Komatiitic Melt Research Articles

Melting relations of peridotite and the density crossover in planetary mantles

Melting relations of primitive peridotite were studied up to 25 GPa. The change of the liquidus phase from olivine to majorite occurs at ∼ 16 GPa. We confirmed the density crossover of the FeO-rich peridotite melt and the equilibrium olivine (Fo 90) at ∼ 7 GPa. Sinking of equilibrium olivine (Fo 95) in the primitive peridotite melt was observed up to 10 GPa. The compression curves of FeO-rich peridotitic and komatiite melts reported in this and earlier work suggest that the density crossover in the Earth's mantle will be located at ∼ 11–12 GPa at 2000°C, consistent with an previous estimation by C.B. Agee and D. Walker. The density crossover can play a key role in the Moon and the terrestrial planets, such as the Earth, Venus and Mars. Majorite and some fraction of melt could have separated from the ascending diapir and sunk downwards at the depths below the density crossover. This process could have produced a garnet-rich transition zone in the Earth's mantle. The density crossover may exist in the FeO-rich lunar mantle at around the center of the Moon. The density crossover which exists at the depth of ∼ 600 km in the Martian mantle plays a key role in producing a fractionated mantle, which is the source the parent magmas of the SNC meteorites.

Komatiite Genesis: Insights Provided by Fe-Mg Exchange Equilibria

Two possible models are available for the generation of komatiitic melts from upper mantle sources: (1) large extents of melting at relatively low pressures and (2) low extents of pseudoinvariant melting at high pressures. For the partial melting of olivine-dominated upper mantle and generation of komatiitic magmas, the [Mg]-[Fe] diagram calculated from olivine-melt data is potentially useful in estimating the physical conditions of melting and in evaluating Fe/Mg ratios of mantle sources. Application of this diagram to a suite of komatiitic amphibolites derived by different extents of melting of similar sources from the Kolar Schist Belt, south India, indicates that (1) their magmas were derived at pressures ranging from 3 to 7 GPa by different extents of melting; (2) melt generation occurred in adiabatically rising deep mantle diapirs; and (3) diapiric mantle sources had higher Fe/Mg ratios than the undepleted Lesotho garnet lherzolite (PHN 1611). Komatiitic rocks from other Archean terranes in the [Mg]-[Fe] diagram suggests a range of pressures and degree of melting that agree with those proposed from high pressure experimental studies. Although some komatiitic rocks have geochemical features and conditions of melting that may require residual garnet in the magma source regions, such geochemical features can also be inherited from sources without much residual garnet. This is possible if the sources had undergone high pressure melt addition at the time of melting.

Velocities and compressibilities of komatiitic melts

Ultrasonic measurements of compressional wave velocities for six komatiitic melts containing 13–30 wt% MgO are reported in the frequency range 3–20 MHz and in the temperature range 1280–1661°C. Melts with less than about 20 wt% MgO have velocities that are 6% lower and relaxed compressibilities, βr, that are 15% higher than melts with more than about 20 wt% MgO. The general trend of increasing velocity with increasing MgO content correlates well with increasing depolymerization (decreasing molar volume) of the melts as reflected in the NBO/T, (CaO + MgO)/Al2O3, and Al/(Al+Si) ratios. The value of dβr/dw (w=wt% MgO) for the high MgO melts is more than four times greater than dβr/dw for the low MgO melts.

The equation of state of a molten komatiite: 1 Shock wave compression to 36 GPa

The equation of state (EOS) of an initially molten (1550°C) komatiite (27 wt % MgO) was determined in the 5–36 GPa pressure range via shock wave compression. Shock wave velocityUsand particle velocityUp(kilometers/second) follow the linear relationshipUs= 3.13(±0.03) + 1.47(±0.03)Up. Based on a calculated density at 1550°C, 0 bar of 2.745±0.005 g/cm3, thisUs‐Uprelationship gives the isentropic bulk modulusKS= 27.0 ± 0.6 GPa, and its first and second isentropic pressure derivatives,K′S= 4.9 ±0.1 andK″S= −0.109 ± 0.003 GPa−1. The calculated liquidus compression curve agrees within error with the static compression results of Agee and Walker (1988) to 6 GPa but is less dense than their extrapolated values at higher pressures. We determine that olivine (Fo94) will be neutrally buoyant in komatiitic melt of the composition that we studied near 8.2 GPa. Clinopyroxene would also be neutrally buoyant near this pressure. Liquidus garnet‐majorite may be less dense than this komatiitic liquid in the 20–24 GPa interval; however, pyropic‐garnet and perovskite phases are denser than this komatiitic liquid in their respective liquidus pressure intervals to 36 GPa. Liquidus perovskite may be neutrally buoyant near 70 GPa. At 40 GPa, the density of shock‐compressed molten komatiite would be approximately equal to the calculated density of an equivalent mixture of dense solid oxide components. This observation supports the model of Rigden et al. (1989) for compressibilities of liquid oxide components. Using their theoretical EOS for liquid forsterite and fayalite, we calculate the densities of a spectrum of melts from basaltic through peridotitic that are related to the experimentally studied komatiitic liquid by addition or subtraction of olivine. At low pressure, olivine fractionation lowers the density of basic magmas, but above 13–14 GPa this trend is reversed. All of these basic to ultrabasic liquids are predicted to have similar densities at 13–14 GPa, and this density is approximately equal to the density of the bulk (preliminary reference Earth model) mantle in this pressure range. This suggests that melts derived from a peridotitic mantle may be inhibited from ascending from depths greater than 400 km.

Re ? Os isotopic constraints on the origin of volcanic rocks, Gorgona Island, Colombia: Os isotopic evidence for ancient heterogeneities in the mantle

The Re — Os isotopic systematics of komatiites and spatially associated basalts from Gorgona Island, Colombia, indicate that they were produced at 155±43 Ma. Subsequent episodes of volcanism produced basalts at 88.1±3.8 Ma and picritic and basaltic lavas at ca. 58 Ma. The age for the ultramafic rocks is important because it coincides with the late-Jurassic, early-Cretaceous disassembly of Pangea, when the North- and South-American plates began to pull apart. Deep-seated mantle upwelling possibly precipitated the break-up of these continental plates and caused a tear in the subducting slab west of Gorgona, providing a rare, late-Phanerozoic conduit for the komatiitic melts.

Evidence from chromium abundances in mantle rocks for extraction of picrite and komatiite melts

ONE of the more important aspects of understanding the geochemical differentiation of the Earth is an accurate knowledge of the compositions of magmas produced by direct melting of the upper mantle, which are called primary magmas1. It is clear that a variety of primary magma compositions can develop within the mantle, depending on the source composition, the presence of volatiles, the extent of melting and the pressure of melt segregation1–4. Proposed primary magma compositions span a wide composition range, from basalts with ∼8–ll% MgO, through picrites with ∼11–20% MgO, to komatiites with ∼20–28% MgO; there is no agreement as to which is the dominant type of primary magma. We suggest here that the abundances of chromium in samples of the Earth's upper mantle indicate that, of the wide variety of possible primary liquids produced in experimental studies of mantle melting, the dominant type of primary liquid actually extracted from the mantle has been picritic to komatiitic, with ≳;15% MgO.

Magmatic contacts between immiscible sulfide and komatiite melts; implications for genesis of Kambalda sulfide ores

Magmatic contacts between immiscible sulfide and komatiite melts; implications for genesis of Kambalda sulfide ores

Comparative Re [sbnd]Os, Sm [sbnd]Nd and Rb [sbnd]Sr isotope and trace element systematics for Archean komatiite flows from Munro Township, Abitibi Belt, Ontario

A series of komatiite flows from Munro Township, Ontario, have Re and Os concentrations that range from 0.545 to 1.27 ppb and 1.00 to 2.20 ppb, respectively. The 187Re/ 186Os in the suite range from 14.05 to 52.37 and the 187Os/ 186Os range from 1.49 to 3.22, giving a crystallization age of2726 ± 93Ma and an of initial 187Os/ 186Os of0.873 ± 0.035. This age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems. The Re Os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks. The initial Os isotopic composition is consistent with the derivation of the melts from a mantle source(s) that originated with a chrondritic ( 187Os/ 186Os) i and evolved with an approximately chondriticRe/Os. Sm Nd data for the same suite cluster about anε Nd value at 2.72 Ga of +2 to +3. Rb Sr isotope data indicate significant mobilization of these elements by the greenschist-grade metamorphism that has altered the rocks. Comparisons of Re and Os concentrations with those of MgO, FeO, Ni, Cr and S suggests that Re was incompatible ( D < 0.01) with the mantle residues and also with the predominantly olivine assemblages that crystallized from the komatiite melts. A lack of correlation between Os and MgO indicates that Os concentrations were not controlled by the presence of olivine in the mantle residue or olivine crystallization within the flows. Os was, however, moderately compatible with the mantle residues ( D > 1). Os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization, and not by the modally predominant silicate minerals.

Geochemical constraints on the evolution of the early continental crust

The most important process affecting both major and trace-element concentrations in the mantle and crust is melting producing silicate liquids which then migrate. Another process whose effects are becoming more apparent is the transport of elements by CO 2 - and H 2 O-rich fluids. Due to the relatively small amounts of fluids involved they have but little effect on the major-element abundances but may severely affect minor- and trace-element abundances in their source and the material through which they travel. The Archaean crust was a density filter which reduced the possibility of komatiite or high FeO melts with relative densities greater than about 3.0 from reaching the surface. Those melts retained in the lower crust or at the crust-mantle boundary would have enhanced the possibility of melting in the lower crust. The high FeO melts may have included the Archaean equivalents of alkali basalt whose derivatives may form an important component in the Archaean crust. The occurrence of ultramafic to basic to alkaline magmas in some Archaean greenstone belts is an assemblage most typical of modern ocean-island suites in continental environments. The rock types in the assemblage were modified by conditions of higher heat production during the Archaean and thus greater extents of melting and melting at greater depths. If modern ocean-island suites are associated with mantle plumes, which even now may be an important way to transport heat upward from the deeper mantle, it is suggested that during the Archaean mantle plumes were an important factor in the evolution of the continental crust. It appears that the Archaean continental crust was of comparable thickness to that of the present based on geobarometeric data. If the freeboard concept applied then, this would suggest that plate tectonics was also an active process during the Archaean. If so, it is probably no more realistic to assume that all Archaean greenstone belts had a similar tectonic setting than to assume that all modern occurrences of basic rocks have a common tectonic setting.

Phanerozoic Peridotitic and Pyroxenitic Komatiites from Newfoundland

Peridotitic and highly magnesian pyroxenitic komatiites, thus far known to occur almost exclusively in the Archean (before 2.5 x 10(9) years ago) terranes, are reported from an Ordovician (0.5 x 10(9) years) ophiolite suite in Newfoundland. Their occurrence as pillow lavas or as chilled dikes, their possession of quench textures and geochemical parameters such as high contents of magnesium oxide, nickel, and chromium and low contents of titanium dioxide and potassium monoxide, low ratios of iron to iron plus magnesium, and values of the ratio of calcium oxide to aluminum oxide of close to unity demonstrate that they were formed through the rapid cooling of a highly mobile komatiitic melt. These features resemble those of many Archean peridotitic-pyroxenitic komatiites and indicate that the Archean-type magmatism did prevail in the younger segments of the earth's history although perhaps in a more erratc manner.