Fossil mammoths from Santa Cruz Island, California

Mammoth remains on Santa Cruz Island, one of the four Northern Channel Islands of California, are very sparse, in marked contrast to those reported from Santa Rosa and San Miguel Islands of the same island group. A probable major reason for this scarcity is that Quaternary deposits are greatly restricted on Santa Cruz Island. It is proposed, contrary to popular opinion, that fossils found on Santa Cruz Island were derived from animals which died on the island, and were not transported there by humans. Reasons for this conclusion are that the size and geological context of the fossils are similar to those of the largest mammoth fossils of Santa Rosa Island, and that, in spite of extensive investigations by many persons, mammoth remains have not been found in middens, either on the islands or on the adjacent mainland.     

The fate of Chernobyl radiocesium in the River Öre catchment,northern Sweden

A¹³⁷Cs-balance for the catchment of the River Öre in central northern Sweden which received about 30 kBq m^–2 of radiocesium from the Chernobyl accident was calculated for the period 1986–1991. Altogether, slightly less than 10% of the total deposition in the catchment was estimated to be exported from the terrestrial parts during this period of time. More than 90% of this loss is transported with the River Öre to the outer sea of the Gulf of Bothnia. The retention in Lake Örträsket which is the only lake along the river course and the Öre Estuary outside the river mouth was thus slightly less than 10%. Nearly all of the radiocesium deposited in the lake is permanently retained in the sediments and successively covered with less radioactive sediment. A considerable export of radiocesium from the estuary to the outer sea takes place due to resuspension and subsequent transport by wind and wave generated currents.     

Ferrobasalts from the Spiess Ridge segment of the Southwest Indian Ridge

Highly vesicular, microporphyritic basaltic rocks have been dredged from the slow-spreading Spiess Ridge segment of the Southwest Indian Ridge. All the samples recovered are hyalocrystalline with plagioclase, clinopyroxene and olivine as phenocryst and microphenocryst phases. Titanomagnetite occurs as euhedral microphenocrysts in some of the more evolved samples. In terms of bulk rock and quench glass chemistry the lavas are characterised by highly evolved compositions(e.g. FeO*=10.3−14.2%;TiO₂=2.0−3.4%;K₂O=0.50−1.1%;MgO=6.0−3.5%;Zr=160−274ppm;Nb=14−32ppm) and can be classified as ferrobasalts. Isotopic and incompatible element ratios of the lavas(e.g.⁸⁷Sr/⁸⁶Sr=0.70325−0.70333;Zr/Nb=8.4−11.3;Y/Nb=2.3−1.4) indicate their strongly “enriched” nature (see also Dickey et al. [6]).

Quantitative major and trace element modelling indicates that most of the compositional variations observed can be attributed to low-pressure fractional crystallisation of plagioclase, clinopyroxene and minor olivine and titanomagnetite. The range in composition can be accounted for by up to 65% fractional crystallisation.

We suggest that the extreme differentiation of the Spiess Ridge lavas is related not to spreading rate, but to rate of magma supply. The basaltic melts appear to have evolved in a newly established zone of magmatic activity, associated with the most recent northward jump of the Bouvet triple junction, where they were effectively isolated from significant admixture of primitive magmas.     

Correlated Nd,Sr and Pb isotope variation in Walvis Ridge basalts and implications for the evolution of their mantle source

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge [13, 14]. The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and consistent with formation at the paleo mid-ocean ridge [14]. The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other.The low ¹⁴³Nd/¹⁴⁴Nd (0.51238), ²⁰⁶Pb/²⁰⁴Pb (17.54), ²⁰⁸Pb/²⁰⁴Pb (15.47), ²⁰⁸Pb/²⁰⁴Pb (38.14) and high⁸⁷Sr/⁸⁶Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan de Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher¹⁴³Nd/¹⁴⁴Nd (0.51270), ²⁰⁶Pb/²⁰⁴Pb (18.32), ²⁰⁷Pb/²⁰⁴Pb (15.52), ²⁰⁸Pb/²⁰⁴Pb (38.77) and lower ⁸⁷Sr/⁸⁶Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing od depleted and enriched end member melts or partial melting of an inhomogenous, variably enriched mantle source. However, observe ZrBaNbY interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.     

Origin of dacites of Taupo Volcanic Zone, New Zealand

Dacites form a relatively small proportion of lavas in the Taupo Volcanic Zone, New Zealand (5km³), and occur mainly on the eastern side. In this paper their origin is considered in terms of three processes: (a) partial melting of crustal rocks; (b) fractional crystallisation of basalt and andesite; and (c) sub-surface mixing of basic and acid magma. Modelling techniques are used to calculate the most acceptable degree of fractional crystallisation and magma mixing to fit major-element data, and these values are used to compare calculated and observed trace-element values. The success or failure of the model is determined by the closeness of the two sets of values. For partial-melt models, trace-element values alone are calculated by the batch-melting equation.Results indicate that White Island dacite can best be modelled by fractional crystallisation; Manawahe by fractional crystallisation plus limited crustal contamination; Maungaongaonga by partial melting of Western Basement greywacke, and Tauhara by partial melting of this greywacke together with minor mixing with a more basic magma. Results from Parekauau and Horohoro indicate that these lavas are unlikely to have formed by any of the processes examined.     

Oceanic magnetic anomaly amplitudes: variation with sea-floor spreading rate and possible implications

A qualitative examination of oceanic magnetic anomalies suggests that their amplitudes increase with the spreading rate of the ocean floor. This suggestion has been investigated quantitatively by inversion of the anomalies. A variance matching technique has been used to calculate the average magnetisation of the crust with crustal thickness held constant or to calculate the thickness with the magnetisation fixed. In spite of variations in the computed intensities of magnetisation and thickness when compared to spreading rates the results show an over-all increase in the intensities with the rate of spreading. The mechanisms responsible for this could include greater irregularity in distribution of magnetic polarities within the crust at slow spreading ridges, greater intensity of magnetisation in crust produced at fast spreading due to initial chemical properties or enhanced hydrothermal alteration, and a dependence of crustal thickness on spreading rate.     

Mesozoic interaction of the Kula plate and the western margin of north america

Oceanic crust west of North America at the beginning of the Jurassic belonged to the Kula plate. The development of the western margin of North America since the Jurassic reflects interaction with the Kula plate, the Kula-Farallon spreading center and the Farallon plate. The Kula plate ceased to exist in the Paleocene and later developments were caused by interaction of the Farallon plate and, subsequently, collision with the East Pacific Rise.At the beginning of the Jurassic, when spreading between North and South America began, the Kula-Farallon-Pacific triple junction moved to the north relative to North America, and the eastern end of the Kula-Farallon spreading center swept northwards along the continental margin.During the Paleocene, Kula-Pacific spreading ceased and the Kula plate fused to the Pacific plate. Throughout the Mesozoic, subduction of the Kula plate took place along the Alaskan continental margin. When the Kula plate joined the Pacific plate a new subduction zone formed along the line of the present Aleutian chain.Wrangellia and Stikinia, anomalous terrains in Alaska and northwestern Canada respectively, were emplaced by transport on the Kula plate from lower latitudes. Hypotheses which require transport of these plates in the Mesozoic from the “far reaches of the Pacific” ignore the problem of transport across either the Kula-Pacific or Kula-Farallon spreading centers. The interaction of the Kula plate and western North America throughout the Jurassic and the Cretaceous should result in emplacement of these terrains by motion oblique to the continental margin. Tethyan faunas in Stikinia must come from the western end of Tethys between North and South America, not the Indonesian region at the eastern end of Tethys.As the northeastern end of the Kula-Farallon ridge moved northward, the sense of motion changed from right lateral shear between the Kula and North American plates to collision or left lateral shear between the Farallon and North American plates. Left lateral shear along zones analogous to the Mojave-Sonora megashear may have been the means by which anomalous terrains were transported to the southeast into the gap between North and South America forming present day Central America. Such a model overcomes the overlap difficulties suffered in previous attempts to reconstruct the Mesozoic paleogeography of Central America.     

Siderophile and chalcophile metal variations in Tertiary picrites and basalts from West Greenland with implications for the sulphide saturation history of continental flood basalt magmas

Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nuussuaq Peninsula in West Greenland, and they include ～1,000 m of picritic lavas and discrete 10- to 50-m-thick members of highly contaminated basalts. On Qeqertarssuaq, the lavas are allocated to the Vaîgat and Maligât Formations of which the former includes the Naujánguit member, which consists of picrites with 7–29 wt% MgO, 80–1,400 ppm Ni, 5.7–9.4 ppb Pt and 4.2–12.9 ppb Pd. The Naujánguit member contains two horizons of contaminated basalts, the Asûk and Kûgánguaq, which have elevated SiO2 (52–58 wt%) and low to moderate MgO (7.5–12.8 wt%). These lavas are broadly characterized by low Cu and Ni abundances (average, 40 ppm Ni and 45 ppm Cu) and very low Pt (0.16–0.63 ppb) and Pd (0.13–0.68 ppb) abundances, and in the case of the Asûk, they contain shale xenoliths and droplets of native iron and troilite. The contaminated basalts from Nuussuaq, the B0 to B4 members, are also usually Ni-, Cu-, and platinum-group elements (PGE)-depleted. The geochemical signatures (especially the ratios of incompatible trace elements such as Th/Nb) of all of the contaminated basalts from Qeqertarssuaq and some of those from Nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas. This suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks, and hence, at a high crustal level. Nickel, Cu, and PGE depletion together with geochemical signatures produced by crustal contamination are also a feature of Siberian Trap basalts from the Noril’sk region. These basalts belong to the 0- to 500-m thick, ～5,000- to 10,000-km³ Nadezhdinsky Formation, which is centered in the Noril’sk Region. A major difference between Siberia and West Greenland is that PGE depletion in the Nadezhdinsky Formation samples with the lowest Cu and Ni contents is much more severe than that of the West Greenland contaminated basalts. Moreover, the volumes of the contaminated and metal-depleted volcanic rocks in West Greenland pale is significant when compared to the Nadezhdinsky Formation; local centers rarely contain more than 15 thin flows with a combined thickness of <50 m and more typically 10–20 m, so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the Nadezhdinsky edifice. The West Greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the Tertiary delta, and so emplacement along N–S structures appears to be a principal control on the distribution of lavas and feeder intrusions. This leads us to suggest that the Greenland system is small and segregation of sulphide took place at high levels in the crust, whereas at Noril’sk, the saturation event took place at depth with subsequent emplacement of sulphide-bearing magmas into high levels of the crust. As a consequence, it may be unreasonable to expect that the West Greenland flood basalts experienced mineralizing processes on the scale of the Noril’sk system.     

Coarsening of three-dimensional structured and unstructured grids for subsurface flow

We present a generic, semi-automated algorithm for generating non-uniform coarse grids for modeling subsurface flow. The method is applicable to arbitrary grids and does not impose smoothness constraints on the coarse grid. One therefore avoids conventional smoothing procedures that are commonly used to ensure that the grids obtained with standard coarsening procedures are not too rough. The coarsening algorithm is very simple and essentially involves only two parameters that specify the level of coarsening. Consequently the algorithm allows the user to specify the simulation grid dynamically to fit available computer resources, and, e.g., use the original geomodel as input for flow simulations. This is of great importance since coarse grid-generation is normally the most time-consuming part of an upscaling phase, and therefore the main obstacle that has prevented simulation workflows with user-defined resolution. We apply the coarsening algorithm to a series of two-phase flow problems on both structured (Cartesian) and unstructured grids. The numerical results demonstrate that one consistently obtains significantly more accurate results using the proposed non-uniform coarsening strategy than with corresponding uniform coarse grids with roughly the same number of cells.