Showing papers on "Partial melting published in 1972"

Water-saturated and undersaturated melting relations in a Paricutin andesite and an estimate of water content in the natural magma

Abstract: Phase relations have been determined for a Paricutin Volcano andesite at pressures to 10 kilobars and for H2O contents in the melt of 2 to 10 weight percent. Runs were made under H2O-saturated and undersaturated conditions. In undersaturated runs a H2O-CO2 fluid phase was always present. Fugacity of H2O in melt, which is directly related to H2O content in the melt, was calculated from thermodynamic data. Plagioclase was found to be the liquidus phase when H2O contents in melt were less than about two percent. With more H2O, orthopyroxene, in some cases joined by olivine, assumes the liquidus. Clinopyroxene crystallizes near the liquidus only for H2O contents greater than five percent. The upper temperature stability limit of hornblende is about 950° C, well below the other silicate liquidi except at H2O-saturated conditions above 5 kb. The geometry of undersaturated liquidi and experimental phase compositions may be compared to the mode and phase compositions of the natural rock. From this comparison, megaphenocrysts of the natural rock are interpreted to have crystallized from a lava which had a water content of 2.2±0.5 percent and a temperature of 1110±40°C. Mass-balance calculations on experimental and natural phase assemblages show that the Paricutin series could not have formed by fractionation at pressures less than 10 kb; rather, it was probably derived by partial melting of subducted basaltic oceanic floor.

Quartz diorites derived by partial melting of eclogite or amphibolite at mantle depths

Abstract: Rare earth elements, Rb, Sr, Ba and K have been determined in tonalite, trondhjemite, dacite, tholeiite and graywacke from the 2700 my old Early Precambrian greenstone-granite terrane of northeastern Minnesota-northwestern Ontario, and also in trondhjemite from the 3550 my old Morton Gneiss, southwestern Minnesota; and the Mesozoic Craggy Peak Pluton, Klamath Mountains, California The Early Precambrian tholeiites have trace element compositions similar to modern oceanic tholeiites, while the quartz dioritic rocks, regardless of age, have total rare earth contents lower than that of tholeiitic basalts, with near chondritic heavy rare earth contents Rb, Sr, Ba and K contents of the quartz diorites are about five times that of oceanic tholeiites, with similar alkali and alkaline earth ratios The Early Precambrian graywacke has a rare earth content intermediate between greenstone and quartz diorite, reflecting its provenance It is proposed that the analyzed quartz dioritic rocks, whether plutonic tonalite, dacite porphyry, gneissic or plutonic trondhjemite, or trondhjemite dikes had similar modes of origin, and were derived by partial melting of amphibolite or eclogite of basaltic or gabbroic composition at depths greater than thirty kilometers, leaving a residue consisting predominantly of garnet and clinopyroxene

Early Precambrian Evidence of a Primitive Ocean Crust and Island Nuclei of Sodic Granite

Abstract: The primordial crust consisted of a stratiform oceanic-type ultrabasic-basic assemblage, relicts of which are retained at the base of Archaean sequences of eastern Transvaal, Rhodesia, and Western Australia. Commonly isochemical metamorphism renders the chemistry of well-retained segments of the meta-igneous rocks significant to their original compositions. Archaean tholeiitic metabasalts comply with the principal criteria for oceanic tholeiites, but tend to have higher Fe/Mg and Mn, and lower Al, Ti, K/Rb, and Fe+ 3 /Fe+ 2 than average recent oceanic tholeiites. These geochemical characteristics can be interpreted in terms of either shallow-level differentiation and (or) a mantle relatively depleted in certain siderophile and transition elements, which possibly indicates a lesser degree of core segregation in the Archean. Metabasalts occurring within basic to acid calc-alkaline volcanic cycles at higher stratigraphic levels than the ultrabasic-basic oceanic assemblages are chemically distinct from metabasalts incorporated in the latter; these rocks have higher K, Sr, Zr, and Y values than recent oceanic tholeiites, and may have originated from high degrees of melting of an underlying oceanic substratum. A model is suggested whereby the evolution from an oceanic crust to greenstone belts proceeded through the rippling of the oceanic crust and the subsidence of linear zones; partial melting below the troughs resulted in the emergence of sodic granites and in cyclic calc-alkaline volcanism. High Ni/Mg and Cr/Mg ratios in the earliest sodic granites support their derivation from the oceanic crust. The granites display cyclic decrease in Na/K, K/Rb, and Sr/ Rb with time, trends which reflect crustal thickening. The plutonism resulted in the superposition of sialic nuclei on early linear structural trends, and in a consequent formation of spatially discrete volcanic-sedimentary depositories, or greenstone belts, in which sequences show an Alpine-type trend of evolution from ophiolites into turbidites and late-stage conglomerates. Notwithstanding this similarity, important differences between Archaean greenstone belts and Alpine or island-arc belts are indicated. Significant similarities exist between this model and the evolutionary pattern of the Fijian archipelago.

Aspects of Magmatic Evolution on Reunion Island

Abstract: As with the Hawaiian islands, the volcanic construction of Reunion can be related to two main phases of activity—a shield-form ing stage of predominantly olivine-basalt composition, and a declining stage comprising more varied products (basalt-trachyte). The older of the two Reunion volcanoes (Piton des Neiges) appears to have completed both these stages, but the younger volcano (Piton de la Fournaise) is still in the shield-forming stage, and is lagging approximately 1.5 Ma* behind Piton des Neiges in its evolution. The chemical data indicate a considerable degree of coherence between the various rock types produced during the different stages of development, and it is concluded that they all stem from essentially the same hypersthene-normative, picritic, primitive magma, generated by partial melting of peridotite in the low-velocity layer of the upper mantle. The shield-forming lavas are believed to represent relatively rapid ascent of this magma, only modified by olivine fractionation, but the declining stage seems to require intermediate-depth fractionation (olivine + pyroxene) to account for its initially nepheline-normative character, followed by high level fractionation (olivine+plagioclase + pyroxene + magnetite, etc.) to produce the hawaiite—mugearite—benmoreite—trachyte sequence. Volume considerations appear to favour an open system of basalt extraction, involving a relatively modest (7 to 15 %) degree of partial melting and continuous replenishment of the mantle source beneath Reunion, rather than a closed system with its restricted basalt potential even if as much as 50 % partial melting is postulated.

Experimental petrology and origin of Fra Mauro rocks and soil

Abstract: Results of melting experiments over the pressure range from 0 to 20 kb on Apollo 14 igneous rocks 14310 and 14072, and on comprehensive fines 14259. It is found that low-pressure crystallization of rocks 14310 and 14072 proceeds as predicted from the textural relationships displayed by thin sections of these rocks. The mineralogy and textures of these rocks are the result of near-surface crystallization. The chemical compositions of these lunar samples all show special relationships to multiply saturated liquids in the system anorthite-forsterite-fayalite-silica at low pressure. Partial melting of a lunar crust consisting largely of plagioclase, low-calcium pyroxene, and olivine, followed by crystal fractionation at the lunar surface, is a satisfactory mechanism for the production of the igneous rocks and soil glasses sampled by Apollo 14. The KREEP component of other lunar soils, may have a similar origin.

Apollo 11 and 12 Mare Basalts and Gabbros: Classification, Compositional Variations, and Possible Petrogenetic Relations

Abstract: On the basis of composition, it is possible to distinguish three major groups of Apollo 12 basaltic rocks: olivine-pigeonite basalts and gabbros, ilmenite-bearing basalts and gabbros, and feldspathic basalts. Two major groups of Apollo 11 basalts are also distinguishable: ophitic ilmenite basalts and intersertal ilmenite basalts. Compositional variations between samples within groups are generally dominated by MgO variations, whereas differences between groups are primarily inverse variations of TiO 2 and SiO 2 or Al 2 O 3 and FeO. Results of fractionation calculations indicate that the MgO variation trends are explained principally by low-pressure fractionation of early-crystallized olivine ± pigeonite ± chrome spinel. The Al 2 O 3 versus FeO trend in the basalts might possibly be explained by near-surface fractionation, but the TiO 2 versus SiO 2 trend is not explainable in this way. Investigations of the latter trend in terms of possible processes of high-pressure fractional melting or fractional crystallization indicate that the compositional variations cannot be the products of simple variations in depth or degree of fractionation. Our data are consistent with the view that the mafic magmas formed by partial melting in the lunar interior, and that near-surface fractionation, with the exception of removal or addition of olivine, has not been extensive.

Some Chemical Data on Members of the Shoshonite Association

Abstract: WHEN attention (Joplin, I965) was drawn to the close resemblance between certain lavas on the south coast of New South Wales, the absarokite-shoshonite-banakite series of Wyoming (Iddings, 1895), and the latites of Sierra Nevada (Ransome, I898), it was suggested that these rocks might be related to the alkali basalts or might even represent a distinct magma-type. A review of world occurrences of shoshonitic rocks (Joplin, I968) has shown that compositions may range from ultramafic absarokites to felsic toscanites and liparites and that there is little iron enrichment. Thus, the shoshonitic rocks have some affinities with the calcalkaline andesitic suite and the suggestion that they may be related to the alkali basalts is wrong. The range in composition from absarokites to liparites suggests that they belong to a differentiation series, but the andesite suite also shows a considerable range in composition and many petrologists consider that members of this suite arise by a complex process involving partial melting rather than by fractional crystallization or by contamination of a basaltic magma (Green and Ringwood, I968; Taylor et al., I969). The shoshonitic rocks may have a somewhat similar origin (Jake~ and White, I969). As the word 'series' seems to imply a differentiation series and it is unlikely that the shoshonitic rocks are the differentiates of a single magma, it is preferable to refer to them as members of the shoshonite association. In I965 it was also pointed out that shoshonitic lavas are associated with regions that were incompletely stabilized or only recently stabilized at the time of their extrusion. This is in harmony with their geographical distribution with regard to the andesites in the island arcs. Shoshonites and monzonites have a very similar composition and because many monzonites are associated with nephelineand melanite-bearing rocks, with or without

Melting of albite at high pressures under conditions

Abstract: The melting of albite has been investigated in the presence of water in the pressure range up to 30 kb. Fixed amounts of water have been contained in the melts so as to satisfy the condition that the melt is saturated with water or PH2O=Pt up to a certain value of pressure, but at higher pressures the melt is undersaturated or PH2O

Melting of albite at high pressures under conditions

Abstract: The melting of albite has been investigated in the presence of water in the pressure range up to 30 kb. Fixed amounts of water have been contained in the melts so as to satisfy the condition that the melt is saturated with water or PH2O=Pt up to a certain value of pressure, but at higher pressures the melt is undersaturated or PH2O