J. W. Boyce

Lunar apatite with terrestrial volatile abundances

The Moon is thought to be depleted relative to the Earth in volatile elements such as H, Cl and the alkalis. Nevertheless, evidence for lunar explosive volcanism has been used to infer that some lunar magmas exsolved a CO-rich and CO2-rich vapour phase before or during eruption. Although there is also evidence for other volatile species on glass spherules, until recently there had been no unambiguous reports of indigenous H in lunar rocks. Here we report quantitative ion microprobe measurements of late-stage apatite from lunar basalt 14053 that document concentrations of H, Cl and S that are indistinguishable from apatites in common terrestrial igneous rocks. These volatile contents could reflect post-magmatic metamorphic volatile addition or growth from a late-stage, interstitial, sulphide-saturated melt that contained ∼1,600 parts per million H2O and ∼3,500 parts per million Cl. Both metamorphic and igneous models of apatite formation suggest a volatile inventory for at least some lunar materials that is similar to comparable terrestrial materials. One possible implication is that portions of the lunar mantle or crust are more volatile-rich than previously thought.

The Lunar Apatite Paradox

Wetted Apatite The long-running story of the dry Moon was rewritten a few years ago when hydrogen-bearing glass spherules were discovered. The highest water contents are found in lunar apatite, at levels suspiciously comparable to the water content of Earth apatites. Boyce et al. (p. 400, published online 20 March; see the Perspective by Anand) now show that the water content of lunar apatite is not a reliable indicator of the abundance of water in mare basalts. The existence of apatite with high water content is an almost inevitable consequence of the loss of tiny amounts of fluorine-rich apatite from a melt and replacement by hydrogen and is thus no indication of a “wet” Moon. Hydrogen-rich apatite crystals in lunar volcanic rocks indicate self-inflicted loss of fluorine from basaltic melts. [Also see Perspective by Anand] Recent discoveries of water-rich lunar apatite are more consistent with the hydrous magmas of Earth than the otherwise volatile-depleted rocks of the Moon. Paradoxically, this requires H-rich minerals to form in rocks that are otherwise nearly anhydrous. We modeled existing data from the literature, finding that nominally anhydrous minerals do not sufficiently fractionate H from F and Cl to generate H-rich apatite. Hydrous apatites are explained as the products of apatite-induced low magmatic fluorine, which increases the H/F ratio in melt and apatite. Mare basalts may contain hydrogen-rich apatite, but lunar magmas were most likely poor in hydrogen, in agreement with the volatile depletion that is both observed in lunar rocks and required for canonical giant-impact models of the formation of the Moon.

Magmatic degassing histories from apatite volatile stratigraphy

Measurements of OH and Cl growth zoning in apatite phenocrysts from the Cerro Galan ignimbrite describe the tumultuous volatile history of a magma chamber in the days to months prior to a supereruption. The chemical stratigraphy in these crystals provides a unique relative time scale not otherwise available when studying pre-eruptive volatiles in ancient magma chambers, and it allows us to temporally correlate the data within each crystal, resulting in a “volatile history.” The five crystals studied show broadly similar volatile histories, but distinctive differences between the various crystals suggest that each recorded a specific perspective on the magma chamber prior to eruption, perhaps correlating to different depths or laterally separated regions within the >1000 km 3 magma chamber. All crystals are consistent with the interpretation of a multistage magmatic history, including a period of residence in a quiescent magma, and a period of rapid growth under strongly varying conditions. Several apparent “events” during this period of growth, consistent with two competing processes of H 2 O degassing and recharge, are recorded by nearly all of these crystals. Such zoning could not have survived for more than ∼400 d prior to eruption, suggesting that during the days to months leading up to a supereruption, apatite records key aspects of the processes that directly lead to and influence these catastrophic events.

Experimental investigation of F, Cl, and OH partitioning between apatite and Fe-rich basaltic melt at 1.0-1.2 GPa and 950-1000 °c

Abstract Apatite-melt partitioning experiments were conducted in a piston-cylinder press at 1.0-1.2 GPa and 950-1000 °C using an Fe-rich basaltic starting composition and an oxygen fugacity within the range of ΔIW-1 to ΔIW+2. Each experiment had a unique F:Cl:OH ratio to assess the partitioning as a function of the volatile content of apatite and melt. The quenched melt and apatite were analyzed by electron probe microanalysis and secondary ion mass spectrometry techniques. The mineral-melt partition coefficients (D values) determined in this study are as follows: DFAp-Melt = 4.4-19, DClAp-Melt = 1.1-5, DOHAp-Melt = 0.07-0.24. This large range in values indicates that a linear relationship does not exist between the concentrations of F, Cl, or OH in apatite and F, Cl, or OH in melt, respectively. This non- Nernstian behavior is a direct consequence of F, Cl, and OH being essential structural constituents in apatite and minor to trace components in the melt. Therefore mineral-melt D values for F, Cl, and OH in apatite should not be used to directly determine the volatile abundances of coexisting silicate melts. However, the apatite-melt D values for F, Cl, and OH are necessarily interdependent given that F, Cl, and OH all mix on the same crystallographic site in apatite. Consequently, we examined the ratio of D values (exchange coefficients) for each volatile pair (OH-F, Cl-F, and OH-Cl) and observed that they display much less variability: KdCl-FAp-Melt = 0.21± 0.03, KdOH-FAp-Melt = 0.014 ± 0.002, and KdOH-ClAp-Melt = 0.06 ± 0.02 . However, variations with apatite composition, specifically when mole fractions of F in the apatite X-site were low (XF < 0.18), were observed and warrant additional study. To implement the exchange coefficient to determine the H2O content of a silicate melt at the time of apatite crystallization (apatitebased melt hygrometry), the H2O abundance of the apatite, an apatite-melt exchange Kd that includes OH (either OH-F or OH-Cl), and the abundance of F or Cl in the apatite and F or Cl in the melt at the time of apatite crystallization are needed (F if using the OH-F Kd and Cl if using the OH-Cl Kd). To determine the H2O content of the parental melt, the F or Cl abundance of the parental melt is needed in place of the F or Cl abundance of the melt at the time of apatite crystallization. Importantly, however, exchange coefficients may vary as a function of temperature, pressure, melt composition, apatite composition, and/or oxygen fugacity, so the combined effects of these parameters must be investigated further before exchange coefficients are applied broadly to determine volatile abundances of coexisting melt from apatite volatile abundances.

Months between rejuvenation and volcanic eruption at Yellowstone caldera, Wyoming

Rejuvenation of previously intruded silicic magma is an important process leading to effusive rhyolite, which is the most common product of volcanism at calderas with protracted histories of eruption and unrest such as Yellowstone caldera (Wyoming), Long Valley caldera (California), and Valles caldera (New Mexico) in the United States. Although orders of magnitude smaller in volume than rare caldera-forming supereruptions, these relatively frequent effusions of rhyolite are comparable to the largest eruptions of the 20th century, and pose a considerable volcanic hazard. However, the physical pathway from rejuvenation to eruption of silicic magma is unclear, particularly because the time between reheating of a subvolcanic intrusion and eruption is poorly quantified. This study uses nanometer-scale trace element diffusion in sanidine crystals to reveal that rejuvenation of a near-solidus or subsolidus silicic intrusion occurred in ∼10 mo or less following a protracted period (220 k.y.) of volcanic repose, and resulted in effusion of ∼3 km3 of high-silica rhyolite lava at the onset of Yellowstone’s last volcanic interval. The future renewal of effusive silicic volcanism at Yellowstone will likely require a comparable energetic intrusion of magma that remelts the shallow subvolcanic reservoir and generates eruptible rhyolite on month to annual time scales.

The chlorine isotope fingerprint of the lunar magma ocean

The unusually heavy Cl of the Moon is related not to degassing of dry magmas but rather to the loss of Cl from the lunar magma ocean. The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free (“dry”) Moon. We have analyzed abundances and isotopic compositions of Cl and H in lunar mare basalts, and find little evidence that anhydrous lava outgassing was important in generating chlorine isotope anomalies, because 37Cl/35Cl ratios are not related to Cl abundance, H abundance, or D/H ratios in a manner consistent with the lava-outgassing hypothesis. Instead, 37Cl/35Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high 37Cl/35Cl in lunar basalts is inherited from urKREEP, the last dregs of the lunar magma ocean. These new data suggest that the high chlorine isotope ratios of lunar basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the Moon’s history. Chlorine isotope variability is therefore an indicator of planetary magma ocean degassing, an important stage in the formation of terrestrial planets.

Inventory of H2O in the ancient Martian regolith from Northwest Africa 7034: The important role of Fe oxides

Water-rich Martian regolith breccia Northwest Africa (NWA) 7034 was analyzed by Fourier transform infrared spectroscopy and transmission electron microscopy to determine the inventory and phase distribution of H2O (used herein to refer to both molecular H2O and OH− structural components in hydrous minerals). Hydrous Fe oxide phases (hydromaghemite and an unidentified nanocrystalline Fe-bearing oxide phase observed with hydromaghemite) and phyllosilicates (saponite) were identified as the primary mineralogic hosts for H2O with a minor contribution from Cl-rich apatite. Based on mass balance calculations and modal abundances of minerals constrained by powder X-ray diffraction and petrography, we can account for the entire 6000 ppm H2O measured in bulk rock analyses of NWA 7034. This H2O is distributed evenly between hydrous Fe-rich oxides and phyllosilicates, indicating that Fe oxides could be as important as phyllosilicates for H2O storage in Martian surface material.

Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust

The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali-rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K-Ar, U-Pb in apatite, and U-Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre-Amazonian high-temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification.

He diffusion in monazite: Implications for (U‐Th)/He thermochronometry

The light rare earth phosphate mineral monazite contains high concentrations of Th and U and thus should have potential as a (U-Th)/He thermochronometer. New 4He diffusion experiments confirm this potential but suggest that the bulk closure temperature may be so strongly compositionally dependent that successful applications will require careful sample characterization prior to (U-Th)/He dating. Three experiments were performed on single crystals of Monazite 554, a material that has been used as a standard for Th-Pb ion microprobe geochronology. The data from each experiment yielded a linear array on an Arrhenius diagram, consistent with simple diffusion behavior. However, the diffusion parameters indicated for each grain were statistically different, indicating closure temperatures (assuming a cooling rate of 10°C/Myr) of 206 ± 24°C, 230 ± 4°C, and 286 ± 13°C. X-ray compositional maps of Monazite 554 illustrate a possible cause of this discrepancy: strong and geometrically complex chemical zoning that varies from grain to grain and likely corresponds to several episodes of growth and dissolution. If He diffusivity in monazite is compositionally controlled, individual crystals with such inconsistent zoning (which is typical of naturally occurring monazites) should have different bulk closure temperatures.

Phosphate-halogen metasomatism of lunar granulite 79215: Impact-induced fractionation of volatiles and incompatible elements

Abstract In the last decade, it has been recognized that the Moon contains significant proportions of volatile elements (H, F, Cl), and that they are transported through the lunar crust and across its surface. Here, we document a significant segment of that volatile cycle in lunar granulite breccia 79215: impactinduced remobilization of volatiles, and vapor-phase transport with extreme elemental fractionation. 79215 contains ∼1% volume of fluorapatite, Ca5(PO4)3(F,Cl,OH), in crystals to 1 mm long, which is reflected in its analyzed abundances of F, Cl, and P. The apatite has a molar F/Cl ratio of ∼10, and contains only 25 ppm OH and low abundances of the rare earth elements (REE). The chlorine in the apatite is isotopically heavy, at δ37Cl = +32.7 ± 1.6‰. Hydrogen in the apatite is heavy at δD = +1060 ± 180‰; much of that D came from spallogenic nuclear reactions, and the original δD was lower, between +350‰ and +700‰. Unlike other P-rich lunar rocks (e.g., 65015), 79215 lacks abundant K and REE, and other igneous incompatible elements characteristic of the lunar KREEP component. Here, we show that the P and halogens in 79215 were added to an otherwise “normal” granulite by vapor-phase metasomatism, similar to rock alteration by fumarolic exhalations as observed on Earth. The ultimate source of the P and halogens was most likely KREEP, it being the richest reservoir of P on the Moon, and 79215 having H and Cl isotopic compositions consistent with KREEP. A KREEP-rich rock was heated and devolatilized by an impact event. This vapor was fractionated by interaction with solid phases, including merrillite (a volatile-free phosphate mineral), a Fe-Ti oxide, and a Zr-bearing phase. These solids removed REE, Th, Zr, Hf, etc., from the vapor, and allowed the vapor to transport primarily P, F, and Cl, with lesser proportions of Ba and U into 79215. Vapor-deposited crystals of apatite (to 30 μm) are known in some lunar regolith samples, but lunar vapor has not (before this) been implicated in significant mass transfer. It seems unlikely, however, that phosphate-halogen metasomatism is related to the high-Th/Sm abundance ratios of this and other lunar magnesian granulites. The metasomatism of 79215 emphasizes the importance of impact heating in the lunar volatile cycle, both in mobilizing volatile components into vapor and in generating strong elemental fractionations.