Report

Asteroidal Water Within Fluid Inclusion-Bearing Halite in an H5 Chondrite, Monahans (1998)

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Science  27 Aug 1999:
Vol. 285, Issue 5432, pp. 1377-1379
DOI: 10.1126/science.285.5432.1377

Abstract

Crystals of halite and sylvite within the Monahans (1998) H5 chondrite contain aqueous fluid inclusions. The fluids are dominantly sodium chloride–potassium chloride brines, but they also contain divalent cations such as iron, magnesium, or calcium. Two possible origins for the brines are indigenous fluids flowing within the asteroid and exogenous fluids delivered into the asteroid surface from a salt-containing icy object.

Over the past three decades, we have become increasingly aware of the fundamental importance of water and aqueous alteration on primitive solar system bodies. Some carbonaceous and ordinary chondrites, long touted as primordial material relatively unchanged since formation, have been altered by interactions with liquid water within the first 10 million years after formation of their parent asteroids (1). Nevertheless, we do not know the location and timing of the aqueous alteration or the nature of the aqueous fluid itself. Workers have attempted to model and understand this aqueous process through analysis of hydrated minerals present in the meteorites and through computer simulations of the alteration process (2). A major impediment to our understanding of aqueous alteration has been the absence of actual samples of aqueous fluids in meteorites. Here, we report the discovery and characterization of aqueous fluid inclusions in an ordinary chondrite.

The Monahans (1998) (hereafter Monahans) ordinary chondrite fell on 22 March 1998 in Monahans, Texas. The fall was witnessed by seven boys, and the first of two stones was recovered immediately. This first stone was carried to the Johnson Space Center and broken open in a filtered-air, clean-room facility <48 hours after the fall, effectively eliminating the opportunity for aqueous or other contamination of the interior of the meteorite (3). The meteorite is a regolith breccia consisting of light and dark clasts set within a gray matrix; the hue is controlled by the shock level. The light lithology has witnessed shock level S2 (5 to 10 GPa), the gray lithology has witnessed S3 (10 to 15 GPa), and the dark lithology has witnessed S4 (15 to 30 GPa) (4). All lithologies are H5, which indicates moderate thermal metamorphism (5). After breaking open the first sample, we noted that the gray matrix contains locally abundant aggregates of purple halite (NaCl) crystals, measuring up to 3 mm in diameter. To the best of our knowledge, megascopic halite has not been previously seen in any extraterrestrial sample, although microscopic halite has been reported in several meteorites (6–8). Backscattered electron imaging of Monahans reveals that crystals of sylvite (KCl) are present within the halite crystals, similar to their occurrence in terrestrial evaporites (9). The purple color of the halite may be caused by exposure to solar and galactic cosmic rays, exposure to beta-decaying 40K in the sylvite (9, 10), or a combination of these effects. The presence of halite and sylvite only within the gray matrix indicates that the halite may have formed at the surface of the asteroid, before final aggregation of the meteorite. However, its deposition probably postdates the most severe thermal metamorphism and shock, because halite is a brittle material that could not have survived these processes.

The three lithologies of Monahans were analyzed separately for noble gases (11), which revealed that Monahans is a regolith breccia. Relative amounts of He, 20Ne, and 36Ar in the gray matrix and dark clasts indicate a major contribution of solar wind–implanted gases (11). The gray matrix lithology contains the highest concentration of solar wind–implanted gas (at standard temperature and pressure, cosmogenic 21Ne and38Ar is 3.0–8 and 3.4 × 10–9 to 4.2 × 10–9 cm3g–1, respectively) (11), indicating that it was pre-irradiated for a few million years on the surface of its parent asteroid during the time of the halite deposition and before its ejection as a meteorite.

One milligram of Monahans halite with sylvite was used for Rb/Sr analysis by mass spectrometry (12). This sample contains 3.75 ± 0.08 parts per million (ppm) of Rb and 0.257 ± 0.006 ppm of Sr, leading to the following ratios:87Rb/86Sr = 43.3 ± 1.6 and87Sr/86Sr = 3.59 ± 0.07. With a87Rb decay constant of 0.01402 giga-annum–1(Ga–1) (12) and an initial ratio of87Sr/86Sr = 0.698972 (13), the calculated Rb-Sr model age for the halite is 4.7 ± 0.2 Ga. The bulk H-group chondrites define an age of 4.58 ± 0.04 Ga (14), which is within the error limits of our reference isochron of 4.56 Ga (Fig. 1). Because the isotopic composition of Sr in the halite and sylvite is very radiogenic, the model age is considered to be the formation age of the halite; it formed early in the history of the Monahans parent asteroid.

Figure 1

87Rb/86Sr and87Sr/86Sr systematics for the Monahans halite and bulk H-group chondrites. The reference 4.56-Ga isochron is calculated for an initial ratio of87Sr/86Sr = 0.698972, determined for angrite LEW86010 (13) and a 87Rb decay constant of 0.01402 Ga–1 (14). Monahans halite is shown as a shaded circle; error bars indicate analytical uncertainties. Data for bulk H-group chondrites (14) are plotted as open squares, and the diagonal line is an isochron fitted to the data for H-group chondrites. Strontium in the Monahans halite is very radiogenic; thus, the halite age is insensitive to uncertainties in the initial 87Sr/86Sr ratio. The model age of 4.7 ± 0.2 Ga calculated for an initial ratio of87Sr/86Sr = 0.69876 ± 0.00040, the average for H-group chondrites (14), is the same as that calculated for the angrite initial 87Sr/86Sr ratio, assumed to be representative of the early solar system.

Fluid inclusions are microsamples of fluid that are trapped as isolated inclusions in the crystal during growth (primary inclusions) or at some later time along a healed fracture in the mineral (secondary inclusions), forming planes (15). Primary and secondary fluid inclusions are found in Monahans halite, with secondary inclusions being predominate (∼80%). The presence of secondary inclusions in the halite indicates that aqueous fluids were locally present after halite deposition, suggesting that aqueous activity could have been episodic.

The inclusions range up to 15 μm in their longest dimension (Fig. 2). At room temperature, a few of the inclusions (∼25%) contain bubbles that are in constant motion, indicating that the inclusions contain a low-viscosity liquid and vapor. The remaining inclusions contain only a single liquid phase. During the cooling of 10 randomly selected inclusions under a microscope, the inclusions solidified (froze) at −45° to −50°C. Thus, the liquid in the inclusions cannot be carbon dioxide, because solid and vapor carbon dioxide cannot coexist at any temperature above the triple point of carbon dioxide, which is −56.6°C. Moreover, the vapor bubble appeared to shrink and in some cases disappear during freezing, consistent with the interpretation that the inclusions contain an aqueous solution (if the inclusions were pure carbon dioxide, the bubble would become larger when liquid carbon dioxide was frozen to form solid carbon dioxide). When the frozen inclusions were heated, melting was initially observed at about −35° to −40°C, indicating that the inclusions may contain divalent cations such as Fe2+, Ca2+, or Mg2+, in addition to Na+ and K+ (16). Given the environment of formation (1), dissolved Fe and Ca are the most likely cause of the lowered first melting temperatures, unlike in terrestrial evaporite environments, in which Ca and Mg cause a reduction in the melting temperatures. The final ice-melting temperature was difficult to determine with any degree of accuracy. The halite sample containing the fluid inclusions described here was not polished (it was simply broken out of the meteorite with a needle) and had been carbon coated for earlier electron microprobe analysis. As a result, the optics on the petrographic microscope were poor. This, combined with the small size of the inclusions and the layer of glass between the microscope objective lens and the sample during cooling, made observation of phase changes difficult. It is estimated that final ice melting occurred at or slightly below −20°C, because at this temperature the vapor bubble moved freely within the inclusions. Below this temperature, the bubble was generally pinned into one place or moved only in a restricted region of the inclusion, presumably because of the presence of ice (or hydrate phases) that could not be discerned optically.

Figure 2

Transmitted light images of fluid inclusions in Monahans halite. (A and B) Primary inclusions, each containing a liquid and a small bubble (indicated by arrows). Scale bars, 5 μm.

The presence of water in the inclusions was confirmed by Raman microprobe analysis (17). Raman spectra of inclusions in Monahans halite show a peak at ∼3400 cm–1, which is diagnostic of aqueous salt solutions (Fig. 3). Pure water gives a broad peak that is skewed to lower wave numbers. With the addition of salt, the peak becomes more symmetrical and narrower (18), as shown by the spectrum for a droplet of NaCl-saturated water collected at the same conditions used for the two inclusions in halite. The Raman spectrum of an area of the halite away from inclusions does not show a peak in the region of 3000 to 4000 cm–1 (Fig. 3), indicating that the species responsible for the peak occurs in the inclusions but not in the halite.

Figure 3

Raman microprobe spectra of Monahans fluid inclusions, halite, and NaCl-saturated water. The two spectra of fluid inclusions in Monahans halite (inclusions 1 and 2) show a broad peak at ∼3400 cm–1 (indicated by vertical dashed line). A spectrum of Monahans halite near inclusion 1 lacks the 3400 cm–1 peak. A spectrum of a droplet of NaCl-saturated water (with a peak at 3400 cm–1) is shown for comparison; the peak at ∼1000 cm–1 in this spectrum is from the glass slide.

The minimum temperature of formation of a given fluid inclusion may be estimated on the basis of the relative size of the vapor bubble in the inclusion at room temperature. Thus, a pure water inclusion with a homogenization temperature of 374°C would contain 69% by volume vapor at room temperature, whereas a pure water inclusion with a homogenization temperature of 200°C would contain ∼15% by volume vapor at room temperature. Fluid inclusions with homogenization temperatures <100°C often fail to nucleate a vapor bubble when cooled to room temperature and remain as single-phase liquid inclusions that are metastable (15). The rarity of vapor bubbles in fluid inclusions in Monahans halite (we only observed 11 bubbles in ∼40 inclusions) suggests a low formation temperature (<100°C), assuming that the formation pressure was low (a few tens of bars at most). The bubbles that are present only occupy ∼5 to 10% by volume of the inclusions and may have resulted from freeze stretching of the inclusions during space exposure (19). The vapor bubbles represent the water vapor in equilibrium with the aqueous solution at room temperature and, as such, are essentially a poor vacuum with a pressure of ∼0.03 bars. No other aqueous fluids, such as CO2, N2, or CH4, were detected during Raman analysis of the inclusions. Detection limits for these species are generally in the range of 1 mole percent in fluid inclusions, indicating that if the gases are present, their concentrations are low.

There have been numerous reports of liquid inclusions in meteorites (20). One of the more widely known studies of fluid inclusions in meteorites, and the one that led to reports of fluid inclusions in meteorites being viewed with skepticism over the past 15 years, was that by Warner et al. (21). In that study, the authors reported the presence of two-phase liquid-vapor inclusions in silicate minerals in diogenite ALHA 77256. The interest in fluid inclusions generated in the planetary science community by this report was short-lived, however, as later investigation of the same samples showed that the inclusions were artifacts from specimen preparation fluids (22). Observations of fluid inclusions within the ordinary chondrites Peetz and Jilin (23) have never been confirmed. The fluid inclusions reported here in the Monahans chondrite are a confirmation that aqueous fluid inclusions exist in some extraterrestrial samples.

Previous studies have indicated that Cl is inhomogeneously distributed in meteorites (24), with the result that the solar abundance of Cl is not well known. Halite was noticed in Monahans because of its purple color and large grain size, which permitted special sample preparation procedures to be employed, preserving the grains. It is possible that halite is commonly present in chondrites but has been overlooked. It is also possible that a considerable fraction of the ubiquitous sulfate and halide efflorescence noted on Antarctic meteorites (25) is derived from dissolution and reprecipitation of indigenous halite (and sulfides), rather than from components introduced from the ice as is commonly assumed.

The most important conclusion of this study is that an actual sample of the aqueous fluid presumably responsible for aqueous alteration of early solar system material has been identified and characterized, and this fluid is a brine. Two possible origins for the brines are (i) indigenous fluids flowing within the asteroid and (ii) exogenous fluids delivered into the asteroid surface from a salt-containing icy object, such as a comet. In either case, the inclusions provide ground truth concerning the nature of water in the early solar system.

REFERENCES AND NOTES

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