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High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions

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Science  08 Jul 2011:
Vol. 333, Issue 6039, pp. 213-215
DOI: 10.1126/science.1204626

Abstract

The Moon has long been thought to be highly depleted in volatiles such as water, and indeed published direct measurements of water in lunar volcanic glasses have never exceeded 50 parts per million (ppm). Here, we report in situ measurements of water in lunar melt inclusions; these samples of primitive lunar magma, by virtue of being trapped within olivine crystals before volcanic eruption, did not experience posteruptive degassing. The lunar melt inclusions contain 615 to 1410 ppm water and high correlated amounts of fluorine (50 to 78 ppm), sulfur (612 to 877 ppm), and chlorine (1.5 to 3.0 ppm). These volatile contents are very similar to primitive terrestrial mid-ocean ridge basalts and indicate that some parts of the lunar interior contain as much water as Earth’s upper mantle.

The Moon is thought to have formed in a giant impact collision between a Mars-sized object and an early-formed proto-Earth (1). Though all of the inner planets, including Earth, are depleted in water and other volatiles when compared with primitive meteorites, the more extreme depletion of volatiles in lunar volcanic rocks has long been taken as key evidence for a giant impact that resulted in high-temperature catastrophic degassing of the material that formed the Moon (2, 3). However, recent work on rapidly quenched lunar volcanic glasses has detected the presence of water dissolved in lunar magmas at concentrations up to 46 parts per million (ppm) (4), and water contents of lunar apatite grains from mare basalts are consistent with similarly minor amounts of water in primitive lunar magmas (57). These results indicate that the Moon is not a perfectly anhydrous planetary body and suggest that some fraction of the Moon’s observed depletion in highly volatile elements may be the result of magmatic degassing during the eruption of lunar magmas into the near-vacuum of the Moon’s surface.

To bypass the process of volcanic degassing, we conducted a search for lunar melt inclusions in Apollo 17 sample 74220, a lunar soil containing ~99% high-Ti volcanic glass beads, the so-called orange glass with 9 to 12 weight % (wt %) TiO2 (8). Melt inclusions are small samples of magma trapped within crystals that grow in the magma before eruption. By virtue of their enclosure within their host crystals, melt inclusions are protected from loss of volatiles by degassing during magma eruption. Melt inclusions have been used for decades to determine pre-eruptive volatile contents of terrestrial magmas from subduction zones (9, 10), hotspots (11, 12), and mid-ocean ridges (13, 14), as well as volatile contents of martian magmas (15, 16). Using standard petrographic methods, we identified nine inclusion-bearing olivine crystals (Fig. 1) and analyzed melt inclusions hosted within (17). The measured water contents of the melt inclusions range from 615 ppm to a maximum of 1410 ppm (Fig. 2); these water contents are up to 100 times as high as the water content of the matrix glass surrounding the olivine crystals (6 to 30 ppm H2O) and the centers of individual volcanic glass beads from the same sample (4). The melt inclusions also contain high concentrations of fluorine (50 to 78 ppm), sulfur (612 to 877 ppm), and chlorine (1.5 to 3.0 ppm) that are 2 to 100 times as high as those of the matrix glasses and individual glass beads from this sample (Fig. 3). Volatile contents corrected for postentrapment crystallization are on average 21% lower than the measured concentrations (17) and represent the best estimate of the pre-eruptive concentrations of volatiles in the 74220 magma.

Fig. 1

(A to F) Optical photographs of olivines A1, A2, N3, N6, N8, and N9 from Apollo 17 sample 74220. Inclusions within circles indicate the inclusions that were imaged in Fig. 2. Scale bars are 10 μm in all photos.

Fig. 2

(A to F) NanoSIMS scanning isotope images of olivines A1, A2, N3, N6, N8, and N9 from Apollo 17 sample 74220, showing the distribution of water within melt inclusions from the olivine grains shown in Fig. 1. The images show the distribution of the isotope ratio 16OH/30Si indicated by the color scale shown in (A), which ranges from dark regions corresponding to low 16OH/30Si ratios (e.g., olivine surrounding melt inclusions), to red regions within melt inclusions with 16OH/30Si ratios approaching 0.25 (corresponding to ~1400 ppm H2O). The color scale is the same in all images, and all images show a scale bar of 1 μm. Rectangular areas are regions of interest within which each isotope ratio is calculated and converted to a concentration.

Fig. 3

(A to C) Volatile abundances for lunar melt inclusions (orange circle with black rims) and matrix glasses (orange circles) from Apollo 17 sample 74220. Melt inclusions show the highest concentrations (>600 ppm H2O) whereas matrix glasses show the lowest concentrations due to degassing (≤30 ppm H2O). The black curves show lunar magma degassing trends, scaled from the volatile-volatile correlations observed in core-rim NanoSIMS data on a lunar glass bead reported by Saal et al. (4); the core-rim data were scaled by multiplying the originally reported data for each element, by the ratio of the highest melt inclusion composition to that of the core composition reported in table 2 of (4). The gray field surrounds data for melt inclusions from the Siqueiros Fracture Zone on the East Pacific Rise, as an example of depleted MORB (24).

There are few descriptions in the literature of melt inclusions contained within olivine from lunar samples, but these existing observations provide important context for the volatile abundances we have observed. Roedder and Weiblen (1820) noted the presence of silicate melt inclusions in the first samples returned from the Apollo 11, Apollo 12, and Luna 16 missions and reported that many primary melt inclusions contained a vapor bubble, requiring dissolved volatiles to have been present in the melt at the time it was trapped within the host crystal. Klein and Rutherford (21) and Weitz et al. (22) found sulfur contents of 600 to 800 ppm, similar to our measurements, and Cl contents of <50 ppm that were limited by electron microprobe detection limits. Reheated Apollo 12 melt inclusions, containing medium-Ti magmas (5 to 6 wt % TiO2), show sulfur contents that are 20% higher than our data on average (23). In our data set, we observe a correlation of all the volatiles with each other (Fig. 3), pointing toward the degassed compositions of the matrix glass rinds and volcanic glass beads (4).

The most important aspect of our volatile data on lunar melt inclusions is their similarity to melt inclusions from primitive samples of terrestrial mid-ocean ridge basalts (MORBs), like those recovered from spreading centers located within transform faults (24); the melt inclusions from 74220 are markedly similar to melt inclusions from the Siqueiros Fracture Zone on the East Pacific Rise, some of the most primitive mid-ocean ridge magmas that have been measured (Fig. 3). These similarities suggest that the volatile signature of the lunar mantle source of the high-Ti melt inclusions is very similar to that of the upper mantle source of MORB.

It is important that we have made these measurements on inclusions from olivine crystals contained within primitive lunar volcanic glasses. These inclusions were quenched within minutes after their eruption (4), providing minimal opportunity for posteruptive hydrogen diffusion out of the inclusions and affording a direct H2O measurement on primary lunar magma samples that have not experienced posteruptive degassing and associated loss of volatiles. The water concentrations that we measured are 20 to 100 times as high as previous direct measurements of the lunar glass beads from this same sample, which was estimated to have suffered 95 to 98% loss of H2O via degassing (4), and they are higher than estimates derived from lunar apatite measurements, which require a 95 to 99% correction for fractional crystallization to estimate primary magma volatile contents (5, 6). Our results are direct measurements on primary lunar magma compositions that require no such extrapolations.

Our melt inclusion data allow us to place some constraints on the volatile content of the lunar mantle source that generated the high-Ti picritic magmas. Using the most water-rich melt inclusion composition after correction for postentrapment crystallization, and an estimation that the high-Ti magmas originated from 5 to 30% batch partial melting with partitioning similar to that of terrestrial mantle-derived melts (17), we estimate lunar mantle volatile concentrations of 79 to 409 ppm H2O, 7 to 26 ppm F, 193 to 352 ppm S, and 0.14 to 0.83 ppm Cl. These estimates overlap most estimates for the volatile content of the terrestrial MORB mantle (2427) and are much higher than previous estimates for the lunar mantle based on the volatile content of lunar apatite (5, 6) and the variation of Cl isotopes in lunar rocks (28), including the sample 74220 that we have studied here. The melt inclusions indicate definitively that some reservoirs within the interiors of Earth and the Moon not only have similar water contents, but also similar contents of fluorine, sulfur, and chlorine associated with this water, a volatile abundance signature shared by both bodies.

These results show that the Moon is the only planetary object in our solar system currently identified to have an internal reservoir with a volatile content similar to that of Earth’s upper mantle, and that previous estimates of the lunar inventory for highly volatile elements are biased to low concentrations owing to the degassed nature of lunar samples thus far studied. The Moon has erupted a wide variety of magmas during its history, and it remains to be seen whether other lunar mantle sources are as volatile rich as the source of Apollo 17 high-Ti magmas. Nevertheless, the hydrated nature of at least part of the Moon’s interior is a result that is not consistent with the notion that the Moon lost its entire volatile inventory to the vacuum of space during degassing after a high-energy giant impact, which would be expected to leave a highly desiccated lunar interior.

If the bulk of the lunar interior has a volatile content similar to our estimate for the high-Ti mantle source, then our results present difficulties for late-accretion models that require volatile delivery to Earth and the Moon after their formation, because these two bodies have very different accretion cross sections that would predict different internal volatile contents. An Earth-Moon similarity in volatiles could indicate that chemical exchange of even the most volatile elements between the molten Earth and the proto-lunar disc might have been pervasive and extensive, resulting in homogenization at the very high temperatures expected after a giant impact; this could have been aided by the presence of a high-temperature convective atmospheric envelope surrounding Earth and the proto-lunar disc as the Moon solidified (29). Alternatively, it is conceivable that a portion of the lunar interior escaped the widespread melting expected in the aftermath of a giant impact and simply inherited the inventory of water and other volatiles that is characteristic of Earth’s upper mantle. Any model for the formation of Earth-Moon system must meet the constraints imposed by the presence of H2O in the lunar interior, with an abundance similar to that of Earth’s upper mantle and with a complement of fluorine, sulfur, and chlorine also present at terrestrial levels. To the extent that lunar formation models predict very different volatile contents of Earth and the Moon, our results on the volatile content of lunar melt inclusions suggest that we lack understanding on some critical aspects of the physics of planetary moon formation by collisional impact.

Our findings also have implications for the origin of water ice in shadowed lunar craters, which has been attributed to cometary and meteoritic impacts (30). It is conceivable that some of this water could have originated from magmatic degassing during emplacement and eruption of lunar magmas (31). These results also underscore the importance of pyroclastic volcanic samples in unraveling the history and composition of the Moon’s interior; indeed, such deposits have been identified and mapped on the surfaces of all the terrestrial planets and many satellites.

Supporting Online Material

www.sciencemag.org/cgi/content/full/science.1204626/DC1

Materials and Methods

Figs. S1 and S2

Tables S1 and S2

References (3237)

References and Notes

  1. Detailed information on melt inclusion identification and analytical methods used in this study, as well as all data, are contained in supporting online materials available at Science Online.
  2. Acknowledgments: This work was supported by the Carnegie Institution of Washington, the NASA LASER and Cosmochemistry programs, the NASA Lunar Science Institute, and the NASA Astrobiology Institute. We thank J. Wang for NanoSIMS assistance, and L. Nittler and Z. Peeters for help with image processing software.
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