Sven Maaløe

The major element composition of the upper mantle estimated from the composition of lherzolites

The compilation of analyses of continental and oceanic spinel Iherzolites show that these two types of Iherzolites have very similar compositions. Their composition range differ from that of African garnet Iherzolites, and the data suggest that the mantle beneath Africa has an anomalous composition. If the composition of the upper mantle may be estimated from that of Iherzolites, the compositions of spinel Iherzolite should form the basis for this estimate. It is suggested that the compositions of spinel Iherzolite represent both undepleted and depleted compositions, and a representative composition for the primitive mantle is proposed on this basis.

Water content of a granite magma deduced from the sequence of crystallization determined experimentally with water-undersaturated conditions

The sequence of crystallization in a biotite-granite from the Bohus batholith of Norway and Sweden, deduced from its texture, was magnetite, plagioclase, microcline, quartz, and finally biotite. Several sequences of crystallization were determined experimentally at 2 kb in the presence of varying only for H2O contents below 1.2% by weight. The rock was fused to a homogeneous glass, and each experiment included samples of finely crushed rock and glass. The samples were reacted in Ag-Pd capsules with measured H2O content in coldseal pressure vessels with NNO buffer. With excess H2O (more than 6.5%) the crystallization interval extends from 865° C to 705° C. In the H2O-deficient region, the solidus temperature remains unchanged as long as a trace of vapor is present, but the liquidus temperature increases as H2O content decreases; with 0.8 % H2O the liquidus temperature is 1125° C, the crystallization interval is 420° C, and a separate aqueous vapor phase is evolved only a few degrees above the solidus at 705° C. The biotite phase boundary increases slightly from 845° C with excess H2O to 875° C with 1% H2O, and it intersects the steep phase boundaries for quartz and feldspars; the sequence of crystallization changes at each intersection point. Similar diagrams at various pressures for related rock compositions involving muscovite, biotite and amphibole will provide grids useful in defining limits for the water content of granitic and dioritic magmas. Applications are considered for the Bohus batholith, other granitic rocks, and rhyolites. The Bohus magma could have been formed by crustal anatexis as a mobile assemblage of H2O-undersaturated liquid and residual crystals with initial total H2O content less than 1.2%, or it could have been derived by fractionation of a more basic parent with low H2O content from mantle or subduction zone, but it could not have been derived from a primary andesite generated from mantle peridotite. We consider it unlikely that the H2O content of large granitic magma bodies exceeds about 1.5% H2O; these magmas are H2O-undersaturated through most of their histories. Uprise and progressive crystallization of magma bodies produces H2O-saturation around margins and in the upper regions of magma chambers. H2O-saturated rhyolitic and dacitic magmas with phenocrysts can be tapped from the upper parts of the magma chambers.

The permeability controlled accumulation of primary magma

The initial accumulation of primary magma occurs just after the mantle has become permeable. The accumulation is caused by the compaction of the residuum, which either may be controlled by the rate of creep, or by the rate of flow of the interstitial melt. Experimental results suggest that the rate of compaction is controlled by the permeability, and a model for the accumulation process is worked out on this basis. The compaction causes the formation of a lower compaction boundary and an upper layer of melt. The ascending mantle of plumes and convection currents will form layers of melt situated 20–100 m apart. The type of partial melting for this accumulation is critical melting.

Geochemical aspects of permeability controlled partial melting and fractional crystallization

Abstract Magma accumulation in the mantle requires that the mantle be permeable. Experimental investigations show that the permeability threshold first will be attained after a certain degree of partial melting. The influence of the permeability threshold on the composition of partial melts is evaluated using the fayalite-forsterite system as an example. In addition the variation in trace element concentrations are calculated for different distribution coefficients. Primary magmas formed by accumulation when a minimal degree of partial melting is required for permeability display a remarkably small variation in composition up to 30% partial melting. It is suggested from REE abundances that primary tholeiitic magmas have been generated by permeability controlled partial melting. The compositions of the primary magmas generated by permeability controlled partial melting will not differ much from the compositions obtained by batch melting, but the degrees of partial melting will differ for similar compositions.

Magma accumulation in the ascending mantle

SUMMARY: The most important factors which control the accumulation of primary magma in the mantle are: (1) The degree of partial melting required for permeability, which is suggested as10–20%. (2) The creep properties of the partially molten mantle, the type of creep probably being Nabarro-Herring creep. (3) The type of flow of the ascending mantle material; the flow may be parallel or divergent dependent on the depth. (4) The depth of extension of the feeder dykes depending on the tensional stress distribution with depth and the rheological properties of the mantle. It is shown that the partially molten mantle becomes stratified into layers of magma and residuum, the layers of residuum being one or several metres thick. The stratified mantle constitutes the primary source of magma. A dispersed source system is formed in ascending convection currents, while a large magma chamber is formed in ascending plumes. The small diffusion rates imply that the melting is of a fractional type rather than batch melting.

Population density and zoning of olivine phenocrysts in tholeiites from Kauai, Hawaii

The population density of olivine phenocrysts of the tholeiites display an exponential variation, which is typical of igneous as well as contact metamorphic rocks. The exponential variation is explained by a new growth probability model, which is consistent with experimental work. The forsterite content of the olivine phenocrysts decreases with decreasing size. Various phenocryst features suggest that the tholeiites first crystallized slowly in a magma chamber, after which they underwent crystallization for a short period of time in a feeder dyke before eruption took place.

Compositional range of primary tholeiitic magmas evaluated from major-element trends

Abstract The major-element trend for Hawaiian tholciites is well defined and may be represented by straight lines in a Bowen diagram. The trend can neither be related to olivine accumulation nor to olivine fractionation. Other phenocryst control of the trend is also unlikely. It is suggested that the range in magnesia content for primary Hawaiian tholeiites is from at least 13% MgO to above 20%, MgO. The major-element trend for abyssal tholeiites suggests that abyssal tholeiites with 8–9% MgO are primary magmas. The total possible range in magnesia content for primary tholeiitic magmas is considered to be from 8–9% MgO to about 20% MgO.

The PT phase relations of a primary oceanite from the Reykjanes peninsula, Iceland

Abstract The field relationships and petrographic features of the oceanites of the Reykjanes peninsula suggest that they might have originated as primary magmas. The principal phase relationships of primary liquids formed by the partial melting of lherzolites are defined. The phase relations obtained for oceanite RE78 between 0 and 30 kbar at dry conditions suggest that the oceanite originated as a primary magma at 25 kbar and 1580°C, and erupted at a temperature near 1300°C.

Delayed fractionation of basaltic lavas

The phenocryst cores of the basaltic lavas from Jan Mayen and Hawaii display a range in compositions. The textural features of the phenocrysts also vary, both euhedral and skeletal phenocrysts are present in the same thin section. Apparently the basaltic magmas underwent crystallization within a temperature interval of 50–200° C before they became fractionated. The fractionates of basaltic lavas are therefore average compositions of the phenocryst assemblages rather than liquidus compositions. This type of fractionation is called delayed fractionation. It is considered that most tholeiitic and alkalic basaltic lavas undergo delayed fractionation.

Shape of ascending feeder dikes, and ascent modes of magma

Abstract The shape of the leading edge of an ascending feeder dike is estimated using the criterion that the direction of the movement of the edge should be perpendicular to the edge. Using this criterion it is shown that the edge can have various elliptic shapes. The ascent of magma within feeder dikes can occur in different manners. The presence of mantle derived nodules in alkalic and nephelinic magmas suggest that these magmas may ascend as a single pulse. The continuous supply to the Kilauea volcano, Hawaii suggest that the tholeiitic magma forms a 20-km high vertical column in the feeder dike, supplied from below by successive pulses of magma.