ranging in age from Upper Cretaceous through Eocene, and likely formed from horizontal dilation, cooling, and erosional unloading associated with 2.8 km of regional uplift and 1.5 km of incision by the Gunnison and Colorado rivers. The vertical fractures create anisotropy in which vertical permeability exceeds horizontal permeability. This enhances vertical flow and depth of penetration of groundwater, favors local flow regimes over regional flow, and results in groundwater temperatures that are elevated by up to 10°C over mean surface temperatures at the location of springs. The uplift and cooling that formed the fractures may also have produced domains of abnormally underpressured pore fluids and natural gas within blocks of low permeability bedrock bounded by the fractures. Pore pressures inside these blocks may be in disequilibrium with the groundwater flow system due to ongoing stress release, and the long time scales required for pressure equilibration in the low permeability strata.     

A mechanism for the production of planetary nebulae

C¹² stars in the range 1.04–1.55M are evolved to simulate the core evolution of the possible precursors of planetary nebulae. The nuclear shell burning in stars above 1.2M advances to within about 0.2M of the surface, where the intense radiation interacts with the surface matter and causes mass loss. Comparison between our theoretical results and observations suggests that this may be a mechanism by which planetary nebulae are formed.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

Formation of iron sulfide nodules during anaerobic oxidation of methane

The biomarker compositions of iron sulfide nodules (ISNs; upper Pliocene Valle Ricca section near Rome, Italy) that contain the ferrimagnetic mineral greigite (Fe₃S₄) were examined. In addition to the presence of specific terrestrial and marine biomarkers, consistent with formation in coastal marine sediments, these ISNs contain compounds thought to originate from sulfate reducing bacteria (SRB). These compounds include a variety of low-molecular-weight and branched alkanols and several non-isoprenoidal dialkyl glycerol diethers (DGDs). In addition, archaeal biomarkers, including archaeol, macrocyclic isoprenoidal DGDs and isoprenoidal glycerol dialkyl glycerol tetraethers are also present. Both SRB and archaeal lipid δ¹³C values are depleted in ¹³C (δ¹³C values are typically less than −50‰), which suggests that the SRB and archaea consumed ¹³C depleted methane. These biomarker and isotopic signatures are similar to those found in cold seeps and marine sediments where anaerobic oxidation of methane (AOM) occurs with sulfate serving as the terminal electron acceptor. Association of AOM with formation of greigite-containing ISNs could provide an explanation for documented remagnetization of the Valle Ricca sediments. Upward migration of methane, subsequent AOM and associated authigenic greigite formation are widespread processes in the geological record that have considerable potential to compromise paleomagnetic records.     

Influence of the saffman force,lift force,and electric force on sand grain transport in a wind–sand flow

Quasi-horizontal trajectories of salting sand grains were found using high-speed video-recording in the desertified territory of the Astrakhan region. The sizes and displacement velocities of the saltating sand grains were determined. A piecewise logarithmic approximation of the wind profile in a quasi-stationary wind–sand flow is suggested, which is consistent with the data of observations and modeling. It was established that, in the regime of stationary saltation, the wind profile in the lower saltation layer of the wind–sand flow depends only slightly on the wind profile variations in the upper saltation layer. The vertical profiles of the horizontal wind component gradient in a quasi-stationary wind–sand flow were calculated and plotted. It was shown using high-speed video recording of the trajectory of a sand grain with an approximate diameter of 95 μm that the weightlessness condition in the desertified territory of the Astrakhan region in a stationary wind–sand flow is satisfied at a height of approximately 0.15 mm. The electric parameters of a wind–sand flow, which can provide for compensation of the force of gravity by the electric force, were estimated. In particular, if the specific charge of a sand grain is 100 μC/kg, the force of gravity applied to the sand grain can be compensated by the electric force if the vertical component of the electric field in a wind–sand flow reaches approximately 100 kV/m. It was shown that the quasi-horizontal transport of sand grains in the lower millimeter saltation layer observed in the desertified territory can be explained by the joint action of the aerodynamic drag, the force of gravity, the Saffman force, the lift force, and the electric force.     

Post-breakup basaltic magmatism along the East Greenland Tertiary rifted margin

Mafic and ultramafic intrusions in East Greenland adjacent to the offshore Greenland–Iceland ridge were emplaced 5–9 My after continental breakup at 55 Ma [1]. Rare earth element (REE) concentrations determined by secondary ion mass spectrometry are reported for cumulus clinopyroxene from these intrusions, and the data are used to estimate REE abundance in equilibrium melts using available partitioning data. Estimated equilibrium melts from intrusions have strongly fractionated REE patterns with Nd/Dy(N) in the range 2 to 5.6 and Yb/Dy(N) 0.55 to 0.92, similar to values for coeval basalts. These melts have markedly higher Nd/Dy(N) than earlier breakup related flood basalts. The moderately low Yb/Dy(N) for the post-breakup volcanism is indicative of residual garnet in the source, while their high Nd/Dy(N) ratios can best be explained by aggregating low degree melts from a light-REE-enriched garnet- and spinel-bearing mantle source. We also report He, Sr, and Nd isotopic data for the intrusions. The highest ³He/⁴He ratios (>10 R/R_a) are found in the samples whose REE data reflect the largest proportion of melts from a garnet-bearing source, and having Sr and Nd isotopic compositions identical with the radiogenic Sr and unradiogenic Nd isotope end of the Iceland compositional field. There is no indication of a MORB-type mantle in the source of the intrusions. We postulate that post-breakup volcanism along the East Greenland coastline reflects the increasing proximity of the mantle plume to the East Greenland continental margin. The low degree of melting at high mean pressure inferred for the parental melts for the intrusions may reflect re-thickening of the lithosphere, which in turn was caused by the vigorous volcanism during breakup, with accompanying depletion of upper mantle and underplating of the crust at the continental margin.     

Pb and other ore metals in modern seafloor tectonic environments: Evidence from melt inclusions

Many modern seafloor tectonic environments are host to hydrothermal systems and associated polymetallic sulfide deposits. Metal transport and precipitation are controlled by magmatic processes such as pre-eruptive degassing and the hydrothermal cycle. The original availability of Pb and other ore metals in a given setting is dependent on concentrations in the original magmatic source or additional enrichment processes. We have examined the Pb budget of melt inclusions from nine modern seafloor settings representing back-arcs, mid-ocean ridges and seamounts. Melt inclusions provide information on the characteristics of parental magmas, including insights into metal budgets. Trace element data in melt inclusions hosted in plagioclase, olivine and pyroxene were obtained by laser-ablation inductively-coupled mass-spectrometry.Results from back-arcs emphasize the impact of slab-subduction and dehydration processes on the chemical characteristics of generated magmas. Volatile- and fluid-mobile element-rich melt inclusions at Manus basin and Okinawa trough reflect a robust contribution of elements from the subducting slab as evidenced by relatively low Ce/Pb ratios. At Bransfield strait, on the other hand, melt inclusions are volatile poor, and fluid-mobile element ratios are similar to mid-ocean ridge values indicating little or no contribution from the slab. High Cu concentrations at Manus basin and Okinawa trough can be explained by fluxing of ferric iron from the subducting slab benefiting the production of sulfate over sulfide.Metal budgets for seamounts located on and nearby the axis of mid-ocean ridge segments appear to be independent of any input of mantle plume material. Results from the southern Explorer ridge (strong lower mantle influence, transitional- and enriched-MORBs), Pito and Axial seamounts (moderate lower mantle influence, transitional-MORBs) and a Foundation near-ridge seamount (little to no mantle influence, normal-MORB) show that, despite similar tectonic environments and varying contributions of mantle plume material, Cu, Zn and Pb values do not vary significantly between the enriched and non-enriched magma components of a given setting.     

SHRIMP zircon dating and Nd isotopic systematics of Palaeoproterozoic migmatitic orthogneisses in the Epupa Metamorphic Complex of northwestern Namibia

The Epupa Metamorphic Complex constitutes the southwestern margin of the Congo Craton and is exposed in a hilly to mountainous terrain of northwestern Namibia, bordering the Kunene River and extending into southern Angola. It consists predominantly of granitoid gneisses which are migmatized over large areas. This migmatization locally led to anatexis and produced crustal-melt granites such as the Otjitanda Granite. We have undertaken reconnaissance geochemical studies and single zircon U–Pb SHRIMP and Pb–Pb evaporation dating of rocks of the Epupa Complex. The granitoid gneisses, migmatites and anatectic melts are similar in composition and constitute a suite of metaluminous to peraluminous, calc-alkaline granitoids, predominantly with volcanic arc geochemical signatures. The zircon protolith ages for the orthogneisses range from 1861 ± 3 to 1758 ± 3 Ma. Anatexis in the migmatitic Epupa gneisses was dated from a melt patch at 1762 ± 4 Ma, and the anatectic Otjitanda Granite has a zircon age of 1757 ± 4 Ma. Migmatization and anatexis therefore occurred almost immediately after granitoid emplacement and date a widespread high-temperature Palaeoproterozoic event at ∼1760 Ma which has not been recorded elswhere in northern Namibia. The Nd isotopic systematics of all dated samples are surprisingly similar and suggest formation of the protolith from a source region that probably separated from the depleted mantle about 2.4–2.0 Ga ago. A major Archaean component in the source area is unlikely.