Claude Herzberg

Melting experiments on anhydrous peridotite KLB-1: Compositions of magmas in the upper mantle and transition zone

Electron microprobe results are reported for liquid and crystalline phases that were synthesized at 5–22.5 GPa in multianvil experiments on anhydrous peridotite KLB-1 [Zhang and Herzberg, 1994]. The results provide information on the partitioning of TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, Na2O, and NiO among liquid and the crystalline phases olivine, modified spinel, garnet, magnesiowustite, and magnesium perovskite. Uncertainties in these partition coefficients stem from quenching problems and from the effects of thermal migration of liquid in a temperature gradient. We have, however, exploited the temperature gradients by determining how the crystalline phase chemistry varies throughout the melting interval from the liquidus to the solidus. This has permitted new constraints to be obtained on the compositions of liquids along the anhydrous peridotite solidus at low melt fractions and at pressures in the 5–18 GPa range. It is demonstrated that the wide range of Al2O3 and CaO/Al2O3 contents in picrites and komatiites can be explained by melt segregation at upper mantle pressures that ranged from 3 to ∼10 GPa. These magmas could have formed by anhydrous melting in plumes with temperatures that were only 100°–200°C higher than ambient mantle below ridges, demonstrating that unusually hot conditions are not required to form komatiites. Primary igneous MgO contents in excess of 26% should be rare, and those that do exist in some komatiites can be explained by advanced melting during adiabatic or superadiabatic ascent, by low Na2O in the source, or by melting in hot plumes from the transition zone and lower mantle. Evidence for deep melting in hot plumes is rather conjectural, but it may be contained in some 2700 Myr komatiites that have high MgO and mantle-like CaO/Al2O3.

Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation

PRIMELT2.XLS software is introduced for calculating primary magma composition and mantle potential temperature (TP) from an observed lava composition. It is an upgrade over a previous version in that it includes garnet peridotite melting and it detects complexities that can lead to overestimates in TP by >100°C. These are variations in source lithology, source volatile content, source oxidation state, and clinopyroxene fractionation. Nevertheless, application of PRIMELT2.XLS to lavas from a wide range of oceanic islands reveals no evidence that volatile-enrichment and source fertility are sufficient to produce them. All are associated with thermal anomalies, and this appears to be a prerequisite for their formation. For the ocean islands considered in this work, TP maxima are typically ~1450–1500°C in the Atlantic and 1500–1600°C in the Pacific, substantially greater than ~1350°C for ambient mantle. Lavas from the Galapagos Islands and Hawaii record in their geochemistry high TP maxima and large ranges in both TP and melt fraction over short horizontal distances, a result that is predicted by the mantle plume model.

Depth and degree of melting of komatiites

High pressure melting experiments have permitted new constraints to be placed on the depth and degree of partial melting of komatiites. Komatiites from Gorgona Island were formed by relatively low degrees of pseudoinvariant melting (< 30%) involving L + Ol + Opx + Cpx + Gt on the solidus at 40 kbar, about 130 km depth. Munro-type komatiites were separated from a harzburgite residue (L + Ol + Opx) at pressures that are poorly constrained, but were probably around 50 kbar, about 165 km depth; the degree of partial melting was <40%. Komatiites from the Barberton Mountain Land were formed by high degrees (∼50%) of pseudoinvariant melting (L + Ol + Gt + Cpx) of fertile mantle peridotite in the 80- to 100-kbar range, about 260- to 330- km depth. Secular variations in the geochemistry of komatiites could have formed in response to a reduction in the temperature and pressure of melting with time. The 3.5 Ga Barberton komatiites and the 2.7 Ga Munro-type komatiites could have formed in plumes that were hotter than the present-day mantle by 500° and 300°, respectively. When excess temperatures are this size, melting is deeper and volcanism changes from basaltic to komatiitic. The komatiites from Gorgona Island, which are Mesozoic in age, may be representative of komatiites that are predicted to occur in oceanic plateaus of Cretaceous age throughout the Pacific [Storey et al., 1991].

Density constraints on the formation of the continental Moho and crust

The densities of mantle magmas such as MORB-like tholeiites, picrites, and komatiites at 10 kilobars are greater than densities for diorites, quartz diorites, granodiorites, and granites which dominate the continental crust. Because of these density relations primary magmas from the mantle will tend to underplate the base of the continental crust. Magmas ranging in composition from tholeiites which are more evolved than MORB to andesite can have densities which are less than rocks of the continental crust at 10 kilobars, particularly if they have high water contents. The continental crust can thus be a density filter through which only evolved magmas containing H2O may pass. This explains why primary magmas from the mantle such as the picrites are so rare. Both the over-accretion (i.e., Moho penetration) and the under-accretion (i.e., Moho underplating) of magmas can readily explain complexities in the lithological characteristics of the continental Moho and lower crust. Underplating of the continental crust by dense magmas may perturb the geotherm to values which are characteristic of those in granulite to greenschist facies metamorphic sequences in orogenic belts. An Archean continental crust floating on top of a magma flood or ocean of tholeiite to komatiite could have undergone a major cleansing process; dense blocks of peridotite, greenstone, and high density sediments such as iron formation could have been returned to the mantle, granites sweated to high crustal levels, and a high grade felsic basement residue established.

Petrology and thermal structure of the Hawaiian plume from Mauna Kea volcano

There is uncertainty about whether the abundant tholeiitic lavas on Hawaii are the product of melt from peridotite or pyroxenite/eclogite rocks. Using a parameterization of melting experiments on peridotite with glass analyses from the Hawaii Scientific Deep Project 2 on Mauna Kea volcano, I show here that a small population of the core samples had fractionated from a peridotite-source primary magma. Most lavas, however, differentiated from magmas that were too deficient in CaO and enriched in NiO (ref. 2) to have formed from a peridotite source. For these, experiments indicate that they were produced by the melting of garnet pyroxenite, a lithology that had formed in a second stage by reaction of peridotite with partial melts of subducted oceanic crust. Samples in the Hawaiian core are therefore consistent with previous suggestions that pyroxenite occurs in a host peridotite, and both contribute to melt production. Primary magma compositions vary down the drill core, and these reveal evidence for temperature variations within the underlying mantle plume. Mauna Kea magmatism is represented in other Hawaiian volcanoes, and provides a key for a general understanding of melt production in lithologically heterogeneous mantle.

Petrological evidence for secular cooling in mantle plumes.

Geological mapping and geochronological studies have shown much lower eruption rates for ocean island basalts (OIBs) in comparison with those of lavas from large igneous provinces (LIPs) such as oceanic plateaux and continental flood provinces. However, a quantitative petrological comparison has never been made between mantle source temperature and the extent of melting for OIB and LIP sources. Here we show that the MgO and FeO contents of Galapagos-related lavas and their primary magmas have decreased since the Cretaceous period. From petrological modelling, we infer that these changes reflect a cooling of the Galapagos mantle plume from a potential temperature of 1,560–1,620 °C in the Cretaceous to 1,500 °C at present. Iceland also exhibits secular cooling, in agreement with previous studies. Our work provides quantitative petrological evidence that, in general, mantle plumes for LIPs with Palaeocene–Permian ages were hotter and melted more extensively than plumes of more modern ocean islands. We interpret this to reflect episodic flow from lower-mantle domains that are lithologically and geochemically heterogeneous.

Generation of plume magmas through time: An experimental perspective

Experimental melting studies indicate that secular variations in the geochemistry of many komatiites were determined by secular variations in the depth of melt segregation in plumes. Higher pressures stabilize garnet relative to olivine and pyroxenes, resulting in komatiites with lower A1203, and higher CaO/Al2O3 and Gd/Yb. The experimental work is consistent with the following pressures of melt segregation: 3–4 GPa for Tertiary and Cretaceous picrites and komatiites; 5–7 GPa for most komatiites with 2.7-Ga ages; and 9–14 GPa for komatiites with 3.5-Ga ages. Melting and melt segregation occurred deeper in the past largely because the Earth was hotter, and komatiites area thermometer of this secular cooling. Depth of melting is critically dependent on internal plume temperature, and the geochemistry of most komatiites can be explained by plumes that were ∼ 200°C hotter than the secular cooling Earth model of F.M. Richter. Hot Archean plumes that were 300–400°C above ambient mantle could have experienced melting in the transition zone and the top of the lower mantle. Geological evidence in support of hot plumes is rare, but the best examples can be found in the 2.7-Ga komatiites from the Boston Township of Ontario and the 3.3-Ga peridotite xenoliths from the Kaapvaal craton.

WERE KOMATIITES WET

The main arguments used to support the concept that komatiites form by melting of hydrous mantle are as follows: (1) Water reduces liquidus temperatures from extreme values to lower, more “normal” temperatures. (2) Some komatiites are pyroclastic and some contain vesicles, features that have been attributed to magmatic volatiles. (3) It is claimed from experimental studies of peridotite melting that the chemical composition of komatiite requires the presence of water, as does their characteristic spinifex textures. Counterarguments are the following: (1) Loss of volatiles as hydrous komatiite approaches the surface should produce degassing textures and structures, which, though not unknown, are rare in komatiites. Degassing should produce a highly supercooled liquid that partially crystallizes to porphyritic magma; komatiites commonly erupt as phenocryst-poor, highly magnesian lavas. (2) Chemical and isotopic compositions of most komatiites indicate that their mantle source became depleted in incompatible elements soon before magma formation. Such depletion removes water, leaving a dry source. (3) The experimental data are at best ambiguous; neither the chemical composition of komatiites, nor the crystallization of spinifex, requires the presence of water. We conclude that although some rare komatiites may be hydrous, most are dry.

Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central orogenic belt, North China craton

The 2.55–2.50 Ga Zunhua and Wutaishan belts within the central orogenic belt of the North China craton contain variably metamorphosed and deformed tectonic blocks of peridotites and amphibolites that occur in a sheared metasedimentary matrix. In the Zunhua belt, dunites comprise podiform chromitites with high and uniform Cr-numbers (88). Peridotites and associated picritic amphibolites are characterized by light rare earth element (LREE)–enriched patterns and negative high field strength element (HFSE: Nb, Zr, and Ti) anomalies. They have positive initial ϵ Hf values (+7.9 to +10.4), which are consistent with an extremely depleted mantle composition. Mass-balance calculations indicate that the composition of the 2.55 Ga mantle beneath the Zunhua belt was enriched in SiO 2 and FeO T compared to modern abyssal peridotites. These geochemical signatures are consistent with a suprasubduction zone geodynamic setting. Metasomatism of the subarc mantle by slab-derived hydrous melts and/or fluids at ca. 2.55 Ga is likely to have been the cause of the subduction zone geochemical signatures in peridotites of the Zunhua belt. In the Wutaishan belt, chromitite-hosting harzburgites and dunites display U-shaped rare earth element (REE) patterns and have high Mg-numbers (91.1–94.5). These geochemical characteristics are similar to those of Phanerozoic forearc peridotites. The dunites might have formed by dissolution of orthopyroxene in reactive melt channels, similar to those in modern ophiolites. However, they differ in detail, and they might be residues of Archean komatiites. Following the initiation of an intra-oceanic subduction zone, they were trapped as a forearc mantle wedge between the subducting slab and magmatic arc. Slab-derived hydrous melts infiltrating through the mantle wedge metasomatized the depleted mantle residue, resulting in U-shaped rare earth element (REE) patterns.

Lithosphere peridotites of the Kaapvaal craton

A large number of garnet peridotite xenoliths from the lithosphere below the Kaapvaal craton are too orthopyroxene-rich and SiO2-rich to have formed as residues from a normal mantle peridotite source by the extraction of basalt or komatiite. However, they can be understood as originally harzburgite rocks (i.e., olivine + orthopyroxene) which formed as either residues or cumulates from a source material that was also enriched in SiO2 compared to normal mantle peridotite or its pyrolite analogue. These harzburgites could have crystallized as cumulates from ultrabasic magmas that were higher in SiO2 than most komatiites, magmas that would have required extensive melting (> 50%) of normal mantle peridotite to form, possibly in a gigantic plume. Alternatively, the harzburgites may have crystallized directly from a high-SiO2 terrestrial magma ocean during the birth of the Earth, or from a high-SiO2 source formed by the mixing of impacted chondritic materials with normal mantle peridotite later in time. Whatever the case, there appear to have been unusual geological circumstances involved in the formation of the continental lithosphere below the Kaapvaal craton.