Edwin Roedder

Fluid inclusion evidence for immiscibility in magmatic differentiation

Abstract After a brief review of recent work on melt inclusions in diamond and in normal magmatic environments, the nature of the fluid-inclusion evidence for various stages of magmatic immiscibility is summarized. During magmatic differentiation by crystal fractionation, from originally low-volatile content silicate melts (yielding normal plutonic rocks), through hydrous silicate melts (yielding pegmatites), to late-stage water-rich fluids (yielding quartz veins and ore deposits), the composition of the residual liquid changes drastically. In some geologic environments, this change in composition of the residual liquid may be continuous, with no phase change in the fluid part of the system. Under most natural conditions, however, it is far more likely to have one or more stages in the evolution during which globules of a separate, new immiscible fluid phase exsolve. The term immiscibility is used here in its more general sense to refer to the existence, at equilibrium, of two or more non-crystalline polycomponent solutions (fluids), differing in properties and generally in composition. I believe that the great bulk of the magmatic inclusions that are being studied today have come about through the intervention of one or more such stages of immiscible fluid separation, i.e., magmatic differentiation by fluid immiscibility. Examples include silicate melt/dense CO2 fluid, silicate melt/sulfide melt, silicate melt/silicate melt, silicate melt/low density vapor (i.e., vesiculation), and, as presented in more detail in this paper, various stages of silicate melt/hydrous saline melt/aqueous fluid/CO2 fluid immiscibility. Generally, only parts of these stages are recorded by the trapping of fluid inclusions, and the interpretation of the inclusion record is commonly ambiguous and difficult, particularly that stage between hydrous silicate melts and the hydrous saline melts (and aqueous solutions) responsible for many ore deposits. The ambiguity results mainly from problems of inclusion origin and nonrepresentative trapping within any given individual inclusion. Thus the probability of trapping of only one, vs. various amounts of two, immiscible fluids, or of trapping solid phases along with the fluid, present severe difficulties. In addition, there is the almost universal superposition of various stages of trapping of fluids within any given sample. These problems can only be approached by very careful petrography and analysis of optimum material, combined with field data and experimental P-V-T-X studies of pertinent systems. Completely unequivocal statements of the course of evolution of the fluids in such natural systems may never be possible.

Uniquely fresh 2.7 Ga komatiites from the Belingwe greenstone belt, Zimbabwe

Extremely fresh Archean komatiite has been discovered near Zvishavane, Zimbabwe. The geochemistry of these rocks opens a window to the Archean mantle and will have profound implications on geophysical and geochemical models of early Earth. In the rocks, olivine is largely pristine and contains inclusions of fresh glass. Surface sampling and a 200-m drill hole have produced complete sections through a series of komatiite flows in which chill zones are preserved and which have MgO content of 17–20 wt%. Some alteration has taken place, but these are nevertheless among the freshest Archean rocks yet found.

Silicate liquid immiscibility in magmas and in the system K2O-FeO-AI2O3-SiO2: an example of serendipity

The concept of silicate liquid immiscibility was invoked early in the history of petrology to explain certain pairs of compositionally divergent rocks, but. as a result of papers by Greig (Am. J. Sci.13, 1–44, 133–154) and Bowen (The Evolution of the Igneous Rocks), it fell into disfavor for many years. The discovery of immiscibility in geologically reasonable temperature ranges and compositions in experimental work on the system K2O-FeO-Al2O3-SiO2, and of evidence for immiscibility in a variety of lunar and terrestrial rocks, has reinstated the process. Phase equilibria in the high-silica corner of the tetrahedron representing the system K2O- FeO-Al2O3-SiO2 are presented, in the form of constant FeO sections through the tetrahedron, at 10% increments. Those sections, showing the tentative relationships of the primary phase volumes, are based on 5631 quenching runs on 519 compositions, made in metallic iron containers in pure nitrogen. Thirteen crystalline compounds are involved, of which at least six show two or more crystal modifica-tions. Two separate phase volumes, in each of which two immiscible liquids, one iron-rich and the other iron-poor, are present at the liquidus. One of these volumes is entirely within the quaternary system, astride the 1:1 K2O:Al2O3 plane. No quaternary compounds as such have been found, but evidence does point toward at least partial quaternary solid solution, with rapidly lowering liquidus temperatures, from K2O·Al2O3· 2SiO2 (‘potash nepheline’, kalsilite. kaliophilite) to the isostructural compound K2O·FeO·3SiO2, and from K2O·Al2O3·4SiO2 (leucite) to the isostructural compound K2O·FeO·5SiO2, Both of these series apparently involve substitution, in tetrahedral coordination. of a ferrous iron and a silicon ion for two aluminum ions. Some of the ‘impurities’ found in analyses of the natural phases may reflect these substitutions. As a result of the geometry of the immiscibility volume located entirely within the quaternary system, compositions near it show a number of phase changes and large amounts of crystallization with small temperature changes, generally in the range 1100–1150 C. Similar low-temperature, high-alkali immiscibility was discovered in a few exploratory runs in the equivalent systems with Rb or Cs substituting for K. But not in those with Li or Na. A review of the compositions and general behavior of systems involving immiscibility, both stable and metastable, and of the evidence for natural immiscibility. indicates that it may be a much more common feature than generally thought. Several examples of natural immiscibility are detailed; most yield a felsic. alkali-aluminosilicate melt and a mafic melt. from a wide variety of generally basaltic parental magmas, both under- and over saturated. Unfortunately, the best line of evidence for immiscibility in terrestrial rocks, a sharply defined meniscus between two compositionally disparate glasses, is by its very nature self-destructing, since it is effectively eliminated by either crystallization or gravitative separation and coalescence into separate magmas. Verification of operation of the exosolutionor ‘splitting’ process on a large scale will probably require careful study of isotopic and trace element partitioning in both laboratory and field.

Metastable superheated ice in liquid-water inclusions under high negative pressure

In some microscopic inclusions (consisting of aqueous liquid and vapor) in minerals, freezing eliminates the vapor phase because of greater volume occupied by the resulting ice. When vapor fails to nucleate again on partial melting, the resulting negative pressure (hydrostatic tension) inside the inclusions permits the existence of ice I crystals under reversible, metastable equilibrium, at temperatures as high as +6.5�C and negative pressures possibly exceeding 1000 bars.

Application of a new Raman microprobe spectrometer to nondestructive analysis of sulfate and other ions in individual phases in fluid inclusions in minerals

Rosascoet al. (1975), reported the first successful application of laser-excited Raman spectroscopy for the identification and nondestructive partial analysis of individual solid, liquid, and gaseous phases in selected fluid inclusions. We report here the results of the application of a new instrument, based on back-scattering, that eliminates many of the previous stringent sample limitations and hence greatly expands the range of applicability of Raman spectroscopy to fluid inclusions. Fluid inclusions in many porphyry copper deposits contain 5–10 μm ‘daughter’ crystals thought to be anhydrite but too small for identification by the previous Raman technique. Using the new instrument, we have verified that such daughter crystals in quartz from Bingham, Utah, are anhydrite. They may form by leakage of hydrogen causing internal autooxidation of sulfide ion. Daughter crystals were also examined in apatite (Durango, Mexico) and emerald (Muzo, Colombia). Valid analyses of sulfur species in solution in small fluid inclusions from ore deposits would be valuable, but are generally impossible by conventional methods. We present a calibration procedure for analyses for SO42− in such inclusions from Bingham, Utah (12,000 ± 4000 ppm) and Creede, Colo. (probably < 500 ppm). A fetid Brazilian quartz, originally thought to contain liquid H2S, is shown to contain only HS− in major amounts.

Silicate liquid immiscibility in lunar magmas, evidenced by melt inclusions in lunar rocks.

Examination of multiphase melt inclusions in 91 sections of 26 lunar rocks revealed abundant evidence of late-stage immiscibility in all crystalline rock sections and in soil fragments and most breccias. The two individual immiscible silicate melts (now glasses) vary in composition, but are essentially potassic granite and pyroxenite. This immiscibility may be important in the formation of the lunar highlands and tektites. Other inclusions yield the following temperatures at which the several minerals first appear on cooling the original magma: ilmenite (?) liquidus, 1210�C; pyroxene, 1140�C; plagioclase, 1105�C; solidus, 1075�C. The glasses also place some limitations on maximum and minimum cooling rates.

Laser-Excited Raman Spectroscopy for Nondestructive Partial Analysis of Individual Phases in Fluid Inclusions in Minerals

Laser-excited Raman spectroscopy has been successfully applied to the identification and partial analysis ofsolid, liquid, and gaseous phases in fluid inclusions. The procedure is no panaceaforproblems ofanalysis offluid inclusions, but some uniquefeatures make it very useful. In particular, the measurement is performed in situ; it is nondestructive; and it can produce qualitative and quantitative data, some of which cannot be obtained otherwise, for samples as small as 10-9 gram. The analysis of fluid inclusions in minerals provides important data related to many mineralogical, geological, and geochemical processes. Inclusions represent samples of the fluids from which the host minerals have crystallized (1) or with which they have reacted (2). They may be trapped either during the growth of the host mineral or at one or more later times. Most samples thus contain more than one generation of inclusions, sometimes of greatly different ages and fluid compositions. During cooling, aftcr the fluid has been trapped, two or more phases gener-

Fluid inclusion analysis—Prologue and epilogue

Abstract This paper summarizes the development of the major qualitative, semiquantitative, and quantitative methods for the determination of the chemical and isotopic composition of fluid inclusions in minerals (excluding melt inclusions), and some of their limitations and applications. Emphasis is placed on the present state of the art and on some of the possibilities for improvements in the future.

Fluid inclusions in quartz crystals from South-West Africa

Abstract Quartz crystals from calcite veins of unknown age in Precambrian metasedimentary rocks at Geiaus No. 6 and Aukam farms in South-West Africa contain both primary and secondary inclusions filled with one or a variable combination of: organic liquid, moderately saline aqueous liquid, dark-colored solid, and vapor. Analysis of these materials by microscopy and by gas chromatography and mass spectrometry shows the presence of constituents of both low and high molecular weights. The former include CH 4 , C 2 H 6 , C 3 H 8 and possibly C 4 H 10 as well as CO, CO 2 , H 2 O, N 2 and H 2 . High molecular weight components are dominantly n-alkanes and isoprenoid hydrocarbons. The n-alkanes range from at least n-C 10 to n-C 33 . Concentrations of n-alkanes larger than n-C 17 decrease regularly with increasing carbon number. An homologous series of isoprenoid hydrocarbons ranging from at least C 14 to C 20 is present in unusually high concentrations. Pristane (C 19 ) is most abundant, and C 17 isoprenoid is least abundant. The molecular composition and distribution of hydrocarbons suggest biological precursors for these components. Consideration of data provided by freezing, crushing and heating experiments suggests that the pressures at the time these in part supercritical fluids were trapped probably exceeded 30–40 atm, and the minimum trapping temperature was about 120–160°C. Both primary and secondary inclusions apparently containing only organic materials were trapped by the growth of the host quartz from aqueous solution. The data obtained neither prove nor preclude Precambrian, Paleozoic or younger sources for the organic materials.

Geobarometry of ultramafic xenoliths from Loihi Seamount, Hawaii, on the basis of CO2 inclusions in olivine

Abstract Abundant fluid inclusions in olivine of dunite xenoliths (∼1–3 cm) in basalt dredged from the young Loihi Seamount, 30 km southeast of Hawaii, are evidence for three coexisting immiscible fluid phases—silicate melt (now glass), sulfide melt (now solid), and dense supercritical CO2 (now liquid + gas)—during growth and later fracturing of some of these olivine crystals. Some olivine xenocrysts, probably from disaggregation of xenoliths, contain similar inclusions. Most of the inclusions (2–10 μm) are on secondary planes, trapped during healing of fractures after the original crystal growth. Some such planes end abruptly within single crystals and are termed pseudosecondary, because they formed during the growth of the host olivine crystals. The “vapor” bubble in a few large (20–60 μm), isolated, and hence primary, silicate melt inclusions is too large to be the result of simple differential shrinkage. Under correct viewing conditions, these bubbles are seen to consist of CO2 liquid and gas, with an aggregate ϱ = ∼ 0.5–0.75 g cm−3, and represent trapped globules of dense supercritical CO2 (i.e., incipient “vesiculation” at depth). Some spinel crystals enclosed within olivine have attached CO2 blebs. Spherical sulfide blebs having widely variable volume ratios to CO2 and silicate glass are found in both primary and pseudosecondary inclusions, demonstrating that an immiscible sulfide melt was also present. Assuming olivine growth at ∼ 1200°C and hydrostatic pressure from a liquid lava column, extrapolation of CO2 P-V-T data indicates that the primary inclusions were trapped at ∼ 220–470 MPa (2200–4700 bars), or ∼ 8–17 km depth in basalt magma of ϱ = 2.7 g cm−3. Because the temperature cannot change much during the rise to eruption, the range of CO2 densities reveals the change in pressure from that during original olivine growth to later deformation and rise to eruption on the sea floor. The presence of numerous decrepitated inclusions indicates that the inclusion sample studied is biased by the loss of higher-density inclusions and suggests that some part of these olivine xenoliths formed at greater depths.