D.H. Green

Experimental melting studies on a model upper mantle composition at high pressure under water-saturated and water-undersaturated conditions

The solidus of a model pyrolite composition is sensitively dependent on water content and has been determined experimentally up to 40 kb, for water-saturated (6% H2O) and water-undersaturated (0.2% H2O) conditions. Pargasitic hornblende is a major subsolidus phase to 29 kb and its breakdown at higher pressure has the effect of sharply depressing the solidus for (pyrolite+0.2% H2O) from ∼1150°C to∼1020°C between 25 and 29 kb. Experiments have been carried out above the solidus to determine the nature of the partial melting process, particularly the nature and composition of the residual phases at a specific pressure, temperature and water content. The presence of siliceous (>58% SiO2), low-magnesium glasses, broadly of andesitic or dacitic character, in experiments quenched at both 10 kb and 20 kb, is shown to be due to growth during quenching of olivine, clinopyroxene, amphibole and mica. However, it is possible in some experiments to use the compositions of the starting mix and analyzed residual phases to calculate the composition of the equilibrium liquid and degree of melting at the particular condition. High degrees of melting under water-saturated conditions at 10 kb yield magnesian, quartz-normative basaltic andesites ∼10%Qz, (1100°C, 28% melting) to quartz tholeiite magmas 5–7% Qz, (1200°C, 32.5% melting), and at 20 kb, yield olivine tholeiite magmas (1100°C, 27–30% melting). Andesitic or dacitic magmas are not products of equilibrium partial melting of pyrolite at P > 10kb under water-saturated conditions but may be derived from parental olivine-poor tholeiites, quartz tholeiites or basaltic andesites ( P < 10kb) by crystal fractionation. Parental magmas of the island arc tholeiitic magma series may originate by partial melting of upper mantle peridotite (pyrolite or residual peridotite of the lithosphere) under water-saturated conditions at ∼5–20kb.

The stability fields of aluminous pyroxene peridotite and garnet peridotite and their relevance in upper mantle structure

An experimental study of the stability fields at high pressure of garnet peridotite and aluminous pyroxene peridotite has been carried out in compositions matching estimates of the average, undifferentiated upper mantle (pyrolite). The appearance of garnet at higher pressures in the pyrolite compositions results from either of two reactions: (1) spinel + orthopyroxene ⇋ olivine + garnet (2) aluminous pyroxene ⇋ garnet + pyroxene (lower alumina). The role of spinel in the lower pressure assemblages is sensitively dependent on temperature and bulk composition. For the pyrolite composition preferred for the upper mantle, spinel is absent above 1300°C and the first appearance of garmet at pressures of 24 kb (1300°C) to 31 kb (1500°C) is due to reaction (2). In this composition garnet does not appear on the pyrolite solidus nor in its melting interval at pressures below 31.5 kb. At temperatures less than 1300°C, garnet appears at 21 kb (1100°C) to 24 kb (1300°C) and develops by reaction (1) at the expense of spinel. The amount of garnet formed by this reaction is dependent on the alumina content of the pyroxenes and throughout the temperature range 1100–1500°C the amount of garnet present increases markedly this is matched by decreasing Al2O3 content of the pyroxenes and allow preliminary estimation of P, T-dependent curves of constant Al2O3 content for orthopyroxene in garnet peridotite assemblages. The experimental data are applied to estimate density and seismic velocity variations along oceanic and continental geothermal gradients in a pyrolite upper mantle. It is emphasized that seismic velocity distributions are sensitively affected by variations in geothermal gradient and by mantle chemical composition, e.g. by variation from pyrolite to residual, refractory, dunite-peridotite. It is suggested that seismic velocity (Vs) variation in an oceanic upper mantle of pyrolite composition may be characterized by two low-velocity channels: (1) a narrow, but sharply defined low-velocity zone at 60–70 km depth caused by mineralogical zoning in the upper mantle; (2) a broader low velocity zone at 120–150 km depth defined primarily by the critical gradient for Vs in the upper mantle but accentuated by mineralogical variations in pyrolite.

The origin of high-alumina basalts and their relationships to quartz tholeiites and alkali basalts

Abstract Experimental crystallization of olivine tholeiite (20% olivine) at 9 kb shows that olivine and to a lesser extent, orthopyroxene are the early crystallizing phases, joined at lower temperatures by clinopyroxene and plagioclase. This contrasts with atmospheric crystallization of olivine first, joined at lower temperatures by plagioclase and clinopyroxene. In high-alumina olivine tholeiite (6% olivine) however, clinopyroxene is the liquidus phase at 9 kb, joined at lower temperatures by plagioclase, while from 0–6.8 kb plagioclase is the liquidus phase joined by olivine and clinopyroxene. Alkali basalt and olivine basalt have olivine and clinopyroxene as important near-liquidus phases at 9 kb, compared with olivine and plagioclase at atmospheric pressure. Quartz tholeiite contains olivine, together with plagioclase and clinopyroxene near the liquidus at atmospheric pressure, but at 4.5 kb and 6.8 kb plagioclase and clinopyroxene alone were evident. These results show that derivation of quartz-normative tholeiites from olivine-normative parent magmas is only possible at depths of less than 15 km, while at depths of 15–35 km high-alumina basalts of tholeiitic or alkalic affinities and only slightly enriched in silica are obtained from olivine tholeiite or olivine basalt parents. At depths of 35–60 km fractionation of olivine-rich magmas is largely governed by aluminous pyroxenes and the derivative liquids trend towards undersaturated alkali basalts. The inter-relationships of the three major magma types, quartz tholeiite, high-alumina basalt and alkali basalt, in such volcanic provinces as Japan are explained by magma segregation or fractional crystallization over specific depth ranges. Thus alkali basalts are derived at 35–60 km depth, high-alumina basalts at 15–35 km and quartz tholeiites at less than 15 km.

Experimental study at high pressures on the origin of olivine nephelinite and olivine melilite nephelinite magmas

The compositions of basaltic rocks containing high pressure xenoliths are used to select olivine nephelinite and picritic nephelinite compositions for study of high pressure melting and crystal fractionation relationships. In the olivine nephelinite composition (26% normative olivine) under anhydrous conditions, olivine is the liquidus phase up to 18 kb and is joined by clinopyroxene at lower temperatures. At 27 kb, clinopyroxene is the liquidus phase and both garnet and clinopyroxene occur in runs near the liquidus at 36 kb. A simlar sequence is found in the picritic nephelinite (36% normative olivine) but olivine persists as the liquidus phase up to 27 kb and garnet is the liquidus phase at 31.5 kb and 36 kb. Orthopyroxene was not observed in the melting interval of either composition under dry conditions. Experiments were carried out in which water added to the sample resulted in lowering of the liquidus temperature by 150–250°C at pressures of 13.5–30 kb. Under these conditions olivine is the liquidus phase up to 18 kb. At pressures of 18–22.5 kb in the olivine nephelinite and 22.5–27 kb in the picritic nephelinite aluminous orthopyroxene is the major near-liquidus phase (at temperatures of 1150°C–1250°C). Minor olivine and garnet accompany the orthopyroxene. Electron microprobe analyses of pyroxenes, olivine and garnet enable calculation of high pressure fractionation trends for both the dry melting and wet melting conditions. While fractionation under dry conditions at high pressure may account for some of the variation among natural undersaturated magmas, the fractionation trend does not conform to the natural chemical variation in the series olivine basanite → olivine nephelinite → olivine melilite nephelinite. However, the fractionation trend at 18–27 kb under ‘wet’ conditions (PH2O < Pload) is dominated by orthopyroxene and separation of orthopyroxene accompanied initially by minor olivine, and at lower temperatures by garnet, produces derivative liquids closely matching the olivine nephelinite → olivine melilite nephelinite series. The results provide evidence for development of the highly undersaturated olivine and melilite nephelinite lavas by either extreme fractionation of picritic magmas or by low degrees of partial melting of the mantle at depths of 60–100 km. An essential factor in their genesis is the presence of small amounts of water so that the magmas are produced at temperatures of 150–250°C below that required for dry magma production.

Archaean greenstone belts may include terrestrial equivalents of lunar maria

Abstract The lower portions of the volcanic sequence of some Archaean greenstone belts include members with crystallized from ultramafic liquids extruded at the earths surface at 1600–1650°C. These liquids are interpreted as products of 60–80% melting of their mantle source composition which implies more catastrophic conditions of mantle melting than obtained in Palaeozoic, Mesozoic or Recent crust-mantle dynamics. Such conditions may be a consequence of major impacts on the surface of the primitive earth. It is suggested that the production of the lunar maria basins was accompanied by similar impacts on the earth and that such terrestrial maria played an important role in early stages of chemical differentiation of the crust and upper mantle. An hypothesis is presented in which some Archaean greenstone belts are interpreted as very large impact scars, initially filled with impact-triggered melts of ultramafic to mafic composition and thereafter evolving with further magmatism, deformation and metamorphism to the present Archaean greenstone belts.

Uranium distribution in ultramafic inclusions from Victorian basalts

The uranium distribution in all phases of nine Iherzolite inclusions from the Newer Volcanics of western Victoria has been determined using fission tracks. Primary clinopyroxene has a mean content of 0.30 ppm U in five of the inclusions, and two contain primary apatite with 35 ppm U. Secondary clinopyroxene and apatite crystallizing in equilibrium with glass formed from partial melting of the Iherzolites have a much lower abundance. Partition coefficients calculated from the uranium contents in these secondary phases and the glass indicate that a liquid in equilibrium with the primary assemblage would have had to contain 25 to 75 ppm U. Since these abundances are at least ten times those of normal basalts, these inclusions could not have formed as accumulates from a basaltic magma, neither are they the residue from a previous episode of complete magma extraction. However other inclusions containing clinopyroxene with a low uranium abundance could be accumulates or residua. A model for the uranium distribution in the upper mantle is based on the uranium abundances in the high U primary assemblage. It is consistent with estimates of the upper mantle uranium content, and the uranium contents of basalt magma series calculated from it are consistent with the reported abundances. The petrology and geochemistry of inclusions illustrate a mechanism for limited wall rock reaction, and suggest that potassium may be moved without uranium.

THORIUM, URANIUM, AND POTASSIUM ABUNDANCES IN PERIDOTITE INCLUSIONS AND THEIR HOST BASALTS.

Abstract The abundances of the radioactive elements uranium, thorium and potassium in six lherzolite inclusions and their host basanites have been determined by neutron activation analysis and γ-ray spectrometry. The lherzolite inclusions have marked differences in K/U, K/Th and smaller differences in Th/U from the basanites. The contents of those elements in lherzolite inclusions are not determined simply by varying degrees of contamination by the host magma. The lherzolites are inferred to be accidental xenoliths in the host magma and their geochemical characteristics are representative of at least some parts of the upper mantle. They are considered to represent residual material left after extraction from mantle pyrolite of an undersaturated basaltic magma or after selective extraction of minor elements by wall-rock reaction processes. Processes of magma extraction from the mantle may lead to strong depletion of K in residual mantle peridotite with very little accompanying depletion in U and Th. The abundances of U, Th and K in the lherzolite inclusions are also compared with various estimates of upper mantle composition based on heat flow data.

Significance of a primitive lunar basaltic composition present in Apollo 15 soils and breccias

Distinctive spherules and fragments of ‘Green Glass’ previously described from Apollo 15 soils, have olivine-rich (30%) magnesian (100MgMg+Fe= 61) picritic compositions. ‘Green Glass’ has been interpreted as a representative composition of a significant lunar rock unit, the actual glass being produced by melting of this unit. Experimental studies of high pressure melting relations in a compositionally similar Apollo 12 basalt (12040) lead to prediction of the nature of liquidus phases of Apollo 15 Green Glass at various pressures. It is argued that the Apollo 15 Green Glass unit was a product of very high degrees of partial melting (30–60%) of pyroxenite (olivine-poor, dominated by subcalcic clinopyroxene) source rock with magma segregation (from residual olivine and orthopyroxene only) occurring at 15 kb, T = 1450°C. The source rock is similar to that predicted for Apollo 11, 12 and 15 mare basalts but the degree of melting is much greater than that postulated for even the more olivine rich mare basalts (e.g. 12009 - 10% melting, magma segregation 10 – 12 kb).

Lead isotope measurements on lherzolite inclusions and host basanites from Western Victoria, Australia

Abstract Lead samples extracted from a group of lherzolite inclusions and from their host basanites have been isotopically analyzed. The lead from the basanites varies only slightly in 206 Pb/ 204 Pb and 208 Pb/ 204 Pb and is similar to average modern terrestrial lead. The lherzolite lead is isotopically different, showing varying degrees of 206 Pb and 208 Pb deficiency. On a 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb diagram, the data appear to follow the average regression line drawn through a world-wide selection of modern basaltic volcanics. The data require separate immediate source regions, differing in U, Th, Pb relationships, for the lherzolites and basanites and demonstrate that the lherzolites cannot have a cognate relationship to their host basanite. A more detailed analysis of the isotopic data suggests that an evolutionary model of at least two stages is required and calculations based on a two stage model suggest formation of the lherzolites from prior source material with higher μ and lower κ values at 2–2.5 billion years ago.

Experimental petrology of Apollo 12 basalts: part 1, sample 12009

Abstract The lunar sample, 12009, is a rapidly quenched basalt with microphenocrysts of olivine (∼7%) and spinel in a cryptocrystalline matrix with many small microlites. The rock is olivine-normative (11%) and comparison of the olivine microphenocryst compositions with the experimentally determined liquidus olivine compositions shows that the rock was originally entirely liquid and that none of the observed olivine results from crystal accumulation. The magma (12009) began crystallizing olivine at ∼1230°C, spinel joined the olivine at ∼1210°C, and pigeonitic clinopyroxene would have appeared at ∼1190°C but sudden quenching of the magma occurred before this temperature was reached. Experimental studies at high pressure on 12009 magma show that olivine ceases to be a liquidus phase at pressures above 8kb and the liquidus clinopyroxene becomes more Ca and Al rich with increasing pressure. Although 12009 is not saturated with orthopyroxene at any pressure, a composition of 12009 + 10% olivine (Fo 75 ) has olivine and orthopyroxene as liquidus phases at 15kb. The data are used to infer partial melting of olivine pyroxenite [orthopyroxene + clinopyroxene + olivine, 100 Mg/Mg + Fe = 75–80] at depths > 200 km within the lunar interior, as the primary source of the maria-filling magmas.