Showing papers on "Incompatible element published in 1984"

Mantle Chemistry and Accretion History of the Earth

Abstract: The chemical composition of the Earth’s primitive mantle (present mantle + crust) yields important information about the accretion history of the Earth. For the upper mantle reliable data on its composition have been obtained from the study of primitive and unaltered ultramafic xenoliths (Jagoutz et al. 1979). Normalized to C 1 and Si the Earth’s mantle is slightly enriched in refractory oxyphile elements and in magnesium. It might be that this enrichment is fictitious and only due to the normalization to Si and that the Earth’s mantle is depleted in Si, which partly entered the Earth’s core in metallic form. Alternatively, the depletion of Si may only be valid for the upper mantle and is compensated by a Si enrichment of the lower mantle.

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 %.

Two magmatic series with island arc affinities within the vourinos ophiolite

Abstract: Within the Vourinos ophiolite evidence of two magmatic series has been preserved in cognate cumulates and in effusive and hypabyssal rocks, which constitute the earlier Krapa sequence and the younger Asprokambo sequence. The Asprokambo dyke basic magmas which are poor in incompatible elements and relatively Ni and Cr rich, bear some resemblance to very low Ti basalts (transitional to boninites) found in subduction related arcs or interarc basins. Krapa series magmas from sills, massive and pillow lavas are best equated with low-K tholeiites of island arc suites. Compositions of Al- and Ti- poor Cpx in lavas from both series are comparable to those in island arc basalts, the Asprokambo Cpx being richer in Ca and Cr than those from Krapa. The large volume of cumulates from the Krapa sequence displays the following crystallization order: Ol±Sp, Cpx, Pl±Opx, Mt. Periodic influx of fresh magma batches into the magma chamber occurred mainly during the formation of the lower cumulates (wehrlite, Ol-clinopyroxenite and melagabbro). The upper cumulates, gabbronorite and leucogabbronorite with minor Mt-bearing gabbronorite, crystallized in the upper levels of a magma chamber which became progressively smaller with time. In the Asprokambo sequence, Ol+Sp, Opx, Cpx, PI and Amph are the successively crystallizing phases. The ortho to heteradcumulates consist of websterite, Pl-websterite, gabbronorite, amphibole bearing leuconorite, diorite and granophyre. In cumulates, especially in the lower Krapa sequence, significant subsolidus reaction was probably induced by the persistence of high geothermal gradients linked to continuous magmatism. Petrological features indicate that the evolution of the Krapa series is more compatible with an intermediate fractional/equilibrium crystallization history in an initially open system, whereas nearly perfect fractional crystallization in closed system may have occurred in the small Asprokambo magma chambers. Chemical variations in the lavas of both series can be explained in terms of crystallization of the observed cumulates. Significantly, the Asprokambo intrusives have igneous Mg-hornblende and vanadium bearing, chromian, aluminous titaniferous magnetite, crystallization of which is responsible for the calcalkaline evolutionary trend of these rocks. Major and trace element modelling necessitates a two stage model for the petrogenesis of the Vourinos parental melts, involving high-degree remelting of previously depleted mantle sources favoured by the influx of subduction derived hydrous fluids. The primary magmas parental to the Krapa and Asprokambo series could have been derived respectively by 20 and 30% equilibrium partial fusion of variably depleted lherzolitic sources, leaving residua having a harzburgitic to dunitic composition.

Rift-zone magmatism: Petrology of basaltic rocks transitional from CFB to MORB, southeastern Brazil margin

Abstract: Compositions of basaltic samples from the southeastern Brazil passive margin (18°–24° S) depict the change from continental to oceanic lithosphere during the opening of the South Atlantic Ocean. Samples studied range from 138 to 105 m.y. old and are from 12 Petrobras drill cores recovered from the coastline to about 150 km offshore in the Espirito Santo, Campos, and Santos basins. Compositions vary, ranging, for example, from 49–54 wt.% SiO2, 0.5–3.0 wt.% TiO2, 0.6–5.0 FeO*/MgO, and 〈1-〉6 La/ Yb(n), but can be grouped: (i) basalts enriched in incompatible elements, such as K (some K2O>2.0 wt.%), Rb (>18 ppm), Zr (>120 ppm), and LREE (some FeO* 16 wt.%; most with SiO2 51–54 wt.%), and resembling Serra Geral continental flood basalts (SG-CFB) of southern Brazil; (ii) basalts less enriched, or transitional, in incompatible elements, having K2O 30). Trace-element ratios (e.g., Zr/Nb, Zr/Y) link the Brazil margin basalts to a heterogeneous mantle (attributed to metasomatic veining) of variably proportioned mixtures of depleted-mantle (N-MORB) and plume (P-MORB, e.g., Tristan hotspot) materials. The various compositions therefore reflect, in part, different zones of melting during the separation of Gondwanaland, where gradual decompression during rifting enabled concurrent melting of upper, more depleted (non- or sparsely-veined) mantle and enriched (densely-veined) mantle. Within the time represented, melting produced enriched, transitional, and depleted magmas that were emplaced subaerially, hypabyssally, and subaqueously; they mark the transition from CFB before rifting and separation (from deeper, enriched mantle) to N-MORB in the S. Atlantic afterwards (from non- or sparsely-veined upper mantle). While P-type mantle components account for the enriched compositions of some basalts (Zr/Nb<8), continental crust is largely responsible for that of others (e.g., Ti/Zr 40–57; La/Yb(n) 5–6, and δ 18O+12.2 in one sample). Some may be contaminated expressions of otherwise T-type basalts free of crustal components. This study identifies CFB to be from sources similar to those for T- and P-type oceanic rocks, where individual CFB magmas may or may not have acquired crustal signatures.

Geochemical and isotopic evidence for the origin of continental flood basalts with particular reference to the Snake River Plain Idaho, U. S. A

Abstract: Voluminous outpourings of olivine and quartz tholeiite cover vast tracts of the western U.S.A. around the Columbia and Snake Rivers. Voluminous eruptive units within each province are petrographically and chemically homogeneous and generally lack significant lateral or temporal variation. These features suggest relatively homogeneous source regions. A possible scenario for the Snake River Plain involves extraction oftholeiitic melts from enriched spinel lherzolite mantle (87Sr/86Sr > 0.7058, 143Nd/144Nd <0.51252) which contains at least a component of 2.5 Ga material. Subsequent fractionation of olivine, plagioclase, apatite and magnetite in crustal magma chambers and simultaneous assimilation of crust (ca. 20 %) accounts for the isotopic variability in the more evolved ferrolatites and ferrobasalts. Unlike the olivine tholeiites these evolved volcanic rocks exhibit all the classic elemental and isotopic correlations consistent with an origin involving combined assimilation and fractional crystallization. Characterization of the mantle source regions of continental flood basalts (c.f.b.) is hampered by uncertainties regarding the extent of modification of the magmas by interaction with the continental crust, and is also plagued by ignorance of the chemical and isotopic heterogeneity of the sub-continental mantle. Evidence from mantle xenoliths (Menzies & Murthy 1980; Erlank et al. 1982) demonstrates that some portions of the sub-continental mantle are both old and relatively enriched in incompatible elements, consistent with suggestions that much of the continental lithosphere may stabilize soon after the last major thermal event registered in the overlying crust (Oxburgh & Parmentier 1978). This may produce mantle of sufficient age that any enrichment in incompatible elements (namely, increase in Rb/Sr and Nd/Sm) will be

Petrology of basalts from the Mohns-Knipovich Ridge; the Norwegian-Greenland Sea

Abstract: Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30′N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K2O, TiO2, and P2O5 contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe2+) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe2+) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C. Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30′ N is also briefly considered. It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform.

Abor Volcanics: further evidence for the birth of the Tethys Ocean in the Himalayan segment

Abstract: The Himalayan region has a record of several phases of igneous activity which indicate a continued N—S orientated tensional regime since at least late Precambrian time. Though the earlier volcanic eruptions were restricted both in time and space, the Late Carboniferous-Triassic activity was widespread in terms of both the time interval and area covered. The Abor Volcanic activity in the Eastern Syntaxial Bend of the Himalayan fold belt is coeval with the initiation of Late Carboniferous—Triassic Panjal Trap volcanism of the Western Syntaxial Bend and eruption of the Sikkim lavas. Chemical characters of the Abor Volcanics indicate (1) a within-plate rift tectonic setting of the lava eruptions, (2) a transitional nature between tholeiitic and alkalic compositions, (3) relative enrichment in incompatible elements, and (4) high Fe-Ti values similar to many such basalts erupted in the regions of active lithospheric rifting. The olivine-free nature, a linear trend for MgO and low Co contents of the studied rocks are suggestive of high pressure olivine fractionation for the generation of the bulk chemistry. However, large variations in the incompatible element abundances and ratios of rocks at a similar fractionation stage discount the evolution of the lava chemistry exclusively by simple fractionation or partial melting processes. Instead, these characters together with the low Zr/Nb ratio and within-plate rift tectonic setting suggests the modification of an upper mantle source region, possibly by fluid phase CO 2 immediately before the partial melting and lithospheric rifting. Combining geology of the region with the geochemistry and inferred tectonic setting of these rocks, the lava eruptions are suggested to have been associated with the 9rift valley stage9 of the Tethys Ocean formation in the present Himalayan region.

Geochemical characteristics and comparison of the basic rocks of the Lizard Complex and the basaltic lavas within the Hercynian troughs of SW England

Abstract: The S Cornish basaltic rocks constitute a separate chemical province relative to the rest of the SW England Hercynian volcanic suites. In particular they are characterized by (a) lower incompatible element abundances, (b) generally lower large-ion-lithophile/high field strength element ratios, and (c) being mildly enriched in the light REE. Within this province some of the basalts have chemical features akin to incompatible element enriched MORB. Tectonically the S Cornish province can be divided into (a) the S Cornish melange zone (N Lizard-Roseland area), and (b) the Mylor sedimentary trough (Penwith-Camborne area), both of which contain localized basaltic lavas. These basalts are compared with the Lizard ophiolite and associated hornblende schists which have compositions ranging from depleted to enriched MORB. The exotic volcanic megaclasts within the melange zone are not chemically related to the ophiolite and as such do not represent the dismembered volcanic portion of the Lizard ophiolite. If the Lizard ophiolite and the hornblende schists represent different segments of a Palaeozoic oceanic area lying to the S, the ocean-floor type Hercynian basalts of S Cornwall are interpreted as an abortive attempt to produce oceanic crust in a rifted continental margin during the early Devonian.

Spanish and Western Australian Lamproites: Aspects of Whole Rock Geochemistry

Abstract: Major, trace and rare earth element data are used to compare Spanish (sanidine bearing) and Western Australian (leucite bearing) lamproites. The Spanish lamproites are separated into the groups of Fuster and co-workers with some modification: verite and fortunite are quartz normative with verites arbitrarily defined as having A1203 indicating potential K richterite, and jumillite has potential olivine ± felspathoid. Chemically, the Western Australian lamproites show higher K2O/AI2O3 ratios. TiO2 is more than 3 times the concentration found in the Spanish lamproites and incompatible elements Ba, Sr, Zr and Nb also show higher levels. The REE chondrite - normalised patterns of the four Spanish groups differ in terms of enrichment (La/Yb ratio) but have similar Eu anomalies. Eu anomalies are not present in the Western Australian rocks which are even more enriched and kimberlitic in character. LREE and incompatible elements do not have a positive correlation with K and may have an independent role in the mantle source. The REE data and the occurrence of spinel lherzolite xenoliths suggest that the Spanish lamproites could have been derived by partial melting of mantle at shallower depths compared with the Western Australian examples which have lower HREE and are associated with diamonds. Six Western Australian lamproites, including three kimberlitic specimens from the same province, have a range of ∈Nd (20 m.y.) = -7.3 to -12.7 further suggesting derivation from a (common) strongly enriched mantle. We conclude that varying degrees of metasomatism of the mantle have taken place prior to lamproite generation.

Basalt geochemistry of the Explorer Ridge area, northeast Pacific Ocean

Abstract: Forty-two fragments of young, fresh basalts, dredged from the Explorer Ridge, Paul Revere Ridge (Fracture Zone), and Dellwood Knolls, have been analysed for 34 major, minor, and trace elements, and 87Sr/86Sr ratios have been determined in seven of the fragments.The Explorer Ridge basalts have major element compositions similar to most mid-ocean ridge basalts (MORB), particularly to the iron-rich basalts of the southern Juan de Fuca Ridge. The basalts exhibit variability in trace element content, largely attributable to crystal fractionation at low pressure. With respect to MORB the incompatible minor and trace elements are weakly to strongly enriched in the Explorer samples, and are most strongly enriched in basalts from Explorer Deep. Adjacent ridge segments erupt basalts with variable incompatible element concentrations and ratios. This could be an indicator of a chemically heterogeneous mantle source, or may be the result of intermittent injection of enriched magmas from a hotspot beneath Explorer Deep...

Magma Mixing in Komatiitic Lavas from Munro Township, Ontario

Abstract: Komatiites at Munro Township, northeast Ontario, show greater LREE depletion and have lower ratios of highly to moderately incompatible elements (e.g. Ti/Sc) than associated komatiitic basalts. These differences indicate that the two magma types are not related to one another by low pressure fractional crystallization: they formed either from mantle sources with slightly different compositions, or from the same source under different conditions of partial melting or high pressure fractionation.

Physical and chemical constraints on the evolution of the Columbia River basalt

Abstract: Physical and chemical constraints require that Columbia River basalt flows were fed from a large (>700 km 3 ) magma reservoir near the crust-mantle boundary and prohibit significant crystal fractionation or crustal assimilation between reservoir and surface. Demonstrable fractionation of plagioclase + olivine ± pyroxene in the reservoir largely obviates the need for an unusual iron-rich pyroxenite source. Variation of Sr and Nd isotope ratios with age in the main series of Columbia River basalt and variation in such incompatible trace-element ratios as Ba/P suggest lower crustal assimilation at the top of the reservoir combined with crystal fractionation. However, decoupling between isotope ratios and incompatible element ratios implies two separate processes in addition to crystal fractionation, of which lower crustal assimilation may be one. The other process may involve the participation of a vertically heterogeneous mantle resulting from metasomatism in the continental lithosphere.

Direct determination of strontium enrichment on grain boundaries in a garnet lherzolite xenolith by proton microprobe analysis

Abstract: The mechanisms by which trace and radiogenic elements are held and transported in the Earth's mantle are of prime importance in studies of basalt petrogenesis and the chemical processing of the mantle. While such mechanics include the migration of melts and the convective physical admixture of differentiated material1, the role of fluids as transport agents has attracted much interest, especially, in regions affected by kimberlitic activity2–4, continental basaltic volcanism5,6, and in areas of subduction-related magmatic activity7,8. The observation that many incompatible elements are readily leached from garnet lherzolites by dilute acid treatment3 has led to the assumption that a proportion of these elements is located on grain boundaries, rather than in solution in the various minerals present. This indirect evidence is reinforced by comparison of the compositions of mineral separates with their bulk parental material4–8,11. Although careful electron microprobe analyses have made it possible to detect minor and trace elements within grains12, the low concentrations involved, together with the high X-ray background of the electron microprobe, have made it impossible to investigate the supposed distribution of incompatible trace elements on grain boundaries. We have measured directly, using a high-resolution proton microprobe9,10, the distribution of Sr and other elements in a garnet lherzolite xenolith. The analyses demonstrate that, in this sample, Sr is preferentially concentrated along grain boundaries.

Petrology, geochemistry and Sr-isotope characteristics of lavas from the area of Commenda (Mts. Vulsini, Italy)

Abstract: Major, trace element and Sr-isotope compositions are reported for a suite of lavas coming from the area of Commenda in the SE Vulsinian district. The analyzed samples have all low silica contents and variable but generally high CaO, MgO and FeOt. Based on K2O% and K2O/Na2O ratio, the rocks from Commenda can be classified as belonging to the Potassic Series (KS) and the High-potassium Series (HKS). The HKS rocks appear to have derived by cristal/liquid fractionation from the most mafic types with separation of olivine and clinopyroxene and then of clinopyroxene + leucite. The most primitive HKS rocks have aphyric texture and high Mg-values, Cr and Ni contents which are close or within the range of values of magmas formed by partial melting of periodititic mantle sources. The KS rocks have lower incompatible element contents as the HKS rocks with similar degree of evolution.

Kimberlite and the Evolution of the Mantle

Abstract: Kimberlites are extremely enriched in the incompatible elements and are therefore important in discussions of the trace-element inventory of the Earth's mantle. Such extreme enrichment implies that kimberlites represent a small degree of partial melting of a mantle silicate or a late-stage residual fluid of a crystallizing cumulate layer. The LIL (large-ion lithophile) elements in kimberlite show that they have been in equilibrium with a garnet-clinopyroxene rich source region, possibly an eclogite cumulate. The LIL contents of kimberlite and MORB (mid-ocean ridge basalts) are complementary. Removal of a kimberlite-like fluid from an eclogite cumulate gives a crystalline residue with the required geochemical characteristics of the depleted source region that provides MORB. Kimberlite and lunar KREEP are remarkably similar in their minor and trace element chemistry. The main differences can be attributed to plagioclase fractionation in the case of KREEP and eclogite fractionation in the case of kimberlite. KREEP has been interpreted as the residual fluid of a crystallizing magma ocean. In a small body the Al_2O_3 content of a crystallizing melt is reduced by rilagioclase crystallization and flotation. In a magma ocean on a large body, such as the Earth, the Al_2O_3 is removed by the sinking of garnet. Kimberlite, in fact, is depleted in eclogite elements including the HREE and Na. This suggests that kimberlite may represent the late-stage residual fluid of a crystallizing terrestrial magma ocean. A buried eclogite cumulate layer is the terrestrial equivalent of the lunar anorthositic crust.

Alkali granite-syenite-carbonatite association in Munnar Kerala, India; implications for rifting, alkaline magmatism and liquid immiscibility

Abstract: Occurrence of carbonatite is reported from the Munnar area, Kerala, where an alkali granite-syenite-carbonatite association is seen emplaced along the intersection zone of the Attur and Kerala fault-lineaments. The carbonatites are of two varieties, a calcite-rich sovite and a very coarse grained, calcite and dolomite bearing alvikite. Higher levels of SiO2, Al2O3 and CaO are characteristic of these as compared to the composition of typical carbonatites. The transition element levels are high whereas the incompatible elements show lower values. The low Sr values, lower amount of apatite and absence of rare metal minerals preclude a primary carbonatite magma. The associated syenite and alkali granite have higher K2O, K2O/Na2O, K/Rb, K/Ba and transition element levels. Petrochemical features suggest the rock association to be a result of separation of an immiscible fraction of less viscous carbonate liquid during cooling and ascent from a more viscous polymerized alkali silicate phase. The pre-requisites for melt equilibration and liquid immiscibility were achieved through volatile degassing related to crustal warping and rifting. The unique alkaline association of Munnar, which shows spatial relationships with deep-seated faults as well as a probable triple-point junction, is suggested to be a signature of late Precambrian alkaline magmatism which manifested in the Indian shield as a precursor to the rifting of the continental margin.

Correlated Sr Isotope and Geochemical Variations in Basalts and Basaltic Andesites from Guatemala

Abstract: The incompatible elements in the high alumina basalts and basaltic andesites of six composite volcanoes in Guatemala are most abundant in the volcanoes of San Pedro, Atitlan and Toliman which lie in the 84 000 year old cauldron of Lake Atitlan. Modelling of the major and trace elements, using the high alumina basalts of Fuego as the parental magma, indicates that the five other volcanoes contain excess incompatible elements but are deficient in Na. We infer the excesses to mean that the magmas have evolved through cycles of magma replenishment and fractional crystallization prior to their delivery to the individual volcanoes, probably in magma ponds at the base of the crust (Carr, 1983). The Na deficiency may reflect either the crystallization of sodic plagioclase as would be expected at high pressures, or the diffusional loss of Na as the magmas rose through continental crust.