Basaltic Partial Melting Research Articles

Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline

The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and NbIY and low ZrINb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group 11 rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher ZrINb, and lower NbIY. Group I11 rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and I1 rocks, and contain elevated Ba contents. Groups 1-111 are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and I1 basalts. Groups I-111 are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline

The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and Nb/Y and low Zr/Nb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group II rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher Zr/Nb, and lower Nb/Y. Group III rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and II rocks, and contain elevated Ba contents. Groups I–III are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and II basalts. Groups I–III are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

Fragments of ancient lunar crust: Petrology and geochemistry of ferroan noritic anorthosites from the Descartes region of the Moon

Clasts of ferroan noritic anorthosite from Apollo 16 breccia 67016 have mineralogical and geochemical affinities with pristine ferroan anorthosites and therefore may represent primary igneous rocks from the lunar crust. The textures of these clasts document a complex history involving magmatic crystallization, brecciation, subsolidus recrystallization, and sulfide metasomatism. Nonetheless, the major, minor, and trace element compositions of their minerals, as determined by electron and proton microprobe, show that the clasts are essentially monomict and belong to the ferroan anorthositic suite of lunar highlands rocks. The low bulk Ni contents, low Ni Co ratios, and ancient SmNd isotopic age (≥4.5 Ga; Alibert et al., 1994) of the clasts are consistent with this interpretation. Strontium and gallium contents of plagioclase in the clasts show that their parental magma was depleted in Ga relative to Sr by a factor of about 30 relative to carbonaceous chondrites. This depletion of Ga, which is both moderately volatile and moderately siderophile, probably reflects volatile loss from the Moon rather than extraction of metal. The parental magma from which these clasts originally crystallized was enriched in Sr by about 4–6X relative to proposed bulk Moon compositions, which implies either that ferroan anorthosites crystallized late in the evolution of a lunar magma ocean, or that their parental magma was a basaltic partial melt. Bulk major and trace element compositions of these clasts are remarkably similar to that of the average upper crust of the Moon, which suggests that igneous ferroan noritic anorthosite such as that found in the North Ray Crater breccias and soils may be a significant primary component of the lunar crust. The ferroan noritic anorthosite clasts in 67016 are unusual among lunar samples for their abundance of troilite, and for the evidence of metasomatic introduction of volatile chalcophile elements such as Cu, Zn, Ni, Sb, and Se. Anhydrous COSCl vapors are the most likely transport agent for these metals, which would be more consistent with an indigenous lunar origin for the volatiles rather than a cometary or meteoritic source.

Ultramafic xenoliths: Clues to Earth's late accretionary history

Fourteen spinel lherzolites, for which extensive trace element data are available, may be divided into three groups depending upon the percentage loss of basaltic partial melt; averages are slightly depleted, −7% melt (Ca/Sa > 0.09); moderately depleted, −13% (0.09 > Ca/Si > 0.06); and strongly depleted, −20% (0.06 > Ca/Si). Rhenium abundances of the groups are correlated with percent depletion, and the intercept of 0.0071 × Cl chondrite corresponding to undepleted mantle is identical to that independently derived from 187Os/186Os in osmiridium of known age. The distribution of Re in mafic and ultramafic rocks is apparently closely related to S and Se abundances. Lherzolite data suggest that the abundances of highly siderophile elements (Os, Re, Ir, Pd, Au) and chalcogenic elements (S, Se, Te) in a primary basaltic melt are significantly higher than those of average oceanic ridge basalts but may be similar to those of trace element‐rich Indian Ocean ridge basalts. The estimated S content (∼1000 ppm) of a primary basaltic melt is compatible with experimentally measured S solubilities at high temperature and pressure. On the basis of lherzolite depletion and using published trace element data for spinel lherzolites and ocean ridge basalts, estimated abundances in ppb (or × 103/Cl chondrite abundances) in pristine upper mantle are Os, 3.1 (5.9); Re, 0.26 (7.1); Ir, 3.4 (7.1); Pd, 4.5 (8.1); Au, 1.01 (8.4); S, 200,000 (3.2); Se, 57 (3.1). The trace element pattern closely resembles that of the CM2 chondrites, but not of the CO3, CV3 or H chondrites. The pattern is a reasonable match, except for Au, with Apollo 17 lunar breccias that contain a “group 2” ancient meteoritic component, suggesting that rather similar objects bombarded earth and moon during the first 600 m.y. of their history.

Liquidus phase relationships on the join anorthite-forsterite-quartz at 10 kbar with applications to basalt petrogenesis

Liquidus phase relationships determined on the join CaAl2Si2O8 (anorthite)-Mg2SiO4 (forsterite)-SiO2 (quartz) at 10 kbar show that increasing pressure causes the forsterite and anorthite primary phase fields to shrink and the spinel, enstatite and silica fields to expand. The boundary line between the enstatite and forsterite fields and that between the enstatite and quartz fields both move away from the SiO2 apex as pressure increases. Therefore, simplified source peridotite would yield simplified basaltic partial melts with decreasing silica as pressure increases, as has been found in other studies. Also, increasing pressure decreases the amount of silica enrichment in residual liquids produced by fractional crystallization. Although anorthite is unstable in simplified peridotite above 9 kbar in the system CaO-MgO-Al2O3-SiO2, it is an important phase in the fractional crystallization of simplified basalts at 10 kbar and probably also in natural basalts.

Thermal convection in magmas generated by hot-plate heating

A condition is derived here for the onset of thermal convection in a layer of silicate partial melt that is generated by suddenly and continuously supplying an anomalous heat flux or by maintaining an anomalous high temperature on a plane at some depth within the Earth so that the overlying material is heated to greater-than-solidus temperatures. The results show that for highly fluid basaltic magmas convection may begin in a layer only 10-m thick, whereas for viscous rhyolitic magmas, at least a 10-km thick layer is required. The fact that thermal convection may occur within such a thin layer of basaltic partial melt may partially explain the homogeneity of large volumes of flood basalts that occur. Since convection within rhyolitic partial melts is unlikely, homogeneous rhyolites must be the result of mixing in magma chambers.

Origin and emplacement of the ophiolite suite: Appalachian ophiolites in Newfoundland

The ophiolite suite is probably generated by axial plate accretion at oceanic ridges, and by diffuse slow spreading in marginal basins behind and within island arc complexes. The dunite/harzburgite component may represent the highest parts of a depleted upper mantle shell, about 16 km thick, from which a basaltic partial melt (approx. 30%) has been withdrawn to form the gabbros, dike swarms, and pillow lavas. Ophiolites have complex internal igneous, structural, and metamorphic relationships that are probably related to processes involved in their generation at ridges and in marginal basins, having no significance in terms of processes within the orogenic belt in which they are finally emplaced. Varying plate accretion rates may have a great influence on the structural and metamorphic patterns of ophiolites. The autochthonous sedimentary cap of ophiolites developed by axial accretion may, rarely, consist of continental margin non-volcanic flysch and salt, but usually comprises chert, argillite, and deep-water limestone: that of ophiolites generated in marginal basins probably consists, in addition to these facies, of andesitic flysch derived from adjacent island arcs. Ophiolites are probably emplaced, in orogenic belts, at consuming plate margins either beneath and behind oceanic trenches (subduction zones) or by thrusting onto continental margins (obduction zones) when a continental margin meets a subduction zone. Metamorphic relationships within and around ophiolite complexes are numerous, probably reflecting processes involved in both genesis and emplacement, and may involve (1) thermal and hydrothermal alteration at the site of origin, (2) blueschist metamorphism in subduction zones, (3) regional high-temperature metamorphism in island arcs, (4) high-temperature aureoles developed around peridotites injected into oceanic crust at time of origin, into the transition region between marginal basins and island arcs or into the regionally metamorphosed assemblages of island arcs, (5) garnet-amphibolites developed along the sole thrusts of obducted peridotites, which may represent transported high-temperature arc metamorphics, oceanic crust, or metamorphic rocks generated during obduction, (6) rodingites developed during low-temperature peridotite diapirism and serpentinization on ridge flanks or during ophiolite emplacement, and (7) amphibolites and eclogites, in subduction melanges, representing transformed oceanic crust ripped up from the downgoing plate beneath trenches. Obducted ophiolites often indicate a narrow time span between origin and emplacement, possibly due to the opening and closing of marginal basins. Evolving ridge/transform/trench and trench/trench/trench triple junctions have great significance for developing diachronous events associated with ophiolite emplacement along continental margins and island arcs. Early Ordovician ophiolites in the Newfoundland Appalachians have complex and variable structural relationships with rocks of the Paleozoic continental margin. Immense obducted ophiolite slices carry transported garnet-amphibolites, and are believed to have developed in a marginal basin behind early Paleozoic island arcs on the northwestern (present) margin of a proto-Atlantic Ocean.