Showing papers on "Partial melting published in 1975"

Lead Isotope Relations in Oceanic Ridge Basalts from the Juan de Fuca-Gorda Ridge Area, N.E. Pacific Ocean

Abstract: Lead isotopic analyses of a suite of basaltic rocks from the Juan de Fuca-Gorda Ridge and nearby seamounts confirm an isotopically heterogeneous mantle known since 1966. The process of mixing during partial melting of a heterogeneous mantle necessarily produces linear data arrays that can be interpreted as secondary isochrons. Moreover, the position of the entire lead isotope array, with respect to the geochron, requires that U/Pb and Th/Pb values are progressively increased over the age of the earth. Partial melting theory also dictates analogous behavior for the other incompatible trace elements. This process explains not only the LIL element character of MOR basalts, but also duplicates the spread of radiogenic lead data collected from alkali-rich oceanic basalts. This dynamic, open-system model of lead isotopic and chemical evolution of the mantle is believed to be the direct result of tectonic flow and convective overturn within the mantle and is compatible with geophysical models of a dynamic earth.

Flows of impact melt at lunar craters

Abstract: Lavalike materials that were emplaced in a fluid state occur in and around lunar impact craters whose diameters range from 3 km to more than 200 km and whose ages span a time interval of at least 3.5 billion years. The distribution of the lavalike deposits conforms to asymmetries of other ejecta from the same craters, and the material is concentrated downrange to distances as great as a crater radius. The character and distribution of the lavalike materials support the idea that they formed by impact melting rather than by volcanism. Returned samples indicate that materials with appropriate physical characteristics are generated by partial melting of feldspathic rocks by impact. The geologic evidence at lunar craters suggests that there is more melt rock in and near the craters than is predicted by experiment and theory.

Rare earth element concentrations in a suite of basanitoids and alkali olivine basalts from Grenada, Lesser Antilles

Abstract: A suite of basanitoids and alkali olivine basalts from Grenada, Lesser Antilles were analyzed for rare earth elements. The REE concentrations of these rocks are characterized by a small variation in the heavy REE (7 to 9 times chondrite) and a large variation in the light REE (17 to 93 times chondrite). Among the possible mechanisms to account for the REE variations, fractional crystallization processes at low and high pressures, and partial melting processes (both batch melting and fractional melting) were examined, using the partition relationships of REE among silicate minerals and melts. It is suggested that the observed REE variations are best explained by variable degrees of batch partial melting, in which garnet is present as one of the solid phases through 2 to 17% melting of a garnet lherzolite parent rock.

Rare earth elements in alpine peridotites

Abstract: The samples from alpine peridotite massifs (Beni Bouchera, Lherz and the Alps) have been analyzed for rare earth elements. The peridotites as a whole are characterized by various degrees of light REE depletion (Ce varies from 1.2 to 0.02 times chondrite) and a small variation in heavy REE (Yb varies about a factor of 2, from 1.3 to 2.2 times chondrite). They show a restricted and regular distribution in a Ce-Yb diagram, giving two types of linear trends for individual massifs (trend A for the Alps and Lizard; trend B for Beni Bouchera and Lherz, branching from trend A). The model calculations of partial melting based on the partition relations of REE among silicate minerals and melts suggest that trend A could represent a series of residua left after partial melting of garnet peridotite despite the fact that there is no garnet observed in the peridotites studied here. It is suggested that trend A would represent a melting event which predated the emplacement of the massifs and occurred at higher pressure (in the presence of garnet) than expected from the present mineralogy. The calculations also suggest that trend B could represent a partial melting event at lower pressures than trend A after the massifs uplifted into spinel peridotite field. It is also suggested that the REE concentrations of the mantle could be estimated as 2–2.5 times chondrite.

A cordierite-bearing granite suite from the New England Batholith, N.S.W., Australia

Abstract: A suite of cordierite-bearing biotite-muscovite intrusive granites in the New England Batholith, New South Wales, outcrops over 3400 sq km and is the largest reported occurrence of granites of this type. Compositionally the granites are close to the low temperature minimum and display only limited chemical variation. The corundum-normative nature of the granites suggests a pelitic sedimentary parentage. Cordierite with an average 100 Mg/ Mg + Fe of 55 occurs as large tabular crystals and is considered to be a refractory phase brought up from the zone of partial melting. The presence of cordierite and the absence of garnet in these granites suggests a partial melting at a pressure maximum of 6 kb, equivalent to 22 km depth.

Petrology of submarine basalts from the central Caribbean: DSDP Leg 15

Abstract: Basalts recovered during leg 15 of the Deep Sea Drilling Project in the central Caribbean are finegrained plagioclase tholeiites which were erupted in the Late Cretaceous, at a time when North America was shifting from South America and Africa, and emplaced as thin flows and sills near the water-sediment interface. The topmost of these units is responsible for seismic horizon B″, and gradually increasing P wave velocities with depth are due to increasing proportions of basaltic rocks relative to interlayered sediments. Basalts from these sites have similar subalkaline characteristics and contain normative hypersthene and 5–15% normative olivine. The major element character of the basalts indicates that they have evolved from more primitive compositions, probably through olivine fractionation. Intersite variations can be explained largely through both olivine and plagioclase fractionation from similar parental liquids. However, olivine is probably not on the liquidus at any pressure for anhydrous melts of these compositions. A small amount of water in the peridotitic source regions may result in olivine being a near-liquidus phase at pressures of 5–8 kbar. Minor element (e.g., TiO2) variations which cannot be accounted for through crystal fractionation reflect variations in the primary liquid compositions which are probably a consequence of degree of partial melting. Plagioclase was the low-pressure liquidus phase in all the samples examined but was quickly followed in the paragenetic sequence by calcic clinopyroxene and finally titanomagnetite. Olivine or pseudomorphs after olivine are not observed. Intratelluric crystals, predominantly calcic plagioclase but more rarely clinopyroxene, can be distinguished by their minor element characteristics. Metastable crystallization trends, the lack of pyroxene exsolution, and the presence of interstitial glass (now altered) attest to the rapid cooling of these liquids. Two distinct REE patterns (LREE (light REE) enriched and LREE depleted, the latter being typical of the majority of the samples) require chemically contrasting upper mantle source regions during the Late Cretaceous in the Caribbean Basin. Restrictions on the permissible degrees of partial melting of a garnet-bearing source preclude derivation of the LREE-enriched basalts in a one-stage process, and it is suggested that the source region acquired a LREE-enriched character prior to the generation of the liquids through 20–30% partial melting. The LREE-depleted liquids were derived from slightly deeper source regions (70–100 km) previously depleted in the LREE. Contrasting tectonic regimes required to produce these REE characteristics conflict with present thinking on the style of Caribbean submarine volcanism.

Geochemistry of upper cretaceous volcanic rocks from the pontic chain, northern turkey

Abstract: Preliminary data on major elements, Cs, Ba, Rb, Pb, Sr, REE, Y, Th, U, Zr, Ht, Sn, Nb, W, Mo, Cr, V, Sc, Ni, Co and Cu contents for eight samples coming from the Upper Cretaceous volcanic belt of the Pontic Chain (Northern Turkey) are reported. SiO, versus K2O relationship shows that the analyzed samples belong to the calc-alkaline and shoshonite series. The calc-alkaline rocks appear to represent two distinct magma types one close in composition to typical island are calc-alkaline magmas and one with high incompatible elements concentration and tractionated heavy REE patterns which suggest a genesis by partial melting at high pressure with a garnet bearing residue. Shoshonitic rocks show Na2O/K2O close to one, high incompatible elements concentration, and TiO2%. Al2O3%, Ni and Co contents, Ni/Co and V/Ni ratios and REE patterns similar to typical island are andesites which suggest for these rocks similar genetical processes as the island are calc-alkaline magmas.

Rare earth element geochemistry of the Fen alkaline complex, Norway

Abstract: Determination of rare earth element (REE) abundances in rocks of the Fen complex has shown that within rocks of the first magmatic series REE abundances increase in the order urtite

Mineral chemistry of peridotite xenoliths from the Lashaine volcano, Tanzania

Abstract: Eight garnet peridotite and ten garnet-free peridotite xenoliths from the Lashaine volcano in northern Tanzania, and one garnet peridotite from the Matsoku pipe in Lesotho, were studied petrographically and by electron-microprobe techniques. The primary assemblages are ol + opx + cpx + ga, ol + opx + cpx + chr, ol + opx + ga + chr, ol + opx + ga, ol + opx + chr, and ol + cpx + chr. With the exception of the ol + cpx + chr assemblage, which is more iron-rich, the composition of each of the primary mineral species is essentially the same for all the xenoliths and is independent of the abundances of the minerals. The primary minerals are Ni-rich very low Ca olivine (Fo92), low Ca, Al, Cr, Ti, Mn,enstatite (Wo1En93Fs7), low Al, Ti, Mn chrome diopside (WO44En52Fs4), chrome pyrope and Mg, Al-rich chromite. Comparisons with phase equilibria studies suggest that the primary assemblages are stable at approximately 1050°C and 50 kbar, equivalent to a depth of approximately 150 km. The primary xenolith assemblages have been locally altered by three types of secondary processes. (1) Garnet has reacted with the adjacent olivine to produce reaction rims of aluminous orthopyroxene plus aluminous clinopyroxene plus spinel. (2) Some chrome-diopsides show marginal or total alteration that may result from secondary melting. (3) Extremely localized partial melting has occurred at some grain boundaries and the interstitial melt quenched to various combinations of olivine, clinopyroxene, orthopyroxene, spinel, phlogopite and glass. Garnet-bearing and garnet-free assemblages differ in bulk composition but have formed under similar physical conditions. The xenoliths derive from a region of the upper mantle that is homogeneous in terms of mineral compositions but heterogeneous, at least on the scale of the Lashaine xenoliths, in mineral proportions and bulk composition. Phlogopite in one garnet peridotite appears to be primary and several peridotites contain regions that have formed by localized melting of K-rich areas. Either the peridotites are not refractory residues from partial melting in the mantle or K-rich material has been added to them after partial melting but within the mantle, before incorporation into the ankaramite host.

Rare earth element geochemistry of kimberlite

Abstract: Abundances of rare earth elements (REE), Sc, Co, Hf, Ta and Th have been determined by neutron activation analysis in kimberlites from the Wesselton pipe, in micaceous kimberlites from the Swartruggens fissure and kimberlite from the Monastery Mine, Ison Creek and Somerset Island pipes. Kimberlites are characterized by a high REE content and have chondrite normalized REE distribution patterns which show extreme fractionation of the REE group, e.g. kimberlite La Yb = 100 ; micaceous kimberlite La Yb = 140 . Distribution patterns are linear, with weak negative Eu anomalies evident in the micaceous types. It is considered that the major sites of REE are apatite, perovskite and carbonate and that Eu anomalies are due to the presence of perovskite or mica. REE distributions are analysed in terms of partial melting and eclogite crystallization models by means of REE crystal-liquid distribution coefficients. Partial melting models indicate that the La Yb ratios of kimberlites and micaceous kimberlites can be produced independently by differing amounts of partial melting ( La Yb ratio (ca. 5) which was produced by extensive (15–20%) partial melting of garnet lherzolite mantle. REE distributions do not provide evidence in favour of any one petrogenetic model, especially with regard to La and Yb abundances; this is considered to be a reflection of errors in the magnitude of the distribution coefficients. Evidence bearing on the possibility of eclogite crystallization from kimberlitic magmas is reviewed (i.e. eclogite distribution, age, mineralogy of kimberlite and of inclusions in diamonds) and is interpreted to indicate that eclogite fractionation is the least likely means of generating kimberlite liquids. The relation of kimberlite magmatism to continental basaltic magmatism is considered in terms of a partial melting model in which the extent of partial melting of the mantle is dependent upon heat flow variations with time. The small volumes of liquid required in partial melting hypotheses are thought to be concentrated into kimberlitic magmas by shearing processes.

The Crust and Upper Mantle of Central Mexico

Abstract: Summary Six strain and inertial seismographs at Queen Creek, Arizona (QC-AZ), recorded six modes of surface waves from a suite of 18 earthquakes near Chiapas, Mexico. The six group-velocity dispersion curves were used in a least-squares inversion to estimate the shear velocity structure of the upper 380km of crust and mantle in central Mexico. Multiple filter analysis of 108 seismograms produced group velocity dispersion curves slower than average continental paths. The range of the average deviation from the mean dispersion curve for the fundamental modes was 0.025 to 0-160 kni s-' for Rayleigh waves and 0.025-0.281 km s-l for Love waves. The inversion models have low velocities that correspond with a representative geotherm and petrologic P-Tdiagrams to indicate partial melting. The 4layer crust is 30 km thick with a high-temperature gradient LVZ in the granitic layer and with a LVZ in the lower 8 km of a basaltic layer resulting from a high geothermal gradient or from partial melting of water saturated rock. The mantle has a 4-8 km thick solid lid and a shallow low velocity zone. The lowest velocities correspond to 10-20 per cent partial melting. A sharp velocity gradient at 70-80 km probably results from both the phase change to garnet pyrolite and the lower extreme of partial water pressure with the disappearance of amphiboles from the host pyrolite. Based on the velocities, 5 per cent anhydrous melting extends to 260 km. From 300 to 380 km temperature gradients and crystal lattice instabilities prior to the olivine-spinel phase change produce another LVZ. A hypothesis is presented that the large volume of low density magma produces a regional vertical force that creates high Aat plateaus as found in central Mexico and the Colorado Plateau. The inversion models have a 7.5s S-wave residual relative to the Canadian Shield model CANSD in agreement with observed US station anomalies.

Chemical Variation Among French Ultramafic Xenoliths—Evidence for a Heterogeneous Upper Mantle

Abstract: Some 200 ultramafic xenoliths and their basaltic hosts from five French localities were studied. New analyses are presented, which show the five host-rocks to be nepheline- and olivine-normative. Seven bulk analyses of xenoliths from four localities, together with analyses of their constituent diopsides and, for six, of their orthopyroxenes, are also presented. Xenoliths from four occurrences appear to have equilibrated at pressures between about 8 to 18 kb at sub-basaltic solidus temperatures. Suites of xenoliths are chemically different. Histograms were used to determine compositions of depleted and ‘undepleted’ upper mantle. A suite of peridotite xenoliths from the Bult-fontein kimberlite pipe is no less depleted in fusible oxides than xenoliths from two French localities. ‘Undepleted’ upper mantle is very similar to ‘pyrolite’ in composition, except that the latter has much higher TiO2, Na2O, and K2O contents. No xenolith encountered in this work has a bulk composition that could yield more than 12% oceanic tholeiite on partial melting.

On the origin of high-Ti mare basalts

Abstract: Analyses were conducted of sixteen Apollo 17 mare basalts for alkali, alkaline-earth, and rare-earth elements, Co, and Sc. The obtained data were utilized in a study concerning the nature of the igneous processes responsible for the chemical variations among the high-Ti, low-K basalts. Estimates were obtained regarding the abundances of the large-ion, lithophile elements in the source regions for the basalts. It is shown that the source regions could plausibly have been produced by processes believed to have occurred during the early history of the moon. Attention is given to chemical variations among the Apollo 17 mare basalts, near-surface (low-pressure) crystal fractionation, partial melting, limits on the extent of partial melting, and a summary of possible events leading to formation of high-Ti mare basalts.

Chemical and isotopic constraints on the origin of low-silica latite and andesite from the Andes of central Peru

Abstract: Low-silica latite highly enriched in large-ion-lithophile elements and moderately potassic low-Si andesite were erupted in central Peru during late Cenozoic time. FeO*/MgO ratios of 0.93 to 1.25 at 53 wt percent SiO/sub 2/ indicate a definitely calcalkalic character. The combination of low FeO*/MgO ratios, low SiO/sub 2/, and high Cr, Co, and Ni with large-ion-lithophile and light rare-earth elements makes it very unlikely that the parent magmas were produced by high-pressure partial melting of subducted ocean-floor basalt (eclogite). The data are more compatible with small degrees of partial melting of ultramafic material or mixtures of basalt and ultramafic mantle. The initial melts probably contained 52 to 53 percent SiO/sub 2/ and had a content of large-ion-lithophile elements nearly as high as that of the rocks. /sup 87/Sr//sup 86/Sr ratios of from 0.7042 to 0.7051 and low to very low Rb/Sr ratios indicate an isotopically variable source region that, at least in part, had earlier been depleted in Rb relative to Sr.

Implications of poikilitic textures in LL-group chondrites

Abstract: Five lithic fragments with poikilitic textures in five LL-group chondrites are examined by microscope and electron microprobe to determine whether the textures have resulted from processes related to impact events, such as thermal metamorphism or partial melting. The bulk and pyroxene compositions of the fragments are determined. The compositional characteristics of minerals in certain fragments are found to indicate an apparent melt origin. It is concluded that impact processes produced the poikilitic textures and that complete melting, partial melting, and solid-state recrystallization all have had some part in producing the textures.

Two Roberts victor cumulate eclogites and their re-equilibration

Abstract: Microprobe and bulk chemical analyses are presented for two eclogites. Both contain relatively Ca-rich garnets (25–45% grossular) and Na-rich (approximately 50% jadeite) tschermakite-poor clinopyroxenes. One specimen is foliated and layered. The layers consist of garnet-clinopyroxene, garnet-clinopyroxene-(kyanite), where all garnets from overgrowths on kyanite, and garnet-clinopyroxene-kyanite. The garnets of the latter layer are richer in Ca and poorer in Mg than those of the other layers though within these layers a cryptic mineral variation is present. Some garnets of the garnet-clinopyroxene layer are zoned. The second specimen is a graphite eclogite. The garnet overgrowths between kyanite and clinopyroxene are considered as part of a general closed system reaction cpx1 ± gt1 ± ky → cpx2 + gt2 ± ky. The composition of cpx1 for the overgrowth layer is calculated from modal and mineral compositions. The garnet-clinopyroxene layer could have been formed from cpx1 and smaller amounts of garnet of rather similar composition. The garnet-clinopyroxene-kyanite layer probably contained different clinopyroxenes and garnets. Textures, mineral compositions and calculated mineral compositions, distribution functions, and appropriate experimental equilibrium curves, allow three stages in the T-P evolution of these rocks to be traced. They were probably formed at 1400–1500°C, 26–28 kbar by cumulate processes from an evolved high P magma. They were subjected to tectonic processes then equilibrating at 750–900°C and 28–32 kbar. The third stage of evolution, considered to represent reaction, partial melting and chemical exchange with an ascending kimberlite magma or fluid, is provisionally assessed at 1000–1100°C, 8–12 kbar.

Rubidium–strontium ages and strontium isotope ratios from the igneous rocks of Leicestershire

Abstract: The south Leicestershire diorites and the Charnwood southern diorites give Rb-Sr isochron ages of 546 ± 22 m.y. and 552 ± 58 m.y., respectively, indicating a late Precambrian to Cambrian age of emplacement. Their respective initial 87 Sr/ 86 Sr ratios of 0.7056 ± 0.0001 and 0.7061 ± 0.0008 agree with analytical error. Despite the large error produced by a small spread of Rb/Sr values an age of 311 ± 92 m.y. for the Charnwood northern diorites indicates that the group is younger than previously thought. The initial 87 Sr/ 86 Sr ratio is 0.7068 ± 0.0004. The Mountsorrel Complex gives an age of 433 ± 17 m.y. and an initial 87 Sr/ 86 Sr ratio of 0.7070 ± 0.0002. This confirms a late Ordovician to early Silurian age for the intrusion. An isochron for the Charnwood porphyroids is not presented because of an extremely small spread at Rb/Sr values. However initial 87 Sr/ 86 Sr ratios are calculated for the group using likely ages between 500 and 700 m.y. The values obtained, 0.7046–0.7049, are significantly lower than any of the other rock groups studied. The initial ratios of the porphyroids, diorites and granodiorites rule out melting or partial melting of ancient high level crustal rocks as a source. Values such as those determined could have been produced by the melting or partial melting of lower crustal and mantle rocks.

Upper-mantle diapirism: Evidence from analogous features in alpine peridotite and ultramafic inclusions in basalt

Abstract: Ultramafic xenoliths in basalt from the western United States are dominantly spinel Iherzolite in which at least four types of gabbroic and pyroxene-rich bands have formed. These bands closely resemble compositional bands in some alpine peridotite, and we infer a similar mode of origin: partial fusion over a range of T-P conditions during diapiric rise of a part of the upper mantle. The sequence of formation of bands, established by crosscutting relationships, is appropriate to crystallization of the products of melting in spinel Iherzolite that has risen from a level of spinel- and garnet-pyroxenite stability in the upper mantle to the lower crust where gabbroids are stable. Both the textures and compositions of the bands have been modified by repeated deformation and partial melting.

29 – light element metasomatism of the continental mantle: the evidence and the consequences

Abstract: The highly potassic volcanics of South West Uganda and the sodi-potassic volcanics of the West Eifel, Germany, represent two distinctive aspects of the highly alkaline volcanism characteristic of uplifted and rifted continental cratons. The sparse lavas are feldspathoidal clinopyroxenites and the associated explosive volcanism provides highly typical ultramafic nodules composed of dark mica + clinopyroxene ± amphibole with titano-magnetite, sphene and apatite. The Eifel suite also includes lherzolites, harzburgites, wehrlites and dunites. Metasomatic textures, plus a mineralogical and chemical transition between spinel lherzolite and alkali clino-pyroxenite, suggest that mica clinopyroxenite nodules represent altered mantle. The existence of metasomatized mantle implied by these nodules is consistent with the nature of the magmatism and its geological context. Calculations show that alteration of anhydrous peridotite mantle to hydrous alkali clinopyroxenite would provide the density contrast required of the anomalous low-density mantle below these regions, producing the observed, geologically long-persistent uplifts. It is proposed that the metasomatism occurs during upward passage of mobile elements through the mantle, accompanying the volatile emission so evident in regions of alkaline volcanism. Close affinities between lavas and nodule suites suggest that the lavas are derived by partial melting of the metasomatized mantle. Highly potassic lavas are generated where the continental geothermal gradient is low, from phlogopite-rich mantle below the level of amphibole stability: sodi-potassic lavas are formed at higher levels, when amphibole-bearing mantle becomes involved in melting. In the oceans highly potassic lavas are not recorded. This accords with the present hypothesis, because under conditions of steep oceanic geotherms phlogopite will persist to depths only slightly greater than that of amphibole: thus there will be very little opportunity for a layer of potassium-enriched mantle to develop, except possibly in the older and cooler parts of the oceanic lithosphere, near the margins.

The Origin of Garnet-Cordierite-Sillimanite Bearing Rocks from Chandos Township, Ontario

Abstract: Garnet-cordierite-sillimanite bearing rocks from the contact aureole of the Precambrian Loon Lake pluton in Chandos Township, southeastern Ontario were analyzed for the major and rare-earth elements. In comparison with the associated Apsley biotite gneisses, they are rich in Al, Mg, and Fe, low in Si, Na, and K and their REE distribution patterns show a depletion of light REE with a negative Eu anomaly. These rocks are probably residuum left after partial melting of biotite gneiss. Leucogranite associated with the GCS rocks may represent the extracted anatectic material. It is suggested that some of the garnet cordierite-sillimanite gneisses which frequently occur in high-grade regionally metamorphosed areas of the Grenville Province of the Canadian Shield may also be of a similar residual origin as proposed by Lal and Moorhouse (1969).

The Role of Partial Melting of Metasediments in the Formation of the Anorthosite-Norite-Syenite Complex, Laramie Range, Wyoming

Abstract: The anorthosite-norite-syenite complex is interpreted as possible refractory residues from partial melting of various metasediments metamorphosed under granulite facies conditions. Deformation of the crystalline residue is correlated with the extensive crushing granulation and brecciation in the residual plagioclase crystals in the anorthosite. The associated younger granitic gneisses of the Sherman Granite complex in contact with the anorthosite and syenite masses could be formed, in part, from the partial melts extracted from the anorthosite-syenite complex. The large volumes of the Sherman Granite require additional sources for the melt. The proposed partial melting of the anorthosite-syenite system may represent only a portion of widespread regional anatexis 1.35 b.y. ago

Petrogenetic implications of uranium abundances in volcanic rocks from southern Peru

Abstract: The content of uranium of andesitic rocks of southern Peru varies according to the distance from the Peru-Chile trench, as has been observed for other trace elements geochemically associated with K. The highest contents found in these rocks may exceed the estimated U content of the upper crust. This enrichment may be related to a variable degree of partial melting and/or crustal contamination of the same source material.