Petrology and Trace Element Geochemistry of the Honolulu Volcanics, Oahu: Implications for the Oceanic Mantle below Hawaii

The Honolulu Volcanics comprises small volume, late-stage (post-erosional)vents along rifts cutting the older massive Koolau tholeüticshield on Oahu, Hawaii. Most of these lavas and tuff of theHonolulu Volcanics have geochemical features expected of near-primarymagmas derived from a peridotite source containing Fo_87–89olivine; e. g. 100 Mg/(Mg + Fe²⁺) >65, >250 p. p. m. Ni,and presence of ultramafic mantle xenoliths at 18 of the 37vents. Consequently, the geochemistry of the alkali olivinebasalt, basanite, nephelinite and nepheline melilitite lavasand tuff of the Honolulu Volcanics have been used to deducethe composition of their mantle source and the conditions underwhich they were generated by partial melting in the mantle. Compositional trends in 30 samples establish that the magmaswere derived by partial melting of a garnet (<10 per cent)Iherzolite source, which we infer to have been carbon-bearing,from analogy with experimental results. This source was isotopicallyhomogeneous (Sr, Lanphere & Dalrymple, 1980; Pb, Sun, 1980;Nd, Roden et al., 1981), and we infer that the source was compositionallyuniform in all major-element oxides except TiO₂, in compatibletrace elements (Sc, V, Cr, Mn, Co and Ni), and in highly incompatibletrace elements (P, Th, La, Ce). However, the source appearsto have been heterogeneous in TiO₂, Zr, Hf, Nb, and Ta, elementsthat were not strongly incompatible during partial melting.Some nepheline melilitite samples may be derived from a sourcewith distinct Sc and heavy-rare-earth-elements (REE) abundances,or which had a phase or phases controlling the distributionof these elements. The relatively limited abundance range for several elements,such as Ti, Zr, Nb, is partly a consequence of the low degreesof melting inferred for the series (2 per cent for nephelinemelilitite, 11 per cent for alkali olivine basalt), which failedto exhaust the source in minor residual phases. We infer thatthese residual phases probably included phlogopite, amphibole,and another Ti-rich phase (an oxide?), but not apatite. In comparison with estimates of a primordial mantle compositionand the mantle source of mid-oceanic-ridge basalt the garnetperidotite source of the Honolulu Volcanics was increasinglyenriched in the sequence heavy REEs, Y, Tb, Ti, Sm, Zr, andHf all <P <Nd <Sr Ce <La <Nb Ta. A multi-stagehistory for the source of the Honolulu Volcanics is requiredbecause this enrichment was superimposed on a mantle that hadbeen previously depleted in incompatible elements, as indicatedby the relatively low ⁸⁷Sr/⁸⁶Sr ratio, high ¹⁴³Nd/¹⁴⁴Nd ratioand low contents of K, Rb, Ba, and Th. The composition of thesource of the Honolulu Volcanics differs from the source ofthe previously erupted tholeiitic shield. The modal mineralogyof the source of the Honolulu Volcanics is not represented inthe upper-mantle xenoliths, e. g. the garnet pyroxenite andolivine-poor garnet Iherzolite included within the lavas andtuff of the unit.     

Bedform climbing in theory and nature

Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.     

Transmission electron microscope study of quartz microstructures produced by aeolian bombardment

The near-surface structure produced by laboratory aeolian bombardment of Brazilian quartz has been studied with transmission electron microscopy (TEM). Thin specimens of the abraded surface were prepared by a one-sided ion etching technique, and control specimens of unabraded material were prepared by standard ion etching methods. The abraded sample exhibited a vastly larger number of defects than the unabraded specimen. The abraded sample contained areas with very little surface disruption, presumably where pieces of material up to several tens of microns in diameter had broken away. Other areas of the abraded surface were extremely disrupted, with open and closed microcracks spaced from a few tens of angstroms to a few microns apart. Electron diffraction indicated significant rotation among submicron blocks bounded by the fractures. These observations suggest that surface disruption occurs primarily by a mechanism involving brittle fracture, rather than plastic deformation. This study demonstrates the feasibility of studying the effects of abrasion on near-surface structure with TEM. The degree and scale of fracturing also suggest a mechanism for forming submicron particles in quantity during aeolian abrasion.     

The relation between the crystallography of quartz, and upturned aeolian cleavage plates

The relation between the crystallography of artificially abraded quartz sand grains and aeolian surface textures has been studied using an X-ray precession camera and the scanning electron microscope (SEM). Crushed Brazilian quartz was sieved to between 250 and 350 μm, eroded in a paddle wheel device which simulated aeolian action at 8 m s^?1 for 3 h, and photographed with the SEM. A typical grain was selected and over 1100 photographs were combined to create a 3 × 3 m photomosaic; because of the large size, it was possible to observe and measure the angular and linear relations between the various features. After the c axis direction in the grain was determined by precession X-ray analysis, the most important aeolian features were related to the crystallography of quartz. The upturned plates or cleavage plates probably represent traces parallel to r(10ī1) and z(01ī1) cleavages in quartz. Blocky areas appear to be an expression of a cleavage parallel to m(10ī0). In addition, plate spacing on thirty-five experimental, modern, and ancient quartz sand grains is fairly constant. Assuming that abrasion in most sedimentary environments acts in the same manner with respect to quartz crystallography, much of the fine silt and clay in sediments and sedimentary rocks may be the result of cleavage following abrasion parallel to the r and z cleavage planes, while the less common blocky particles are probably the result of cleavage parallel to the m planes.     

The influence of bulk shape factors on settling velocities of natural sand-sized sedimentary suites

Settling rates of natural sand-size particles are influenced, to some extent, by their shapes and this may be an important factor in using settling rates to estimate grain size. In order to gauge the sensitivity of this influence, two natural sand populations from the Mesozoic Nubian Sandstones of Southern Israel, with a high probability of being similar in their bulk shape characteristics, were examined in ¼φ sieved fractions for their shape characteristics and settling rates. Fine surface features (roundness and surface roughness) were evaluated using Fourier shape analytical methods. Significant differences in bulk shape were detected and their influence on settling rates was measured empirically in a settling tube. The most marked differences were in the coarse grain sizes and, to a lesser extent, in the intermediate sizes. Sampling of raw settling data at closely-spaced time intervals yielded high-resolution grain size frequency plots which were usually polymodal in nature. Subtle shape contrasts, which are an important influence on settling rates, are thus an important consideration when working at this level of sensitivity. Natural sand populations which have followed a more varied provenance or process pathway could be expected to have even greater contrasts in settling rates than the samples analysed here. Thus it is recommended that the bulk shape factor should be taken into account in order to minimize errors in the conversion of settling times to grain size. An easy method, outlined in this paper, is through the establishment of an empirically derived calibration curve for each individual suite of sand undergoing analysis. Sieved ¼φ samples, derived from a split of the total composite sample undergoing analysis, forms the basis of the calibration and hence a correction factor converting sieve diameters to true diameters must be applied. In this research, nominal section diameters were obtained optically through an image analyser.     

Subsolidus and Partial Melting Reactions in the Quartz-excess CaO+MgO+Al2O3+SiO2+H2O System under Water-excess and Water-deficient Conditions to 10 kb: Some Implications for the Origin of Peraluminous Melts from Mafic Rocks

Experimental results up to 10 kb pressure are presented on thestability of amphibole in the quartz-excess CaO+MgO+Al₂O₃ (CMASH)system under H₂O)-excess and H₂O deficient conditions. Amphiboleis stable above the solidus under H₂O-excess conditions whereasunder H₂O-deficient conditions dehydration melting of amphibole-bearingassemblages defines the solidus. The successive appearance ofamphibole, talc, and zoisite with increasing pressure considerablymodifies the plagioclase-pyroxene-garnet-kyanite reactions documentedexperimentally in the CaO+MgO+Al₂O₃+SiO₂ system for gabbro-granulite-eclogitetransitions. Although both clino pyroxene and cordierite (withanorthite+orthopyroxene+quartz) may melt eutectically at oneatmosphere to form diopside-normative and corundum-normativemelts respectively, at higher pressures under H₂O-excess conditionsthe peritectic melting of mafic rock compositions produces corundum-normativeliquids together with either clinopyroxene or amphibole. Dehydrationmelting produces melts which are not corundum-normative. Thesedata are used to discuss the origins and evolution of contrastingbasalt-andesite-dacite-rhyolite volcanic suites and graniticplutons, many of whose silicic variants are corundum-normativein character, such as the Toba luff ignimbrites, Indonesia (Beddoc-Stephenset al., 1983) and I-type granite minimum melts (White &Chappell, 1977). In contrast, it is proposed that for the Cascadesbasalt-andesite-dacite-rhyolite suite the ortho pyroxene-plagioclase-quartzthermal divide was maintained up to rhyolite compositions, therebyprohibiting the derivation of corundum-normative rocks fromdiopside-normative parent magmas. The deduced reaction relations between pyroxenes, amphibole,plagioclase, quartz, and liquid are used to explain the absenceor extreme scarcity of hydrous phases in some hydrous magmas.These phase relations can also explain the development of laterplagioclase overgrowths on resorbed plagioclase cores in graniticintrusives, and the general absence of resorption and overgrowthsin chemically equivalent extrusive rocks. A theoretical analysis of the partial melting of forsterite-bearingassemblages in the CaO+MgO+Al₂O₃+SiO₂+H₂O system shows thatunder H₂O-excess conditions partial melting may generate corundum-normative(but low SiO₂) melts from a peridotite source at shallow depths.     

Stability Relations of the Ferruginous Biotite, Annite

Annite, KFe₃AISi₃O₁₀(OH)₂ a member of the iron biotites andthe ferrous analogue of phlogopite, has been synthesized andits phase relations have been determined as functions of temperature,fugacity of oxygen (fo₂), and total pressure (P_totalPH₂O+PH₂).A method for controlling fo₂at high total pressures is described,and data for the ‘oxygen buffers’ used are given.Buffers range from quartz+iron+fayalite assemblages (low fo₂)to magnetite-hematite assemblages (high fo₂). Optical propertiesand unit-cell dimensions of synthetic annites depend on theconditions of synthesis. By recalculating published analyses of natural iron-rich biotitesit can be shown that one cannot assume a constant hydrogen contentfor such biotites. Oxidation may have occurred by drying at115?C. Octahedral occupancy therefore cannot be calculated fromsuch data. Phase relations of annite are presented in 2,070 and 1,035 barsections. Depending on fo₂-T values annite was found to decomposeto one of the following assemblages: hematite+ sanidine, magnetite+sanidine,fayalite+leucite+kalsilite, iron+sanidine. All decompositionsare dehydration and redox reactions and are sensitive to changesin fH₂0 and fo₂ (or fH₂0 and fH₂). At 2, 070 bars total pressureannite+magnetite+sanidine can coexist between 425?C and 825?C, depending upon the magnitude of fo₂. In the presence of quartz the stability field of annite is morerestricted. Phase equilibria in the system KAlSiO₄–SiO₂–Fe–O₂–H₂have been summarized schematically. Wherever possible, thermodynamic extrapolations are made totest the internal consistency of the data. Enthalpies of formationare calculated for both annite and phlogopite. Ranges of fo₂values in nature as well as mechanisms for changes in fo₂ areinvestigated. It is useful to distinguish between assemblageswhich are internally buffered with respect to fo₂changes andthose which are not buffered. The applications of individualreactions involving annite to specific geologic problems arediscussed with respect to igneous, metamorphic, and sedimentaryrocks.     

Oxidation During Magmatic Differentiation, Finnmarka Complex, Oslo Area, Norway: Part 2, The Mafic Silicates1

Electron-microprobe analyses are presented for pyroxene, amphibole,and biotite from monzonite, granodiorite, and granite at Finnmarka,Norway. Compositional trends measured in biotite, present inall three rock types, and in amphibole, present in the monzoniteand granodiorite, are markedly atypical and are interpretedas reflecting crystallization under progressively more oxidizingconditions. The average Fe/Fe + Mg for biotites from successivelymore silicic rock types changes from 0.64 0.35 0.28, and foramphiboles changes from 0.58 in the monzonite to 0.29 in thegranodiorite. Analyses of selected areas within amphibole grains in the granodioriteshow marked chemical variations, although single-crystal X-rayphotographs are sharp and do not reveal multiple phases. Onthe basis of 33 such analyses, four coupled substitutions areidentified as operative; the most unusual finding is the relationof 1 Ti cation to 4 Al^IV cations in the unit cell. Variationswithin individual amphibole grains of the granodiorite resemblechanges noted in evolution of amphibole composition from monzoniteto granodiorite and are interpreted as reflecting progressiveoxidation. Consideration of these data for the mafic silicates, data forthe opaque oxides, and the extensive formation of sphene inthe granodiorite, has allowed development of schematic reactionsand an overall picture of magmatic environment and evolutionat Finnmarka. Crystallization apparently took place at PH₂oof 1000 bars or less and a temperature of about 700 °C.The trend of oxidation during differentiation is more extremethan any heretofore reported. Amphiboles, as well as biotites,may participate in oxidation reactions and may reflect the oxidation-reductionprocesses that occurred during magmatic evolution.