Undersaturated Basalts Research Articles

Predominantly recycled carbon in Earth's upper mantle revealed by He-CO2-Ba systematics in ultradepleted ocean ridge basalts

Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, and provide constraints on the proportions of mantle carbon that are juvenile (primordial) vs. tectonically recycled. Here we present new results for submarine basalt glasses, erupted within the Garrett Transform Fault (GTF) and to its north and south along the East Pacific Rise (EPR). Analyses of 3He/4He, and He and CO2 concentrations in vesicles and glass were performed, through a series of crushing and melting experiments plus FTIR spectroscopy. Trace elements were analyzed by laser ablation ICP-MS.The GTF basalts provide further tests for the origin of volatile-undersaturated basalts using CO2/Ba and CO2/Nb systematics. CO2 is highly correlated with Ba and Nb in basalts erupted in the transform domain (n = 13, including 5 undersaturated basalts) and does not show the variability expected from mixing between undegassed and variably degassed melts. Rather, the melts appear to originate from a heterogeneous mantle source that was variably depleted through partial melting, and limited mixing of melts is involved in their generation. The CO2/Ba and CO2/Nb weight ratios of 106 ± 8 and 308 ± 27, respectively, are similar to values determined previously for a global suite of undersaturated mid-ocean ridge basalts (MORBs).The ridge and transform domains have distinct 3He/4He ratios. Along the nearby EPR, 3He/4He = 8.5 - 9.1 RA, while within the Garrett Transform Fault 3He/4He = 9.2 - 10.1 RA. These two basalt populations are also distinct in their Pb-Sr-Nd isotope compositions based on earlier regional studies. The distinct populations result from partial melting of two different mantle source compositions. Melting of depleted mantle containing a small amount (∼1 to 5%) of enriched, ancient heterogeneities occurs beneath the EPR. Melting of ultradepleted mantle (in which the heterogeneities have been removed by earlier melting beneath the EPR) occurs beneath the GTF. This explains the distinction between intra-transform and spreading ridge domains for 3He/4He, if the heterogeneities were enriched in U, Th and He and had low 3He/4He as would be found in tectonically recycled material. The enriched mantle component sampled by the EPR basalts has molar CO2/3He =2×109, and it dominates the CO2/3He ratio generally ascribed to the upper mantle source for mid-ocean ridge basalts. In contrast, the ultradepleted MORB mantle component sampled by the GTF basalts has CO2/3He =3×108 or less. This indicates that most of the carbon in Earth's upper mantle originates from tectonic recycling.

Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts – Implications for sulfur cycle in subduction zones

To add to our understanding of sulfur cycle in subduction zones in general and constrain the effects of slab-released sulfate (SO42−) on the magma genesis and sulfur transport from sub-arc mantle to the arc volcanoes in particular we carried out an experimental study. High pressure-temperature, piston-cylinder experiments were carried out in Au-Pd capsules at 0.5–3 GPa and 1050–1325 °C to investigate (a) the effect of variable sulfur concentration from 0 to ~2 wt%, dissolved as SO42− on the stability field of a primary arc basalt with ~4 wt% H2O and determine (b) the sulfur content at anhydrite saturation (SCAS) of hydrous mafic magmas. Speciation of sulfur in the silicate melt was confirmed to be SO42− by S Kα X-ray peak position using electron microprobe. S-free hydrous clinopyroxene liquidus at 2 GPa is ~25 °C hotter than the hydrous clinopyroxene liquidus with ~0.1 wt% S in the liquidus melt as SO42− and the liquidus depression with further S-enrichment to anhydrite saturation (~2 wt% S) can be fitted by a power function ΔT (°C) = 26.52(±3.48)(Smelt in wt%)0.24(±0.06). Anhydrite-saturated experiments show that SCAS increases with increasing temperature and CaO content of melt and decreases with increasing SiO2 content of the melt. Previous SCAS models based mostly on lower P-T experiments and/or on silicic melt compositions can't capture our new experimental SCAS data. A new SCAS parameterization was developed using previous and our new experimental data. Calculations using our new parameterization and assuming 200–500 ppm S in the arc mantle show that <10% hydrous melting of mantle wedge would exhaust anhydrite, if present. Therefore, anhydrite even if present is expected to be exhausted by partial melting in the mantle wedge and parental basaltic melt will extract similar amount of S via mantle melting irrespective of the presence of sulfide or sulfate at subsolidus conditions. The S content, as dissolved SO42−, of hydrous arc basalts produced by 10–30% melting will be 500–4000 ppm, which is comparable to the melt inclusion S contents from various arcs. The sulfate undersaturated basalts may assimilate crustal sulfate and lead to high observed SO2 flux at the arcs, known as the “excess S”.

Comparative Petrology of the Montpelier and Roseland Potassic Anorthosites, Virginia

Abstract The Montpelier and Roseland plutons are the southernmost, and among the youngest and smallest, examples of massif anorthosite in North America. They occur in separate lithotectonic terranes, but strong similarities between them suggest common factors in their petrogenesis. Both massifs represent “alkalic anorthosite” dominated by antiperthitic plagioclase that yield reconstituted K-rich compositions averaging ∼An 26 Or 19 and ∼An 29 Or 16 , respectively. Concentrations of Rb (∼10–30 ppm), Sr (∼1100–1300 ppm), and Ba (∼800–1400 ppm) are likewise high in both, and Montpelier is unique in being the only known anorthosite in which Ba levels exceed those of Sr. Montpelier and Roseland are the most potassic of all massif anorthosites on Earth and their feldspar compositions appear unique among plutonic igneous rocks. In addition to such alkalic compositions (and in contrast to most other massif anorthosites), quartz is a common accessory mineral, belying any direct link to undersaturated alkali basalt. Furthermore, compositions of co-occurring plagioclase and pyroxene in Montpelier and Roseland (%An versus %En) lie well off trends defined for basaltic intrusions. Another distinctive feature of Montpelier and Roseland is their Fe,Ti-oxide assemblage of ilmenite + rutile, which contrasts with the ilmenite ± magnetite assemblage typical of most mafic intrusions. This ilmenite + rutile assemblage also contrasts with the widespread occurrence of hemo-ilmenite in similarly young, alkalic anorthosites in the Grenville Province of Quebec ( e.g ., St-Urbain, Labrieville, Château-Richer, Mattawa). Such differences indicate that the Virginia anorthosites crystallized under low f O 2 conditions compared to their Quebec counterparts, despite higher silica activity. We suggest two alternatives for the origin of the compositionally distinct magmas that gave rise to these anorthosites: (1) melting of an atypically alkalic source; or (2) significant contamination of mantle-derived magmas by evolved crust. Regardless of mechanism, the origin of these anorthosites is probably linked to their timing as late- to post-tectonic intrusions, and may be related to orogen-scale extension following crustal thickening.

Comparative Geochemical Study of the Tertiary and Quaternary Lavas of Western Senegal and the Cape Verde Islands: Geodynamic Implications

The western area of Senegal was the site of a basic volcanism in the Tertiary and Quaternary. That volcanism had caused the formation of alkaline olivine basalts, basanites and nephelinites from microlitic and porphyritic texture to dolerite texture. The geochemistry of major elements shows that the Quaternary basalts are more aluminous and more siliceous, but they contain less magnesium, less iron and less calcium than the Tertiary basalts. Two types of basalt occur here, based on the characteristics of the major elements in the Tertiary lavas: the highly undersaturated basalts of the Cape Verde Peninsula and the slightly undersaturated basalts of the Thiès Plateau. The characteristics of the trace elements reveal shallow interactions due to the contamination and/or assimilation, and to the fractional crystallization, but not due to the heterogeneity of the source. The strong enrichment in light rare earth elements (LREE) and the slight enrichment in heavy rare earth elements (HREE), the anomalies which are negative in K and Pb and positive in Nb, Ta and Eu of the Tertiary and Quaternary basalts of Western Senegal and similar anomalies of the same elements (Pb not analyzed) described in the Cape Verde islands, allow us to establish links between the petrogenetic mechanisms that had caused the formation of the basalts of this Islands and Western Senegal. Moreover, the volcanic activity in Western Senegal (of continental lntraplate type) and that of the Cape Verde Islands (oceanic intraplate type) are probably related to the hot spot located on the vertical of the Cape Verde.

Petrology, geochemistry and magnetic properties of Sadara sill: Evidence of rift related magmatism from Kutch basin, northwest India

Mafic volcanic rocks of the Mesozoic Kutch basin represent the earliest phase of Deccan volcanic activity. An olivine-clinopyroxene-plagioclase-phyric undersaturated basalt occurs as a sill near Sadara in the Pachham upland, Northern Kutch. The Sadara sill is deformed and emplaced along faults. The sill is alkaline in character and is transitional between basalt and basanite. Compared to primitive mantle, the Sadara sill is enriched in Sr, Ba, Pb and LREE but depleted in Nb, Cr, Y, Cs and Lu. Fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill. IRM and L-F test experiments and mineral analyses show titano-magnetite as the major remanence carrying magnetic mineral. AF and thermal demagnetizations of the Sadara sill yielded a mean ChRM direction as D=315.6°, I=−43.0° ( α 95=9.78; k=25.38) and the corresponding VGP at 25°S; 114.6°E (d p/d m=6.58°/11.6°). The Sadara sill pole is significantly different from those of the Deccan (65 Ma) and the Rajmahal Traps (118 Ma) but is close to the Cretaceous poles of 85–91 Ma rock units from southern India. This suggests a pre-Deccan age for the sill.

A clinopyroxene geobarometer for basaltic systems based on crystal-structure modeling

The crystal chemical response of basalt clinopyroxene to increasing pressure was investigated by means of crystal-structure simulation (a procedure that enables modeling of the structural parameters of a clinopyroxene of known chemistry without requiring direct X-ray diffraction analysis) using available experimental chemical data. Pressure proved the main physical variable governing clinopyroxene behavior in a magmatic environment. The general internal consistency of the simulation data permitted construction of an empirical geobarometer based on the relationship of cell volume (Vc) vs M1-site volume (VM1). The straightforward geobarometric formulation in the absence of direct X-ray analysis is: P(kbar) = 698.443 + 4.985-AlT - 26.826-Fe M1 2+ - 3.764-Fe3+ + 53.989-AlM1 + 3.948-Ti + 14.651.Cr - 700.431.Ca - 666.629.Na - 682. 848-MgM2 - 691.138-Fe M2 2+ - 688.384-Mn - 6.267-(MgM2)2 -4.144-(Fe M2 2+ ) where: (Fe M1 2+ -MgM2)/(Fe M2 2+ -MgM1) = e**(0.238-R3+ + 0.289.CNM - 2.315), CNM = Ca + Na + Mn, and R3+ = A1M1 + Fe3+ + Ti + Cr, with cations in atoms per formula unit. The geobarometer reproduces experimental pressures within α 2 kbar (= 1 σ max. dev. ≤ 5 kbar; N = 29) in the range 0-24 kbar and is applicable to near-liquidus C2/c clinopyroxenes crystallized from basaltic melts in the absence of garnet (excepting high-Al2O3 basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-preserved cumulate pyroxenites. If the above restrictions are not wholly satisfied, the Vc vs VM1 plot can also be used qualitatively to deduce the relative pressure conditions of clinopyroxenes forming from similar batches of magma. The structural simulation of experimental data also provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene at high pressure. Within the considered compositional space at given P-T, αCaO and αSi02 in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyroxenes from high-er-CaO or more undersaturated basalts have higher VM2, VT and Vc and lower VM1. For basalts with normal contents of A12O3 (< 18 wt%), variations of major elements in the melt do not reduce the accuracy of the geobarometer.

Rare earth element contents and multiple mantle sources of the transform-related Mount Edgecumbe basalts, southeastern Alaska

Pleistocene basalt of the Mount Edgecumbe volcanic field (MEF) is subdivided into a plagioclase type and an olivine type. Olivine basalt crops out farther inboard from the nearby Fairweather transform than plagioclase basalt. Th/La ratios of plagioclase basalt are similar to those of mid-ocean-ridge basalt (MORB), whereas those of olivine basalt are of continental affinity. The olivine basalt has higher 87Sr/86Sr ratios than the plagioclase basalt.We model rare earth element (REE) contents of the olivine basalt, which resemble those of transitional MORB, by 10–15% partial melting of fertile spinel–plagioclase lherzolite followed by removal of 8–13% olivine. Normative mineralogy indicates melting in the spinel stability field. REE contents of an undersaturated basalt (sample 5L005) resemble those of Mauna Loa tholeiite and are modelled by 5–10% partial melting of fertile garnet lherzolite followed by 10% olivine removal. Plagioclase basalt resembles sample 5L005 in REE contents but is lower in other incompatible-element contents and 87Sr/86Sr ratios. Plagioclase basalt either originated in depleted garnet lherzolite or is a mixture of sample 5L005 and normal MORB; complex zoning of plagioclase and colinear Sc and Th contents are consistent with magma mixing.We conclude that olivine basalt originated in subcontinental spinel lherzolite and that plagioclase basalt may have originated in suboceanic lithosphere of the Pacific plate. Lithospheric melting seemingly requires vertical flow of mantle material, although there is no direct evidence at the MEF for crustal extension that might provide a mechanism for mantle advection. In any case, most MEF magmas are subalkaline because of moderately high degrees of partial melting at shallow depth.

Gabbroic and Pyroxenite Layers in the Tinaquillo, Venezuela, Peridotite: Succession of Melt Intrusions in a Rising Mantle Diapir

The Tinaquillo alpine-type peridotite (Venezuela) contains an unusual abundance of pyroxenite, gabbroic granulite, and homblendite layers (about 15% of the massif). Of the anhydrous mafic layers, the metaclinopyroxenites and spinel websterites have cumulate-type compositions, with high MgO contents (14 to 18%). They originated in the upper mantle, in the spinel lherzolite facies and underwent the same two deformation events as the peridotite. Their petrography, mineralogy, and geochemistry are typical of the Al-augite suite pyroxenite layers in subcontinental lherzolite massifs and alkaline basalt nodules. Orthopyroxenite layers, characterized by low-alumina content, could represent earlier Cr-diopside suite pyroxenites. In contrast, the gabbroic hornblende granulites were only affected by the second deformation-recrystallization episode, at about 7 kb, have lower MgO contents (7-11%), and are enriched in Fe, Ti, and incompatible elements. Their normative compositions are those of tholeiitic and mildly undersaturated basalts. Based on their chemistry, three groups of hornblende granulites have been defined: (1) Group A form a moderate-P (5-7 kb) sequence enriched in iron and silica; (2) Group B is richer in silica, magnesium, and chromium but poorer in iron than group A. Their normative compositions are compatible with an origin as primary melt at about 8-10 kb; (3) Garnet-hornblende granulites are iron-rich, mildly alkaline rocks, that could result from a high P (>7-8 kb) evolution of a tholeiitic magma. Trace element concentrations in the gabbroic granulites suggest that they intruded the massif during the early stage of the formation of a back-arc basin.

Sr [sbnd]Nd [sbnd]Pb isotopic evolution of Gran Canaria: Evidence for shallow enriched mantle beneath the Canary Islands

We report the Sr, Nd and Pb isotopic compositions (1) of 66 lava flows and dikes spanning the circa 15 Myr subaerial volcanic history of Gran Canaria and (2) of five Miocene through Cretaceous sediment samples from DSDP site 397, located 100 km south of Gran Canaria. The isotope ratios of the Gran Canaria samples vary for 87Sr/ 86Sr: 0.70302–0.70346, for 143Nd/ 144Nd: 0.51275–0.51298, and for 206Pb/ 204Pb: 18.76–20.01. The Miocene and the Pliocene-Recent volcanics form distinct trends on isotope correlation diagrams. The most SiO 2-undersaturated volcanics from each group have the least radiogenic Sr and most radiogenic Pb, whereas evolved volcanics from each group have the most radiogenic Sr and least radiogenic Pb. In the Pliocene-Recent group, the most undersaturated basalts also have the most radiogenic Nd, and the evolved volcanics have the least radiogenic Nd. The most SiO 2-saturated basalts have intermediate compositions within each age group. Although the two age groups have overlapping Sr and Nd isotope ratios, the Pliocene-Recent volcanics have less radiogenic Pb than the Miocene volcanics. At least four components are required to explain the isotope systematics of Gran Canaria by mixing. There is no evidence for crustal contamination in any of the volcanics. The most undersaturated Miocene volcanics fall within the field for the two youngest and westernmost Canary Islands in all isotope correlation diagrams and thus appear to have the most plume-like (high 238U/ 204Pb) HIMU-like composition. During the Pliocene-Recent epochs, the plume was located to the west of Gran Canaria. The isotopic composition of the most undersaturated Pliocene-Recent volcanics may reflect entrainment of asthenospheric material (with a depleted mantle (DM)-like composition), as plume material was transported through the upper asthenosphere to the base of the lithosphere beneath Gran Canaria. The shift in isotopic composition with increasing SiO 2-saturation in the basalts and degree of differentiation for all volcanics is interpreted to reflect assimilation of enriched mantle (EM1 and EM2) (cf. [1]) in the lithosphere beneath Gran Canaria. This enriched mantle may have been derived from the continental lithospheric mantle beneath the West African Craton by thermal erosion or delamination during rifting of Pangaea. This study suggests that the enriched mantle components (EM1 and EM2) may be stored in the shallow mantle, whereas the HIMU component may have a deeper origin.

Pressure dependence of melt viscosities on the join diopside-albite

Abstract The pressure dependence of melt viscosities on the join diopside-albite has been studied using falling-sphere viscometry. The five melt compositions investigated are: diopside, Ab 25 Di 75 , Ab 50 Di 50 , Ab 75 Di 25 and albite. Experiments were performed at 1500° and 1600°C and at pressures of 5, 10, 15, 20 and 25 kbar. The positive and negative pressure dependence of the viscosity of diopside and albite, respectively, were confirmed. All intermediate compositions show an initial decrease in viscosity with increasing pressure; however, melt of Ab 25 Di 75 composition passes through a minimum viscosity at approximately 12 kbar and 1600°C. This behavior is analogous to the variation in the viscosity of water with pressure at low temperature. It is suggested that the three-dimensional, fully polymerized, albite structure dominates flow at low pressures. With increasing pressure, disruption of this structure and decrease in the average size of the flow units leads to domination by the diopside structure. The variation in viscosity with composition along the join at one atmosphere can be adequately modelled using the Adam and Gibbs (1965) configurational entropy model with an additional two-lattice configurational entropy of mixing term. The pressure dependence of viscosity in the diopside-albite system, however, cannot be predicted by the model, because there is an absence of information on the pressure dependence of the model parameters. It is probable that relatively polymerized magmas (e.g. rhyolites to SiO 2 -saturated basalts) show a negative pressure dependence of viscosity to depths where they originate in the lower crust or upper mantle. In contrast, the most depolymerized, naturally-occurring melts, such as strongly SiO 2 -undersaturated basalts and picrites, may exhibit a viscosity minimum. The viscosity of these melts may be sufficiently high at depths within the upper mantle to inhibit their segregation, rise and eventual eruption at the surface.

Remarques sur la genèse et la cristallisation des magmas basaltiques

Fractional crystallization may not be the most appropriate model to explain the evolution of the very first liquids in a basaltic series. The model exposed in this paper, based upon a continual requilibrium of liquids with a solid paragenesis, leads to a simultaneous crystallization of olivine (with clinopyroxene or plagioclase) and resorption of orthopyroxene. The genesis of liquids at depths and their evolution due to decompression and cooling could be formulated in the same ways : a more or less perfect equilibrium liquid-solid under decreasing pressure and temperature. Continued equilibrium down to low pressures would produce saturated basalts, interrupted equilibrium would produce undersaturated basalts.

K‐Ar ages of Cainozoic volcanic suites, Bowen‐St Lawrence Hinterland, North Queensland (with some implications for petrologic models)

Abstract Thirty K‐Ar dates on Cainozoic volcanic rocks lying at the north end of the Bowen Basin suggest that several episodes of volcanism took place at major structural weaknesses. The oldest volcanism (ca 54 m.y.) was located outside the basin structure. The main volcanism (Nebo and East Clermont Provinces) extended from early Oligocene (34–35 m.y.) to mid‐Cainozoic time (21–22 m.y.?). Isolated Pliocene activity is tentatively suggested by dates on Mt St Martin (ca 3 m.y.). Dating of the Nebo central volcano (31–33 m.y.) supports the model of Wellman & McDougall, with volcanic activity related to migration of Australia northwards over a mantle magma source. Consideration of the Nebo dates with those of other central volcanoes in north Queensland, suggests that central felsic activity was surrounded by broad zones of peripheral eruptives, petrologically zoned from outer undersaturated basalts to inner saturated basalts. These zones (super provinces) delineate the size and profile of underlying magma sou...

Geochronology of Gran Canaria, Canary Islands: Age of shield building volcanism and other magmatic phases

Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km3 was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (∼100 km3) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago. Following a long hiatus there was resurgence of volcanism with eruption of about 100 km3 of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km3. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria. The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.

Composition and origin of Deccan basalts

The Deccan basalts are essentially composed of saturated tholeiitic lavas. However, undersaturated basalts, nephelinites, carbonatites, intermediate and acid differentiates have also been encountered in parts of western India (Upper Traps). Such unusual rocks are broadly aligned in two major rift zones in western India, the Narbada-Son and Cambay grabens and the faulted west coast. These rocks are younger than the earliest tholeiitic eruptions of central India (Lower Traps), but there are evidences of renewed eruptions of tholeiitic basalts in parts of western India. The earliest eruptions of Deccan basalts of quartz tholeiite composition have been derived by high degrees of partial melting of peridotite at moderate depth (37–41 km). The undersaturated basalts and nephelinites were possibly generated by low degrees of partial melting of garnet peridotite in the low velocity zone along the two tectonically active belts (rifts) in western India. The undersaturated basalts were formed prior to the break-up of Gondwanaland, when a concentration of pressure developed at the tectonically weak zones. The renewed eruptions of saturated tholeiites are thought to be post-tectonic, resulting from the release of stress when the tectonic events had ceased.

A Discussion on volcanism and the structure of the Earth - Silica saturation in cenozoic basalt

Cenozoic basalts are conspicuously either over- or undersaturated with respect to silica. Undersaturated basalts are strongly dominant in the mediterranean areas and in oceanic islands other than Hawaii. Oversaturated basalts are strongly dominant in Hawaii and throughout the circumoceanic environment. Both over- and undersaturated basalts are abundant on the continents. Silica-saturated basalts are dominant only in the submarine ridges, where fresh representatives of either of the other saturation types are still unknown. Experimental work suggests that initial or early melting fractions of basalts or their plutonic equivalents or precursors would usually be either over- or undersaturated regardless of the saturation state of the starting material. The bimodal distribution of silica saturation in Cenozoic basalts is thus compatible with, and provides little basis for preference among, a variety of Earth models in which basaltic lavas are presumed to be early melting fractions of pre-existing solid materials.

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.

Rare earth element distribution in lavas and ultramafic xenoliths from the Comores Archipelago, Western Indian Ocean

Lavas and included xenoliths from the Comores Archipelago have been analysed for the rare earth elements (REE) La-Lu. Among basaltic lava types fractionation of REE rock/chondrite distribution patterns is more extreme with greater SiO2 undersaturation and contents of incompatible elements. Enrichment and slight fractionation of REE in the rock series basanite-phonolite is considered compatible with a model of fractional crystallisation at low pressures involving mainly olivine and clinopyroxene, and to a much lesser extent, plagioclase. Apatite is probably effective in curtailing further enrichment of REE. High level fractional crystallisation and eclogite fractionation at depth appear unlikely causes for the relative enrichment of light REE (La-Eu) in the undersaturated basalts. This effect is more probably due to mineralogical control during partial melting in the upper mantle. Lherzolite xenoliths are poor in REE, exhibiting a slight relative depletion in the light REE. These patterns are interpreted as those of possible mantle material subjected to small degrees of partial melting, although not necessarily related to those melts erupted as lava flows at the surface.

Mafic and ultramafic inclusions associated with undersaturated basalt on Kod Ali Island, Southern Red Sea

The island of Kod Ali is a small diatreme cone lying close to the Ethiopian coast at latitude 13° 57′ N, longitude 41° 49′ E. This volcanic structure, produced by geologically Recent volcanic activity, consists, almost entirely, of pyroclastic debris. Evidence of a liquid phase is confined to the thin veneers of markedly alkalic olivine basalt which coat many of the abundant gabbroic, pyroxenitic and lherzolitic fragments found on the island. As silicic inclusions are completely absent and as the alkalic liquid phase and the “tholeiitic” inclusions are seemingly genetically unrelated, it is proposed that the latter might have been derived from a tholeiitic plutonic layered sequence that underlies the tholeiitic basalts of an “oceanic” Red Sea floor.

Field evidence and K, Rb, Sr data bearing on the origin of the Mt. Taylor volcanic field, New Mexico, U.S.A.

The Mt. Taylor volcanic field straddless the southern margin of the San Juan Basin in the southeast corner of the Colorado Plateau. Central activity resulted in the extrusion of rhyolite, dacite alkali andesite, and basaltic andesite flows, from a limited and now deeply eroded area. Coeval with central activity alkali basalt magma erupted from more remote centres, and local activity has apparently continued until a few thousand years ago or less. Major element and Rb, Sr, K 2O abundances allow the following rock types to be recognized: early basanite flows and pipes; mildly undersaturated basalts; ‘big-feldspar’ basalts; basaltic-andesites; alkali andesites; dacites; rhyolites. There is marked composition gap between basalts and basaltic andesites. K/Rb, Rb/Sr and 87Sr/ 86Sr data suggest that both alkali olivine basalt and the hypersthene normative ‘big-feldspar’ basalts are mantle derived whereas andesites, dacites, and rhyolites are not. It is suggested that the rhyolites have arisen by partial melting of the granite basement.

Syntectonic Recrystallization of Olivine and Modes of Flow in the Upper Mantle

Olivine in dunite, peridotite, and olivine powder has been recrystallized syntectonically during compression tests at temperatures from 950°C to 1350°C, confining pressures from 5 to 30 kb, and strain rates from 10 −3 to 10 −8 /sec. For intragranular recrystallization of the dunite, the orientations of the new grains are controlled by the host orientation. For inter-granular recrystallization and total recrystallization of the dunite and of olivine powder, the orientations of the grains are controlled dominantly by the stress. The fabric produced over the entire range of physical conditions is a [010] maximum parallel to σ 1 and [001] and [100] girdles in the σ 2 = σ 3 plane. With few exceptions, and allowances for the experimental requirement that σ 2 = σ 3 , fabrics in olivine tectonites are similar to those produced experimentally. Such probable upper mantle source materials as ultramafic nodules in diamond-bearing kimberlites and in under-saturated basalts and “Alpine-type” peridotites show these same preferred orientations. We propose, therefore, that Syntectonic recrystallization is an important or dominant mode of flow in the upper mantle at temperatures above about 500°C. The implications of this conclusion, as they bear on seismicity and seismic anisotropy, are considered in light of the two most seriously regarded hypotheses advanced to explain motions of spreading plates of the lithosphere.