Tibor Gasparik

Comparative compressibilities of majorite-type garnets

Relative compressibilities of five silicate garnets were determined by single-crystal x-ray diffraction on crystals grouped in the same high-pressure mount. The specimens include a natural pyrope [(Mg2.84Fe0.10Ca0,06) Al2Si3O12], and four synthetic specimens with octahedrally-coordinated silicon: majorite [Mg3(MgSi)Si3O12], calcium-bearing majorite [(Ca0.49Mg2.51)(MgSi)Si3012], sodium majorite [(Na1.88Mgp0.12)(Mg0.06Si1.94)Si3O12], and an intermediate composition [(Na0.37Mg2.48)(Mg0.13Al1.07 Si080) Si3O12]. Small differences in the compressibilities of these crystals are revealed because they are subjected simultaneously to the same pressure. Bulk-moduli of the garnets range from 164.8 ± 2.3 GPa for calcium majorite to 191.5 ± 2.5 GPa for sodium majorite, assuming K′=4. Two factors, molar volume and octahedral cation valence, appear to control garnet compression.

Experimentally determined compositions of diopside-jadeite pyroxene in equilibrium with albite and quartz at 1200–1350°C and 15–34 kbar

Abstract Equilibrium compositions of diopside-jadeite pyroxene coexisting with albite and quartz were experimentally determined at 25 different P-T conditions, using an electron microprobe for analysis. The new data and the 600°C data of Holland (1983) provided the following mixing properties of the diopside (Di)-jadeite (Jd) solid solution (J, K): G xs = X Jd X Di [12600 − 9.45 T + (12600 − 7.6 T )( X Jd − X Di ) − (21400 − 16.2 T )( X Jd − X Di ) 2 ]. The Di-Jd solution is close to ideal above 1000°C but immiscible below 565°C. The Di-Jd solvus is slightly asymmetric with the crest at composition Di 42.4 Jd 57.6 . Excess enthalpy is positive but smaller than indicated by the enthalpy of solution measurements of Wood et al. (1980). Disorder in the Di-Jd solution is significantly smaller than complete disorder implied by the ionic two-site model.

MELTING OF JADEITE TO 16.5 GPA AND MELTING RELATIONS ON THE ENSTATITE-JADEITE JOIN

Abstract A split-sphere anvil apparatus (USSA-2000) was used to determine the melting curve of jadeite (NaAlSi2O6) from 2.4 to 16.5 GPa, and the melting relations on the enstatite (Mg2Si2O6)-jadeite join at 2.8, 4, and 6 GPa. The melting temperatures of jadeite increase more rapidly with pressure than the known melting temperatures of any other mantle material and exceed the melting temperatures of enstatite around 13 GPa. The melting curve of jadeite was fitted with the Simon equation, as follows: P (GPa) = 2.5 + 2.01[T(K)/1543)3.7− 1]. Melting on the enstatite-jadeite join is peritectic in the investigated pressure range, with the peritectic composition becoming more magnesium-rich with increasing pressure. The melting changes to azeotropic at pressures higher than 8 GPa. Thus, while the fractionation at lower pressures can proceed to the most sodium-rich compositions, the fractionation at pressures greater than 8 GPa is controlled by the composition of the minimum on the azeotrope. This could have important implications for the fractionation of the upper mantle, applicable to fractional crystallization in a magma ocean.

Phase equilibrium and calorimetric study of the transition of MnTiO3 from the ilmenite to the lithium niobate structure and implications for the stability field of perovskite

The phase boundary between MnTiO3 I (ilmenite structure) and MnTiO3 II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO3 I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔHo=5300±1000 J·mol−1, ΔSo = 1.98 ±1J·K−1· mol−1. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol−1. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate, ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO3 is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms of crystal structure and lattice vibrations.

Geology of the Precambrian rocks between Elizabethtown and Mineville, eastern Adirondacks, New York

The northern part of the Elizabethtown and Port Henry quadrangles, which includes the largest surficial exposure of olivine metagabbro in the Adirondack Precambrian rocks, was mapped at a scale of 1:15,840. The area is dominated by meta-igneous rocks of four types, which probably intruded in the following sequence (from earliest to latest): granitic gneiss, anorthosite, garnet-pyroxene gneiss, and metagabbro. The metagabbroic complex is a multiple intrusion forming a basinlike structure, with the central part still covered by a roof complex of granitic gneisses. The limited scale of the in situ differentiation could not produce the observed range of compositions; therefore, differentiation prior to intrusion is postulated. The observed differentiation trend toward lower silica content can be explained by pyroxene fractionation. Intrusive relationships are evident from locally preserved chilled margins in gabbros and from the development of hybrid zones in the surrounding granitic gneisses; these zones are especially common in the roof complex. Granulite facies metamorphism produced garnet-bearing mineral assemblages, but igneous assemblages and textures are still well preserved. The garnet-pyroxene gneiss is characterized by the metamorphic assemblage plagioclase + garnet + clinopyroxene and by the presence of blue plagioclase mega-crysts. The gneiss forms intrusive sill-like bodies, usually in contact with anorthosite. In some places, the contact is transitional, suggesting a possible comagmatic origin.

Titanium in ureyite: A substitution with vacancy

Solubility of Ti4+ in ureyite (cosmochlor, NaCrSi2O6) was experimentally studied at 1 atmosphere and ≈1000°C, using sodium disilicate as flux. Microprobe analyses indicate that at low titanium concentrations the substitution of titanium in ureyite is almost exclusively in the M1 site, coupled with a vacancy in the M2 site. At higher TiO2 contents, a small additional amount enters the tetrahedral site. If the solubility of titanium is similar in jadeite and acmite, the □TiSi2O6 substitution could contribute significantly to the vacancy content of natural titanium bearing omphacites.

System MgO-SiO2

The two oxides, MgO and SiO2, account for 85 % of the Earth’s mantle. Thus, the phase relations in the system MgO-SiO2 (MS), shown in Figs. 2.1 and 2.2, are fundamental for understanding the mineral composition of the mantle. There are five enstatite polymorphs (Mg2Si2O6): protoenstatite, orthoenstatite, low clinoenstatite, high-T clinoenstatite, and high-P clinoenstatite, which can coexist in silica undersaturated compositions with forsterite, wadsleyite, or ringwoodite (Mg2SiO4). At high pressures, high-P clinoenstatite breaks down to majorite at the temperatures above 1,600 °C, and to wadsleyite + stishovite or ringwoodite + stishovite at lower temperatures. Akimotoite and MgSiO3 perovskite appear at progressively still higher pressures. The MgSiO3 perovskite can coexist with periclase (MgO) in the lower mantle.

System MgO-Al2O3-SiO2

The three-component chemical system MgO–Al2O3–SiO2 (MAS) accounts for 89 % of the Earth’s mantle. Phase relations in the MAS system are fundamental for understanding the mineral composition of the mantle; other components only modify but do not qualitatively alter these phase relations. The importance of the MAS system has been recognized early, as is evident from the large number of experimental studies carried out in this system over a time period of several decades. However, the quality of these studies varies substantially. While the phase relations for spinel and garnet peridotites have been studied repeatedly and are known in detail, the equilibrium relations for other MAS compositions are known only partially and often from older studies which have not been re-investigated by more modern experimental and analytical techniques. The approach used here builds on the detailed experimental work carried out for the spinel and garnet peridotites, and extends this information to the whole MAS system by maintaining internal consistency. Earlier experimental studies were reviewed to obtain the full understanding of the complex phase relations at low pressures, and to derive an internally consistent set of parameters. These parameters were then used to calculate phase relations for the whole MAS system in the T-P range corresponding to the crust and upper mantle.

System CaO–MgO–Al 2 O 3 –SiO 2 Saturated with Silica

The silica-saturated CMAS system includes assemblages with anorthite, kyanite, and quartz or coesite. Clinopyroxene coexisting with such silica-rich phases can dissolve an excess of silica charge balanced by vacancies in the M2 site. Such “non-stoichiometric” pyroxenes were first reported by Eskola (1921) and synthesized by Mao (1971). Other natural occurrences were reported by Smyth (1980) and McCormick (1986). The compositions of clinopyroxene with vacancies can be extrapolated to the end-member Ca0.5AlSi2O6. Following Khanukova et al. (1976 a), Gasparik and Lindsley [2] started to use for this component the name Ca-Eskola pyroxene (CaEs), and reported the synthesis of a pyroxene along the CaTs-CaEs join with up to 40 mol % of CaEs.

Composition and Structure of the Earth’s Interior

The primary goal of the research at very high pressures is the interpretation of the Earth’s structure, as revealed by seismic observations, in terms of the mineral and chemical composition. A major progress in the study of the Earth’s interior has been made in the last 20 year, primarily due to advances in petrology, geophysics, and geochemistry. However, our understanding of the Earth’s interior lags severely behind the accumulation of the facts, because it is hampered and distorted by the many myths and beliefs inherited from the past. The origins of many of these views cannot be even traced anymore, others were proposed in the distant past on the basis of meager or nonexistent evidence. Some represent only a clever scheme to compensate for the lack of facts and the inability to obtain them. Yet, despite the advances that make possible now to obtain those facts, the old views still figure prominently in the current understanding of the Earth’s interior, and stand in the way of the real progress made possible by the accumulation of the new evidence.