Showing papers on "Basalt published in 1985"

The magma ocean concept and lunar evolution

Abstract: The model of lunar evolution in which the anorthositic plagioclase-rich oldest crust of the moon is formed over a period of 300 Myr or less by crystallization as it floats on a global ocean of magma tens or hundreds of km thick is examined in a review of petrological and theoretical studies. Consideration is given to the classification of lunar rocks, the evidence for primordial deep global differentiation, constraints on the depth of the molten zone, the effects of pressure on mineral stability relationships, mainly-liquid vs mainly-magmifer ocean models, and the evidence for multiple ancient differentiation episodes. A synthesis of the model of primordial differentiation and its aftereffects is presented, and the generalization of the model to the earth and to Mars, Mercury, Venus, and the asteroids is discussed.

Sources of oceanic basalts: Radiogenic isotopic evidence

Abstract: Oceanic basalts can be subdivided into five distinct groupings on the basis of their Sr, Nd, and Pb isotope composition. These classes, represented by mid-ocean ridge basalts (MORB) and St. Helena, Kerguelen, the Society Islands, and the Hawaiian Islands, may represent different but internally heterogeneous mantle reservoirs or merely distinct groups within which chemical evolution has proceeded in a similar manner. Little systematic geographic distribution of volcanism tapping these sources is apparent. Depletion has been most important in the evolution of the MORB-type reservoirs, whereas crustal recycling has dominated the evolution of sources of the Kerguelen and Society types. Primitive mantle is identifiable in the Sr, Nd, Hf, and Pb isotope characteristics of the Hawaiian source only. The evolution of St. Helena-type sources remains enigmatic.

Trace element and isotopic geochemistry of lavas from Haleakala Volcano, east Maui, Hawaii: Implications for the origin of Hawaiian basalts

Abstract: Haleakala volcano on East Maui, Hawaii, consists of a tholeiitic basalt shield which grades into a younger alkalic series that was followed by a posterosional alkalic series. Tholeiitic, transitional, and alkalic basalts range widely in Sr and Nd isotopic ratios (from mid-ocean ridge basalt to bulk earth ratios) and incompatible element (P, K, Rb, Sr, Zr, Nb, Ba, REE, Hf, Ta, and Th) abundances, but isotopic ratios and incompatible element abundance ratios (e.g., Ba/La, Nb/La, La/Ce, La/Sm) vary systematically with age. The youngest series (posterosional alkalic lavas) has the highest Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/144Nd ratios and the lowest 87sr/86sr ratios, whereas the oldest series (dominantly tholeiitic basalts) has the lowest Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/144Nd ratios and the highest 87sr/86sr ratios. The most striking features of the trace element and isotopic data are the inverse correlations between isotopic ratios and parent/daughter abundance ratios in the Sr and Nd systems. Although some of the geochemical variations can be explained by shallow level fractional crystallization (e.g., alkali basalt to mugearite [Chen et al., 1984, and manuscript in preparation, 1985]), the temporal geochemical trends require a major role for mixing. We propose a model in which melts from a diaper interact with incipient melts of its wall rocks, presumed to be oceanic lithosphere. Because of motion between the lithosphere and mantle hot spot the relative contribution of melts from the diapir (mantle plume) material to the lavas decreases with time; consequently, with decreasing age the basalts become more enriched in incompatible trace elements and acquire Sr and Nd isotopic ratios which overlap with mid-ocean ridge basalts. This model quantitatively explains the isotopic ratios and incompatible trace element abundances in representative samples from the three Haleakala volcanic series. On the basis of the degrees of melting inferred for the mixing components we conclude that the lower lithosphere and much of the asthenosphere beneath Hawaiian volcanoes are involved in creating these volcanoes.

East maui, hawaii' implications for the origin of hawaiian basalts

Abstract: Haleakala volcano on East Maui, volcanism which has been attributed to plate Hawaii, consists of a tholeiitic basalt shield migration over a mantle hot spot (e.g., Wilson, which grades into a younger alkalic series that 1963; Morgan, 1971, 1972). Recent studies of was followed by a posterosional alkalic series. Hawaiian basalts show that Hawaiian volcanoes Tholeiitic, transitional, and alkalic basalts have distinct geochemical characteristics which range widely in Sr and Nd isotopic ratios (from require distinct mantle source compositions mid-ocean ridge basalt to bulk earth ratios) and (e.g., Leeman et al., 1977; Tatsumoto, 1978; incompatible element (P, K, Rb, St, Zr, Nb, Lanphere et al., 1980; Basaltic Volcanism Study Ba, REE, Hf, Ta, and Th) abundances, but iso- Project (BVSP), 1981; Stille et al., 1983). If topic ratios and incompatible element abundance the mineralogy and composition of the mantle ratios (e.g., Ba/La, Nb/La, La/Ce, La/Sm) vary sources for each Hawaiian volcano can be esti- systematically with age. The youngest series mated, constraints on the relative importance (posterosional alkalic lavas) has the highest of various mantle regions (e.g., lithosphere, Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/I44Nd asthenosphere, and lower mantle) to Hawaiian ratios and the lowest 87Sr/86Sr ratios, whereas volcanism may be obtained which will lead to an the oldest series (dominantly tholeiitic basalts) understanding of the mantle processes which has the lowest Rb/Sr, Ba/La, Nb/La, La/Ce, and create hot spot volcanism. 143Nd/144Nd ratios and the highest 87Sr/86Sr However, interpretation of geochemical dif- ratios. The most striking features of the trace ferences between Hawaiian volcanoes is only element and isotopic data are the inverse meaningful if the geochemical variations within correlations between isotopic ratios and individual volcanoes are well defined. Systema- parent/daughter abundance ratios in the Sr and tic studies of stratigraphically controlled Nd systems. Although some of the geochemical samples from mature Hawaiian volcanoes are variations can be explained by shallow level required to define the mantle source com- fractional crystallization (e.g., alkali basalt to positions involved in the generation of an mugearite (Chen et al., 1984, and manuscript in individual volcano, and such data are necessary preparation, 1985)), the temporal geochemical for several volcanoes before intervolcano trends require a major role for mixing. We pro- geochemical differences can be realistically pose a model in which melts from a diapir interpreted in terms of mantle processes. Stu- interact with incipient melts of its wall rocks, dies of stratigraphically controlled samples presumed to be oceanic lithosphere. Because of encompassing a major time fraction of the erup- motion between the lithosphere and mantle hot tion history of a mature Hawaiian volcano are spot the relative contribution of melts from the difficult because older lavas are deeply buried, diapir (mantle plume) material to the lavas and when exposed on seacliffs or in canyons, decreases with time; consequently, with they are commonly highly altered; however, decreasing age the basalts become more geochemical studies of stratigraphically enriched in incompatible trace elements and controlled samples are now being done on Mauna acquire Sr and Nd isotopic ratios which overlap Loa (Rhodes, 1983), Mauna Kea (West and Gar- with mid-ocean ridge basalts. This model quan- cia, 1982; Kwon and Wise, 1983; Frey et al., titatively explains the isotopic ratios and 1984), Kohala (Feigenson et al., 1983), East incompatible trace element abundances in repre- Molokai (Beeson, 1976; Clague and Beeson, 1980; sentative samples from the three Haleakala Clague et al., 1983) and Haleakala (Chen and volcanic series. On the basis of the degrees Frey, 1983; Chen et al., 1984, this paper; West of melting inferred for the mixing components and Leeman, 1984). The first-order evolu- we conclude that the lower lithosphere and tionary characteristics of Hawaiian volcanoes much of the asthenosphere beneath Hawaiian are known: the main shield is constructed of volcanoes are involved in creating these volca- tholeiitic basalt which is usually overlain by noes.

Komatiites I: Eruption and Flow

Abstract: Because of their high eruption temperatures and ultrabasic composition, komatiite lavas had low viscosities, which typically ranged from 0-1 to 10 Pa s. A major consequence of this low viscosity is that most lavas erupted as turbulent flows. An analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms"1 and Reynolds numbers much greater than the critical value of 2000. The lavas would have remained turbulent for most or all of their subsequent flow and emplacement. Typical horizontal flow rates are estimated to range from 0-5 to 100 m2 s~' per unit width of flow. Such turbulent lava flows would have lost their heat by convection to the surroundings, at rates which are orders of magnitude greater than the rates for laminar flows, which cool by conduction. A quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 °C hr"' to a few °C hr"1, while the flows remained turbulent. These rates are in an appropriate range to cause phenomena such as high nucleation rates, strong supersaturation of the lava, delayed nucleation of olivine, and skeletal or dendritic crystal morphologies. Komatiites often flowed over ground composed of rocks with lower melting temperatures. It is proposed that the turbulent lavas melted the ground to form deep thermal erosion channels. Melting rates at the lava source are calculated at several metres per day, and deep troughs with depths of several metres to hundreds of metres and lengths of several kilometres probably formed. Laboratory experiments performed to simulate thermal erosion show qualitative agreement with the theory with channel depth decreasing downstream. The experiments also revealed that the channel margins become undercut during thermal erosion to form overhanging sides of the channel. Some sinuous rilles observed in the mare regions of the Moon are thought to have formed by thermal erosion (Hulme, 1973). They provide analogues of the channels postulated to form in komatiite eruptions, where conditions were in fact more favourable for thermal erosion. An assessment of the role of olivine crystals, precipitated in the cooling, turbulent flows, indicates that they will remain in suspension until the lava has come to rest. Contamination of komatiite lava by underlying rock can be as much as 10 per cent. Some illustrative calculations show how the major element and trace element compositions of residual melts can be significantly modified by combined assimilation and fractional crystallization in a moving flow. Assimilation of tholeiitic basalt into a komatiite can cause incompatible trace element ratios, such as Ti/Zr and Y/Zr, and the rare earth patterns of derivative lavas, to vary substantially. Some of the variations in such geochemical parameters, which are often ascribed to mantle heterogeneity, also could have resulted from assimilation of the ground. Assimilation could have modified the isotope geochemistry of lava suites and led to apparent ages which differ from the true eruption age. The thermal erosion model also provides an explanation of the formation of some nickel sulphide ores found at the bottom of thick komatiite flows. It is proposed that ores can form by assimilation of sulphur-rich sediment, which combines with Ni from the komatiite to form an immiscible liquid.

Large-scale regional units in the depleted upper mantle revealed by an isotope study of the South-West Indian Ridge

Abstract: Isotope analyses of ocean island basalts have demonstrated the existence of regional geochemical provinces, the Indian Ocean being distinguishable from the North Atlantic and Pacific oceans. Equivalent analyses of basalts from the South-West Indian Ridge now indicate that the upper mantle under the Indian Ocean also has a distinct isotope composition. Thus, the upper mantle itself must be composed of several discrete provinces which may relate to the convection pattern.

Jorullo Volcano, Michoacán, Mexico (1759–1774): The earliest stages of fractionation in calc-alkaline magmas

Abstract: Between 1759 and 1774, Jorullo Volcano and four associated cinder cones erupted an estimated 2 km3 of magma which evolved progressively with time from early, hypersthene-normative, primitive basalts to late-stage, quartz-normative, basaltic andesites. All lavas contain <6 vol% phenocrysts of magnesian olivine (Fo90-70) with Cr-Al-Mg-spinel inclusions, and microphenocrysts of plagioclase and augite; late-stage basaltic andesites also carry phenocrysts of plagioclase, augite, and rare orthopyroxene, hornblende pseudomorphs, and microphenocrysts of titanomagnetite. Olivine-melt compositions indicate liquidus temperatures ranging from 1,230° C to 1,070° C in the early- and late-stage lavas, respectively; $$f_{{\text{O}}_{\text{2}} } $$ was about 0.6 log units above the Ni-NiO buffer in the early lavas but increased to 2.5 log units above Ni-NiO in the late lavas, perhaps through groundwater-magma interaction. Smooth major and trace element compositional trends in the lavas can be largely modeled by simple crystal fractionation of olivine, augite, plagioclase, and minor spinel. La, Ce, and other incompatible elements (Rb, Sr, Ba, Hf, Th, Ta), however, are anomalously enriched in the latestage lavas, whereas the heavy rare earth elements (Dy, Yb, Lu) are anomalously depleted. The modeled crystal fractionation event must have occurred at lower-crustal to upper-mantle pressures (8–15 kb), although the crystals actually present in the Jorullo lavas appear to have formed at low pressures. Thus, a two-stage crystallization history is implied. Despite the presence of granitic xenoliths in middle-stage lavas from Jorullo, bulk crustal assimilation appears to have played an insignificant role in generating the compositional trends among the lavas. As MgO decreases from 9.3 to 4.3 wt% through the suite, Al2O3 increases from 16.4 to 19.1 wt%. Most highalumina basalts reported in the literature have 18 to 21 wt% Al2O3, but are too depleted in MgO, Ni, and Cr to have been generated directly through mantle partial melting. These high-alumina basalts have probably undergone significant fractionation of olivine, augite, plagioclase, and spinel from primitive parental basalts similar to the early Jorullo lavas. Such primitive basalts are rarely erupted in mature arcs and may be completely absent from mature stratovolcanoes. Cerro La Pilita is a late-Quaternary cinder and lava cone centered just 3 km south of Jorullo. The primitive trachybasalts of Cerro La Pilita, however, are radically different from the Jorullo basalts. They are nepheline normative with high concentrations of K2O (>2.5 wt%), P2O5 (>0.9 wt%), Ba (1,200 ppm), Sr (>2,000 ppm), and many other incompatible elements, and contain crystals of hornblende and apatite in addition to olivine, spinel, augite, and plagioclase. The magmas of these two neighboring volcanoes cannot be related to one another by any simple mechanism, and must represent fundamentally different partial melting events in the mantle. The contrasts between Jorullo and Cerro La Pilita demonstrate the difficulty in defining simple relationships between magma type and distance from the trench in the Mexican Volcanic Belt.

Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel, FRG)

Abstract: The Quaternary foidites and basanites of the West Eifel (Germany) contain optically and chemically heterogeneous clinopyroxenes, some of which occur as discrete zones within individual crystals: Most clinopyroxene phenocrysts are made up of a core and a normally zoned comagmatic titanaugite mantle Most cores are greenish pleochroic and moderately resorbed (fassaitic augite) Some are pale green and strongly resorbed (acmitic augite) Cores of Al-augite composition and of Cr-diopside derived from peridotite xenoliths are rare The fassaitic augites are similar in trace element distribution pattern to the titanaugites, but are more enriched in incompatible elements The acmitic augites, in contrast, are clearly different in their trace element composition and are enriched in Na, Mn, Fe and depleted in Al, Ti, Sr, Zr A model for polybaric magma evolution in the West Eifel is proposed: Primitive alkali basaltic magma rises through the upper mantle precipitating Al-augite en route It stagnates and differentiates near the crust/mantle boundary crystallizing Fe-rich fassaitic augites The magma differentiated at high pressure is subsequently mixed with new pulses of primitive magma from which the rims of pyroxene are crystallized Sporadic alkali pyroxenite xenoliths are interpreted to represent cumulates of cognate phases formed within the crust and not metasomatized upper mantle material (Lloyd and Bailey 1975)

Geodynamic mixing in the mesosphere boundary layer and the origin of oceanic islands

Abstract: Since the isotopic heterogeneity of ocean island basalts (OIB) is of variable character and requires billions of years to evolve, we propose to explain the observed heterogeneity by mixing between delaminated continental lithosphere, a continental crustal component, depleted mantle and pristine mantle. Such mixing occurs in a boundary layer located at the bottom of the upper mantle from which ascending blobs rise.

Petrology and geochemistry of basalts from the American-Antarctic Ridge, Southern Ocean: implications for the westward influence of the Bouvet mantle plume

Abstract: Ridge segments and fracture zones from the American-Antarctic Ridge have been systematically dredge sampled from ∼4° W to ∼18° W. Petrographic studies of the dredged basalts show that the dominant basalt variety is olivine-plagioclase basalt, although olivine-plagioclase-clinopyroxene basalt is relatively common at some localities. Selected samples have been analysed for major and trace elements, rare earth elements and Sr and Nd isotopes. These data show that the majority of samples are slightly evolved (Mg#=69-35) N-type MORB, although a small group of samples from a number of localities have ‘enriched’ geochemical characteristics (T- and P-type MORB).

Geochemistry and Petrogenesis of Amphibolites, Kolar Schist Belt, South India: Evidence for Komatiitic Magma Derived by Low Percentages of Melting of the Mantle

Abstract: The Kolar Schist Belt of the Dharwar Craton of South India is an Archean greenstone belt dominated by metavolcanic rocks. The mafic metavolcanic rocks occur as komatiitic and tholeiitic amphibolites. The komatiitic amphibolites occur along the margins of the N-S trending, synformal belt. They are much less abundant than the tholeiitic amphibolites and have 14 to 21-3 wt. per cent MgO. The komatiitic amphibolites from the west/central part of the belt have two distinctive REE patterns: (1) those enriched in the middle to light REE but depleted in Ce relative to Nd; and (2) those with patterns that are convex up, i.e. depleted in both light and heavy REE, although more depleted in the light REE. Associated tholeiites have light REE depleted to flat REE patterns. Komatiitic and tholeiitic amphibolites from the eastern part of the belt have enriched light REE patterns. The tholeiitic amphibolites from the Kolar Schist Belt are similar to the TH I and TH II types of Archean tholeiites of Condie (1981). The komatiitic amphibolites are similar to komatiites and komatiitic basalts of Barberton Mountainland, but have higher FeO and TiO 2 abundances and lower Yb/Gd ratios. The petrogenetic interpretations for these rocks are based primarily on a modification of the MgO-FeO diagram of Hanson & Langmuir (1978), and modelling of Zr, Ni and REE. All of the rocks have undergone some fractionation. While the modelling does not give accurate temperatures, pressures, compositions and extents of melting of the mantle sources for the various amphibolites, it does present an approach which can be used for estimating these parameters. For example, the komatiitic amphibolites appear to be derived from melts generated by 10 to 25 per cent melting of the mantle over a range of depths and temperatures greater than 80 km and 1575°C. The variation in the P-T conditions of magma generation is possibly due to adiabatic melting in mantle diapirs with a range of FeO/MgO ratios. If the tholeiitic amphibolites are derived from similar mantle sources (it is not clear that they are), their parent melts may have been generated by similar extents of melting, but at depths of less than 80 km. The komatiitic amphibolites from the west central part of the belt were generated from light REE depleted mantle, whereas those from the eastern part of the belt appear to have been generated from light REE enriched mantle. The sources for the komatiitic amphibolites in both areas were significantly enriched in FeO relative to pyrolite. Thus, a light REE depleted and a light REE enriched source appear to have provided mafic volcanics with similar major element chemistry to this belt during its evolution.

Melting of peridotite to 14 GPa and the genesis of komatiite

Abstract: It is generally accepted that the Earth's upper mantle is made of peridotite1,2. In volatile-absent conditions, partial melting of mantle peridotite close to the solidus yields a variety of basaltic melts that are progressively more olivine-normative with increasing depth3,4. At 3–4 GPa (30–40 kbar), the highest pressures so far investigated, the melts are picritic with up to 20 wt% MgO (ref. 5). Basaltic magmas that are dominant on the Earth's surface today, therefore, are considered to have been generated at relatively shallow depths ( 30 wt% MgO. Based on these results, we describe a model for the genesis of komatiite magmas by partial melting at depths of 150–200 km in the Archaean upper mantle.

A model for mixing basaltic and dacitic magmas as deduced from experimental data

Abstract: It has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours. The presence of phenocrysts larger than 0.5 mm increased considerably the efficiency of mixing. Flow patterns in the experimental system were visualized using Pt spheres, which indicated that convection occurs in basalt melt, but not in dacite melt. The Reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10−3 and 10−6, respectively. Mixing proceeds initially by mechanical mixing of the two magmas in a large scale, but later by coupling interfacial convection and mutual diffusion. Thus, depending on the depth where vesiculation and following disruption of the magma occurs, banded pumice, homogeneous pumice and homogeneous andesite lava are erupted. The observed textures of mixed rocks of Plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model.

Igneous Petrogenesis of the Yakuno Ophiolite (Japan) in the context of the diversity of ophiolites

Abstract: An ophiolite complex includes three major members: basaltic volcanics, mafic-ultramafic cumulates, and residual peridotite. From the aspect of igneous petrology, three distinct types are recognized among the associations of the three members: (Liguria type) alkalic basalt, plagioclase-type cumulates, and lherzolite; (Yakuno type) high-alumina tholeiite, clinopyroxene-type cumulates, and clinopyroxene-bearing harzburgite; (Papua type) low-alumina tholeiite, orthopyroxene-type cumulates, and clinopyroxene-free harzburgite. In the light of recent experimental studies, the three types represent cogenetic, complementary products of low ( 30%) degrees of partial melting in the lherzolitic source mantle, respectively. The cumulates of the Yakuno ophiolite show structural and chemical continuity to the underlying residual peridotite, and were recrystallized at high pressures (5–10 kb). They originated in a deep, “soft-floored” magma chamber directly overlying the partially melted residual harzburgite, from which the magma was extracted. The three members of the Yakuno ophiolite were cogenetically formed through a magmatic event induced by a moderate degree of partial melting in the mantle.

Volcanism and metallogenesis of axial and off-axial structures on the East Pacific Rise near 13 degrees N

Abstract: A fast-spreading segment (12 cm/yr (super -1) ) of the East Pacific Rise near 12 degrees 50' N, explored in detail by surface ship and manned submersible, displays intense hydrothermal activity. The rise crest (about 1,500 m in width) with a regional depth of 2,600 m is occupied at its center by an axial graben ( 1.3%); titania-alkali-enriched olivine (Ti-K) basalts with high TiO 2 (>1.4%) and high alkali content (Na 2 O > 3.0%; K 2 O > 0.3%) occur on the southeastern seamount. The fractionated tholeiites are believed to have been derived from primitive melts during differentiation which took place within a magma reservoir--asymmetrical with respect to the axis of the graben--which supplied both axial and off-axial lava flows. The Ti-K-enriched basalts are believed to have been produced from an alkali-enriched magmatic source.

The platinum-group element and gold contents of the marginal rocks and sills of the Bushveld Complex

Abstract: Previous work has defined three suites of sills and marginal rocks adjacent to and below the Rustenburg Layered Suite of the Bushveld Complex, namely an ultramafic (MgO approximately 30%), a magnesian basaltic (MgO approximately 12%), and a tholeiitic suite (MgO approximately 7%). These three suites are thought to represent magmas which have intruded and been incorporated into the Rustenburg Layered Suite at various times during its formation. The elements Ir, Ru, Rh, Pd, Pt, and Au have been analyzed in 17 samples representative of the three suites. The ultramafic sills have the highest total noble metal content (46 ppb) followed by the magnesian basaltic suite (37 ppb) and the tholeiitic suite (21 ppb). The ultramafic sills are characterized by a relatively fiat chondrite-normalized platinum-group element pattern with an average Pt/Ir ratio of 3.52 and a pronounced Rh anomaly. The magnesian basaltic and tholeiitic suites both have steep positive chondrite-normalized patterns with average Pt/Ir ratios of 16.14 and 40.5, respectively, and no Rh anomaly. Modeling done using incompatible trace elements and petrographic mixing suggests the ultramafic sills represent crystal mushes (of cumulate olivine and chromite) that were intruded into the floor during the crystallization of the lower portion of the Rustenburg Layered Suite. The enrichment of Ir, Ru, and Rh in the ultramafic sills appears to be associated with the cumulate enrichment of olivine or chromite in these sills.The chondrite-normalized patterns found for the Merensky Reef and the Platreef are similar to those of the magnesian basaltic magma. The pattern found for the UG-2 is much flatter and in some respects very similar to that found for the ultramafic sills.On a worldwide basis the platinum-group element contents of the magnesian basaltic and tholeiitic magmas generally fall in the range of basalts and chill zones of other layered suites. The magnesian basaltic magma does have one of the highest Pt contents measured for a magma.

Petrological and geochemical variations along Iceland's Neovolcanic Zones

Abstract: Petrological, geochemical, and geophysical gradients along the SE volcanic zone in Iceland imply systematic variations in melting and crystallization conditions and in magma supply and eruption rates. At the southern tip of the zone, in Vestmannaeyjar, alkali basalt magmas are generated by small degrees of melting under a thick lithosphere. Farther north, in the Hekla-Katla region, greater degrees of melting result in the generation of transitional basalt magmas. Magma supply rates exceed eruption rates, and melts begin to accumulate at the base of the crust, as indicated by magnetotelluric evidence. Uniform rare earth element patterns in the Hekla-Katla basalts may be explained by homogenization in the melt accumulation zone or by uniform melting conditions. Infrequent replenishment of magma reservoirs in this region leads to mixing of compositionally diverse magmas and, consequently, to basalts with diverse phenocryst compositions and textures. Even farther north, in central Iceland, the melting anomaly associated with the SE zone has developed to the same degree as it has beneath the SW axial rift zone, leading to similar magmatic conditions. High magma supply rates and low cooling rates inhibit fractionation and lead to the eruption of voluminous olivine tholeiites. In these areas a broad spectrum of melt compositions is generated by variable degrees of melting over a wide depth range. The compositional diversity, e.g., in large ion lithophile element enrichment, is masked somewhat by reequilibration and mixing of melts on ascent and in the melt accummulation zone. Compositional diversity may be preserved, however, in the melt accummulation zone in a lateral direction away from the rift axis since distal parts of the melt zone are fed only by melts segregating at greater depths. The variations in magmatic conditions along the SE zone, which are analogous to those inferred along propagating rifts, may be related to a mantle blob that ascended beneath central Iceland 2–3 m.y. ago, spread out laterally and triggered a southward propagating rift.

Structure and eruptive mechanisms at Surtsey Volcano, Iceland

Abstract: Glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of Surtsey volcano. Despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island. The tuff includes abundant accretionary lapilli and tuff vesicles, indicating that it was all deposited subaerially. During the growth of the tuff rings, repeated hydromagmatic explosion cycles began with a series of intermittent tephra-finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours. This nozzle-like continuous activity produced eruption columns 100–250 m in diameter and 500–2000 m in height which probably quarried several hundred metres below the ground surface. The continuous-uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down. During construction of the tuff rings, concentric faults repeatedly downdropped a funnel-like structure (400–800 m in diameter) several hundred metres, thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level. Ring dykes later intruded upward along these faults and fed small lava flows. Heat in the surface tephra probably originated primarily from these shallow intrusions.

The influence of subduction processes on the geochemistry of Japanese alkaline basalts

Abstract: The composition of volcanic rocks erupted in complex plate tectonic settings can provide information on the nature of the underlying mantle. We show here that the geochemistry of alkali basalts from Japan and eastern Asia varies systematically with distance from the Japanese island-arc. Samples from northeastern Japan, relatively close to the Japan Trench, are enriched in K, Sr, Ba and Rb and depleted in Ta, Nb and Ti as compared with samples from southwestern Japan. Both sets show an island-arc influence on their composition, but alkali basalts from still further west (Korea and northeastern China) have chemistries which are indistinguishable from ocean island basalts. We suggest that the northeastern island-arc type of alkali basalts were derived from a ‘normal’ upper mantle source altered by fluids or melts released from the underlying subducted Pacific plate. The extent of this island-arc-related alteration decreases with distance from the trench.