Showing papers on "Basalt published in 1992"

Chemical subdivision of the A-type granitoids:Petrogenetic and tectonic implications

Abstract: The A-type granitoids can be divided into two chemical groups. The first group (A1) is characterized by element ratios similar to those observed for oceanic-island basalts. The second group (A2) is characterized by ratios that vary from those observed for continental crust to those observed for island-arc basalts. It is proposed that these two types have very different sources and tectonic settings. The A1 group represents differentiates of magmas derived from sources like those of oceanic-island basalts but emplaced in continental rifts or during intraplate magmatism. The A2 group represents magmas derived from continental crust or underplated crust that has been through a cycle of continent-continent collision or island-arc magmatism.

Petrological systematics of mid-ocean ridge basalts: Constraints on melt generation beneath ocean ridges

Abstract: Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath ocean ridges, and contain much information concerning melt formation, melt migration and heterogeneity within the upper mantle. MORB major element chemical systematics can be divided into global and local aspects, once they have been corrected for low pressure fractionation and interlaboratory biases. Regional average compositions for ridges unaffected by hot spots ("normal" ridges) can be used to define the global correlations among normalized Na20, FeO, TiO2 and Si02 contents, CaO/Al 203 ratios, axial depth and crustal thickness. Back-arc basins show similar correlations, but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships between FeO, Na 20 and depth over distances of 1000 km. Local variations in basalt chemistry from slowand fast-spreading ridges are distinct from one another. On slow-spreading ridges, correlations among the elements cross the global vector of variability at a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the elements are distinct from both global and slow-spreading compositional vectors, and involve two components of variation. Spreading rate does not control the global correlations, but influences the standard deviations of axial depth, crustal thickness, and MgO contents of basalts. Global correlations are not found in very incompatible trace elements, even for samples far from hot spots. Moderately compatible trace elements for normal ridges, however, correlate with the major elements. Trace element systematics are significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions of the MAR are very depleted in REE, with little variability; hot spots cause large long wavelength variations in REE abundances. Normal EPR basalts are significantly more enriched than MAR basalts from normal ridges, and still more enriched basalts can erupt sporadically along the entire length of the EPR. This leads to very different histograms of distribution for the data sets as a whole, and a very different distribution of chemistry along strike for the two ridges. Despite these differences, the mean Ce/Sm ratios from the two ridges are identical. Existing methods for calculating the major element compositions of mantle melts [Klein and Langmuir, 1987; McKenzie and Bickle, 1988; Niu and Batiza, 1991] are critically examined. New quantitative methods for mantle melting and high pressure fractionation are developed to evaluate the chemical consequences of melting and fractionation processes and mantle heterogeneity. The new methods rely on new equations for partition coefficients for the major elements between mantle minerals and melts. The melting calculations can be used to investigate the chemical compositions produced by small extents of melting or high pressures of melting that cannot yet be determined experimentally. Application of the new models to the observations described above leads to two major conclusions: (1) The global correlations for normal ridges are caused by variations in mantle temperature, as suggested by Klein and Langmuir [1987] and not by mantle heterogeneity. (2) Local variations are caused by melting processes, but are not yet quantitatively accounted for. On slower spreading ridges, local variations are controlled by the melting regime in the mantle. On the EPR, local variations are predominantly controlled by ubiquitous, small scale heterogeneites. Volatile content may be an important and as yet undetermined factor in affecting the observed variations in major elements. We propose a hypothesis, similar to one proposed by Allegre et al [1984] for isotopic data, to explain the differences between the Atlantic and Pacific local trends, and the trace element systematics of the two ocean basins, as consequences of spreading rate and a different distribution of enriched components from hot spots in the two ocean basins. In the Atlantic, the hot spot influence is in discrete areas, and produces clear depth and chemical anomalies. Ridge segments far from hot spots do not contain enriched

Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry

Abstract: Chromian spinel in volcanic rocks is a potential discriminant for magma chemistry. The TiO2 content of spinel, compared at similar Fe3+/(Cr + A1 + Fe 3+) ratios, can distinguish island arc basalts from intraplate basalts. MORB spinels are low in this ratio and are intermediate for the TiOe level at comparable Fe 3+ ratios. Spinels from back-arc basin basalts, although similar in TiO2/Fe 3+ ratio, are more enriched in Fe 3+ than the MORB spinels. Spinels in the oceanic plateau basalts are distinctly lower in TiO2 than other intraplate basalt spinels and even slightly lower in TiOa than the MORB spinels. The data were successfully applied to estimate the kind of the magma from which spinelbearing cumulates, especially dunites, were formed. Original magma chemistry of altered or metamorphosed volcanics in which spinels survive can also be estimated by the chemistry of relict spinel alone. It is possible to estimate the magma type of source volcanics for detrital spinel particles of volcanic derivation. K E Yw o RI~ s : chromian spinel, TiO2 content, MORB, island-arc basalt and andesite, intraplate basalt, magma chemistry.

The age of parana flood volcanism, rifting of gondwanaland, and the jurassic-cretaceous boundary.

Abstract: The Parana-Etendeka flood volcanic event produced ∼1.5 x 10 6 cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New 40 Ar/ 39 Ar data combined with earlier paleomagnetic results indicate that Parana flood volcanism in southern Brazil began at 133 ± 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Parana flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary.

Primary magmas of mid-ocean ridge basalts 2. Applications

Abstract: Variable initial mantle composition and extent of depletion during dynamic melting processes strongly influence compositions of primary basaltic magmas. The descriptions of the equilibria that pertain to melting in the upper oceanic mantle presented in the companion paper (Kinzler and Grove, this issue) are used to estimate the major element compositions and temperatures of aggregate primary magmas of mid-ocean ridge basalt (MORB) generated in the adiabatically upwelling mantle beneath oceanic spreading centers. Primary MORB magmas with high Na2O abundances that are produced from more fertile mantle compositions or represent initial melts of a depleted spinel-lherzolite have higher SiO2 and Al2O3 and lower MgO, FeO, and CaO abundances, relative to low-Na2O primary magmas. Na2O abundance variation in the mantle source during polybaric, near-fractional melting processes causes melt compositions to vary significantly. The total extents of depletion achieved by the decompression melting process to yield the observed variation in major elements of MORB range from ∼ 6 to 18%; the range of mean pressures of melting is relatively narrow, 8–15 kbar; the total range modeled for the adiabatic, near-fractional melting process is 4 to 25 kbar. Aggregate primary magmas of MORB are not picritic, nor do they resemble sampled primitive MORB (MORB with MgO > 9.0 wt%). Much of the variation in major element composition observed in sampled MORB can be explained by melting a similar depleted MORB-mantle source. The ambient temperature range of the upper mantle beneath the global ridge system required to explain the observed chemical variations is 1475°–1315°C.

Primary magmas of mid-ocean ridge basalts 1. Experiments and methods

Abstract: This paper reports experiments carried out between 9 and 16 kbar (0.9–1.6 GPa) using natural, primitive mid-ocean ridge basalt compositions and synthetic analogs of mid-ocean ridge basalts to investigate the effects of pressure, temperature, and variable bulk composition on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and plagioclase or spinel. For this low-variance, five-phase assemblage, equations involving pressure, melt NaK # ([Na2O+K2O]/[Na2O+K2O+CaO]; weight ratio), melt Mg # (Mg/[Mg+Fe2+]; total iron as Fe2+), and weight percent TiO2 in the melt predict temperature and major element compositions of magmas produced by melting spinel and plagioclase lherzolites at upper mantle pressures. The equations are estimated using a selected set of data from this experimental study and published experimental studies that report compositions of glasses coexisting with olivine, low-Ca pyroxene, augite, and plagioclase and/or spinel. An experimental test of a liquid compositionally similar to melts produced in a subset of peridotite-basalt sandwich experiments is presented. The composition tested was reported as multiply saturated (with orthopyroxene + augite + spinel + olivine) in the sandwich experiment, but it does not crystallize these phases at the conditions of the experiment. We exclude this liquid and the subset it represents (data from Fujii and Scarfe [1985] and Falloon and Green [1987]) from the data set used to constrain the melting equilibria. With estimates of the melt NaK #, melt Mg #, and weight percent TiO2 of the melt the quantitative descriptions of the melting equilibria can be used to predict the temperatures and major element compositions of melts from lherzolite. Methods are described for estimating these compositional parameters with the nonmodal batch melting equation (for Na2O, CaO, K2O, and TiO2) and a mass balance calculation (for MgO and FeO) from the initial composition and phase proportions of the mantle source, the amount of melt produced and the nature of the melting process, and the stoichiometric coefficients of the mantle melting reaction.

Recent chemical weathering of basalts

Abstract: Relative bulk leach rates of the major elements Na, Ca, Mg, Fe, Al, Ti, and Si from studied basaltic weathering profiles are not greatly affected by primary mineralogy or by bulk composition of the parent basalt, probably because major silicate phases are weathered at grossly similar rates. By contrast, trace element leach rates may be effectively controlled by their mineralogical siting in parent basalts. Release rates of trace elements, including potentially toxic heavy metals, are confidently predicted only where bulk compositions, mineralogical and modal analyses of the parent basalt are available. Ti is the most conservative major element in the basaltic profiles studied. Fe and Al are leached from soil zones and from weathering profiles subjected to extreme chemical weathering. Both Fe and Al, however, are effectively conserved in recent, aerated profiles that have suffered moderate to extensive (but not extreme) weathering. Generally, Al is more extensively leached than Fe(III), an observation supported both by bulk compositional changes of profiles and by compositions of associated weathering solutions. Climatic conditions do not affect greatly the gross, relative release rates of major elements from weathered basalts. By implication mechanisms of chemical weathering are similar regardless of climate. Climate, however, affects dramatically the relative rates of chemical and physical weathering (that is, leaching and erosion). The diverse clay mineral suites observed in soils of various climates may result more from establishment of different steady state conditions in each climatic regime (chemical weathering versus erosion) than from different weathering-reaction mechanisms. During weathering of the Baynton Basalt, the order of susceptibility of primary phases is olivine > glass > plagioclase > clinopyroxene > Fe-Ti-oxide. Glass, plagioclase, and clinopyroxene are weathered at grossly similar rates. Field observations, local equilibrium, and kinetic considerations indicate that there is no fixed order of mineral susceptibility to weathering and no fixed leaching order of elements from basalts. Only after classification of a basalt according to bulk composition and model proportions can generalizations be made in these regards.

The geochemistry of young volcanism throughout western Panama and southeastern Costa Rica: an overview

Abstract: Oblique aseismic subduction below western Panama and southeastern Costa Rica has produced Recent arc-related volcanism. The aseismicity is probably related to the subduction of relatively hot oceanic lithosphere. The volcanism throughout this region over the past 2 Ma has been quite distinct, consisting of felsic magmas (andesites to rhyolites but mainly dacites) with geochemical signatures suggesting a metamorphosed basaltic source. It is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge. Rocks with slab-melt geochemistries and associated with young subducted crust have been termed adakites elsewhere. The young adakite melts are sometimes associated with a few rare young high-Nb basalts, but there is no obvious genetic link between them through differentiation. High-Nb basalts may also be derived from the partial melting of the subducted oceanic crust. High-Nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the Catalina Schist, California. Alternatively, the high-Nb basalts may be partial melts of phlogopite-rich mantle that has previously reacted with adakite magmas. Eruption of adakites and high-Nb basalts was preceded by a 2-3 Ma period of relative quiescence. Prior to this, there was a 7 Ma period of calc-alkaline volcanism typical of the present-day magmatism (associated with a distinct Benioff zone) found throughout the Central American arc. The abrupt transition in volcanism with time from an early calc-alkaline sequence to a later adakite-high-Nb basalt sequence may record a change in the tectonic setting of western Panama and southeastern Costa Rica over the past 12 Ma.

Australian volcanic-hosted massive sulfide deposits; features, styles, and genetic models

Abstract: The Cambro-Ordovician period has yielded the major development of volcanic-hosted massive sulfide (VHMS) deposits in Australia, resulting in about 12 million metric tons of contained metal (copper, lead, and zinc), concentrated in the Mount Read Volcanics (Tasmania) and the Mount Windsor Volcanics (Queensland), The Archean (4 million metric tons of metal) and the Silurian (3.5 million metric tons of metal) constitute the next important episodes while the Devonian and Permian have produced isolated deposits. The deposits range from Cu type to Zn-Cu type to Zn-Pb-Cu type. The Zn-Cu-type deposits are restricted to the Archean and Silurian, whereas the Cu type and Zn-Pb-Cu type occur sporadically throughout the time span from early Archean to Permian. In general terms, the major VHMS-bearing districts are calc-alkaline in character and display a thick basal pile of rhyolitic volcanics (1-3 km thick) including lavas, epiclastics, and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite,dacite, andesite, basalt, and sediments. The major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence. There is a wide range in variability of styles of Australian VHMS deposits including mounds, pipes, sheets, layered deposits, stacked deposits, stockwork and disseminated deposits, distal reworked deposits, and cyclic layered deposits. Although these various styles show a range of features, there is a consistent theme across the spectrum of metal zonation, alteration mineralogy, alteration chemistry, sulfur isotopes, macrotextures, microtextures, and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit. The classic mound-style deposits, such as Hellyer, have a series of key features, subsets of which are represented in the other deposit styles. The mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent. Growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures, leading to zone refining and lead-zinc-silver-gold enrichment in the outer and upper parts of the mound, Departure from the classic mound style of deposit is related principally to three key factors: the chemistry of the ore fluid (salinity, temperature,f02, and aH2S), the nature (permeability and chemistry) of the volcanic pile, and the sea-Hoor environment (sea-Hoor topography and ..seawater depth). These factors control the aspect ratio of the deposit, the extent of stringer zone development, the degree of subsea-floor replacement mineralization, the nature and spatial development of footwall alteration, and the development and style of related distal mineralization. Sulfur isotope studies of Australian VHMS deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit, whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks. This conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences. The input of metals directly from volcanic magma chambers is not precluded by the available data, and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume, rising to mix with convective seawater fluids which contribute lead, zinc, silver, and gold leached from the footwall volcanics and basement rocks. The relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins.

Chemical stratigraphy of the Paraná lavas (South America): classification of magma types and their spatial distribution

Abstract: A new classification scheme has been developed to assign the lava flows of the Parana continental flood basalt province (South America) into geochemically distinct magma types, with six basaltic major and trace element abundances and/or ratios. By mapping out the spatial distribution of these magma types within the lava sequences, it has been possible to determine the internal stratigraphy of the lava pile on a regional scale. Previous studies on road profiles traversing the well-exposed coastal Serra Geral escarpment of southern Brazil are summarised together with results from some new sampled sections. More widespread stratigraphical investigations of the Parana lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks. However, access to drill-core chippings from nine boreholes in the central Parana region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions. The borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north, which suggests that the main locus of magmatism moved northwards with time within the Parana basin. This migration could be related to the northward propagation of rifting during the initiation of the South Atlantic Ocean. Maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes, although suggesting that the shift in magmatism may have been towards the northwest. On the basis of geochemical similarities between magma types and their inferred stratigraphical relationships, it is proposed that the Parana can be divided into two principal magmatic centres: (1) an older one in the south, comprising the Gramado, Esmeralda and Urubici magma types; and (2) a younger one, developed about 750 km to the north, formed by the Pitanga, Paranapanema and Ribeira magma types.

Dehydration melting and the generation of continental flood basalts

Abstract: CONTINENTAL flood basalt provinces represent important magmatic events, which may contribute significantly to the generation of new crust. In a typical flood basalt province, large volumes of magma are erupted in a short time: for example, at least 106 km3 in 2–3 Myr in the Parana–Etendeka province of southeast Brazil and northern Namibia. In many areas flood basalts are associated with mantle plumes, but details of their origin, such as the site of major-element melting, remain unresolved. Recent authors1–3 have assumed that partial melting took place at the anhydrous peridotite solidus, and thus concluded that during continental extension, more than 95% of the erupted magmas are generated in the sublithospheric upper mantle. In this situation, the distinctive isotope and trace element geochemistry of the basalts is attributed not to this process, but to the addition of low-degree partial melts scavenged from the overlying lithosphere4. Here we present an alternative model in which, in the presence of small amounts of water (∼0.4%), the observed quantities of melt may be generated entirely within the mantle lithosphere. As rifting proceeds, however, the basalts acquire an increasingly 'asthenospheric' chemistry, as melts from the asthenosphere come to dominate those from the lithosphere.

Open system melting and temporal and spatial variation of peridotite and basalt at the Atlantis II Fracture Zone

Abstract: In situ ion microprobe analysis of trace and rare earth elements in discrete diopsides in abyssal peridotites from nine transform dredge hauls from the Atlantis II Fracture Zone (All FZ) shows that these samples have a wide range of trace element contents close to the total range found for the entire Southwest Indian Ridge. Though the spread in analyses is large, the average composition of the peridotites is close to that reported for the All FZ by Johnson et al. (1990) and lies at the relatively undepleted end of the spectrum for SW Indian Ridge residual mantle peridotites. A sharp break in peridotite diopside composition and modal mineralogy occurs across the transform, suggesting that it acts as a boundary for different melting regimes and initial mantle compositions. The difference in peridotite compositions is mirrored in spatially associated basalts, which lie on separate parallel liquidus trends in the normative ternary pyroxene-olivine-plagioclase. Basalts from the east side of the transform have higher normative plagioclase contents, indicating that they may be products of lower degrees of mantle melting than basalts from the western side, consistent with greater depletion of peridotites from the western wall or a more depleted initial composition. Basalts from the eastern wall also have consistently lower Fe8.0 and higher Na8.0 than basalts from the western wall and lie parallel to the global along-ridge Fe8.0 − Na8.0 trend (Klein and Langmuir, 1987) and orthogonal to the local melting paths of Klein and Langmuir (1989). Our data provide strong evidence for segmentation of the melting regime, with major mantle discontinuities occurring at transform offsets at slow spreading ridges. Peridotites analyzed along the eastern wall of the fracture zone also show a systematic change in composition with latitude and, with the older peridotites from the median tectonic ridge, define a systematic change in the degree of melting of the mantle occurring beneath the paleoridge axis over the last 11 m.y. Emplacement of mantle showing the lowest degree of melting, or the least depleted parental mantle composition, corresponds roughly to the time of crystallization of Ocean Drilling Program site 735B gabbros. Melting is modeled as a non-steady state, discontinuous process with 0.1–0.5 vol % aggregated melt retained in the porous residue (open system melting). The range in degree of open system melting for the combined suite of All FZ peridotites is 8–20%. Such a large systematic variation would appear to require a dynamically significant change with time, either in the initial temperature and/or a large compositional difference of the mantle beneath the paleoridge axis. This in turn suggests that in the relative reference frame of the ridge axis, mantle flow was non-steady state. This could reflect episodic mantle diapirism beneath the ridge axis or, alternatively, that the ridge axis has moved over a zone of enhanced upflow in the underlying mantle that was fixed in the absolute hotspot mantle reference frame.

A basalt trigger for the 1991 eruptions of Pinatubo volcano

Abstract: THE eruptive products of calc-alkaline volcanos often show evidence for the mixing of basaltic and acid magmas before eruption (see, for example, refs 1, 2). These observations have led to the suggestion3 that the injection of basaltic magma into the base of a magma chamber (or the catastrophic overturn of a stably stratified chamber containing basaltic magma at its base) might trigger an eruption. Here we report evidence for the mixing of basaltic and dacitic magmas shortly before the paroxysmal eruptions of Pinatubo volcano on 15 June 1991. Andesitic scoriae erupted on 12 June contain minerals and glass with disequilibrium compositions, and are considerably more mafic than the dacitic pumices erupted on 15 June. Differences in crystal abundance and glass composition among the pumices may arise from pre-heating of the dacite magma by the underlying basaltic liquid before mixing. Degassing of this basaltic magma may also have contributed to the climatologically important sulphur dioxide emissions that accompanied the Pinatubo eruptions.

How deep do common basaltic magmas form and differentiate

Abstract: This work investigates the potential of experiments on peridotite melting to provide quantitative estimates of the temperature and pressure at which basaltic melts were formed. As the primary chemical signal is systematically obscured by crystal fractionation, the first part of this study is devoted to the analysis of the main differentiation trends in the basaltic series from the major geodynamic sites: mid-ocean ridges, hot spots (both intraplate and near the ridge axis), continental floods, and orogenic boundaries. Two main sorts of primary magmas can be identified: (1) the parent magmas of mid-ocean ridge, continental flood, and orogenic tholeiites which are poor in Ti and Fe, rich in Al and, to a lesser extent, Na and (2) the parent magmas of within-plate ocean island basalts (OIB) which have the opposite characteristics. The importance of plagioclase during postmelting fractionation increases in the following order: OIB, island arc tholeiites (IAT), low-Ti continental flood basalts (CFB), high-Ti CFB, and mid-ocean ridge basalts (MORB) corresponding to differentiation pressure decreasing from >10 to ∼5 kbar. Ocean islands that formed near ridge axis have parent magmas and differentiation conditions intermediate between those of MORB and within-plate OIB. Along-ridge variations in TiO2 and Na2O contents in MORB are ascribed to mantle heterogeneities and suggest pervasive infiltration of an OIB component. The chemical compositions of synthetic melts in equilibrium with peridotite have been fitted not as a function of the degree of melting, which is neither available from the sandwich experiments nor independent of the fertility of the source, but as a function of temperature (mean error, 40°C) and pressure (mean error, 2.7 kbar). Although fractionation of multimineral assemblages cannot be accounted for with precision, the effect of individual minerals, plagioclase, clinopyroxene, and olivine can be calculated. Assuming that primary melts are in equilibrium with a mantle olvine Fo90–92, it is suggested that MORB segregated from their mantle source at ∼15 ± 5 kbar. Shield-stage ocean island basalts (Hawaii, Gough, Reunion) and Solomon Island picrites segregated at 20–30 kbar, whereas even greater depths are suggested for Tahiti lavas. The estimated thickness of the molten layer from which the lavas evolve depends on the degree of melting assumed. It is ∼50 km under the ridges, and 20 km or even less at the base of the lithosphere under intraplate volcanoes.

Lithospheric thickness as a control on basalt geochemistry

Abstract: Variations in the thickness of the lithosphere are likely to influence the trace element compositions of basaits by controlling the distribution of melt and the minerals in the melting assemblage in the asthenosphere. Examples of systematic trace element variations that appear to be related to the thickness of the lithosphere are found in both oceanic and continental basalts. In the oceans, lithospheric thickness is closely related to age, and its control is manifested by a relation between trace element composition and age of basement ocean crust in Atlantic ocean-island basalts. Beneath continents, lithospheric thickness is less age sensitive, but it varies widely in response to extensional tectonics. Contemporary hot spots, which have been implicated in the generation of ancient flood basalts, show variations in the rare earth element content of their associated basalts which apparently reflect changes in lithospheric thickness between the time of flood-basalt eruption and the present.

Groundmass crystallization of Mount St. Helens dacite, 1980 1986: a tool for interpreting shallow magmatic processes

Abstract: The 1980–1986 eruption of Mount St. Helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale. Groundmass plagioclase size measurements are coupled with measured changes in matrix glass, plagioclase and Fe−Ti oxide chemistry to document increasing groundmass crystallinity, and thus to better constrain proposed physical models of the post-May 18, 1980 magmatic reservoir. Measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth (approximately 10-9 cm/s) of groundmass plagioclase occurred immediately subsequent to May 18. Relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10-11 cm/s; plagioclase growth rates then decreased to <1x10-11 cm/s through 1986. Changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst-rim chemistry, although the matrix glass composition appears to have remained approximately constant from 1981–1986 after a rapid compositional change from May 18 until the end of 1980. Plagioclase microphenocrysts show increasingly more complex zoning patterns with time; microphenocryst-core compositions are commonly positively correlated with crystal size. Both of these observations indicate continuous groundmass plagioclase growth through 1986. Magmatic temperatures estimated from Fe−Ti oxide pairs are approximately constant through 1981 at eruption temperatures of ∼ 930°C and at log fO2 of -10.8; by 1985–1986 oxide temperatures decreased to ∼ 870°C. Chemical and textural changes can be explained by: (1) rapid degassing and crystallization in response to the intrusion of magma into a shallow (<4.5 km) reservoir toward the end of the May 18, 1980 eruption; (2) continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir. Growth rates (and consequent chemical changes) appear to decrease at the end of 1980—this is coincident with the change in eruption style from explosive eruptions, sometimes followed by dome growth, to solely extrusive (dome-building) events, and can be explained by the expected viscosity increase of both degassing and increasing crystallinity. The model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies (Casadevall et al. 1983; Gerlach and Casadevall 1986 a, b), patterns of crater deformation (Chadwkck et al. 1988) and post-1980 seismicity (Endo et al. 1990), although it does not explain the experimental data of Hill and Rutherford (1989) on the growth rate of amphibole reaction rims. Textural measurements on Mount St. Helens dacite can also be used to evaluate crystallization kinetics in silicic magmas, systems for which experimental data is almost non-existent. Plagioclase growth rates are 5–10 times slower than estimated plagioclase growth rates in basaltic systems, a result consistent with the higher viscosity of a more silicic melt. Furthermore, patterns of textural change (both average crystal size and number density) are similar to those observed during the 1984 Mauna Loa eruption by Lipman and Banks (1987), suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates, such that the rate of crystallization scales with the melt viscosity.

Southwestern limits of Indian Ocean Ridge Mantle and the origin of low 206Pb/204Pb mid-ocean ridge basalt: Isotope systematics of the central Southwest Indian Ridge (17°–50°E)

Abstract: Basalts from the Southwest Indian Ridge reflect a gradual, irregular isotopic transition in the MORB (mid-ocean ridge basalt) source mantle between typical Indian Ocean-type compositions on the east and Atlantic-like ones on the west. A probable southwestern limit to the huge Indian Ocean isotopic domain is indicated by incompatible-element-depleted MORBs from 17° to 26°E, which possess essentially North Atlantic- or Pacific-type signatures. Superimposed on the regional along-axis gradient are at least three localized types of isotopically distinct, incompatible-element-enriched basalts. One characterizes the ridge between 36° and 39°E, directly north of the proposed Marion hotspot, and appears to be caused by mixing between hotspot and high ∈Nd, normal MORB mantle; oceanic island products of the hotspot itself exhibit a very restricted range of isotopic values (e.g., 206Pb/204Pb = 18.5–18.6) which are more MORB-like than those of other Indian Ocean islands. Between 39° and 41°E, high Ba/Nb lavas with unusually low 206Pb/204Pb (16.87–17.44) and ∈Nd (−4 to +3) are dominant; these compositions are not only unlike those of the Marion (or any other) hotspot but also are unique among MORBs globally. Incompatible-elementenriched lavas in the vicinity of the Indomed Fracture Zone (∼46°E) differ isotopically from those at 39°–41°E, 36°–39°E, and both the Marion and Crozet hotspots. Thus, no simple model of ridgeward flow of plume mantle can explain the presence or distribution of all the incompatible-element-enriched MORBs on the central Southwest Indian Ridge. The upper mantle at 39°–41°E, in particular, may contain stranded continental lithosphere, thermally eroded from Indo-Madagascar in the middle Cretaceous. Alternatively, the composition of the; Marion hotspot must be grossly heterogeneous in space and/or time, and one of its intrinsic components must have substantially lower 206Pb/204Pb than yet measured for any hotspot. The origin of the broadly similar but much less extreme isotopic signatures of MORBs throughout most of the Indian Ocean could be related to the initiation of the Marion, Kerguelen, and Crozet hotspots, which together may have formed a more than 4400-km-long band of juxtaposed plume heads beneath the nearly stationary lithosphere of prebreakup Gondwana.

Metasomatism of the sub-arc mantle inferred from trace elements in Philippine xenoliths

Abstract: ISLAND arc basalts are thought to derive from the melting of the wedge of mantle overlying the subducting slab1; hence, the composition of this mantle wedge, and the nature of any metasomatic fluids or magmas introduced from the slab, have been the subject of much speculation2–6. Most of the evidence bearing on these questions has been indirect, coming from the chemistry of island arc lavas, or from mantle xenoliths originating elsewhere than immediately below the arc7,8. Here we report trace element data for mantle xenoliths from the Luzon arc (Philippines)9, which come from the mantle wedge directly below the arc-front volcanoes. The trace element patterns reflect metasomatism of a relatively depleted mantle (more depleted than the source of mid-ocean-ridge basalts), and identify the metasomatizing agent as a hydrous fluid, rather than a melt. We suggest that the depletion of high-field-strength elements in arc magmas originates from a combination of low initial concentrations in the mantle before metasomatism, and the inability of hydrous fluids to transport these elements.

Solidification and Morphology of Submarine Lavas: A Dependence on Extrusion Rate

Abstract: The results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity. The experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter, the ratio of the time taken for the surface to solidify, and a time scale for lateral flow. For submarine basalts a solution of the cooling problem (which is dominated by conduction in the lava but convective heat transfer in the water) and estimates of lava viscosities place this parameter within the empirically determined 'pillowing' regime over a wide range of extrusion rates. This results is consistent with the observation that pillow basalts are the most common products of submarine eruptions. Smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusion of basaltic magma. Still higher eruption rates in regions of low topographic relief may produce submarine lava lakes. Minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type.