The effects of cooling rate on texture and pyroxene chemistry in DSDP Leg 34 basalt: A microprobe study

The postcumulus evolution of a portion of the Bushveld Complex that includes the Merensky reef is inferred from the study of a continuous 56 m drill core. The core penetrated the basal orthopyroxenites of the Merensky and Bastard units, the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the Merensky pyroxenite. Detailed profiles of major, minor and rare earth element (REE) contents of clinopyroxene and orthopyroxene were determined by electron and ion probe. Good correlations exist between textural and lithological variations and the REE contents of the pyroxenes. Specifically, enrichments in pyroxene REE abundances are observed in the basal pyroxenites of the Merensky and Bastard units relative to the underlying and overlying rocks. In the pyroxenites the Nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm (≈90 × chondrite), and Nd/Yb ratio is in the range 8 to 25. These extreme enrichments in REE are not accompanied by large variations in major element contents. Computations of the compaction parameters relevant to the conditions of crystallization of the Bushveld Complex combined with a consideration of cooling history confirm the importance of compaction as a post-cumulus process. This analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile. A model for the development of the Atok section is developed. The Merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late-stage, hydrous and incompatible-element enriched melt, which was thus trapped in the underlying Merensky pyroxenite. The flow was driven by compaction of the 350+ meter-thick section of predominantly norite beneath the anorthosite. The introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution, recrystallization and REE enrichment of the rock forming minerals, and in the formation of the main lithologic features of the Merensky pyroxenite. Further upward percolation of the interstitial melt through the Merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite. This melt accumulated in and metasomatized the Bastard pyroxenite in the same manner as the Merensky pyroxenite, having again been trapped by the overlying Bastard anorthosite.

The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content. The alkali olivine basalts undersatured in SiO2 and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize. The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.

Dioritic plugs (< 1 km across) are common associates of the late Caledonian, post‐tectonic granites of the Scottish Highlands. These contain a very wide range of rock types from ultramafic through mesocratic diorites to anorthositic and granitic. These rocks form steeply dipping, wall‐parallel layers and zones within the plugs. Outer layers are shown to form first, inner layers later. Mafic zones are composed of the minerals seen as phenocrysts in chilled margins and have the chemical characteristics of cumulates. The inner margins of the mafic zones reflect the onset of plagioclase crystallization. Core zones also show inward variations in mineralogy and texture which define vertical, wall‐parallel cylinders. These variations are ascribed to fractional crystallization accompanying accretion of rock onto the pipe walls. Cores in different intrusions may be mafic, mesocratic, or leucocratic, which indicates vertical zonation in the bodies. The mafic cores additionally show that recharge with less evolved magma occurred in some cases. The few larger (> 1 km diameter) intrusions show examples of layering and lamination dipping at low to moderate angles. These indicate the beginning of a change from wall‐dominated to floor‐dominated crystallization as intrusion diameter increases. A comparison is made with the nature and origin of layering in other intrusions.

Tissue trace and major element levels in organophosphate insecticide fenthion (Lebaycid ®) toxicity in rats: Prophylactic and therapeutic effect of exogenous melatonin

The forest floor at high elevation spruce-fir sites from southern Vermont, U.S. to the Gaspe Peninsula, Quebec, Canada was sampled and analyzed in 1979 and re-sampled and analyzed in 1996 to study temporal changes in the impacts of atmospheric pollutants. We determined organic matter mass, pH, and concentrations of Al, Ca, Fe, K, Mg, Na, Cd, Cu, Hg, Pb, and Zn for the litter (L = fresh litter plus Oi horizon) and fermentation plus humic horizons (F+H) (= Oe plus Oa horizons) of the forest floor. There were no trends for Al or Fe concentrations in the 1979 or 1996 L along the transect. Several sites had significantly lower Al and Fe values in 1996 than in 1979, likely indicating less mineral soil in the 1996 samples. The 1996 concentrations of Ca in L increased along the transect from 0.22% dry weight (dw) in Vermont to 0.60% dw in Quebec. Concentrations of Mg in L were relatively constant along the transect. Neither Ca nor Mg changed at sites from 1979 to 1996, indicating unchanged base status. Concentrations of Cd did not vary spatially along the transect but decreased at all sites from 1979 to 1996. Cu and Zn did not vary spatially or with time. In 1979, the concentration of Hg in L ranged between 150 and 300 μg kg−1 dw, with no spatial gradient. By 1996, Hg concentrations were 25 to 50% lower in L, with decreases generally proportional to the concentration in 1979. The concentration of Pb in 1979 L decreased significantly from 200 mg kg−1 dw in southern Vermont to 60 mg kg−1 dw in Quebec. By 1996, the Pb concentration in L ranged between 32 and 66 mg kg−1 dw with no spatial trend along the transect. Decreases in Pb concentrations at sites were proportional to the absolute value in 1979. The concentrations of Cd, Hg, and Pb have declined in litter from 1979 to 1996, indicating a decline in atmospheric deposition. Higher Hg and Pb accumulation rates to the southwest are suggested for the past as indicated by (F+H) concentrations and inventories of Hg and Pb. The decline of Pb in L is consistent with the decreased use of leaded gasoline starting in the 1970s; the declines in Cd and Hg probably reflect lower emissions over the same period. Declining concentrations of Cd, Hg, and Pb in L parallel those documented in recent lake and peat sediments in the northeastern United States.

It has long been known that as a result of frequent deposition the mineralogy of Mississippi River sands remains constant in time and space along the river. There has never been a comparable, detailed mineralogical study of the vastly more abundant muddy sediment of the river. To evaluate the magnitude of variation in Mississippi River fine-fraction mineralogy, suspended sediment was collected at 3 stations across the river during July and November 1980. Samples were disaggregated, and without pretreatment, separated into 4 sizes: 62.5-7.8 µm; 7.8-2.0 µm; 2.0-0.49 µm; and < 0.49 µm. Using calibration curves prepared from pure minerals with an internal standard, XRD determinations of the mineralogy were obtained with a high precision. Minerals quantified were, in order of decreasing abundance: smectite, illite, kaolinite, quartz, plagioclase, alkali feldspar, chlorite, and amphibole. The largest source of variation in suspensate mineral abundance was sediment size. Smectite, for example, comprised 50% of the fine clay, but was not present as silt. ANOVA revealed significant temporal variation in the abundance of smectite and illite within the finest sizes and lateral variation in quartz and feldspar abundance in coarser sizes. Discriminant function analysis determined that the total mineralogy of river-suspended sediment varied significantly with time. This temporal variation in mineralogy apparently results from differences in soil mineralogy in drainage basins of the major tributaries of the river, coupled with t e time-varying contributions of the tributaries.

Emplacement pressures of the 2350 Ma old, high-Mg tholeiite dyke suite in the Vestfold Hills block have been previously estimated to be 7-8 kbar. New data suggest that previous estimates are too high. The depth of emplacement has been estimated using two different approaches: (1) Geobarometry based on aluminium exchange between clinopyroxene and plagioclase and (2) examination of high pressure liquidus mineralogy of the high-Mg tholeiites. Geobarometry applied to groundmass assemblages (clinopyroxene + plagioclase) indicates across-dyke variations. Samples from the chilled margins give higher pressure estimates (max. 7 kbar) than more slowly cooled samples from the dyke interior (1 to max. 5 kbar, corresponding to a depth of ∼ 3-16 km). The variations of the liquidus mineralogy (opx + ol) from chilled margins and from dyke interior samples are consistent with the results derived from the Al-exchange geobarometry. It is inferred that pressure estimates from the dyke interior (1 to max. 5 kbar) represent the ambient emplacement pressures, whereas pressure estimates derived from phenocryst assemblages (ol + opx) in chilled margins may represent crystallization pressures prior to dyke emplacement. The results suggest a much shallower emplacement depth (1-max. 5 kbar) of the 2350 Ma old tholeiites than previously thought (7-8 kbar)

The geochemistry of modern sediments from the Gulf of Paria—I The relationship between the mineralogy and the distribution of major elements

Comprehensive characterization of Al-doped ZnO thin films deposited in confocal radio frequency magnetron co-sputtering

Synopsis In the western part of the Midland Valley of Scotland, Carboniferous volcanic activity is considered to have been initiated at c . 335 Ma with extrusion of the first lavas of the Clyde Plateau Volcanic Formation (CPF). On the island of Little Cumbrae, in the Firth of Clyde, the basal flow of the CPF lies disconformably on sandstones and conglomerates of the upper part of the Clyde Sandstone Formation. However, at the south end of adjacent Great Cumbrae, the lower part of the Clyde Sandstone Formation (Tournaisian) is host to a series of thin concordant igneous bodies that resemble sills but lack features such as chilled margins. Some display fine bedding and lamination near the base and top. These rocks are interpreted as tuffs with chemical affinities to alkali basalts such as those of the CPF on the neighbouring island of Little Cumbrae and in south Bute. Their major element content has been modified by addition of Ca and volatiles, possibly during ascent of magma through caliche-bearing rocks and water-rich sediments. The tuffs have chondrite-normalized rare earth element patterns similar to those of basaltic rocks forming the lower part of the CPF. They provide new evidence of Tournaisian volcanic activity in the western part of the Midland Valley, thus bringing it into line with that in parts of the Southern Uplands and Scottish Borders region. The Bessy's Port tuffs occur near the base of the Clyde Sandstone Formation whereas, on Little Cumbrae, the first lava of the CPF is near its top. Early magmatic activity is also indicated by the occurrence of peperites associated with mafic dykes intruding mudstones of the Lower Foul Port Member, near the base of the Kinnesswood Formation, in southwestern Great Cumbrae, and by basaltic tuffs in the Ascog Member of the Clyde Sandstone Formation (or basal part of the Birgidale Formation) in east–central Bute.

Abstract: The Karaj Dam basement sill (KDBS) shows layering between upper and lower chilled margins. Geochemically, the KDBS belongs to high-K calc-alkaline series and displays the geochemical characteristics related to an active continental margin environment. The rocks of the chilled margins are gabbroic in composition and porphyritic, becoming coarse-grained toward the center of the sill. The layers are dominated by gabbro, monzogabbro, and monzodiorite, formed from the lower to the upper chilled margin, with gradational transitions between rock types. The KDBS shows an S-shaped profile of MgO concentration from the base to the top of the sill. In situ crystallization is indicated by an approximately constant modal abundance of pyroxene and plagioclase, a narrow range of major element contents in the main lithologies and variable trace element concentrations. Calculated densities of successive cumulates and estimated Rayleigh number also confirm extraction of the residual liquid by compositional convection at the crystallization front and existence of turbulent flow in the main magma chamber. Disequilibrium crystallization, caused by saturation gradient thermal diffusion and to some extent Soret effect (Thermal migration) in the marginal series, is also indicated by an unexpected overall enrichment trend in MgO concentration and average Mg# of pyroxenes from the chilled margin (olivine-bearing gabbro) to marginal gabbro.

The Horoman peridotite complex, Hokkaido, Japan is divided into Lower and Upper zones on the basis of contrasting geological features. The complex recorded a consecutive decompression history in chemical zoning of pyroxenes and plagioclase in plagioclase lherzolite, which is interpreted to have been derived from garnet lherzolite by subsolidus decompression reactions. In the Lower Zone, and earlier decompression history is clearly preserved in large pyroxene porphyroclasts, which show marked M-shaped Al zoning characterized by low Al concentration at the core (Al=0.12/6 oxygens), gradual increase toward the marginal region, and rapid decrease toward the rim. The Ca content in the core is nearly constant (Ca=0.03/6 oxygens) with slight increase toward the margin followed by abrupt decrease toward the rim. The Al and Ca contents in the core of orthopyroxene in plagioclase lherzolite from the Upper Zone (Al=0.22, Ca=0.055/6 oxygens) are much higher than those for the Lower Zone, and the Al content typically decreases monotonously from the core to the rim with several exceptions that show poorly developed M-shaped zoning profiles. The earliest P-T conditions, inferable from the core compositions of pyroxenes are 900–950°C and ∼20 kbar for the Lower Zone and 1100–1150°C and ∼20 kbar for the Upper Zone. The increase of Al from the core to the margin is inferred to have resulted from nearly adiabatic decompression from these conditions into spinel peridotite facies. The complex experienced further decompression from the spinel stability field into the plagioclase stability field, which is inferred from plagioclase zoning in fine-grained aggregates composed mostly of plagioclase, chromite spinel, and olivine with minor pyroxenes. The Na-Ca ratio of each plagioclase grain decreases from the core to the rim, suggesting continuous decompression reaction producing olivine and plagioclase from pyroxenes and spinel. The sharp increase in Ca content toward the rim indicates that fairly rapid cooling associated with decompression is necessary to form and preserve the marked zoning. The sharp decrease in Al and Ca contents toward the rim of orthopyroxene was also formed during this final ascent of the complex. The systematic changes of the mineralogic and petrographic features that are gradational between the Lower and Upper zones suggest that the Horoman complex retains a temperature variation from the upper mantle. The Upper Zone is interpreted to have followed a higher temperature decompression path than the Lower Zone and probably represents a relatively hotter portion of a mantle diapir ascending from a depth greater than 60 km in the upper mantle.

The Trafalgar Plutonic Suite intruded metasedimentary rocks of the Goldenville and Halifax groups in the northeastern part of the Meguma terrane of southern Nova Scotia at about 374 Ma, based on previously published U–Pb and 40Ar/39Ar mineral ages. Using field and petrographic observations, the suite is divided into 20 different plutons on the combined basis of variations in grain size (fine, medium, or coarse), texture (equigranular or porphyritic) and modal mineralogy (quartz diorite/tonalite, granodiorite, monzogranite, and syenogranite). The granodiorite, monzogranite, and syenogranite plutons are relatively uniform in composition with little variation in mineralogy or chemistry within each pluton or between plutons of the same lithology. In contrast the quartz diorite/tonalite plutons show mineralogical and chemical variation, both within and between plutons. The granodiorite, monzogranite, and syenogranite plutons closely resemble other peraluminous granitoid plutons characteristic of the Meguma terrane. The quartz diorite/tonalite plutons are varied but chemically resemble minor Devonian mafic intrusions elsewhere in the Meguma terrane. Like other plutons of the Meguma terrane, the Trafalgar Plutonic Suite has chemical characteristics of volcanic-arc to syn-collisional granitoid rocks and likely has experienced extensive contamination by metasedimentary material as documented by previous studies of plutons in the Meguma terrane. The minor quartz diorite/tonalite plutons are additional examples of the mafic rocks that have been proposed in tectonic models of the Meguma terrane to have facilitated melting of the lower crust to generate granodioritic parent magmas, followed by crystal fractionation and extensive contamination by metasedimentary material.

The role of cooling rate in the origin of high temperature phases at the chilled margin of magmatic intrusions

Mineral chemistry of the Neoproterozoic Alaskan-type Akarem Intrusion with special emphasis on amphibole: Implications for the pluton origin and evolution of subduction-related magma