Showing papers on "Basalt published in 1984"

Phase equilibrium controls on the tholeiitic versus calc‐alkaline differentiation trends

Abstract: This paper discusses some of the factors that allow tholeiitic and calc-alkaline differentiation trends to be generated from a common basaltic parent melt, and calculates liquid lines of descent, for the two divergent trends. Phase proportions and compositions measured in 1-atm experiments on natural basalts are used as input to calculate the tholeiitic trend, and high-pressure phase proportions, inferred from phase equilibrium studies on natural compositions, are used to calculate the calc-alkaline trend. An important control on the development of the contrasting tholeiitic versus calc-alkaline trends is the proportions of olivine, plagioclase, and pyroxene that crystallize from the basaltic parent melt. The tholeiitic trend is produced by fractional crystallization of a basalt magma at low pressures. The crystallization sequence is olivine, followed by plagioclase, followed by augite. Plagioclase dominates the assemblage, and the three-phase crystallization continues to a reaction point where olivine+liquid react to augite, plagioclase, and pigeonite. Total iron in the liquid increases throughout this crystallization process, and a dramatic increase in iron and a mild decrease in SiO2 occur at the reaction point. The calc-alkaline trend develops when olivine, calcic plagioclase, and augite crystallize in nearly equal mass proportions. This phase assemblage precipitates under conditions of moderate pressure and water undersaturation in the middle to upper crust and produces a gentle increase in total iron, an increase in SiO2 and a decrease in MgO in derivative liquids. Liquids derived by such a process can evolve to andesites and dacites by continued crystallization involving plagioclase, orthopyroxene, pigeonite, augite, and magnetite. The assimilation of a crustal component by a fractionating basaltic melt and the mixing of basaltic liquids with siliceous residual liquids produced by either fractionation or fractionation aided by assimilation are additional processes that assist in the production of the calc-alkaline trend.

The Pliocene seamount series of La Palma/Canary Islands

Abstract: A Pliocene submarine series of alkali basaltic pillow lavas, hyaloclastites, and breccias (A), a sheeted dike swarm (B), and a basal suite of gabbro and ultramafic rocks (C) from La Palma (Canary Islands) is interpreted as a cross section through an uplifted seamount. This series has been tilted to its present orientation of 50°/230° (plunge and azimuth), probably by upwarping due to intrusions in the central portion of the island. The basal plutonic complex (C) also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series. The plutonic suite (C) is overlain abruptly by more than 1800 m of sills (B), 0.4–1 m thick on average, with minor screens of lavas and breccias. Extrusives (A) form a 1750 m thick sequence of pillow lavas, breccias, and hyaloclastites. The clastic rocks increase in abundance upward and are of four main types: (1) breccias, consisting of partly broken pillows, formed nearly in situ, (2) heterolithologic pillow fragment breccias, (3) hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards, the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows, (2) and (3) being interpreted to have been resedimented, and (4) pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal, highly vesicular “pillows” and lava stringers and local bombs, probably formed by cracking and “bleeding” of gas-rich expanding pillow lava and some shallow submarine/subaerial lava fountaining. The extrusive series is chemically and mineralogically crudely zoned, with the most differentiated rocks (metatrachytes and mugearites) at the base and most picritic lavas occurring near the top of the series. Subsequent to emplacement, the entire extrusive and intrusive series has been hydrothermally altered, the lower part to greenschist and the upper part to smectite—zeolite facies mineral assemblages. The La Palma succession, combined with evidence from surface studies of seamounts, suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions. The nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth, the critical depth depending on magma and thus volatile composition. Changes in slopes of a seamount influence depositional processes. Based on these factors, at least three major depositional sites develop as a seamount grows: summit, flank, and apron facies. Nonexplosive, extrusive processes prevail in the Deep Water Stage, dominantly producing pillow lavas (75%). These consist of individual pillow volcanoes up to 200 m high, with large pillows near the base and decreasing pillow size toward the top of a volcano. Pillow breccias, and pillow fragment breccias comprise approximately 20% of this facies. The deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material. The Shallow Water (shoaling) Stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles, about 800 m for the alkali basaltic seamount of La Palma, resulting in formation of mainly clastic rocks (70%): in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount, by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining. Resedimented, heterolithologic pillow fragment breccias, lapilli breccias, and hyaloclastites are deposited on the flanks and aprons of the seamount. Pillow lavas comprise < 30% of these deposits.

Petrology of Young Pacific Seamounts

Abstract: Twenty-two young seamounts near the East Pacific Rise (EPR) consist predominantly of Ol-Hy normative basalt chemically identical to depleted mid-ocean ridge basalts (MORB). This basalt is systematically more primitive than the EPR lavas and is supplied directly to the seamounts from the mantle under the ridge, ridge-transform intersections, and fracture zones. The seamounts also contain small volumes of differentiated basalt with low Mg # (Mg # = Mg/Mg + Fe2+) which fractionate from basalt parents of high Mg # mainly by processes of fractional crystallization. Seamounts also contain Ne normative alkali basalts, which are enriched in light rare earth, other incompatible trace elements, and volatiles, as well as a complete spectrum of transitional basalts of intermediate composition. We suggest that this spectrum of primitive basalt types forms owing to magma mixing during melt segregation in the region of melting of a chemically, isotopically, and perhaps mineralogically heterogeneous mantle. Melting probably takes place at about 100 MPa in the stability field of plagioclase or plagioclase-spinel, and the extent of partial melting is highly variable: relatively low to produce alkalic basalts and relatively high to produce the tholeiitic MORB. All basalt types, however, are probably hybrids produced by mixing in the region of melt segregation. The thermal and mechanical regime which results in the magma diversity observed on young seamounts is evidently also present at very slow spreading ridges but not below fast-spreading ridges. Normal ridge crest segments and fracture zones (seamount volcanism) thus exhibit a range of thermochemical characteristics which differ markedly from plume/hotspot volcanism.

Tapping of magmas from ubiquitous mantle heterogeneities: An alternative to mantle plumes?

Abstract: Long-lived heterogeneities in the mantle are indicated by Nd-Sr-Pb isotopic systematics. Explanations for these variations have invoked either distinct layers in the earth that are preferentially tapped by some source regions (such as plumes rising from the base of the mantle) or small-scale heterogeneities which are ubiquitously distributed through the mantle and tapped differentially in various source regions. Consideration of the thermal and mechanical aspects of the latter hypothesis provides an explanation for the differences between mid-oceanic ridge basalts and off-axis volcanism. The melting behavior in a region with heterogeneities is modified by lateral conduction of heat, so that the first melted regions are tapped preferentially at off-axis volcanoes where small degrees of melting occur. A small (less than a few kilometers), easily melted heterogeneity draws heat from its surroundings as it melts during ascent. An increase of melting by a factor of 2 over adiabatic ascent is probable in the most easily melted regions if the melted and unmelted regions have comparable volume. The increase is larger in the earliest stages if melting is confined to small isolated heterogeneities. The depleted iso-topic ratios of mid-oceanic ridge basalts can be explained as follows. Heterogeneities with enriched components melt before those with depleted components as material ascends beneath the ridge. Then the enriched melts are removed from the source region before mid-oceanic ridge basalts are tapped. The first enriched components to melt enter fractures that carry them upward and outward away from the source region of basalts at the ridge axis. This occurs because the direction of shear strain in the upwelling is such that the axis of tension plunges at 45° away from the ridge axis and dikes of melt propagate up perpendicular to the axis of tension. In contrast with the ridge, volcanoes away from the ridge axis preferentially tap the earlier melted components. At the greater source depths of these volcanoes only non-mid-oceanic ridge basalt (MORB) components are significantly molten. Another enriched off-axis source region is the layer formed by the first enriched magma which moved laterally and avoided the source region of basalts at the ridge axis. For small-scale ubiquitous heterogeneities to explain the differences between mid-oceanic ridge basalts and off-axis volcanism, the MORB component must produce a greater volume of melt than the other components. If isotopic mass balances preclude the MORB component from being this abundant in the whole mantle, then the lower mantle is probably a separate reservoir from the upper mantle. In that case, it is conceivable that passive ubiquitously distributed blobs derived from the lower mantle at earlier times act as small-scale heterogeneities. Both this blob model and the small-scale heterogeneity model are possible alternatives to active mantle plumes.

Alteration of the upper oceanic crust, DSDP site 417: mineralogy and chemistry

Abstract: Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage. 2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H2O, increase of Fe3+/FeT, and possibly some losses of Ca and Mg occurred during this stage. 3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H2O, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost. 3b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe3+/FeT and H2O occurred. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H2O, and CO2 in the rocks. 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage.

The volcanic‐tectonic cycle of the FAMOUS and AMAR Valleys, Mid‐Atlantic Ridge (36°47′N): Evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections, AMAR 3

Abstract: The 1978 AMAR expedition extended the investigation of the Mid-Atlantic Ridge that was begun by the 1973–1974 FAMOUS expedition from the northern end of the FAMOUS rift (the original FAMOUS area) to the narrow central and southern end of the FAMOUS rift (Narrowgate region) and into the broad AMAR valley south of Fracture Zone B. Available field and geochemical data allow us to characterize in detail the volcanic-tectonic cycle for these two segments of the mid-ocean ridge system. A dynamic model of the crust-forming processes within these two rift valley segments is deduced from (1) variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins, (2) composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution, and (3) detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor. Each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition. The decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits. Intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes, the common evidence of crystal resorption, and the apparent disequilibrium between megacrysts, phenocrysts, and their enclosing glass. This indicates control by a crustal magma chamber. Liquid and crystal compositions and textural relations can best be explained by a three-stage crystallization history: (1) crystallization of olivine, plagioclase, and clinopyroxene onto the floor of a magma chamber, (2) partial resorption of mineral phases by a superheated, undersaturated liquid above the floor, (3) intratelluric crystallization of olivine and plagioclase (± clinopyroxene) during rise of melt to the surface through the conduit system above the magma chamber. A magma chamber model is suggested for the AMAR-FAMOUS rift valleys that satisfies both seismic and geological constraints. We propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment (AMAR and Narrowgate regions) but that these thin and terminate near the intersecting fracture zones. Expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization. The FAMOUS-Narrowgate rift is currently in a contraction period so that in the northern FAMOUS region, primitive liquid has erupted at Mount Venus and Mount Pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the Narrowgate region.

Regional O-, Sr-, and Pb-isotope relationships in late Cenozoic calc-alkaline lavas of the Andean Cordillera

Abstract: Quaternary-Recent volcanism in the Andes has occurred in three regions: 45–33°S, 28–16°S and 2°S–5°N, each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics. Isotope ratios of O and Sr are lowest and those of Pb least variable in the southern volcanic zone (SVZ) where medium-K lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent. The SVZ lavas were probably derived from an asthenospheric mantle source above a shallow Benioff zone. Parental basaltic magmas rose largely unmodified through thin continental crust, where differentiation occurred by low-pressure crystal fractionation without concurrent modification of isotopic composition. The slight enrichment of 206 Pb in the northern volcanic zone (NVZ), where the crust is thin and the Benioff zone deep, suggests a greater subduction-zone component for parental medium-K magmas. The slight 18 O enrichment suggests a small amount of lower crustal interaction. Isotope ratios of O and Sr are highest and those of Pb most variable in the central volcanic zone (CVZ). Parental magmas in the CVZ were probably generated within the same mantle source region as those in the SVZ and NVZ. Subsequently, during transit through the exceptionally thick continental crust of the central Andes, high-K magmas were produced by a combination of bulk contamination in the lower crust and later low-pressure fractional crystallization-assimilation processes in the upper crust which altered both chemical and isotopic compositions.

Behavior of Alkalies during Diffusive Interaction of Granitic Xenoliths with Basaltic Magma

Abstract: A series of three experiments was made at ∼125°C and 10 kbar in which a 1-gram reservoir of molten oceanic tholeiite was maintained in contact with one end of a 6.5 mm diameter cylinder of partially-molten, fine-grained granite. Detailed microprobe analysis (450 spots total) of the quenched samples revealed progressive basalt/granite interaction as run duration increased from 0.75 to 8 to 24 hours. Interdiffusion of melt components is limited for structure-controlling species such as $$Si^{4+}$$ and $$Al^{3+}$$ and for divalent cations, which all mix over a zone less than 0.5 mm wide in 24 hours at run conditions. In contrast, K and Na are extremely mobile, resulting in considerable uptake of K by the basalt melt and eventual loss of Na from the basalt to the granite by "uphill" diffusion. The chemical diffusivity of K in the basalt can be accurately estimated at $$3 \times 10^{18} cm^{2}/sec$$ for the specific experimental conditions used. Although this value is somewhat lower than previous estimates K o...

Influence of degassing on oxidation states of basaltic magmas

Abstract: Intrinsic oxygen fugacity measurements on minerals from mafic rocks that cool at pressures greater than one atmosphere indicate that the Earth's interior is in a more reduced state than are the erupting tholeiitic magmas. This review suggests that specific oxidation–reduction equilibria within magmas and between magma and vapour can account for the characteristic oxygen fugacities of most lavas. A model is presented to illustrate how an initially highly reduced mafic magma may oxidize by degassing of C-species in the conditions of the shallow crust.

Areal distribution and age of low-K, high-alumina olivine tholeiite magmatism in the northwestern Great Basin

Abstract: Field, petrographic, chemical, and geochronologic information has led to the identification and characterization of a widespread low-K, high-alumina olivine tholeiite (HAOT) magma type in the northwestern Great Basin. This basalt covers at least 22,000 km 2 and is estimated to represent a total volume of at least 650 km 3 . The time period over which HAOT lavas were erupted extends from late Miocene to Holocene (10.5–0 m.y. B.P.). This interval overlaps with the timing of Snake River, Cascade, and northwestern Basin and Range volcanism but distinguishes HAOT from the main pulse of Columbia River volcanism (∼15 m.y. B.P.). Furthermore, three major pulses of HAOT magmatism are suggested from the geochronology of this study: 0 to 2.5 m.y. B.P., 3.5 to 6 m.y. B.P., and 7 to 10 m.y. B.P. The distinctive holocrystalline, nonporphyritic, and diktytaxitic texture, the low incompatible-element concentrations, and the high MgO/FeO* of HAOT serve to distinguish this basalt from other basalts of the northwestern United States. The low incompatible-element signature accentuates the similarities between HAOT, mid-ocean–ridge basalts, and back-arc–basin basalts. These similarities, combined with the HAOT chronology, support the idea that the processes giving rise to extensional tectonism and HAOT magmatism in the northwestern Great Basin are similar to those acting in active back-arc–spreading regions.

Geology and geochemistry of early arc-volcanic rocks from Guam

Abstract: The island of Guam is located at the southern end of the Mariana fore-arc and was the site of early arc volcanism between 43 and 32 m.y. ago. Two volcanic units were erupted during the early evolution of Guam: the late middle Eocene Facpi Formation and the late Eocene to early Oligocene Alutom Formation. The Facpi Formation is composed largely of interbedded boninite series pillow lavas, pillow breccias, and dikes, although arc tholeiite series rocks cap the formation in some areas. The Alutom Formation is composed of interbedded volcanic breccias, tuffaceous sandstones, lava flows, and sills, calc-alkaline, arc tholeiite, and boninite series compositions are all found in the Alutom formation. Primitive boninite series rocks are relatively high in SiO 2 , MgO, and Ni and low in A1 2 O 3 and TiO 2 contents compared to basaltic rocks from the arc tholeiite series. Additionally, they have low Ti/Zr ratios and high K/Zr, Rb/Zr, and Ba/Zr ratios. Although no calc-alkaline basalts are found in the early arc formations of Guam, basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc-alkaline series. Among the analyzed samples here are a few with anomalously high Y and rare-earth element (REE) contents. Chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase, clinopyroxene, and orthopyroxene. The arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase, clinopyroxene, orthopyroxene, and magnetite. The composition of the least silicic calc-alkaline series andesite can be modeled by fractional crystallization of olivine, clinopyroxene, plagioclase, and magnetite from a transitional basalt. Derivation of more silicic calc-alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende. All basalts from the early volcanic series of Guam were erupted in an arc setting. The boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels, whereas the arc tholeiite and calc-alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels. We suggest that because the calc-alkaline series parent basalts most likely were richer in incompatible high field-strength (HFS) elements and SiO 2 than were the tholeiitic parents, they were generated by lower degrees of melting.

Relative contribution of crust and mantle to flood basalt magmatism, Mahabaleshwar area, Deccan Traps

Abstract: three formations on the basis of the trace elements Sr, Ba, Rb, Zr and Nb. The lowermost unit, the Poladpur Formation, is characterized by high Ba, Rb, and Zr/Nb, and low Sr. These features are accompanied by high K and Si, high and variable 87Sr/86Sr initial ratios (0.7043-0.7196), and low and variable eNd values (+2.6 to - 17.4). The formation is interpreted as having developed by contamination of the overlying Ambenali magma-type with ancient granitic crust, with simultaneous fractionation of a gabbroic mineral assemblage. The more basic members of the formation are found towards the base of the succession and are more contaminated than the upper flows. The succeeding Ambenali Formation, characterized by the Ambenali magma type, has low Ba, Rb, Sr and Zr/Nb, and low and rather uniform 87Sr/86Sr initial ratios (0.7038-0.7043) coupled with high and relatively uniform eNd (+4.7 to +6.4). It is interpreted as being essentially uncontaminated and derived from a mantle source with a history of slight trace-element enrichment relative to m.o.r.b.-source. The uppermost group of flows, the Mahabaleshwar Formation, is, like the Poladpur, enriched in Ba, Rb, K and Si relative to the Ambenali, but has lower Zr/Nb and higher Sr. 87Sr/86Sr initial ratios (0.7040-0.7056) are slightly higher than in the Ambenali, and eNd lies in the range + 7.1 to -3.0. In this formation Sr correlates positively with the other incompatible elements and with 87Sr/86Sr initial ratios. This is in strong contrast to the relations observed in the Poladpur, and we believe that the behaviour of Sr may be a simple pointer to the distinction between mantle and crustal contributions. Assuming that late-stage crystal fractionation processes can be allowed for, if Sr correlates positively with elements such as K, Rb and Ba then mantle enrichment processes are clearly implied. Conversely, as for example in the Poladpur, if the correlation is negative, crustal contamination is suspected because Sr is unlikely to behave as an incompatible element in most crustal derived melts or fluids because of buffering by residual plagioclase. Furthermore, the relative uniformity of the Mahabaleshwar Formation, the position on the Sr and Nd isotope diagram close to the 'mantle array', the fact that in terms of both incompatible element concentrations and isotopes the rocks are similar to tholeiites from oceanic islands such as Hawaii and Kerguelen, are all factors that reinforce the conclusion that these are mantle derived magmas which have suffered insignificant crustal contamination. They are, however, derived from a mantle which is trace-element enriched relative to the Ambenali source. Thus in the succession as a whole the crustal contribution appears to be small. Maximum amounts of contamination in the Poladpur Formation are difficult to determine but the average amount is probably in the region of 6-12 percentage mass. The whole sequence therefore contains a crustal contribution of about 2-3 %.

Craters, calderas, and hyaloclastites on young Pacific seamounts

Abstract: Craters, calderas, and bedded hyaloclastites are commonly associated with seamounts. New Sea MARC 1 side-looking sonar data for the summit of MOK seamount located near the East Pacific Rise (EPR) at 10°N show that MOK has a large caldera consisting of three coalesced circular depressions. These data also reveal many features such as stepped crater walls, talus deposits, ring faults, intracaldera flows, small cones, lava tubes and channels, and other features. Results of an ALVIN submersible dive in the crater of seamount “F” near the EPR at 21°22′N, 108°37′W revealed the presence of bedded hyaloclastite deposits; bedded hyaloclastites were also recovered by dredging of six other volcanoes all with summit craters. The bedded hyaloclastites are thin blankets of several types of basalt glass shards in a matrix of bottom sediment, clay, and ferromanganese minerals. The glass shards are chemically homogeneous and similar to midocean ridge basalts and transitional basalt lavas typical of young seamounts. Hyaloclastite deposits are crudely inverse graded with tabular, platy shards parallel to bedding. The characteristics of bedded hyaloclastites on seamount suggest an origin by rapid eruption rate, explosive mixing of magma and seawater, followed by rapid transport and deposition, such as may occur during submarine lava fountaining.