Hypersthene-normative Basalts Research Articles

Heterogeneous nickel isotope compositions of the terrestrial mantle – Part 2: Mafic lithologies

We report stable Ni isotope compositions (δ60/58Ni, relative to SRM986) for mafic lavas with a range of −0.16‰ to +0.20‰ (n = 44), similar to that of peridotite samples. Ocean island basalts (OIB) have been analysed from Iceland (n = 6), the Azores (n = 3), the Galápagos Islands (n = 2), and Lōʻihi, Hawaii (n = 1). Samples from Iceland (average δ60/58Ni = +0.13 ± 0.16‰, 2s, n = 7) display the greatest range in Ni isotope compositions from a single OIB location in this work, of +0.01‰ to +0.23‰. Samples from the Azores (average δ60/58Ni = −0.10 ± 0.10‰, 2s) and Galápagos (average δ60/58Ni = −0.01 ± 0.04‰, 2s) are generally isotopically lighter. The single Lōʻihi sample has a δ60/58Ni of +0.17‰. The lightest analysed bulk rock δ60/58Ni in this work, −0.16‰, is from the Azores island, Pico. Enriched mid ocean ridge basalts (E-MORB), which have (La/Sm)N > 1, are isotopically lighter than normal type MORB (N-MORB), as shown by data from the Mid Atlantic Ridge (n = 9) and East Pacific Rise (n = 3). All E-MORB average δ60/58Ni = +0.00 ± 0.06‰ (2s, n = 7), whereas N-MORB average δ60/58Ni = +0.14 ± 0.10‰ (2s, n = 5).A suite of 15 mafic samples from the Cameroon Line, comprising lithologies ranging from nephelinites to hypersthene-normative basalts, have Ni isotope compositions that are identical within analytical uncertainty (average δ60/58Ni = +0.08 ± 0.06‰, 2s). Similarly, MORB samples display no relationship between δ60/58Ni and geochemical indicators of degree of partial melting or fractional crystallisation. Host lavas for two previously analysed ultramafic xenolith suites have δ60/58Ni identical to the average δ60/58Ni of their respective xenolith suites. This is consistent with previously published evidence from peridotites and komatiites that Ni isotopes are not greatly fractionated by melting. Therefore, mafic rocks may preserve the δ60/58Ni of their mantle source. Sampling a greater volume of mantle, their average Ni isotope composition +0.07 ± 0.17‰ (2s, n = 44) may also be a better representation of the Bulk Silicate Earth (BSE), than estimates based purely on peridotites.The δ60/58Ni of MORB co-varies with La/Sm, Rb/Sr, europium anomaly (Eu/Eu*), and K2O/(K2O + Na2O). The relationships between these parameters and δ60/58Ni are consistent with mixing between two model endmembers. One could be depleted MORB or depleted MORB mantle (DMM) with a relatively heavy Ni isotope composition; the other a more enriched endmember that has isotopically lighter δ60/58Ni. The link between lighter δ60/58Ni and enriched lithologies in the mantle is further supported by published evidence of light Ni isotope compositions associated with some pyroxenite xenoliths. However, the curvature of the apparent mixing arrays defined by basalts is hard to reconcile with admixing of geochemically enriched but isotopically fractionated oceanic crustal lithologies. High [Ni] enriched magmas such as kimberlites may be a closer match to the enriched endmember. However, this needs further study.

Insights into the lithosphere to asthenosphere melting transition in northeast Africa: Evidence from the Tertiary volcanism in middle Egypt

Throughout the northeast African and Arabian plates, small-volume basaltic volcanism has erupted sporadically over the last ~35Myr. While magmatic activity associated with the Afar plume and subsequent rifting of the Red Sea, Gulf of Aden and East African rift provide a mechanism for melt generation in those areas, the origin of melt elsewhere in the region has remained enigmatic. We have identified two types of intraplate basalt eruptions from middle Egypt, which we suggest were generated by distinct melting processes: sub-alkaline, hypersthene-normative basalts and basaltic andesites (2.8–4.9wt% total alkalis); and nepheline-normative alkali basalts and trachybasaltic flows (4.9–5.5wt% total alkalis). Estimations of the primary melt compositions suggest about twice as much fractionation in the sub-alkaline basalts (F=32–58%, dominantly clinopyroxene and olivine removal) in comparison to the alkaline basalts (F=18–30%, dominated by clinopyroxene removal). Application of the geochemical model HAMMS1 generates results which suggest the alkaline basalts are likely derived from low degrees of partial melting (F<2%) of a pyroxenite-bearing mantle source at moderate potential temperatures (TP) ~1386–1449°C, and pressures >3.7GPa. These conditions require melting at or below the base of lithospheric mantle. In contrast, the sub-alkaline basalts exhibit negative K-anomalies for all fractionation-corrected melts, suggesting a source with a K-bearing phase (likely amphibole), located within lithospheric mantle, constrained by the stability of amphibole to be <3GPa and 1170°C. We suggest that the enrichment in the lithospheric mantle is best explained by the percolation of fluids/or hydrous melts either from the Late Neo-Proterozoic subduction zone during accretion the Arabian-Nubian Shield, or from an old mantle plume that impinged the base of the lithosphere during the Phanerozoic. Destabilization of these enriched metasomes was likely a function of diffuse stretching associated with the opening of the Red Sea at ca. 30Ma, thermal perturbation from a slightly warmer asthenosphere, and the passage of asthenospheric melts through the lithospheric mantle. The strong correlation between these small-volume lavas and regional structural lineaments (e.g., Baharyia NE-SW Fault), and smaller NW-SE fractures associated with the Red Sea system, suggests that the eruption of lavas in the region is controlled by melt focusing into the lithosphere and favorable stress conditions in the upper crust. Thus, while the potential for melt generation may be present beneath much of north-east Africa, the rarity at which volcanism is discovered at the surface reflects what may be an unusual convergence of circumstances.

Volcanism from fissure zones and the Caldeira central volcano of Faial Island, Azores archipelago: geochemical processes in multiple feeding systems

AbstractMagmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized by variable degrees of metasomatism, both at local and regional scales. Hawaiites evolved from basalts through 30–40% fractional crystallization of mafic phases plus some plagioclase, in deep reservoirs, at about 430–425 MPa (~ 15 km). The Caldeira central volcano rocks range from basalts to trachytes. Basalts, produced under similar conditions as fissure basalts, evolved to trachytes through large degrees of polybaric fractional crystallization (100–760 MPa; i.e. ~ 3.6–26 km), involving olivine, clinopyroxene, feldspar and minor quantities of amphibole, biotite, apatite and oxides. In contrast, mafic magmas from the fissure zones were erupted directly onto the surface from magma reservoirs mainly located at the crust–mantle boundary.

Minor- and trace-element sector zoning in synthetic enstatite

Minor- and trace-element sector zoning is nearly ubiquitous in pyroxene grown from basaltic melts using 1 atm gas-mixing furnaces. Mineral/melt partition coefficients can differ between sectors by more than a factor of five. A synthesized hypersthene-normative basalt composition was doped with Al, Cr, and Ti using a matrix of doping experiments, to understand better the mechanisms behind the formation of sector zoning. Isothermal and controlled-cooling experiments were conducted. The {010} sectors contain higher enrichments of Al, Cr, and Ti than {100} sectors, at the expense of Mg and Si. Evaluation of these relative sector enrichments indicates that sector zoning results from several kinetic mechanisms, which exert influence even when attempting laboratory conditions that approximate equilibrium: (1) rapid growth rate, not necessarily synonymous with rapid cooling; (2) crystal-chemical control by means of charge-coupled substitutions; and (3) diffusion limitations of Si tetrahedra in the melt boundary layers respective to the different crystal faces.

Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai'i

A diverse range of crustal xenoliths is hosted in young alkali basalt lavas and scoria deposits (erupted 3–5 ka) at the summit of Hual alai. Leucocratic xenoliths, including monzodiorites, diorites and syenogabbros, are distinctive among Hawaiian plutonic rocks in having alkali feldspar, apatite, zircon and biotite, and evolved mineral compositions (e.g. albitic feldspar, clinopyroxene Mgnumber 67–78). Fine-grained diorites and monzodiorites are plutonic equivalents of mugearite lavas, which are unknown at Hual alai. These xenoliths appear to represent melt compositions falling along a liquid line of descent leading to trachyte—a magma type which erupted from Hual alai as a prodigious lava flow and scoria cone at 114 ka. Inferred fractionating assemblages, MELTS modeling, pyroxene geobarometry and whole-rock norms all point to formation of the parent rocks of the leucocratic xenoliths at 3–7 kbar pressure. This depth constraint on xenolith formation, coupled with a demonstrated affinity to hypersthenenormative basalt and petrologic links between the xenoliths and the trachyte, suggests that the shift from shield to post-shield magmatism at Hual alai was accompanied by significant deepening of the active magma reservoir and a gradual transition from tholeiitic to alkalic magmas. Subsequent differentiation of transitional basalts by fractional crystallization was apparently both extreme—culminating in >5.5 km of trachyte—and rapid, at 2.75 · 10 m magma crystallized/year.

Plume-Lithosphere Interactions in the Generation of the Basalts of the Kenya Rift, East Africa

Major and trace element and Sr–Nd–Pb isotopic data for mafic volcanic rocks are used to assess the number of mantle plumes contributing to the Tertiary–Holocene magmatism of the Kenya Rift Valley, current estimates of which vary from none to three. Rocks ranging in composition from nephelinite to hypersthene-normative basalt have been sampled from three lithospheric zones: the Tanzanian craton, the craton margin reworked during the late Proterozoic, and the Mozambique mobile belt. The magmas are interpreted as the products of variable degrees of partial melting within the spinel–garnet peridotite transition zone. Trace element and isotopic compositions from all three zones are broadly similar to those of oceanic island basalts, but there is considerable compositional variation, which is related to a strong overprint from the lithosphere on plume-derived melts. Sr and Nd isotopic ratios provide the only clear distinction between magmatic rocks from the three lithospheric domains. Within each setting, mafic magmatism has tended to become less silica undersaturated with time, and at any one locality magmatism has migrated towards the centre of the rift. Magmas may have formed as a result of the infiltration of plume-derived melts into the base of the lithosphere. The extent of interaction of inferred plume melts with the lithosphere has not varied systematically in time or space. The plume component appears to be similar to the source of oceanic island basalts.

Two mantle plumes beneath the East African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts

Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recent basalts (MgO>4 wt%) from the axial regions of the Kenya Rift. Samples have compositions ranging from hypersthene-normative basalt through alkali basalt to basanite and are a subset of a larger database in which compositions extend to nephelinite. A broadly negative correlation between Zr/Nb (<2–7) and Ce/Y (1–8) indicates derivation from a garnet-bearing mantle source region as a result of <3% melting. Isotope ratios in basalts from the axial regions of the Kenya Rift have 143Nd/ 144Nd=0.51300 to 0.51255, 87Sr/ 86Sr=0.7030 to 0.7055 and 206Pb/ 204Pb<18 to >20, broadly similar to values from OIB. The Kenya Rift cuts through basement of different ages and aspects of the composition of mafic magmas reflect the anisotropy of the underlying lithosphere. Specifically, those basalts from that part of the rift underlain by the Tanzanian craton (TC) have higher Ce/Y and lower Zr/Nb ratios than those erupted through the Panafrican Mozambique belt (MB) implying an origin either at greater depth or from a more trace element-enriched source region. Samples erupted through the zone of reactivated craton margin (RCM) share the characteristics of mafic lavas from both the craton and the mobile belt. MB samples have 143Nd/ 144Nd=0.5130–0.5127, 87Sr/ 86Sr=0.7030–0.7035 and 206Pb/ 204Pb=18.3–19.8, defining a steep negative trend on the Nd–Sr diagram and plotting close to the NHRL on conventional Pb isotope diagrams. By contrast TC and RCM samples have 143Nd/ 144Nd=0.5124–0.51275, 87Sr/ 86Sr=0.7035–0.7056 and 206Pb/ 204Pb=17.6–21.2, defining flat-lying arrays on Nd–Sr plots and a much greater scatter and spread on Pb isotope diagrams, with many analyses plotting above the NHRL. Both groups of analyses trend towards a common end member on a plot of 143Nd/ 144Nd against 87Sr/ 86Sr, at 143Nd/ 144Nd∼0.51275 and 87Sr/ 86Sr∼0.7035. These values are suggested to reflect the isotopic characteristics of the sub-lithospheric Kenyan mantle, inferred to be the Kenya mantle plume. Comparison with data from Afar suggest that the Kenya plume is distinct from the Afar plume, implying that the east African Rift is underlain by at least two distinct mantle plumes. Eocene and Oligocene basalts from southern Ethiopia bear a closer resemblance to the Kenyan basalts than to those from Afar and the Ethiopian plateau, suggesting that the Kenya plume has been active for at least 45 Ma. Migration of magmatism from southern Ethiopia southwards is consistent with the northeastward migration of the African plate over the Kenya plume over the past 50 My.

Geochemical and isotopic characteristics of lower crustal xenoliths, San Francisco Volcanic Field, Arizona, U.S.A

A wide variety of xenoliths has been entrained in Miocene-to-Recent alkali olivine and hypersthene-normative basalts in the San Francisco Volcanic Field (SFVF), northern Arizona, U.S.A. Based on petrography, mineralogy, bulk rock chemistry and Sr-Nd isotopic characteristics, SFVF xenoliths can be divided into two major groups: cumulates and granulites. The cumulates are genetically related to the Cenozoic volcanic rocks and represent under- and/or intraplated additions to the crust of the Colorado Plateau. Assemblages are mafic to ultramafic and are dominated by clinopyroxene-orthopyroxene-plagioclase-spinel-amphibole-olivine. The granulites are probably Proterozoic in age, mafic-to-intermediate/felsic in bulk composition, either two pyroxene-plagioclase-spinel or plagioclase-alkali feldspar-quartz-magnetite-amphibole-biotite assemblages. Many of the granulites show evidence of partial melting. Some high SiO2, very high RbSr glasses are close in composition to erupted rhyolites, and probably represent end-member melts that have interacted with basalt to produce a variety of hybrid intermediate lavas. The major element, trace element and Sr-Nd isotope geochemistry is highly variable in the SFVF xenoliths. Extremely high Ba contents and BaNb of a number of the granulites are equivalent to values characteristics of modern supra-subduction zone magmas. The considerable variation of chemical and isotopic composition depends upon mineral proportions, assemblages and chemistry. Isotopically, three end-members can be identified within the granulites: (i) lowest 87Sr86Sr (0.702870) with low 143Nd144Nd (0.511541, ϵNd-21.4); (ii) high 87Sr86Sr (0.711069) with the lowest 143Nd144Nd (0.511434, ϵNd-23.5); (iii) highest 87Sr{86}Sr (0.715306) with low 143Nd144Nd (0.511793, ϵNd-16.5). Two important age ranges deduced from the isotopic data probably relate to episodes of crustal-growth beneath the SFVF (1.88 ± 0.33 Ga and Cenozoic). Thermobarometric calculations assuming equilibrium show that the xenoliths are derived from the lower crust (0.6–1.3 GPa, 850–1050 °C). The average SFVF lower crust is mafic in composition. In the absence of partial lithospheric delamination, the lower crust may become mafic with time due to under- and intraplating of continental crust by mafic magmas derived from the mantle.

Geochemistry and Petrogenesis of the Early Tertiary Lava Pile of the Isle of Mull, Scotland

Major and trace element analyses and strontium isotope ratios are presented for twenty-four samples of lavas and plugs from the early Tertiary lava pile in Mull. The samples were selected on the basis of petrographic freshness from a large collection from outside the hydrothermally altered “zone of pneumatolysis” which occupies the central region of the volcanic complex. Most of the analyses yield normative hypersthene and we argue that these are essentially unaltered magmatic compositions. The analytical data indicate that the samples may be divided into three groups on the basis of major element chemistry, initial 87Sr/86Sr ratios and correlations between lithophile element contents. Group I comprises an alkaline series (basalt-hawaiite-mugearite) with extremely low initial 87Sr/86Sr ratios (≦0.7030) and generally low lithophile element contents. Apart from their alkalinity and high Sr and Zr contents these samples have affinities with abyssal tholeiites. Group II contains hypersthene normative basalts with more tholeiitic characteristics but (as in the case of the Skye Main Lava Series) the more evolved rocks are trachytes. This group is characterized by more normal levels of lithophile element concentrations and relatively high initial 87Sr/86Sr ratios of about 0.7055. Group III is less clearly defined and contains basalts that are generally sparsely olivine-phyric and in most chemical respects fall between Group I and Group II-including initial 87Sr/86Sr ratios (0.7033 to 0.7043). They may represent mixtures of Group I and Group II type sources or magmas. Groups I and II appear to be similar, respectively, to the relatively sodic iron-rich and the relatively potassic ironpoor silica enrichment trends distinguished in the Skye Main Lava Series. In the Group I magma series the behaviour of Y and Sr relative to other incompatible elements can only be explained by differential partial melting of a deep garnet-lherzolite mantle source. Fractional crystallization has undoubtedly occurred at some stage during the ascent of these magmas from the mantle, as indicated by the behaviour of Ni and Cr, but has not been a major factor in the production of evolved magma compositions. The Group II magmas appear to have originated from a source more enriched in lithophile elements, and a relatively shallow (< 50 km) plagioclase-lherzolite mantle source is suggested for these magmas because they have Sr/Ba ratios between one and two orders of magnitude lower than those characteristic of Group I. Rb-Sr systematics suggest that the vertical heterogeneity of the mantle which was largely responsible for the chemical differences between these three groups may have existed for a very long time prior to Tertiary magmatism.

Nature of alkalic volcanic rock series

The alkalic rocks are here regarded as a category in a classification of igneous rock series (rock associations) and not as a class in petrographic systematics. The alkalic series as a whole are characterized by higher Na2O+K2O content than the subalkalic series in the alkali vs. SiO2 diagram. At least three different trends (types) of differentiation appear to exist in large-scale alkalic volcanic associations. One (here designated as the Kennedy trend) starts from weakly nepheline-normative basalt and shows increasing normative nepheline with advancing fractionation to reach a phonolitic composition. Another (here called the Coombs trend) starts from hypersthene-normative basalt and shows increasing normative hypersthene and then normative quartz with advancing fractionation to reach a comenditic composition. Besides these two trends, it seems that many alkalic associations exist which show a differentiation trend starting from nepheline-normative basaltic composition and leading to hypersthenenormative, and then to quartz-normative compositions (here designated as the straddle-B type). Alkalic rocks of these three trends are higher not only in Na2O+K2O but also in Rb, Ba, Sr and Zr than subalkalic rocks. The alkalic basalts as a whole are characterized by higher contents of such elements and not by any degree of silica undersaturation. It is widely believed that alkalic rocks are characterized by the presence of normative nepheline as well as by the absence of orthopyroxene and pigeonite. Indeed such a relationship holds for the Kennedy trend, but it is not always valid for other types of alkalic associations. Some alkalic rocks of the Coombs trend and straddle-B type have quartz (or other silica minerals) and orthopyroxene and pigeonite.

Rifting of a tethyan continent — Rare earth evidence of an accreting plate margin

Rifting of a continent in the Tethys ocean was associated with two forms of volcanism initially identified by Hynes (1972). An early light rare earth element (LREE)-enriched magma accompanied rifting of the continental crust and subsidence of a marginal carbonate platform. The early basalts are high K 2O, nepheline-normative basalts, associated with silic igneous rocks, and carrying olivine pseudomorphs. A later or contemporaneous LREE-depleted magma is associated with the active formation of sea floor in a marginal embryo ocean basin. The ophiolite basalts are low K 2O, hypersthene-normative basalts containing feldspar laths and pyroxene subhedra. Similar transitions or changes in extrusives are evident in present-day embryo oceans and at the edges of rifted continental margins which surrounded larger ocean basins. Genesis of the tholeiites can be related to 10–30% partial fusion of foliated mantle lherzolites a sample of which adheres to the base of the Othris ophiolite. The alkalic basalts require either a fractionation model, or a more LREE-enriched source perhaps similar to the Ataq lherzolites, since the “tholeiite source lherzolite” can only produce alkalic basalts at low degrees of melting.

Geological and geochemical studies of the sintra alkaline igneous complex, portugal

The Sintra igneous complex, Portugal was an important centre of activity in late Cretaceous times. The great proportion of thealkaline rocks are felsic and include five large quartz syenite intrusions and trachyandesite, trachyte and alkali rhyolite lavas and dykes, most of which are oversaturated. Mafic rocks are sparse, but vary widely from alkaline and highly undersaturated types containing high K2O, TiO2 and Ba, similar to the contemporaneous Lisbon lavas, to hypersthene normative trachybasalts and one hypersthene normative basalt. The various magma types are intimately associated and a well-developed netveined complex of alkali gabbro, monzonite and syenite is recognised at Cabo da Roca. A study of the dyke distributions, intersections and orientations suggest a close propinquity of both oversaturated and undersaturated and of both felsic and matic magmas.

Trace-element distribution in lavas from Anjouan and Grande Comore, western Indian Ocean

The elements Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr and Ba have been determined by X-ray fluorescence for 65 basaltic and differentiated lavas from Anjouan, while Sc, V, Cr, Co, Ni, Cu, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba and Hf have been determined by spark-source mass spectrometry for selected lavas from Anjouan and Grande Comore, the most recently formed of the Comores Archipelago. Basaltic lavas studied range through nephelinite, basanite, alkali basalt and hypersthene-normative basalt, while differentiated lavas belong mainly to the trends: alkali basalt - trachyte and basanite - phonolite. The results indicate that during magmatic fractionation behaviour of large-ion elements such as Rb, Sr, Y, Zr, Ba and Nb is controlled by size/charge criteria, resulting in their exclusion from crystallising phases until the late trachytic and phonolitic stages. These elements are clearly fractionated by amphibole, plagioclase and alkali feldspar. Variation of transition elements due to crystal-liquid differentiation is largely in accord with the predictions of crystal field theory. The behaviour of Zn is not readily accounted for. Fractionation of K/Rb, K/Cs and probably Zr/Nb and discrepancies in abundance levels of large-ion elements between the main basaltic types are best accounted for in terms of high-pressure processes and probably also reflect inherent features of source-region geochemistry, coupled with the effects of variable partial melting.

Evolution of basaltic and differentiated lavas from Anjouan, Comores Archipelago

Anjouan is one of four volcanic islands comprising the Comores Archipelago. Three (arbitrarily defined) categories of basic magma are recognised on Anjouan: ‘hypersthenenormative’, ‘alkalic’ and ‘basanitic’, which appear in that order with an eruptive sequence involving 1) shield construction, 2) peripheral fissure-controlled activity, and 3) rejuvenescent (posterosional) eruptions. Differentiated magmas have evolved within the three chemical groupings, and trend mainly towards undersaturated trachyte and phonolite. These trends are considered to have developed by initial removal of olivine and clinopyroxene, followed by Fe-Ti oxides, apatite and amphibole from arrested liquid pools within and beneath the volcano. The appearance of feldspar on the liquidus was clearly inhibited by the high contents of normative diopside in most Anjouan magmas, although late stage plagioclase fractionation is probably responsible for development of peralkaline phonolites at shallow depths, assisted eventually by alkali feldspar. Lherzolite-xenolith-bearing lavas are likely to be directly mantle-derived liquids. Three analysed representatives with 100 · Mg/Mg + Fe2+ (atomic) ratios approaching 70 are characteristically rich in normative diopside and may confirm the suggestion of primitive ankaramitic melts in mantle regions. High pressure fractional crystallisation may involve fractionation of ‘eclogite’, orthopyroxene or clinopyroxene, with or without olivine. It is believed however that such processes do not adequately explain the compositional change from hypersthene-normative basalt towards basanite, as they imply unacceptable degrees of Fe-enrichment. Problems of large ion element enrichment and fractionation would also remain, while the high contents of Cr and Ni place limits on the extent of crystal fractionation. A partial melting model of magma genesis is able to explain the main features of basalt geochemistry, especially if the upper mantle low velocity zone is regarded as having been severely fractionated due to upward migration of large ion elements. An ‘open’ system of magma production in the context of regional plate movement, and the implied decoupling between lithosphere and asthenosphere, accomodates realistic degrees of partial melting and allows a greater potential volume of mantle available for melting than possible ‘closed’ systems. The model also accounts for the migratory pattern of Comores volcanism.

The origin of Pliocene-Holocene basalts of New Mexico in the light of strontium-isotopic and major-element abundances

Strontium isotope ratios of nepheline-normative basalts of New Mexico are similar to those of oceanic basalts and these undersaturated basalts have probably originated deep in the mantle under all structural provinces in the state. The hypersthene-normative basalts have highly variable Sr-isotope ratios (0.7029–0.7078) indicating either an origin in mantle enriched in radiogenic Sr or crustal contamination which is likely for one flow with variable 87Sr/ 86Sr ratios. The most acceptable hypothesis for the origin of the hypersthene-normative basalts requires that they be formed primitively in the mantle at a shallow depth with a Sr content low enough to reflect a change in the Sr ratios by small additions of crustal material of higher Sr-isotope ratios.