Amounts Of Partial Melting Research Articles

The viscosity of a partially molten layer in a paleo-orogenic plateau

Orogenic plateaus (e.g., Tibet, Altiplano) are characterized by broad, flat-top topography at high elevation and significantly increased crustal thickness. Partial melt is thought to weaken the middle crust of orogenic plateaus, and thus reduce the viscosity of the crust; however, the amount of partial melt and the magnitude of associated weakening remain unconstrained. The New England Appalachians represent an exposed mid- to lower crustal section of a paleo-orogenic plateau, similar to modern-day Tibet. In this study, we utilize the relationship between the spacing of deformation bands and the compaction length to constrain mid-crustal shear viscosity in a late Devonian migmatite. We find that the viscosity of the middle orogenic crust in the paleo-orogenic plateau of the New England Appalachians is 1017–18 Pa∙s at ∼3–9% melt. This finding is consistent with geophysical models of orogenic channel flow and provides field-based evidence for a significant rheologic transition at low melt-fraction. Our results suggest that the key elements for the formation of a weak, mid-crustal layer in orogenic plateaus are an influx of water and temperatures near the hydrous granite solidus.

Provenance Response to Rifting and Separation at the Jan Mayen Microcontinent Margin

The Eocene-Miocene successions recovered at DSDP sites on the Jan Mayen Ridge (NE Atlantic) and on the adjacent East Greenland margin provide a sedimentary record of the rifting and separation of the Jan Mayen Microcontinent from East Greenland. A combination of palynology, conventional heavy mineral analysis, single-grain major and trace element geochemistry and radiometric dating of amphibole and zircon has revealed a major change in sediment provenance took place at the Early/Late Oligocene boundary corresponding to a prominent seismic reflector termed JA. During the Eocene and Early Oligocene, lateral variations in provenance character indicate multiple, small-scale transport systems. Site 349 and Kap Brewster were predominantly supplied from magmatic sources (Kap Brewster having a stronger subalkaline signature compared with Site 349), whereas Site 346 received almost exclusively metasedimentary detritus. By contrast, Late Oligocene provenance characteristics are closely comparable at the two Jan Mayen sites, the most distinctive feature being the abundance of reworked Carboniferous, Jurassic, Cretaceous and Eocene palynomorphs. The Site 349 succession documents an evolution in the nature of the magmatic provenance component. Supply from evolved alkaline magmatic rocks, such as syenites, was important in the Middle Eocene and lower part of the Early Oligocene, but was superseded in the later Early Oligocene by mafic magmatic sources. In the latest Early Oligocene, the presence of evolved clinopyroxenes provides evidence for prolonged magmatic fractionation. Initial low degrees of partial melting led to generation of alkaline (syenitic) magmas. The extent of partial melting increased during the Early Oligocene, generating basaltic rocks with both subalkaline and alkaline compositions. Towards the end of the Early Oligocene, the amount of partial melting and magma supply rates decreased. In the Late Oligocene, there is no evidence for contemporaneous igneous activity, with scarce magmatic indicator minerals. The provenance change suggests that the hiatus at the Early/Late Oligocene boundary represents the initiation of the proto-Kolbeinsey Ridge and separation of the Jan Mayen Microcontinent from East Greenland.

Tectonomagmatic development of the Eocene Pasevh pluton (NW Iran): Implications for the Arabia-Eurasia collision

Abstract The Pasveh pluton is one of a number of intrusive complexes in the northwestern part of the Sanandaj-Sirjan Zone (SSZ), NW Iran. The major part of this complex is composed of high-K, calc-alkaline granitic rocks. The geochemical data indicate that the granitic rocks, although compositionally similar, are not cogenetic and can be divided into three groups. The first group (mesocratic granitoids) is represented by the largest outcrop in the Pasveh pluton. It is mainly composed of metaluminous magnesian monzodiorite to granodiorite with abundant MME and gabbroic enclaves. Zircon LA–ICP–MS U–Pb dating yielded Eocene crystallization ages (ca. 40 Ma) for this group. The monzodiorite (eNdt = +2.0 to +2.5), granodiorite (eNdt = +1.5 to +1.6), MME and gabbroic enclave (eNdt = +3.3 to +4.7) have distinct Nd isotopic compositions. The second group is composed of Eocene (39.7 ± 0.6 Ma), weakly peraluminous monzogranites that have Carboniferous and Cretaceous inherited zircons. Leucocratic granitic dikes of adakitic composition comprise the third group of Pasveh silicic rocks. The dikes are slightly younger (37.5 ± 0.3 Ma) than host rocks and cross-cut the mesocratic granitoids. Our results indicate that the underplating of basaltic magmas related to extensional stress in the overlying Iranian plate during the Eocene induced partial melting of the lower crust. Differences in the amount of partial melting of the source rocks are likely responsible for the volume and heterogeneity of their melt compositions. Subsequent to melt generation, it is possible that the silicic magmas mixed with mafic magmas in variable proportions to produce the hybridized rocks and enclaves. The intrusion of leucogranite dikes after the emplacement of the Pasveh pluton suggests that during or slightly after Neo-Tethys subduction there was another period of magmatism that was likely related to the melting of the subducted slab.

Stability and migration of slab-derived carbonate-rich melts above the transition zone

We present a theoretical model of the stability and migration of carbonate-rich melts to test whether they can explain seismic low-velocity layers (LVLs) observed above stalled slabs in several convergent tectonic settings. The LVLs, located atop the mantle transition zone, contain small (∼1 vol%) amounts of partial melt, possibly derived from melting of subducted carbonate-bearing oceanic crust. Petrological and geochemical evidence from inclusions in superdeep diamonds supports the existence of slab-derived carbonate melt, which may potentially explain the origin of the observed melt in the LVL. However, the presumptive reducing nature of the ambient mantle can be an impediment to the stability of carbonated melt. To reconcile this apparent contradiction, we test the stability and migration rates of carbonate-rich melts atop a stalled slab as a function of melt percolation, redox freezing, amount of carbon supplied by subduction, and the metallic Fe concentration in the mantle. Our results demonstrate that carbonate-rich melts in the LVL can potentially survive redox freezing over long geological time scales. We also show that the amount of subducted carbon exerts a stronger influence on the stability of carbonate melt than does the mantle redox condition. Concentration dependent melt density leads to rapid melt propagation through channels while a constant melt density causes melt to migrate as a planar front. Our calculations suggest that the LVLs can sequester significant fractions of carbon transported to the mantle by subduction.

Petrogenetic link between amphibolites and the banded iron formation of the Yishui region in the North China Craton: implications for Neoarchean plume tectonics

ABSTRACTAmphibolites have a genetically close relationship with banded iron formations (BIFs) in the North China Craton (NCC). The Yishui amphibolites in the NCC occur interbedded with Algoma–type Yishui BIFs as related wall rocks. A reconstruction of the amphibolite protolith was conducted based on the results of petrologic, mineralogic, and geochemical analyses. The Yishui amphibolites consist of actinolite, ferrohornblende, albite, orthoclase, biotite, quartz, magnetite with minor pyrite, titanite, and ilmenite. Their chemical compositions are mainly SiO2, Fe2O3T, CaO, and Al2O3 with subordinate TiO2, MgO, Na2O, K2O, P2O5, and MnO. The chondrite–normalized rare earth element diagram, characterized by enriched light rare earth elements (La/YbCN = 19.51–24.05) with insignificant Eu and Ce anomalies, shows coherent trends. The primitive mantle–normalized multi–element spider diagram is enriched in large ion lithophile elements, high field strength elements, and light rare earth elements which are related to a mantle source. The results of this study, combined with previous literature data, indicate that the Yishui amphibolite protoliths had intraplate alkaline basalt affinities and were derived from an ocean island basalt–type mantle source with no contamination. The results also suggest that the basalts were primarily the product of small amounts of partial melting. Based on the results, it is considered that a mantle plume model is the most appropriate tectonic model as it better explains the amphibolite geochemical signature. Furthermore, this model can provide crucial information regarding the Archaean NCC tectonic evolution and can demonstrate the temporal and spatial relationships between the BIFs and wall rocks.

Geochemical and Sr–Nd isotopic constraints on the mantle source of Neoproterozoic mafic dikes of the rifted eastern Laurentian margin, north-central Appalachians, USA

Abundant and widely distributed unmetamorphosed mafic dikes intrude Mesoproterozoic rocks of the New Jersey Highlands. The age of the dikes is imprecisely known but interpreted to fall between 615 and 576 Ma, which is consistent with the range of ages of mafic dikes from Labrador and Nova Scotia south to Pennsylvania that were emplaced along the rifted eastern Laurentian margin. New Jersey Highlands dikes are a few cm to 18 m wide and have lengths of as much as several km. They have sharp, largely discordant contacts against enclosing Mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors. Columnar joints are present locally and suggest emplacement at a shallow crustal level. Geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high TiO 2 , P 2 O 5 , Zr, Nb, Y, and La/Yb, and low MgO, Cr, and Ni. TiO 2 contents define high-Ti and low-Ti dikes that differ in high field strength elements (HFSE) and light rare earth elements (LREE) but overlap in abundances of most other elements. Dike magma evolved in an ascending mantle plume of OIB-like asthenosphere from enriched higher TiO 2 compositions to more depleted lower TiO 2 compositions. Subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation. Sr–Nd isotope values of both dike compositions overlap and are characterized by ε Nd (T) of + 1.5 to + 3.8 and initial 87 Sr/ 86 Sr ranging from 0.7032 to 0.7077. Higher Sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous, high 87 Sr/ 86 Sr lithospheric mantle having EMI or EMII-like geochemical characteristics. Dikes form tabular structures that have long segments striking an average of N44°E and short segments striking about east–west. Their regional geometries form right-stepping, rhomb-shaped patterns due to emplacement into rift-related dilational fractures likely formed through a combination of southeast-directed extension and strike-slip shear stresses. Geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event. They, along with other mafic dikes in the north-central Appalachians, were emplaced in a within-plate tectonic setting along the rifted margin of eastern Laurentia, prior to opening of the Iapetus Ocean. • Mantle sources of Neoproterozoic mafic dikes were modeled from Sr–Nd isotopes and geochemistry. • Dikes formed from partial melting of a garnet-free, enriched OIB-like source. • Geochemical similarity to other regional Neoproterozoic dikes along the eastern Laurentian margin • Dikes emplaced from single magmatic event during rifting that formed the Iapetus Ocean

Head-to-tail transition of the Afar mantle plume: Geochemical evidence from a Miocene bimodal basalt–rhyolite succession in the Ethiopian Large Igneous Province

Miocene rhyolites and basalts from the Ethiopian Large Igneous Province (LIP) crop out in north Shewa on the central Ethiopian plateau. The volcanics were emplaced at the onset of development of the Main Ethiopian Rift and postdate eruption of the Oligocene Ethiopian continental-flood basalt (CFB) province by ~ 15 Ma. Variations in major- and trace-element chemistry indicate that fractionation of the north Shewa parental magmas occurred at ~ 15 km depth; this is consistent with geophysical studies which have predicted the presence of gabbroic intrusions in the mid to upper parts of the underlying ~ 50 km thick crust. High-to-moderate [La/Nb] n (1.1–1.4) and Ce/Pb ratios (17–19.5) for the basalts, together with mantle δ 18O values for the rhyolites, suggest that the north Shewa magmas contain only minimal contributions of crustal melts. The formation of the Ethiopian CFB province has been associated with the sub-lithospheric impact of the Afar mantle plume ‘starting-head’ whereas recent volcanic activity in the Main Ethiopian rift has been linked to lithospheric extension and adiabatic decompression melting in the steady-state ‘tail’ of the plume. Rare-earth element (REE) inversion modelling of Oligocene, Miocene and Recent mafic magmas from the Ethiopian LIP has allowed us to determine changes in the amount of partial melting that accompanied the head-to-tail transition of the Afar plume. The results also provide information on the approximate depth of melting and mantle potential temperature and allow us to assess how these have varied both spatially and temporally. The REE inversion results suggest that, since the Oligocene, the amount of adiabatic decompression melting has decreased two-fold, from 13 to 7%, and the potential temperature of the Afar mantle plume has decreased from ~ 1450 to 1350 °C. The ~ 15 Ma north Shewa basalts formed by slightly lower degrees of partial melting (5%) than those in the present-day rift, presumably due to the presence of thicker lithosphere (60 km) prior to the main phase of rifting during which the lithosphere thinned to ~ 50 km at the rift axis.

Partial melting in an upwelling mantle column

Decompression melting of hot upwelling rock in the mantle creates a region of partial melt comprising a porous solid matrix through which magma rises buoyantly. Magma transport and the compensating matrix deformation are commonly described by two-phase compaction models, but melt production is less often incorporated. Melting is driven by the necessity to maintain thermodynamic equilibrium between mineral grains in the partial melt; the position and amount of partial melting that occur are thus thermodynamically determined. We present a consistent model for the ascent of a one-dimensional column of rock and provide solutions that reveal where and how much partial melting occurs, the positions of the boundaries of the partial melt being determined by conserving energy across them. Thermodynamic equilibrium of the boundary between partial melt and the solid lithosphere requires a boundary condition on the effective pressure (solid pressure minus melt pressure), which suggests that large effective stresses, and hence fracture, are likely to occur near the base of the lithosphere. Matrix compaction, melt separation and temperature in the partially molten region are all dependent on the effective pressure, a fact that can lead to interesting oscillatory boundary-layer structures.

Petrotectonic evolution and melt modeling of the Penon Blanco arc, central Sierra Nevada foothills, California

The Penon Blanco arc of the Jurassic-Triassic arc belt in central California is composed of the Jasper Point Formation, Penon Blanco Formation, and coeval Don Pedro intrusive suite, all exposed in the core of the Cotton Creek anticline. The Jasper Point Formation consists of ∼900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts. It passes upward into the Penon Blanco Formation, which is made up of ∼700 m of crystal-lithic basaltic tuff, 1–3.5 km of augite-rich volcaniclastic rocks, and up to 3.5 km of massive to brecciated flows of augite-phyric basalt. The Penon Blanco Formation is paraconformably overlain by the Oxfordian-Kimmeridgian Mariposa Formation, which provides a minimum age of juxtaposition for the Penon Blanco arc against the inboard Calaveras Complex. New geochemical data from the Penon Blanco arc show that the two volcanic suites are geochemically distinct. Jasper Point basalts are tholeiitic and are characterized by high large ion lithophile element (LILE) abundances, moderate high field strength element (HFSE) and heavy rare earth element (HREE) abundances, and low Ti/V ratios. Penon Blanco basalts are calc-alkaline, have higher LILE abundances, lower HFSE and HREE abundances, and lower Ti/V ratios. Geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel-bearing mantle source at 30–60 km depth by low to moderate amounts of partial melting (∼3%–5% for Jasper Point and 5%–7.5% for Penon Blanco). The geochemical modeling and field data suggest that the Jasper Point basalts are similar to normal mid-ocean-ridge basalt (N-MORB) and were associated with forearc rifting, while the Penon Blanco basalts represent the transition to arc volcanism and head-on subduction. This model is consistent with interpretations for a single ensimatic arc on the Jurassic margin of North America.

Volcanism and volatile recycling on a one‐plate planet: Applications to Venus

The surface of Venus displays volcanic features indicating eruption of lavas with a wide range of viscosities. We present numerical experiments showing that lithospheric gravitational instabilities can produce lavas with compositions consistent with the range of volcanic forms seen on Venus. The presence of incompatible elements and their oxides (specifically, water, carbon dioxide, and alkali elements) in trace‐ to percent‐level concentrations in the Venusian mantle allows the formation of a variety of magmatic source regions. The pressure and temperature paths that the dense lithospheric materials travel as they sink into the Venusian mantle indicate that the lithospheric material may devolatilize as it sinks, enriching the surrounding upper mantle, or it may itself melt. These processes can produce magmas with a variety of compositions and viscosities, potentially consistent with the range of Venusian volcanic forms. These processes also suggest that Venus may recycle incompatible elements internally. Indeed, if Venus began with an internal volatile content, then no amount of partial melting can make it entirely volatile‐free even in the absence of recycling into the interior. These models therefore suggest geodynamic processes that can produce a range of magmatic activity and retain some interior volatiles on a one‐plate planet.

Velocity–density relationships and modeling the lithospheric density variations of the Kenya Rift

We have devised pressure-dependent and temperature-dependent linear velocity–density relationships from the crustal rocks in the Kenya Rift and applied the relationships to the crustal velocity model derived from the KRISP90 refraction experiment to compute densities and gravity anomalies. Our results indicate that pressure and temperature considerations reduce the misfit between the observed and the calculated gravity anomalies in the areas of large lateral pressure and temperature variations. Using a separate relationship involving velocity and density contrasts for the mantle of the Kenya Rift, the KRISP teleseismic velocity model, and the variation of the observed gravity anomalies, we derive a d ρ/d V p value of ∼0.05 (Mg s m −3 km −1) which results in the density contrast of ∼0.03 Mg m −3 at ∼100 km depth beneath the rift and its flanks. Using the magnitude of the velocity and density contrasts, we conclude that the mantle at the depth of 100 km beneath the rift could be ∼200°C to 375°C hotter than the adjacent lithosphere (and much of it is considered subsolidus) and contains partial melt up to 2.5%. The amount of partial melt in the region of the Moho upwarp would be greater because both velocities and densities are lower in that region. Our results are consistent with the upwarping of the thermal boundary layer representing the lithosphere–asthenosphere boundary and the consequent partial melting of the lithosphere.

ORIGIN OF PLAGIOCLASE LHERZOLITE FROM THE NIKANBETSU PERIDOTITE COMPLEX, HOKKAIDO, NORTHERN JAPAN

ORIGIN OF PLAGIOCLASE LHERZOLITE FROM THE NIKANBETSU PERIDOTITE COMPLEX, HOKKAIDO, NORTHERN JAPAN

Controlled partial melting during isobaric and isothermal processing of dipcoated Bi-2212/Ag tapes

A detailed study has been made of the control and optimization of partial melting of dipcoated Bi2Sr2Ca1Cu2O8+δAg0.1 (Bi-2212) tapes using reduced oxygen partial pressures. A coulometric titration technique has been employed to vary the oxygen partial pressure in a region of the phase diagram corresponding to binary melting, and the amount of partial melting has been quantified. Using this information, tapes have been processed using both isothermal and isobaric techniques. An optimum processing route was determined which combined isothermal and isobaric processes. Highly aligned material at the point of optimum melting was obtained.

Experimental constraints on differentiation and H2O abundance of calc-alkaline magmas

Partial melting experiments were conducted on a natural high-alumina basalt from the Higashi-Izu volcanoes (47.4 wt.% SiO2; 8.3 wt.% MgO) with 1 and 2 wt.% total H2O at 0.5 and 1 GPa, 1125-875°C. The partial melts produced segregate by capillary action into crimped parts of the capsule, preventing chemical modification of the melt during quenching. Liquid compositions in 1 wt.% total H2O runs at 0.5 GPa mimic the chemical variations of calc-alkaline andesites and dacites associated with the basalt. The residual phase assemblages from 1 wt.% total H2O experiments are consistent with the observed phenocrysts in two pyroxene andesite and hornblende (hb)-orthopyroxene (opx) dacite. In contrast, a clinopyroxene (cpx)-hb rhyolite assemblage is produced in the 2 wt.% total H2O experiments. Although no such cpx-hb rhyolite is found in either Northeast Japan or the Izu-Mariana arc, this type of rhyolite is found in Central America, which is also characterized by high H2O contents in the associated basaltic melts ([1]).The abundance of SiO2 in residual liquids increased from 49 to 54 wt.% in the 25°C interval between 1125° and 1100°C in the 1 wt.% total H2O system, and from 63 to 71 wt.% in the 50°C interval between 975° and 925°C. The first gap between basalt and andesite is generated by a sudden change in the amount of partial melt, and the second gap between andesite and dacite is produced by the formation of hornblende by the consumption of pyroxene, because the SiO2 abundance of hornblende is less than that of pyroxene. These two reaction relations result in the existence of two compositional gaps, between basalt and andesite, and between andesite and dacite, which are recognized in the Higashi-Izu volcano group and many other calc-alkaline volcanoes.

Seismic constraints on a model of partial melts under ridge axes

In a recent global scale seismic study, the correlation between S wave velocity under ridge axes and spreading rate was pointed out. The correlation is strong for depths to about 70 km, but it diminishes below this depth. We present the correlation plots at four depths, 38, 66, 90, and 110 km, for which correlation is strong at 38 and 66 km but is weak at 90 km and is almost nonexistent at 110 km. We present a model to explain this behavior, which includes a thermal conduction model for the development of lithosphere and a simple melt percolation. Thermal effects on S wave velocity are assumed to be accounted for entirely by the plate cooling (thermal conduction) model. We point out that the thermal model under this assumption predicts asymptotically no correlation between S wave velocity and spreading rate, specifically for spreading rate larger than about 3 cm yr−1. This contradicts the correlation observed in the data at shallow depths. The existence of partial melt is thus required to explain the observed behavior at 38 and 66 km depths. We start from four basic equations that govern the distribution of partial melt and derive the relation between the amount of partial melt and the spreading rate. We adopt a simple power law relation between permeability (k) and porosity (ƒ) by k(ƒ) = k0ƒn, where k0 and n are constants and assume that pores are filled with melt. We then set up an integral relation between S wave velocity and spreading rate. The final formula indicates that the gradient in the correlation plots is the inverse of the power (1/n) in the permeability‐porosity relation, thus enabling us to constrain n as well as k0 from seismic data. The data also have some sensitivity to the depth to solidus. We show that (1) the depth to solidus is probably within the range 60–100 km and (2) if the power n is n = 2–3, then k0 = 10−8 ‐ 10−10 m2. These parameters predict that porosity and fluid velocity are 1–2% and about 0.5 m yr−1, respectively. The depth to solidus is consistent with previous estimates by petrological data but is perhaps the first and direct seismological evidence of partial melt from surface wave data. Analytical forms for the dependence on depth and spreading rate of porosity, fluid velocity within permeable rocks, and ascent times of magma are also obtained.

Core formation in asteroids

Chemical arguments based on the S concentrations in the metallic magmas that formed magmatic iron meteorites and on the lack of pyroxene in pallasites suggest that core formation in asteroids was accompanied by >40% melting of the associated silicates. The existence of plagioclase‐free residues from partial melting, such as lodranite meteorites, indicates that metal did not segregate from silicates when the amount of partial melting was only 15%. Thus, core formation in asteroids requires substantial melting of an asteroid. High interfacial energies between metallic melts and silicates require that metallic spherules sink through partially molten silicates to form cores in asteroids and planetesimals. The globule‐sinking mechanism is not very efficient because silicate crystal‐liquid mixtures have high yield strengths, making it difficult for the globules to sink. Calculations suggest that the melt fraction must reach >50% before globules can sink. Alternatively, porous flow of molten metal is possible when the amount of metal is high enough to form an interconnected network, about 50%; this can be achieved by high initial abundances of metal or by gradual removal of silicate melt, thereby enriching the residue in the metal‐sulfide assemblage. This also requires removal of 50% silicate partial melt, though not necessarily all at once. Such high amounts of melting were not always attained in asteroids, so many asteroids probably consist of partially differentiated silicates and metallic masses that did not segregate to a core. S asteroids might represent such partially differentiated bodies.

The geology and geochronology of a Proterozoic trachyandesite plug, Murchison Province, Yilgarn Block, Western Australia

A discrete, roughly circular, steep‐sided magnetic anomaly outlines a buried trachyandesite plug at Gearless Well in the Murchison Province of the Yilgarn Block. Agglomeratic texture indicates high level emplacement and probable venting. The mildly alkaline, silica‐saturated, heterogeneous trachyandesite is markedly alumina‐deficient and enriched in large ion lithophile elements and, to a lesser extent, high field strength elements. A seven‐point whole rock linear alignment on the Rb/Sr isochron diagram of samples from one drill hole is interpreted as a two‐component mixing line of meaningless age. Samples from another drill hole indicate the presence of a third component. Two two‐point biotite‐whole‐rock measurements yield similar ages which pool to give 2188 ± 11 Ma, the estimated time of emplacement of the trachyandesite plug. Samples defining the seven‐point line were produced by small to moderate amounts of partial melting of LIL‐enriched (large ion lithophile elements) upper mantle, and the line is ...

Rare earth abundances and origin of some Indian lamprophyres

SummaryAbundances of rare-earth elements (REE) and Th, Ta, Hf, Sc, Co, Ba determined by instrumental neutron activation analysis show similarity between lamprophyres and other potassic rocks including kimberlite. The Indian lamprophyres have fractionated REE patterns with enrichment of light REE. The light REEs show more variation than the heavy REEs within the group.Lamprophyres occurring in intimate association with the alkaline basalt may be genetically related. The trace element characteristics of the abundant lamprophyre dykes in granitic terrain are unlikely to be generated by assimilation of alkaline basalt by granitic crust. Small amounts of partial melting in mantle pockets enriched in incompatible elements as a result of metasomatic processes would explain the elemental concentration observed in the lamprophyres.

Geochemical features of some Archaean and post-Archaean high-magnesian-low-alkali liquids

High-magnesian-low-alkali liquids are found as mafic lavas ranging in age from Archaean to Gainozoic. The most magnesian lavas are represented by Archaean spinifex textured peridotitic komatiites, and in this study these liquids are used as a comparative base for younger, less magnesian liquids. The post-Archaean lavas fall into three categories: (1) the Gape Smith (Proterozoic) - Baffin Bay (Gainozoic) group, (2) the low-Ti ophiolitic basalts of Cyprus, which represent remelting of a sequentially depleted source, and (3) the boninite group, which are the products of (wet?) melting of a source that had previously experienced depletion and addition of incompatible element enriched phases. With the use of parameters such as Al 2 O 3 /TiO 2 , Sc/Zr, Ti/V, a comparison of Archaean komatiites with the younger high magnesian lavas indicates that the bulk of the variation seen in these rocks types can be interpreted in terms of the amount of partial melting and nature of residual phases. However, some of the variability that occurs within individual lava provinces (particularly among the light rare earth elements) is best explained by a heterogeneity superimposed on a previously homogeneous source. The abundance of high-magnesian liquids declines sharply after the Archaean as does the maximum MgO content achieved by the lavas.

Partial Melting and Conductivity Anomalies in the Upper Mantle

INVESTIGATIONS with arrays of magnetometers recording geomagnetic variation fields have shown the presence of large anomalies in electrical conductivity under the western United States1-3. Under the Basin and Range Province and under the Southern Rocky Mountains of Colorado, the regions of enhanced conductivity are in the upper mantle. The depth is poorly defined by the magnetometer array data, but is between 50 and 400 km4,5. At about 400 km the mantle conductivity rises steeply with increasing depth in the Earth as a whole6-7. Conductive structures include an elongated ridge on the surface of the conductive mantle (Southern Rockies) and a step-plus-ridge (Basin and Range). The conductivity structure is strongly correlated with heat flow8·9 and with velocities and absorption of seismic waves10. The regional variations of P and S velocities in the upper mantle beneath North America can best be understood11 in terms of a small amount of partial melting in the low velocity zone, at depths between 50 and 150 km, under the western region.