Paul C. Hess

Implications of liquid-liquid distribution coefficients to mineral-liquid partitioning

The effect of silicate liquid structure upon mineral-liquid partitioning has been investigated by determining element partitioning data for coexisting immiscible granitic and ferrobasaltic magmas. The resulting elemental distribution patterns may be interpreted in terms of the relative states of polymerization of the coexisting magmas. Highly charged cations (REE, Ti, Fe, Mn, etc.) are enriched in the ferrobasaltic melt. The ferrobasaltic melt is relatively depolymerized due to its low SiO ratio. This allows highly charged cations to obtain stable coordination polyhedra of oxygen within the ferrobasaltic melt. The granitic melt is a highly polymerized network structure in which Al can occupy tetrahedral sites in copolymerization with Si. The substitution of Al+3 for Si+4 produces a local charge imbalance in the granitic melt which is satisfied by a coupled substitution of alkalis, thus explaining the enrichment of low charge density cations, the alkalis, in the granitic melt. P2O5 increases the width of the solvus and, therefore, the values of the distribution coefficients of the trace elements. This effect is attributed to complexing of metal cations with PO4−3 groups in the ferrobasaltic melt. The values of ferrobasalt-granite liquid distribution coefficients are reflected in distribution coefficients for a mineral and melts of different compositions. The mineral-liquid distribution coefficient for a highly charged cation is greater for a mineral coexisting with a highly polymerized melt (granite) than it is for that same mineral and a depolymerized melt (ferrobasalt). The opposite is true for low charge density cations. Mineralliquid and liquid-liquid distribution coefficients determined for the REEs indicate that fractionated REE patterns are due to mineral selectivity and not the state of polymerization of the melt.

A model for the thermal and chemical evolution of the Moon's interior: implications for the onset of mare volcanism

Crystallization of the lunar magma ocean creates a chemically stratified Moon consisting of an anorthositic crust and magma ocean cumulates overlying the primitive lunar interior. Within the magma ocean cumulates the last liquids to crystallize form dense, ilmenite-rich cumulates that contain high concentrations of incompatible radioactive elements. The underlying olivine-orthopyroxene cumulates are also stratified with later crystallized, denser, more Fe-rich compositions at the top. This paper explores the chemical and thermal consequences of an internal evolution model accounting for the possible role of these sources of chemical buoyancy. Rayleigh-Taylor instability causes the dense ilmenite-rich cumulate layer and underlying Fe-rich cumulates to sink toward the center of the Moon, forming a dense lunar core. After this overturn, radioactive heating within the ilmenite-rich cumulate core heats the overlying mantle, causing it to melt. In this model, the source region for high-TiO2 mare basalts is a convectively mixed layer above the core-mantle boundary which would contain small and variable amounts of admixed ilmenite and KREEP. This deep high-pressure melting, as required for the generation of mare basalts, occurs after a reasonable time interval to explain the onset of mare basalt volcanism if the content of radioactive elements in the core and the chemical density gradients above the core are sufficiently high but within a range of values that might have been present in the Moon. Regardless of details implied by particular model parameters, gravitational overturn driven by the high density of magma ocean Fe-rich cumulates should concentrate high-TiO2 mare basalt sources, and probably a significant fraction of radioactive heating, toward the center of the Moon. This will have important implications for both the thermal evolution of the Moon and for mare basalt genesis.

Chemical dieferentiation of a convecting planetary interior: Consequences for a one plate planet such as Venus

Partial melting to generate the crust of a planet creates compostionally buoyant residual mantle. In the absence of mantle flow associated with plate tectonics, this buoyant, refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface. In this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer. As the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases. As less depleted mantle with less positive compositional buoyancy is added, negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy. The depleted layer then sinks into and mixes with the convecting interior. The top of the convecting mantle then moves to a shallower depth, larger degrees of melting resume, and a new depleted layer accumulates. This accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planets evolution with a period of 300–500 Myr. On Venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 Myr. We speculate that the mechanism described above may explain this episodic global resurfacing of Venus.

Chassigny petrogenesis - Melt compositions, intensive parameters, and water contents of Martian (?) magmas

Abstract The SNC meteorites are a class of eight basaltic achondrites that have extremely young crystallization ages (≤ 1.3 Ga). Chassigny, a unique SNC meteorite, is a dunite containing Fo68 olivines and rare poikilitic Ca-pyroxenes. It is one of the most primitive SNC meteorites and thus is most likely to reveal information about the SNC basalt source region. This study presents a detailed examination of partially crystallized melt inclusions in cumulus olivine grains in Chassigny. These trapped melts are argued to be representative samples of the melt that existed when Chassigny crystallized. This melt has been modified, however, by interaction with the host olivine and by closed-system crystallization. The phases inside the melt inclusions include hydrous Ti-rich amphibole (kaersutite), biotite, two pyroxenes, and rhyolitic and alkali feldspar glasses. All phases have been extensively analyzed with an electron microprobe. The new mineralogical information is combined with a complementary experimental study of kaersutite/melt equilibria. This combined data set can be used to formulate a system of linear equations which can then be solved to determine the composition of the originally trapped melt. This calculation reveals that the trapped melt was an FeO-rich and Al2O3-poor basalt. The melt composition is shown to be consistent with the crystallization sequence both inside the melt inclusions and in the rock matrix. The major element chemistry of this liquid closely resembles terrestrial boninite lavas. The new data also allow the intensive conditions (temperature, total pressure and water fugacity) of Chassigny crystallization to be estimated. Two-pyroxene geothermometry indicates that equilibration temperatures were ∼1000 ± 50°C, although amphibole does not coexist with melt until T = 960°C. The trapped liquid initially contained 1.5 wt% dissolved water. Crystallization of anhydrous phases caused water to buildup in the trapped melt. Prior to kaersutite crystallization, the melt must have contained at least 4 wt% dissolved water, suggesting that a minimum of 1.5 kbar total pressure is required. A maximum total pressure cannot be inferred, but all available data are consistent with low pressure (⪡5 kbar) crystallization. Finally, X(H2O) in the fluid in the melt inclusion is estimated to have been at least 0.8 in order to stabilize amphibole without plagioclase, implying that the water fugacity was ∼ 1480 bars. The existence of hydrous melts and conditions appropriate for amphibole crystallization suggest that evolved, SiO2-rich lavas exist on the Chassigny parent body (Mars).

The role of P2O5 in silicate melts

Abstract Phase equilibria data in the systems SiO 2 -P 2 O 5 , P 2 O 5 -M x O y , and P 2 O 5 -M x O y -SiO 2 are employed in conjunction with Chromatographic and spectral data to investigate the role of P 2 O 5 in silicate melts. Such data indicate that the behavior of P 2 O 5 is complex. P 2 O 5 depolymerizes pure SiO 2 melts by entering the network as a four-fold coordinated cation, but polymerizes melts in which an additional metal cation other than silicon is present. The effect of this polymerization is apparent in the widening of the granite-ferrobasalt two-liquid solvus. In this complex system P 2 O 5 acts to increase phase separation by further enrichment of the high charge density cations Ti, Fe, Mg, Mn, Ca, in the ferrobasaltic liquid. P 2 O 5 also produces an increase in the ferrobasalt-granite REE liquid distribution coefficients. These distribution coefficients are close to 4 in P 2 O 5 -free melts, but close to 15 in P 2 O 5 -bearing melts. The dual behavior of P 2 O 5 is explained in a model which requires complexing of phosphate anions (analogous to silicate anions) and metal cations in the melt. This interaction destroys Si-O-M-O-Si bonds polymerizing the melt. The higher concentration of Si-O-M-O-Si bond complexes in immiscible ferrobasaltic liquids relative to their conjugate immiscible granite liquids explains the partitioning of P 2 O 5 into the ferrobasaltic liquid.

Mineralogy of the mafic anomaly in the South Pole-Aitken Basin: Implications for excavation of the lunar mantle

The mineralogy of South Pole-Aitken Basin [SPA] (the largest confirmed impact basin on the Moon) is evaluated using five-color images from Clementine. Although olivine-rich material as well as basalts rich in clinopyroxene are readily identified elsewhere on the farside, the dominant rock type observed across the interior of SPA is of a very noritic composition. This mineralogy suggests that lower crust rather than the mantle is the dominant source of the mafic component at SPA. The lack of variation in observed noritic composition is probably due to basin formation processes, during which extensive melting and mixing of target materials are likely to occur.

Polymer model of silicate melts

The distribution of silicate species in silicate melts is analyzed using the techniques of conventional polymer chemistry. The structure of a melt is dominated by the SiO4 monomer at low silica compositions. With increasing SiO2, the silicate species polymerize forming larger branch and ring structures. Infinitely branched species have a high probability of forming once 13 of the singly bonded oxygens of the silicon atoms become doubly bonded to two silicon atoms. In most binary silicate melts, this point is reached at mole fractions NsiO2 between 0.28 and 0.44. The extent of polymerization of a melt can be expressed as a constant, K. This constant can, in turn, be correlated to the field strength of the cation that is added as an oxide. Cations of high field strength form the most polymerized melts. These highly polymerized melts are characterized by liquid immiscibility at high silica compositions.

Rutile solubility and titanium coordination in silicate melts

Abstract The solubility of rutile has been determined in a series of compositions in the K 2 O-Al 2 O 3 -SiO 2 system ( K ∗ = K 2 O (K 2 O + Al 2 O 3 ) = 0.38–0.90), and the CaO-Al 2 O 3 -SiO 2 system ( C ∗ = CaO (CaO + Al 2 O 3 ) = 0.47–0.59 ). Isothermal results in the KAS system at 1325°C, 1400°C, and 1475°C show rutile solubility to be a strong function of the K ∗ ratio. For example, at 1475°C the amount of TiO 2 required for rutile saturation varies from 9.5 wt% ( K ∗ = 0.38 ) to 11.5 wt% ( K ∗ = 0.48 ) to 41.2 wt% ( K ∗ = 0.90 ). In the CAS system at 1475°C, rutile solubility is not a strong function of C ∗ . The amount of TiO 2 required for saturation varies from 14 wt% ( C ∗ = 0.48 ) to 16.2 wt% ( C ∗ = 0.59 ). The solubility changes in KAS melts are interpreted to be due to the formation of strong complexes between Ti and K + in excess of that needed to charge balance Al 3+ . The suggested stoichiometry of this complex is K 2 Ti 2 O 5 or K 2 Ti 3 O 7 . In CAS melts, the data suggest that Ca 2+ in excess of A1 3+ is not as effective at complexing with Ti as is K + . The greater solubility of rutile in CAS melts when C ∗ is less than 0.54 compared to KAS melts of equal K ∗ ratio results primarily from competition between Ti and Al for complexing cations (Ca vs . K). TiK β x-ray emission spectra of KAS glasses ( K ∗ = 0.43–0.60 ) with 7 mole% added TiO 2 , rutile, and Ba 2 TiO 4 , demonstrate that the average Ti-O bond length in these glasses is equal to that of rutile rather than Ba 2 TiO 4 , implying that Ti in these compositions is 6-fold rather than 4-fold coordinated. Re-examination of published spectroscopic data in light of these results and the solubility data, suggests that the 6-fold coordination polyhedron of Ti is highly distorted, with at least one Ti-O bond grossly undersatisfied in terms of Paulings rules.

The metamorphic paragenesis of cordierite in pelitic rocks

A petrogenetic grid is constructed for mineral assemblages occurring in metapelitic rocks, particularly those involved in the paragenesis of cordierite. The most useful assemblages for estimating pressures and temperatures are staurolite-cordierite, cordierite-biotite-Al2SiO5 and cordierite-hypersthene. Cordierite is stable with kyanite, sillimanite or andalusite. At high pressures cordierite is Mg-rich so that pelitic rocks typically do not contain the phase. Cordierite is stable at temperatures less than 500° C but does not commonly appear in metapelitic rocks until the garnet-chlorite, chlorite-staurolite or chlorite-Al2SiO5 tie-lines are broken. At high metamorphic grades, the assemblage garnet-hypersthene-cordierite indicates relatively low pressures, and the assemblage hypersthene-cordierite-sillimanite relatively high pressures. It is clear however, that the absence of cordierite is of little use in characterizing a metamorphic facies unless an alternate mineral assemblage can be shown to be more stable.

Petrogenesis of lunar troctolites

Models of petrogenesis for lunar troctolites are severely constrained by the high Mg* (>88) values of the most primitive members of the Mg-rich suite. Melts derived by partial melting of olivine cumulates to the lunar magma ocean have high Mg* but are strongly undersaturated with respect to plagioclase. Assimilation of lunar crust cannot bring these melts to the plagioclase liquidus without unduly reducing the Mg* values of olivine. Partial melts derived by melting a Iherzolite primitive interior have higher normative plagioclase contents but require sources with Mg* > 90. More terrestrial-like Mg* (∼88) values are permitted if rising thermal plumes are capable of entraining the most Mg-rich cumulates of the magma ocean. The Mg* values of lunar troctolites may, therefore, reflect a hybrid source composed of primitive mantle and Mg-rich cumulates of the magma ocean. Higher Mg* values may also arise from the reduction of small amounts of FeO to Fe. Alternatively, the parent magmas to lunar troctolites may be impact melts of the anorthositic crust and the Mg-rich dunite cumulates of the magma ocean; the latter were brought to the upper mantle during the overturn of the unstable cumulate pile of the magma ocean.