The Significance of Multiple Saturation Points in the Context of Polybaric Near-fractional Melting

Experimental petrologists have successfully located basalticliquid compositions parental to mid-ocean ridge basalt thatare, within experimental resolution, multiply saturated withthree-phase harzburgite or four-phase lherzolite assemblageson their liquidus at some elevated pressure. Such an experimentalresult is a necessary consequence of any paradigm in which eruptedbasalts derive from single-batch primary liquids that equilibratewith a mantle residue and undergo no subsequent magma mixingbefore differentiation and eruption. Here we investigate whether,conversely, such evidence of multiple saturation is sufficientto exclude dynamic melting models wherein increments of meltare mixed after segregation from residues, during melt transportor in magma chambers. Using two independent models of crystal–liquidequilibria to simulate polybaric near-fractional peridotitemelting, we find that aggregate liquids from such melting processescan display near-intersections of liquidus surfaces too closeto distinguish experimentally from exact multiple saturationpoints. Given uncertainties in glass compositions, fractionationcorrections, experimental temperature and pressure conditions,and achievement of equilibrium, these results suggest that polybaricmixtures can in fact masquerade as mantle-equilibrated single-batchprimary liquids. Multiple saturation points on the liquidussurfaces of primitive basalts do, however, preserve informationabout the average pressure of extraction of their constituentincrements of liquid. KEY WORDS: mantle melting; basaltic volcanism; experimental igneous petrology; thermodynamic modelling; inverse method     

Large-scale glacial thrusting and related processes in North Dakota

Large-scale thrusting by the glacial ice occurred in many parts of the Interior Plains of North Dakota, Alberta, and Saskatchewan in late Wisconsinan time. Thrust features are especially prominent in parts of North Dakota. Many of the topographic features of the glaciated landscape of North Dakota formed, either entirely or in part, by the thrusting mechanism, and many individual ice-thrust features have been recognized. The ice-thrusting process was also related to fluting by the glacier, and, as a result, drumlins and other types of fluted features are commonly closely associated with ice-thrust topography. Thrusting by the glacier was dependent primarily on groundwater conditions beneath the ice; thrusting occurred only where hydrologic conditions were appropriate. Most of the individual ice-thrust features that have been recognized are located over discrete aquifers, and the sizes and shapes of the thrust features are dependent upon the sizes and shapes of the aquifers.     

Evolution of Miocene fluvial environments, eastern Potwar plateau, northern Pakistan

The Miocene-Pliocene Siwalik Group records changing fluvial environments in the Himalayan foreland basin. The Nagri and Dhok Pathan Formations of this Group in the eastern Potwar Plateau, northern Pakistan, comprise relatively thick (tens of metres) sandstone bodies and mudstones that contain thinner sandstone bodies (metres thick) and palaeosols. Thick sandstone bodies extend for kilometres normal to palaeoflow, and are composed of large-scale stratasets (storeys) stacked laterally and vertically adjacent to each other. Sandstone bodies represent single or superimposed braided-channel belts, and large-scale stratasets represent channel bars and fills. Channel belts had widths of km, bankfull discharges on the order of 10³ cumecs and braiding parameter up to about 3. Individual channel segments had bankfull widths, maximum depths, and slopes on the order of 10² m, 10¹ m and 10^?4 respectively, and sinuosities around 1-1. These rivers are comparable to many of those flowing over the megafans of the modern Indo-Gangetic basin, and a similar depositional setting is likely. Thin sandstone bodies within mudstone sequences extend laterally for on the order of 10² m and have lobe, wedge, sheet and channel-form geometries: they represent crevasse splays, levees and floodplain channels. Mudstones are relatively bioturbated/disrupted and represent mainly floodbasin and lacustrine deposition. Mudstones and sandstones are extremely disrupted in places, showing evidence of prolonged pedogenesis. These ‘mature’ palaeosols are m thick and extend laterally for km. Lateral and vertical variations in the nature of their horizons apparently depend mainly on deposition rate. The 500 m-thick Nagri Formation has a greater proportion and thicker sandstone bodies than the overlying 700 m-thick Dhok Pathan Formation. The thick sandstone bodies and their large-scale stratasets thicken and coarsen through the Nagri Formation, then thin and fine at the base of the Dhok Pathan Formation. Compacted deposition rates increase with sandstone proportion (0-53 mm/year for Nagri, 0-24 mm/year for Dhok Pathan), and palaeosols are not as well developed where deposition rates are high. Within both formations there are 100 m-scale variations (representing on the order of 10⁵ years) in the proportion and thickness of thick sandstone bodies, and tens-of-m-scale alternations of thick sandstone bodies and mudstone-sandstone strata that represent on the order of 10⁴ years. Formation-scale stratal variations extend across the Potwar Plateau for at least 100 km, although they may be diachronous: however, 100-m and smaller scale variations can only be traced laterally for up to tens of km. Alluvial architecture models indicate that increases in the proportion and thickness of thick sandstone bodies can be explained by increasing channel-belt sizes (mainly), average deposition rate and avulsion frequency on a megafan comparable in size to modern examples. 100-m-scale variations in thick sandstone-body proportion and thickness could result from ‘regional’ shifts in the position of major channels, possibly associated with ‘fan lobes’on a single megafan or with separate megafans. However, such variations could also be related to local changes in subsidence rate or changes in sediment supply to the megafan system. Formation-scale and 100-m-scale stratal variations are probably associated with interelated changes in tectonic uplift, sediment supply and basin subsidence. Increased rates of hinterland uplift, sediment supply and basin subsidence, recorded by the Nagri Formation, may have resulted in diversion of a relatively large river to the area. Alternatively, changing river sizes and sediment supply rates may be related to climate changes affecting the hinterland (possibly linked to tectonic uplift). Climate during deposition of the Siwalik Group was monsoonal. Although the deposits contain no direct evidence for climate change, independent evidence indicates global cooling throughout the Miocene, and the possibility of glacial periods (e.g. around 10-8 Ma, corresponding to base of Nagri Formation). If the higher Himalayas were periodically glaciated, a mechanism would exist for varying sediment supply to megafans on time scales of 10⁴-10⁵ years. Although eustatic sea-level changes are related to global climatic change, they are not directly related to Siwalik stratigraphic changes, because the shoreline was many 100 km away during the Miocene.     

Late‐Quaternary palaeoclimatic research in Fennoscandia – A historical review

Birks, H. J. B. & Seppä, H. 2010: Late‐Quaternary palaeoclimatic research in Fennoscandia – A historical review. Boreas, Vol. 39, pp. 655–673. 10.1111/j.1502‐3885.2010.00160.x. ISSN 0300‐9483. Ideas and understanding of Late‐Quaternary climatic history, especially in the Lateglacial and the postglacial, were initiated in Fennoscandia in the 19th century with pioneering studies on peat stratigraphy, megafossils and macrofossils. This review traces the history of palaeoclimatic research and the development of current ideas about climatic changes, with particular reference to the Lateglacial and its rapid climatic fluctuations, such as the Younger Dryas period, and to the Holocene, with its thermal maximum and shifts in precipitation.     

Salinity reflux and dolomitization of southern Australian slope sediments: the importance of low carbonate saturation levels

Anomalously saline waters in Ocean Drilling Program Holes 1127, 1129, 1130, 1131 and 1132, which penetrate southern Australian slope sediments, and isotopic analyses of large benthic foraminifera from southern Australian continental shelf sediments, indicate that Pleistocene–Holocene meso‐haline salinity reflux is occurring along the southern Australian margin. Ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50‰ salinity. A well‐flushed zone at the top of all holes contains pore waters with normal marine trace element contents, alkalinities and pH values. Dolomite precipitation occurs directly below the well‐flushed zone in two phases. Phase 1 is a nucleation stage associated with waters of relatively low pH (ca 7) caused by oxidation of H₂S diffusing upward from below. This dolomite precipitates in sediments < 80 m below the sea floor and has δ¹³C values consistent with having formed from normal sea water (? 1‰ to + 1‰ Vienna Pee Dee Belemnite). The Sr content of Phase 1 dolomite indicates that precipitation can occur prior to substantial metastable carbonate dissolution (< 300 ppm in Holes 1129 and 1127). Dolomite nucleation is interpreted to occur because the system is undersaturated with respect to the less stable minerals aragonite and Mg‐calcite, which form more readily in normal ocean water. Phase 2 is a growth stage associated with the dissolution of metastable carbonate in the acidified sea water. Analysis of large dolomite rhombs demonstrates that at depths > 80 m below the sea floor, Phase 2 dolomite grows on dolomite cores precipitated during Phase 1. Phase 2 dolomite has δ¹³C values similar to those of the surrounding bulk carbonate and high Sr values relative to Phase 1 dolomite, consistent with having formed in waters affected by aragonite and calcite dissolution. The nucleation stage in this model (Phase 1) challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water.     

Relict magnesian calcite oolite and subsidence of the Amazon shelf

Oolitic sediments occur over large portions of the outer continental shelf off northernmost Brazil and southern French Guiana. Carbon-14 dates, stable isotope values and analogies with similar deposits from other areas in the world indicate that the ooids were formed in shallow water (possibly protected hypersaline environments) during low stands of sea level. In contrast to both modern ooids and other relict ooids, however, the Amazon shelf ooids are composed entirely of magnesian calcite (12 mol % MgCO₃) and possess radial crystal microstructures. A comparison of water depths and ages of the Amazon ooids with the generally accepted eustatic sea level curve indicates that the Amazon shelf subsided more than 100 m 16-21 thousand years ago. Prior to and subsequent to the subsidence, however, the outer shelf remained relatively stable.     

The Signature of Amphibole in Mafic Alkaline Lavas, a Study in the Northern Canadian Cordillera

Despite evidence for its involvement, the importance of amphibolein controlling the compositions of mafic alkaline magmas remainsunder-appreciated. Relatively small variations in large ionlithophile elements (LILE) with respect to other incompatibleelements, such as light rare-earth elements (LREE) or Th, requirethat amphibole was an important residual phase during the productionof Late Tertiary to Recent olivine nephelinite (Ol-NEPH) magmasbeneath the northern Canadian Cordillera. The erupted maficmagma types define a continuous array from Ol-NEPH to hypersthene-normativeolivine basalt (Hy-NORM AOB). The overall compositional arrayhas a sense of curvature which is counter to binary mixing,but can be modeled by two distinct linear melting trends: onefrom Ol-NEPH to basanite (BASAN) compositions, during whichamphibole controlled the composition of the melt, and the ratiosof LILE/LREE change significantly, but the ratios of high fieldstrength elements (HFSE) remain relatively constant; the otherfrom BASAN to Hy-NORM AOB corresponding to the melting of alherzolite assemblage, following the exhaustion of amphibole,across which the ratios of LILE/LREE remain relatively constant,but the ratios of HFSE change significantly. Other intraplate alkaline suites, such as those of the HawaiianIslands, show similar evidence for the involvement of residualamphibole in the genesis of Ol-NEPH to BASAN magmas. The meltingof any amphibole-bearing mantle assemblage is likely to be atwo-step process, regardless of whether the amphibole is segregatedas veins or distributed interstitially. In a water-undersaturatedenvironment, the first stage of melting is controlled by thebreakdown of amphibole, which produces silica-saturated liquidsbelow 12 kbar and silica-undersaturated liquids at greater depths,with little contribution from other mineral phases. In the secondstage, following the exhaustion of amphibole, the major elementcompositions of subsequent melts change rapidly to equilibratewith a lherzolite mineralogy, but the incompatible trace-elementcharacteristics of the former amphibole persist. KEY WORDS: amphibole; mafic alkaline magmas; northern Canadian Cordillera; trace elements ^*Corresponding author     

Intriguing climatic shifts in a 90 kyr old lake record from northern Russia

A 22 m long sediment core from Lake Yamozero on the Timan Ridge in northern Russia has provided evidence of intriguing climatic shifts during the last glacial cycle. An overall shallowing of the lake is reflected in the lower part of the cores, where pollen indicates a transition from glacial steppe vegetation to interstadial shrub-tundra. These beds are capped by a well-defined layer of compact clay deposited in relatively deep water, where pollen shows surrounding spruce forests and warmer-than-present summer temperatures. The most conservative interpretation is that this unit represents the last interglacial period. However, a series of Optical Stimulated Luminescence (OSL) dates suggests that it corresponds with the Early Weichselian Odderade interstadial (MIS 5a). This would imply that the Odderade interstadial was just as warm as a normal interglacial in this continental part of northern Europe. If correct, then pollen analysis, as a correlation tool, is less straightforward and the definition of an interglacial is more complex than previously thought. We discuss the validity and possible systematic errors of the OSL dates on which this age model is based, but conclude they really indicate a MIS 5a age for the warm period. Above the clay is an unconformity, most likely reflecting a period of subaerial exposure implying dry conditions. Deposition of silt under fluctuating cold climates in the Middle Weichselian continued until a second gap in the record at c . 40 kyr BP. The lake basin started to fill up again around 18 kyr BP.