General Characteristics of Geofluids

Chapter Outline

The subdivision of everyday matter into solid, liquid, and gas (vapor) is also maintained in chemical and physical terminology. An ideal gas is characterized by perfect disorder at the molecular level. Its counterpart is the crystal structure of a solid—the most ideally ordered package of atoms and molecules. In contrast, an exact and concise definition of liquid is sometimes ambiguous. Liquids can be most easily classified in terms of cohesive (attractive and repulsive) forces keeping them together. One can distinguish between ionic liquids or molten salts, metallic liquids composed of ions and moving electrons, molecular liquids fixed by van der Waals forces, and waterlike liquids, where hydrogen bonds are the dominant attractive forces. Glass represents a compromise between crystals and liquids. Quartz crystals as well as quartz glass are fixed by electrostatic bonds between silica and oxygen. Unlike in quartz, these bonds have uneven lengths in silicate melts, resulting in some physical properties reminiscent of those of typical liquids (Moore, 1979).

The subdivision of matter into solid, liquid, and vapor is sufficient for the specific and narrow range of thermodynamic conditions on the Earth's surface. Distinguishing between liquid and vapor may be rather problematic in the Earth's crust and mantle, which is at increased temperature and pressure. Hence, geologists prefer the term fluid to characterize the aggregation state of the matter with properties of liquids and gases. Silicate melt at high temperature (> 700 °C) also has properties typical of liquids, but this term cannot be applied to a solidified magma on the Earth's surface. Dynamic viscosity—the ability of matter to flow due to an oriented differential strain—is a limiting physical quantity that divides liquids from solids.

Having an active supply of atoms and molecules during the crystallization of solids is the second fundamental feature of a fluid. The main fluid compounds can be directly involved in the structure of the precipitating solid, for example, during crystallization of halite from NaCl-oversaturated aqueous fluid and crystallization of rock-forming minerals from cooling silicate melt. In most cases, however, the fluid serves only as a transporting medium for soluble mineral-forming compounds, as during precipitation of insoluble sulfides from aqueous solutions by decomposition of soluble thiocomplexes. Except for some metamorphic reactions exchanging ions in a solid state, almost all terrestrial and extraterrestrial solids originate in a fluid medium as indicated by fluid inclusions identified in literally all minerals, including diamonds and meteorites.

1.1 Brief History of Geofluid Observation and Research

The first reference to what were probably fluid inclusions in a mineral is found in the Natural History of Gaius Plinius Secundus (Pliny the Elder) written about 75 A.D. (Kesler et al., 2013). The first scientific document about fluid inclusions was written by Abu Reikhan al-Biruni (972–1048 A.D.). The Uzbek scholar described “inclusions in form of air bubbles and water droplets” in quartz, sapphire, and other minerals in his book Precious Stones. He was the first to attribute these inclusions to “sap of the Earth,” from which minerals originated by “lithification.” Al-Biruni thereby defined for the first time the primary focus of the study of fluid inclusion: deciphering the origin of minerals and rocks. Apart from this, al-Biruni attributed the spontaneous cracking of gemstones during cutting and polishing to fluid inclusions and recommended their removal from minerals using fine drilling (Lemmlein, 1950). The first systematic descriptions of fluid inclusions in precious stones were made by Ahmad al-Tifashi in Cairo and Albertus Magnus, Archbishop of Cologne, in the 13th century (Kesler et al., 2013).

Sporadic observations of “moving bubbles” in quartz were described by Boyle (1672, 1673) and Scheuchzer (1723), but true scientific interest in fluid inclusions is dated at the outset of the 19th century. Davy (1822) made the first attempts to determine the chemical composition of fluid inclusions in quartz crystals. He tried to open the inclusions by drilling under water, oil, and mercury, and described different behavior among the liberated gas bubbles. Brewster (1823) observed within some fluid inclusions two immiscible liquids (water and carbon dioxide), and first reported on halite “squares” in an aqueous phase. Moreover, Brewster isolated insoluble daughter crystals from inclusions and identified them as calcite. Brewster (1845) was the first to study the behavior of solid phases in fluid inclusions on heating. The observations performed by Davy and Brewster significantly supported the neptunistic hypothesis of mineral origin.

A factual revolution and turning point in the study of fluid inclusions is connected with Henry Clifton Sorby. In his classic paper, Sorby (1858) was the first to describe melt inclusions in volcanic rocks (Figure 1.1) and to prove experimentally that the liquid phase in most inclusions is represented by water. He specifically described samples from ore deposits and drew conclusions concerning ore formation that remained scientifically unfashionable for many years. Sorby heated minerals containing fluid inclusions in a sealed test tube, had the condensed vapor frozen out, and determined its crystallographic shapes and temperature of melting. During these experiments, he noticed precipitation of another phase, provisionally identified as NaCl or KCl. Beyond that, he crushed quartz crystals and leached them in pure water to analyze soluble components in the inclusion fluids. He observed thermal expansion of salt-containing aqueous solutions in thin capillaries. These observations led him to conclude that the coefficient of thermal expansion of the sealed liquid must be one or two times greater than that of the host mineral, resulting in the formation of vapor bubbles in the aqueous inclusion on cooling. Consequently, he proposed a method of estimation of the crystallization temperature of minerals by heating the trapped fluid inclusions until the bubbles disappear. In such a way, Sorby defined the essential principle of the fluid inclusion thermometry. Sorby together with Buttler described in 1869 multiphase inclusions in emerald and spinel, and conducted experiments with homogenization of inclusions in sapphire. They found out that vapor bubbles in CO2-containing inclusions always diminished at temperatures lower than 31 °C.

Substantial progress in knowledge on material composition of fluid inclusions was achieved at the end of the 19th century. Phillips (1875, cited in Hein, 1990) used a paraffin bath to observe changes in fluid inclusions on heating and compared his results with those obtained by Sorby. He concluded that an inclusion population in the same sample exhibits various liquid-to-vapor ratios and so also had different homogenization temperatures. According to this, he questioned Sorby's assumptions about the possibility to determine crystallization temperatures of minerals by measuring homogenization temperatures. Neither Phillips nor other scientists who challenged Sorby's hypotheses could know of the existence of various inclusion generations trapped at different times, which led to their reaching erroneous...