Lower Melt Rates Research Articles

U-series disequilibria in Kick’em Jenny submarine volcano lavas: A new view of time-scales of magmatism in convergent margins

We present data for U and its decay series nuclides 230Th, 226Ra, 231Pa, and 210Po for 14 lavas from Kick’em Jenny (KEJ) submarine volcano to constrain the time-scales and processes of magmatism in the Southern Lesser Antilles, the arc having the globally lowest plate convergence rate. Although these samples are thought to have been erupted in the last century, most have ( 226Ra)/( 210Po) within ±15% of unity. Ten out of 14 samples have significant 226Ra excesses over 230Th, with ( 226Ra)/( 230Th) up to 2.97, while four samples are in 226Ra– 230Th equilibrium within error. All KEJ samples have high ( 231Pa)/( 235U), ranging from 1.56 to 2.64 and high 238U excesses (up to 43%), providing a global end-member of high 238U and high 231Pa excesses. Negative correlations between Sr, sensitive to plagioclase fractionation, and Ho/Sm, sensitive to amphibole fractionation, or K/Rb, sensitive to open system behavior, indicate that differentiation at KEJ lavas was dominated by amphibole fractionation and open-system assimilation. While ( 231Pa)/( 235U) does not correlate with differentiation indices such as Ho/Sm, ( 230Th)/( 238U) shows a slight negative correlation, likely due to assimilation of materials with slightly higher ( 230Th)/( 238U). Samples with 226Ra excess have higher Sr/Th and Ba/Th than those in 226Ra– 230Th equilibrium, forming rough positive correlations of ( 226Ra)/( 230Th) with Sr/Th and Ba/Th similar to those observed in many arc settings. We interpret these correlations to reflect a time-dependent magma differentiation process at shallow crustal levels and not the process of recent fluid addition at the slab–wedge interface. The high 231Pa excesses require an in-growth melting process operating at low melting rates and small residual porosity; such a model will also produce significant 238U– 230Th and 226Ra– 230Th disequilibrium in erupted lavas, meaning that signatures of recent fluid addition from the slab are unlikely to be preserved in KEJ lavas. We instead propose that most of the 238U– 230Th, 226Ra– 230Th, and 235U– 231Pa disequilibria in erupted KEJ lavas reflect the in-growth melting process in the mantle wedge (reflecting variations in U/Th, daughter–parent ratios, fO 2, and thermal structure), followed by modification by magma differentiation at crustal depths. Such a conclusion reconciles the different temporal implications from different U-series parent–daughter pairs and relaxes the time constraint on mass transfer from slab to eruption occurring in less than a few thousand years imposed by models whereby 226Ra excess is derived from the slab.

Antarctic subglacial conditions inferred from a hybrid ice sheet/ice stream model

Subglacial conditions of large polar ice sheets remain poorly understood, despite recent advances in satellite observation. Major uncertainties related to basal conditions, such as the temperature field, are due to an insufficient knowledge of geothermal heat flow. Here, a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique. Additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots, as well as the distribution of subglacial lakes. In this way, geothermal heat-flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available. Results of the sensitivity experiments show that 55% of the grounded part of the Antarctic ice sheet is at pressure melting point. Calculated basal melt rates are approximately 65 Gt year − 1 , which is 3% of the total surface accumulation. Although these sensitivity experiments exhibit small variations in basal melt rates, the impact on the ice age of basal layers is quite important. In areas that are characterized by relatively low melt rates, this may lead to standard deviations up to 50% (in a model run over a period of 10 6 years). Subglacial water flow is concentrated underneath the large outlet glaciers of Antarctica. The larger subglacial lakes are perched at the head of these subglacial water systems. An exception, however, is the lake system of the Recovery Ice Stream Catchment, that receives a substantial amount of subglacial melt water from the upstream catchment.

The temperature regime of Fimbulisen, Antarctica

AbstractNumerical simulations of the temperature regime of the ice shelf Fimbulisen, Antarctica, are presented. A vertical temperature profile (S1) of Fimbulisen has been measured at the extension of Jutulstraumen, in which the temperature decreases with depth. The three-dimensional steady-state temperature field was computed by a finite-element technique. Horizontal flow velocities and surface accumulation rates were derived from observations. The basal melt rate distribution arose from an assumption of balance in the mass continuity equation. The computed basal melt rate distribution (ab) indicates that the highest basal melt rates, up to 15 m a 1 occur at the inflow gate of Jutulstraumen, and low basal melt rates (<0.6ma 1) occur in the slower moving parts. Where the ice shelf overhangs the continental shelf, ab ~1.2ma−1 . The resulting temperature field indicates that Fimbulisen consists of a cold middle part, built up by the extension of Jutulstraumen, and warmer ice masses in slow-moving areas to the west and east. Furthermore, model runs were set up in which the atmospheric temperatures increased in +1 K steps. The results suggest that the warming effectively increases the temperatures throughout the ice column in the slower-moving parts, therefore enhancing shear at the margins of the extension of Jutulstraumen.

Petrography, geochemistry and geodynamic environment of potassic alkaline rocks in Eslamy peninsula, northwest of Iran

Eslamy peninsula, 360 km2 in area, is located in the eastern coast of Urmieh lake in the northwest of Iran. This peninsula is a complex stratovolcano with a collapsed center, which is elevated due to later intrusions of sub-volcanic masses with trachytic to microsyenitic composition. The composite cone consists of a sequence of leucite tephrite, tephrite, leucite basanite, basanite and related pyroclastic rocks. Magmatic activities in the Eslamy peninsula begin with potassic alkaline to ultrapotassic and basic, silica-undersaturated shoshonitic rocks and they are followed by intrusions of lamprophyric dykes and end with acidic magmatism including trachytic, microsyenitic, syenitic and phonolitic domes. The original magma of the Eslamy peninsula rocks has a potassic alkaline nature (Roman type) rich in LREE and LILE and depleted of HREE. These characteristics suggest that the origin of magma can be from deep mantle with a garnet lherzolite composition, a low partial melting rate which has been contaminated by crustal materials in its way up. Fractional crystallization of olivine, diopsidic clinopyroxene and leucite played an important role in the evolution of magmas. Scrutinizing the geodynamic environment of Eslamy peninsula rocks in discrimination diagrams indicates that these rocks must have been formed in a post-collision magmatic arc setting.

Contribution of the melting stage to the evolution of the morphology and chemical conversion of immiscible polyamide/polyethylene blends in twin‐screw extruders

AbstractThis study investigates the effects of processing conditions (the screw speed, throughput, barrel temperature, and screw configuration) on the chemical conversion and morphology evolution of polyamide/polyolefin blends along a twin‐screw extruder. Polymer samples were collected rapidly at specific barrel locations with a special sampling device for subsequent chemical and morphological characterization. Increasing the screw speed or using more restrictive screw modules at the beginning of the melting zone promoted a faster reaction and better dispersion along the extruder. Increasing the throughput or decreasing the barrel temperature slowed the evolution of the morphology and chemical conversion along the extruder because of the lower melting rate. As soon as melting started, the chemical reaction took place. However, high chemical conversion rates required extensive melting, that is, significant interface generation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Use of fat replacers in the production of reduced‐calorie vanilla ice cream

AbstractThe purpose of this research was to determine the effects of different fat replacers on the sensory and texture attributes of vanillin‐flavored ice cream. Inulin is a fairly new fat replacer which has been proven to be a health promoter. Two vanilla ice cream formulations were produced using different ratios of inulin and Simplesse® as fat replacers to provide better flavor and textural properties. Aspartame and acesulfam‐K were used as sweeteners in the low‐calorie ice creams. Sensory and instrumental analyses were used to evaluate the texture quality of light (4.55%) (control group), low‐fat (2%) and fat‐free vanilla ice creams. The experimental mixes were significantly more viscous than the control mix due to the water binding capacity of carbohydrate‐ and protein‐based substances. The texture scores of the ice creams (sensory evaluation) paralleled the viscosity values. Ice creams manufactured with fat replacers were free of cooked‐milk flavor. The energy value of ice cream was reduced compared to full‐fat ice cream (207 kcal/100 g): by 75% for non‐fat ice cream, 71% for low‐fat ice cream and 41% for light ice cream. An increase in total solids in the mix resulted in better texture, a lower melting rate and better shape retention.

A glacier respires: Quantifying the distribution and respiration CO2 flux of cryoconite across an entire Arctic supraglacial ecosystem

This paper quantifies the mass distribution of cryoconite at the glacier scale using photographic surveys conducted either at ground level, or at 20 m elevation using a novel uninhabited aerial vehicle (UAV). Image acquisition allowed three key deposits to be quantified: cryoconite holes, cryoconite in streams (“stream cryoconite”), and dispersed cryoconite granules (detectable only in the ground‐based images). Cryoconite was found all over the snow‐free parts of the glacier in one or more of these forms, covering about 0.42% (4600 kg km−2 dry weight) as holes and stream cryoconite deposits (>0.25 cm2 and thus visible in the UAV images), or 1% (10600 kg km−2) when smaller dispersed granules were included (using the ground images). No spatial patterns in the distribution of cryoconite cover were apparent, although cryoconite holes were far more common than stream cryoconite at high altitude due to lower melt rates. Measurements of respiration and bacterial carbon production estimated from in situ incubations of cryoconite–water mixtures indicated rates of 1.174 ± 0.182 (1 standard deviation) and 0.040 ± 0.019 μg C g−1 h−1, respectively. The respiration measurements then yielded glacier‐wide CO2 fluxes for 1998 and 2000 of 6.3 and 5.1 kg C km−2 a−1 when the loci and duration of activity were defined using the UAV images and a degree day model, respectively. These fluxes increased to 14 and 12 kg C km−2 a−1 when the dispersed cryoconite detected in the ground‐based images were also considered. The measurements therefore show that cryoconite ecosystems clearly have the capacity to impact upon carbon cycling in glacial environments.

Suspended sediment from the Gangotri Glacier: Quantification, variability and associations with discharge and air temperature

To understand the sediment delivery variation for a Himalayan Glacier (Gangotri Glacier, Garhwal Himalayas) and to determine its relationship with discharge and air temperature, suspended sediment samples and discharge data were collected near the glacier snout (4000 m) for four melt seasons during the period 2000–2003. These data were used to estimate suspended sediment concentration (SSC), suspended sediment load (SSL), sediment yield and erosion rate in the glacier melt stream (Bhagirathi). The monthly distribution of suspended sediment and its variability from year to year have been examined. Mean monthly SSC for May, June, July, August, September and October were found to be 1942, 2063, 3658, 2551, 734 and 136 mg l −1, respectively. Maximum SSC in meltwater was observed in July followed by August. It was found that the cumulative percentage delivery of SSC precedes discharge throughout the melt season. Mean monthly total SSL for May, June, July, August, September and October during the study period was found to be 149, 423, 1220, 746, 143 and 5×10 3 ton, respectively. The strong variability is found in SSL ( C v=1.1) than SSC ( C v=0.8) because computation of SSL includes both discharge ( C v=0.6) and SSC. Delivery response of SSL in terms of percentage of total load is less in the early part of the melt season than in the later stage in comparison to that of discharge. This may be due to the fact that in the beginning of the melt season low melt rate conditions prevails and thus, the low discharge velocity could not flush out stored glacial sediment. It has been observed that 59–64% of the sediment passed through the channel by the time 50% of the total discharge passed. The average suspended sediment yield for the whole melt season from the study area was estimated to be about 4834 ton km −2 and corresponding erosion rate was 1.8 mm. The relationship between mean monthly SSC and discharge ( R 2=0.99) is much better than the daily SSC and discharge ( R 2=0.40) because variability of both parameters is averaged-out on monthly scale. Mean monthly SSC and mean monthly SSL provide a good exponential relationship with mean monthly air temperature. These results are relevant for planning and management of water resources in the high altitude areas and for designing hydropower projects.

Deglaciation dynamics following the Little Ice Age on Svalbard: Implications for shaping of landscapes at high latitudes

The late culmination of the Little Ice Age (LIA) on Svalbard allows a detailed reconstruction of the landscape's response to the subsequent climatic warming. The study area comprises a small glacier (400–1000 m a.s.l.), on the south side of Adventfjorden (78°11′N) that was polythermal during the LIA and turned into a passively down-wasting cold-based ice-mass prior to 1936. Reconstruction of the formation and decay of ice-cored moraines and the shifting courses of glacial meltwater shows that sediment transport during deglaciation occurred in a slow, stepwise fashion with glacial landforms and sediments being slowly replaced by fluvial morphologies and slope-waste products. The key controlling factors are melting rates, aspect and surface gradients. A low melting rate and slow reworking of glacial debris promote the formation of a lag or “pavement” on low-gradient surfaces and debrisfall deposits along steeper slopes. Both products easily may be misinterpreted as a result of weathering and non-glacial processes.

Melting and freezing beneath the Ross ice streams, Antarctica

AbstractWe have estimated temperature gradients and melt rates at the bottom of the ice streams in West Antarctica. Measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed. Our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow. We find that ice-stream tributaries and the inland ice account for about 87% of the total melt generated beneath the Ross ice streams and their catchments. Our estimates indicate that the ice plains of Whillans Ice Stream and Ice Stream C (even when active) have large areas subject to basal freezing, confirming earlier estimates that import of water from upstream is necessary to sustain motion. The relatively low melt rates on Whillans Ice Stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future, possibly within this century. While there are pockets of basal freezing beneath Ice Streams D and E, there are larger areas of basal melt that produce enough melt to more than offset the freezing, which is consistent with inferences of relatively steady flow for these ice streams over the last millennium.

Influence of emulsifiers on ice cream produced by conventional freezing and low-temperature extrusion processing

Ice cream at six different levels of emulsification was produced by freezing in a conventional scraped surface freezer and with a serial configuration of a conventional freezer followed by a low-temperature extruder. The aim was to examine the influence of emulsifiers on the process, since both emulsifier addition and low-temperature extrusion may have similar effects on promotion of colloidal structure in ice cream. Ice cream samples from both processes were analyzed for stiffness at draw, fat destabilization indices, melting performance, and microstructure. Low-temperature extrusion generally promoted enhanced fat destabilization, however, fat particle size and solvent extractable fat showed different dependencies on emulsification level from the two processing systems. Although solvent extractable fat reached high levels with increasing emulsification, fat particle size data suggested that fat agglomerate size was controlled by mechanical shearing. Significant difference between the two systems was seen also in the meltdown test, where melting rates for unemulsified and slightly emulsified mixes led to a very low melting rate and high shape retention in extruded ice cream. Scanning electron microscopy detected generally smaller air bubbles for extruded ice creams. Enhanced fat structuring around the air bubble and into the serum phase was also shown for unemulsified extruded samples.

Energy balance and runoff seasonality of a Bolivian glacier

The runoff of Zongo Glacier (Bolivia, 16°S) shows an appreciable seasonal variability, with low discharges in the dry season (May to August) and high values in the humid season (October to March). Incoming radiation, temperature and precipitation are poorly correlated with discharge and cannot explain the hydrological seasonality and the glacier's response to climate variability in the Tropical Andes. Since 1996, energy balance measurements have been carried out in the vicinity of the mean equilibrium line (5150 m a.s.l.) on Zongo Glacier (2.1 km 2). Comparisons are made with proglacial stream discharges recorded at the main hydrometric station. Each component of the energy balance (net radiation, turbulent heat fluxes, heat transfer into the ice and heat supplied by precipitation) is derived separately from the measurements, and the variability throughout the year is evaluated. Radiation and turbulent fluxes dominate the surface energy balance. Sensible heat flux is small and does not show a significant seasonal change. Latent heat flux is highly variable with low values during the accumulation season and high values during the dry period. This high sublimation loss during the dry season causes well-developed penitents at the glacier surface. In conclusion, incoming energy throughout the year is constant, with no large variations, and humidity controls the balance of this energy between sublimation and melting. During the accumulation season, sublimation is reduced because of a low gradient of vapour pressure and the energy supplied by radiation is directly consumed by melting, explaining why discharge is high. During the dry period, a large part of the energy supplied by radiation is used to sublimate snow or ice and therefore, energy available for melting is low, which leads to low melt rates. Due to the important role of humidity, tropical glaciers are likely to be the climatic indicators the most sensitive to climatic changes like the greenhouse effect.

Subglacial processes of the Scandinavian Ice Sheet in Fennoscandia inferred from flow-parallel features and lithostratigraphy

Subglacial processes of the last Scandinavian Ice Sheet in Fennoscandia were investigated on two different scales: a detailed case study in southwestern Finland, and a generalized study in the central parts of the ice sheet. Previously reconstructed time-transgressive glacial flow patterns were correlated with different types of glacial flow-parallel features and lithostratigraphic units. The comparison of orientations of each flow-parallel feature with relative ages of different flow patterns enabled estimations of relative distances of the orientation-generating processes in relation to the receding ice margin. This method was also used to infer sedimentary environments of each lithostratigraphic facies identified. Generalized lithostratigraphic sequences were compiled in Fennoscandia using sections described previously. In areas of ice streams a sandy till showing mature deformation structures forms till sheets, drumlins and some ribbed moraines. The facies is overlain by occasional glaciofluvial deposits and partly washed deformation till both indicating abundant excess water at the ice-bed interface. In interlobate areas, glaciofluvial material is under- and overlain by a silty till showing ductile deformation structures. In places, uppermost thin clayey till, interpreted to be a subglacial melt-out till, mantles underlying till and subglacial glaciofluvial deposits. The characteristics of different units are controlled by ice-flow velocities, subglacial hydrology and deformation type of the substratum. The time-transgressive origin of till beds and intervening washed sediment, as well as divergent ice-flow patterns, is emphasized. Ice flow was concentrated in streams which generated marginal fan-shaped lobes. Interstream areas of low basal velocities and melting rates were left between adjacent ice streams. In areas of ice streams, fast basal ice-flow velocities continuously generated heat and meltwater which flowed at first in the sediment and closer to the margin also in thin discontinuous sheets at the ice-bed interface. Excess water may have subsequently formed a linked cavity system and channelized to a tunnel network initiating esker deposition or incision of tunnel channels. Most processes operated continuously during deglaciation, with spatial and temporal differences in their activity. The mode of subglacial drainage changed periodically during deglaciation, and was probably controlled by the volume of subglacial meltwater available. In some periods, sheet flow of basal water deposited major marginal complexes, and subsequently tunnel networks were re-established. Esker pattern and sedimentation rates of proglacial varved clays indicate that tunnel drainage of subglacial water was partly episodic.

Geochemical and isotopic disequilibrium in crustal melting: An insight from the anatectic granitoids from Toledo, Spain

In the Hercynian Anatectic Complex of Toledo (ACT) the anatectic granitoids include leucogranitic leucosomes, leucogranitic massifs, and restite‐rich granites. They show a broad range of initial Sr and Nd isotope ratios from 0.711 to 0.720 and 0.51164 to 0.51203, respectively, which clearly indicate the absence of isotopic homogenization in the melts. Broadly, the ranges reflect the isotopic variation of the metapelitic protoliths. If crustal melting occurs under water‐undersaturated conditions, as is the case of the ACT, the generated melts do not isotopically and chemically equilibrate with the granulitic residuum. The preservation of heterogeneities could arise through a number of processes. (1) duration of the process:in which the presence of melts with disequilibrium features, and the high solid content of several of the granites in the ACT point to a very short‐lived magmatic system, (2) limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization, and (3) magma dynamics in which the low melting rates, the high restite and low water contents, and the silica‐rich composition of these granites, together with the small volume and high viscosity values of the melts, seriously restrict the physical processes leading to homogenization.

Variations of near-surface firn density in the lower accumulation area of the Greenland ice sheet, Pâkitsoq, West Greenland

AbstractFirn-density variations have been studied in the lower accumulation area of the Greenland ice sheet (1440-1620 m a.s.l.) near Pâkitsoq, West Greenland. The main control on density in the near-surface firn layer (of 5-10 m thickness) is the formation of ice layers by the refreezing of meltwater that reaches depths of 2-4 m below the surface. The density variations are described by the ratio of annual surface meltMto the annual accumulationC. The ratioM/Cis about 0.6 at the run-off limit (at about 1400 m a.s.l. in the study area) where refreezing of meltwater transforms snow into impermeable ice. The mean density of near-surface firn decreases with elevation, reflecting a decrease in melt with elevation. There is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface, which reflects lower melt rates and/or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer. The formation of such low-density firn may have partially contributed to the 1978-85 thickening of the ice sheet observed by satellite-radar altimetry. Near-surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring.

Variations of near-surface firn density in the lower accumulation area of the Greenland ice sheet, Pâkitsoq, West Greenland

AbstractFirn-density variations have been studied in the lower accumulation area of the Greenland ice sheet (1440-1620 m a.s.l.) near Pâkitsoq, West Greenland. The main control on density in the near-surface firn layer (of 5-10 m thickness) is the formation of ice layers by the refreezing of meltwater that reaches depths of 2-4 m below the surface. The density variations are described by the ratio of annual surface melt M to the annual accumulation C. The ratio M/C is about 0.6 at the run-off limit (at about 1400 m a.s.l. in the study area) where refreezing of meltwater transforms snow into impermeable ice. The mean density of near-surface firn decreases with elevation, reflecting a decrease in melt with elevation. There is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface, which reflects lower melt rates and/or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer. The formation of such low-density firn may have partially contributed to the 1978-85 thickening of the ice sheet observed by satellite-radar altimetry. Near-surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring.

Chemical nonhomogeneity of the electro-slag remelting steel measured by EPMA

The chemical and structural homogeneity of the electro-slag remelting (ESR) alloyed structural Cr-Ni-Mo steel (18H2N4MA) is investigated with electron probe microanalysis. The level of nonhomogeneity is expressed in terms of microsegregation coefficients (Cmax/Cmin of an investigated element) [1]. Microsegregation coefficients are higher for molybdenum than for chromium and vary from 1.5 to 2.05 for Mo and from 1.13 to 1.3 for Cr. Both at low ( 400 kg/h) melting rates, the increase of microsegregation is more pronounced for both elements investigated.

Mineral chemistry of ultramafic tectonites and ultramafic to gabbroic cumulates from the major oceanic basins and Northern Apennine ophiolites (Italy) — A comparison

The paper presents the mineralogy of upper-mantle tectonites and associated lower-crustal ultramafic and mafic cumulates from the Jurassic Northern Apennine ophiolites (NAO) in a comparison assay with equivalent rocks from the major oceanic basins (MOB: East Pacific Rise and Atlantic fracture zones, Legs 37 and 45). Among the mantle rocks both tectonite lherzolites and harzburgites have been so far widely found in MOB whereas lherzolites largely prevail in NAO. The Mg# [ Mg# = Mg ( Mg + Fe 2+) ] range in mafic minerals in NAO tectonites is restricted to high values (Ol: 88–90; Opx: 90–92; Cpx: 91–93). Cr-spinels show large variation of the Cr# [ Cr ( Cr + Al) ] ratio found in spinels from lherzolites and very rare Cpx-bearing harzburgites (Cr# 10 and 15, respectively). The varied compositions largely reflect the relative greater degree of partial melting that affected MOB tectonites compared to NAO tectonites. From a restored stratigraphic log of the NAO cumulates the order of appearance of the cumulus phases both in NAO and MOB cumulates is: Ol + Chr, Pl, Cpx, Opx, Fe/1bTi-oxides, Ap. The sequence gave rise to the following main lithotypes: dunites, troctolites, minor wehrlites, Ol-gabbros, gabbronorites, ferrogabbros and ferrodiorites. The olivine from early ultramafic cumulates has high Fo content (Fo 90 for NAO, Fo 91 for MOB) with a NiO content ranging from 0.21 to 0.31 wt.%. Cr-diopside generally shows higher TiO 2 than found in tectonite peridotites. The coexisting spinel is typically a Ti-chromite with high Cr# (43–60). The most Mg-rich olivine coexisting with An-rich plagioclase in the most primitive cumulates are Fo 88 and An 72 and were in equilibrium with a basaltic melt. These facts coupled with the lack of intense plastic deformation in the early cumulates allow us to exclude a high-pressure fractionation system. Formation of very little volume percent of amphibole indicates a very low H 2O activity in the magma. Starting from the early fractionating cumulus phases a recalculated primary magma composition is proposed. From these calculations it is concluded that NAO cumulates derived from a tholeiitic basalt liquid similar to primitive MOB liquid except for a higher Na 2O content. In particular present-day analogues are found among high Na 2O mid-ocean ridge basalts from the Mid-Cayman Rise and Southwest Indian Ridge. The composition of calculated primary magmas indicated for NAO: (1) a much lower degree of melting than average MOB, also sustained by evident differences in residual peridotites; (2) a crustal thickness of ∼ 2–3 km, also supported by the reconstructed maximum thickness of crustal sequences of 2 km; (3) a spreading axis with an average water depth of ∼ 5 km. By analogy with the Mid-Cayman Rise and Southwest Indian Ridge, half spreading rates of <1 cm a −1 are suggested. Picritic parental magma can be excluded for NAO. We propose an Atlantic/Indian-type or Mid-Cayman Rise rift for the origin of NAO. This ridge would be cut by several fracture zones explaining the highly dismembering of the ophiolites sequences, the low partial melting rate involved and the Na 2O enrichment character of the cumulates and related lavas.

Role of the Molecular Sublayer in the Melting or Freezing of Sea Ice

Abstract In an earlier paper, a second-moment turbulence closure model was applied to the problem of the dynamic and thermodynamic interaction of sea ice and the ocean surface mixed layer. An overly simplistic parameterization of the molecular sublayers of temperature and salinity within the mixed layer was used. This paper investigates the use of a more recent parameterization by Yaglom and Kader which is supported by laboratory data. A relatively low melt rate results in the case where ice overlays warm water. This agrees with some recent observations in the interior of the marginal ice zone. A surface heat sink drives the freezing case which, due to the large difference in heat and salt molecular diffusivities, produces a strong supercooling effect. This is converted into an estimate of frazil ice production through a simple scheme. The model results provide an explanation for high frazil ice concentrations observed in the Arctic and Antarctic.