Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.

Gradients in silicic magma chambers: Implications for lithospheric magmatism

Moon and Earth : compositional differences inferred from siderophiles, volatiles, and alkalis in basalts

Paleomagnetic directions have been determined on Late Cretaceous and Miocene rocks in the Nohi area, in the eastern part of southwest Japan, in an effort to clarify within-block deformation of the rotated part of southwest Japan. Twenty-two sites in the area are found to have a reliable primary magnetic component through progressive demagnetization tests using thermal and alternating field methods. Tiltcorrected paleomagnetic data in the Nohi area are divided into three stages based on geologic age: Late Cretaceous, early Miocene (23–15 Ma), and middle Miocene (12–10 Ma). The formation-mean direction of 10 sites obtained from Late Cretaceous Nohi rhyolite is D=17.0°, I=49.8°, and α95=6.1°. Neogene rock units underlain by Nohi rhyolite show two distinct formation-mean directions: D=13.0°, I=52.7°, and α95=7.2° for eight sites of early Miocene, and D=−2.5°, I=55.6°, and α95=6.6° for four sites of middle Miocene. Differential rotation is proposed between the eastern part of southwest Japan including the Nohi area and the central part of southwest Japan, which is known to have rotated clockwise through about 50° around 15 Ma. The spatial distribution of contemporaneous paleomagnetic data in southwest Japan suggests that two differentially rotated parts are connected in the western neighborhood of the Nohi area. It is concluded that the Nohi area had rapidly rotated counterclockwise through 48° relative to the central part of southwest Japan between 15 and 12 Ma. Together with previous paleomagnetic data from early Miocene rocks in the Kanto area, which is situated to the east of the Nohi area, the present results suggest that differential rotation of the eastern part of southwest Japan is attributed to the formation of northward cuspate shapes of pre-Neogene terranes which constitute the framework of the Japan arc. One probable explanation of the formation of cusps is a crustal indentation caused by the collision of landmasses on the Philippine Sea plate against the Japan arc in the middle Miocene.

Differential rotation of the eastern part of southwest Japan inferred from paleomagnetism of Cretaceous and Neogene rocks

Partition of Fe2+ and Mg between coexisting (Mg, Fe)2SiO4 spinel and (Mg, Fe)SiO3 pyroxene was investigated at pressures 80 and 90 kbar and at temperatures 840 and 1050° C, using tetrahedral-anvil type of high pressure apparatus. Olivine-spinel solid solution equilibria in the system Mg2SiO4-Fe2SiO4 were discussed in the light of the partition reaction. Partition of Fe2+ and Mg in both olivine-spinel and pyroxene-spinel systems can not be regarded as that between ideal solid solutions. By applying the simple solution model for the partition of Fe2+ and Mg, sign of the heat of mixing was estimated to be positive for all olivine, spinel and pyroxene. Relative concentration of Fe2+ in spinel in the pyroxene-spinel system is likely to cause some change in the chemical composition of modified spinel (β) or spinel (γ) in the transition zone of the mantle. A considerable change is also expected in the transition pressure of β to (β + γ) and (β + γ) to γ.

Partition of magnesium and iron between Olivine and spinel, and between pyroxene and spinel

Petrology of the Nohi Rhyolite and its related granitoids: A Late Cretaceous large silicic igneous field in central Japan

Cs/Rb ratio was examined on the Nohi rhyolite complex in Central Japan, which is composed mainly of rhyolitic ignimbrite. The variation in the Cs/Rb ratio of the magma through crystallization differentiation was estimated using a Rayleigh fractionation model and available distribution coefficients between phenocryst minerals and co-existing groundmass. It was found that the K/Rb ratio of this complex decreased from 273 to 125, but the Cs/Rb ratio remained constant through the differentiation process. The initial Cs/Rb ratio of this magma is 0.030 ± 0.005. The value is very close to the average Cs/Rb ratio of the ignimbrite in the Taupo region, Northern New Zealand, but is distinctly heigher than that of ocean floor basalts. To estimate the effect of contamination the Rb-Sr whole rock isochron method was used.

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Significance of Cs/Rb ratios in volcanic rocks as exemplified by the Nohi Rhyolite complex, Central Japan

The partitioning of Mg, Fe and Co between coexisting olivine and orthopyroxene found in peridotite inclusions is examined from the thermodynamic point of view, using a binary and a ternary regular solution models. The following two exchange reactions are considered: MgSi0.5O2+FeSiO3 = FeSi0.5O2 +MgSiO3 (1) and MgSi0.5O2+CoSiO3=CoSi0.5O2+MgSiO3. (2) The apparent partition coefficients are respectively: RT·lnK'Fe.Mg=-ΔG1-(2XMg01-1)(W01P+α01)+(2XopxMg-1)(WopxP+αopx) (3) and RT·lnK'Co.Mg=-ΔG2+(Δα+ΔWP)XFe (4) Using (3) and (4), we can estimate the P-T conditions at equilibrium of peridotite inclusions, if we know the numerical values of the apparent partition coefficients. Examination of the equilibrium relations in a peridotite inclusion from a South African kimberlite and in an lchiomegata lherzolite leads to the conclusion that the garnet peridotite is formed at higher pressure and lower temperature than the lherzolite.

https://www.jstage.jst.go.jp/article/geochemj1966/4/3/4_3_111/_pdf

Pressure-temperature conditions for the formation of peridotite inclusions. —An application of a regular solution model to partitioning of Mg, Fe and Co between coexisting olivine and orthopyroxene—

Takashi Matsumoto

Papers

Significance of Cs/Rb ratios in volcanic rocks as exemplified by the Nohi Rhyolite complex, Central Japan

Pressure-temperature conditions for the formation of peridotite inclusions. —An application of a regular solution model to partitioning of Mg, Fe and Co between coexisting olivine and orthopyroxene—

Partition of Al, Mg and Si between coexisting Ca-rich, and Ca-poor pyroxenes.