P. K. Carpenter

Detection of structurally bound hydroxyl in fluorapatite from Apollo Mare basalt 15058,128 using TOF-SIMS

Abstract Fluorapatite grains from Apollo 15 Mare basalt 15058,128 were analyzed by Raman spectroscopy, Raman spectral imaging, time-of-flight secondary ion mass spectrometry (TOF-SIMS), field emission scanning electron microscopy (FE-SEM), and electron probe microanalysis (EPMA) in an attempt to detect structurally bound OH- in the fluorapatite. Although OH- could not be definitively detected by Raman spectroscopy because of REE-induced photoluminescence, hydroxyl was detected in the fluorapatite by TOF-SIMS. The TOF-SIMS technique is qualitative but capable of detecting the presence of hydroxyl even at trace levels. Electron microprobe data indicate that on average, F and Cl (F+Cl) fill the monovalent anion site in these fluorapatite grains within the uncertainties of the analyses (about 0.07 ± 0.01 atoms per formula unit). However, some individual spot analyses have F+Cl deficiencies greater than analytical uncertainties that could represent structural OH-. On the basis of EPMA data, the fluorapatite grain with the largest F + Cl deficiency constrains the upper limit of the OH- content to be no more than 4600 ± 2000 ppm by weight (the equivalent of ~2400 ± 1100 ppm water). The TOF-SIMS detection of OH- in fluorapatite from Apollo sample 15058,128 represents the first direct confirmation of structurally bound hydroxyl in a lunar magmatic mineral. This result provides justification for attributing at least some of the missing structural component in the monovalent anion site of other lunar fluorapatite grains to the presence of OH-. Moreover, this finding supports the presence of dissolved water in lunar magmas and the presence of at least some water within the lunar interior.

EPMA Standards: The Good, the Bad, and the Ugly

The technique of electron-probe microanalysis (EPMA) is remarkable because a set of primary calibration standards allows accurate microanalysis in multiple phases over a wide range of concentration. Other analytical techniques require calibration curves and characterization of secondary standards which have limited applicability outside of the calibration range. Significant advances in microprobe instrumentation and correction algorithms over the past 50 years have resulted in the ability to better perform quantitative analysis of materials. EPMA depends fundamentally on primary calibration standards and verification of accuracy by analysis of secondary standards. Standards should be homogeneous on the micron scale, well characterized, widely available to international laboratories, and similar in composition to typical samples in order to minimize reliance on correction algorithms. Most standards fail to satisfy one or more of these criteria, and indeed, the technique of EPMA is deficient with respect to diverse standards.

Considerations for the Acquisition of Very Large Area EDS Spectral Image Mosaics

Modern SEMs, EDS detectors, and microanalysis software have enabled the acquisition of EDS spectral images over very large areas so that entire samples may be described by composition and phase maps (e.g., [1]). Here, we investigate what count richness (X-ray counts/pixel) is needed to meaningfully extract different map types (counts, quantitative, and phase maps) from spectral image mosaic datasets.

Compositional Mapping by EPMA and µXRF

We present methods combining backscattered-electron (BSE) mosaic imaging, quantitative spot-mode electron-probe microanalysis (EPMA), and quantitative compositional mapping by EPMA and micro-xray fluorescence (μXRF) to provide a framework for detailed analysis of terrestrial and lunar samples. BSE imaging provides a base map for the characterization of samples by EPMA. Recent developments in image stitching provide a convenient method of processing BSE mosaic image sets. Characterizing cm-sized samples by EPMA and μXRF provides complementary information about sample chemistry.

Connecting Lunar Meteorites to Source Terrains on the Moon

The number of named stones found on Earth that have proven to be meteorites from the Moon is approx. 180 so far. Since the Moon has been mapped globally in composition and mineralogy from orbit, it has become possible to speculate broadly on the region of origin on the basis of distinctive compositional characteristics of some of the lunar meteorites. In particular, Lunar Prospector in 1998 [1,2] mapped Fe and Th at 0.5 degree/pixel and major elements at 5 degree/pixel using gamma ray spectroscopy. Also, various multispectral datasets have been used to derive FeO and TiO2 concentrations at 100 m/pixel spatial resolution or better using UV-VIS spectral features [e.g., 3]. Using these data, several lunar meteorite bulk compositions can be related to regions of the Moon that share their distinctive compositional characteristics. We then use EPMA to characterize the petrographic characteristics, including lithic clast components of the meteorites, which typically are breccias. In this way, we can extend knowledge of the Moons crust to regions beyond the Apollo and Luna sample-return sites, including sites on the lunar farside. Feldspathic Regolith Breccias. One of the most distinctive general characteristics of many lunar meteorites is that they have highly feldspathic compositions (Al2O3 approx. 28% wt.%, FeO <5 wt.%, Th <1 ppm). These compositions are significant because they are similar to a vast region of the Moons farside highlands, the Feldspathic Highlands Terrane, which are characterized by low Fe and Th in remotely sensed data [4]. The meteorites provide a perspective on the lithologic makeup of this part of the Moon, specifically, how anorthositic is the surface and what, if any, are the mafic lithic components? These meteorites are mostly regolith breccias dominated by anorthositic lithic clasts and feldspathic glasses, but they do also contain a variety of more mafic clasts. On the basis of textures, we infer these clasts to have formed by large impacts that excavated and mixed rocks from depth within the lunar crust and possibly the upper mantle. One of the key questions is whether the mafic materials are ferroan or magnesian, which remote sensing does not clearly distinguish, and if mafic, whether they might contain mantlederived components such as olivine (dunite). Many but not all have mainly ferroan mafic components, consistent with a ferroan crustal source that is complementary to the ferroan anorthositic suite and that represents primary magma-ocean-derived feldspathic crust. Meteorites such as ALH 81005 [5] and Shir 161 [6], however, contain coarse-grained magnesian mafic clasts (Fig. 1a) derived from deeply seated and melted material associated with impact basins. Comparison to LP gamma-ray data [2] supports an origin for magnesian feldspathic meteorites such as these (e.g., Shir 161) as shown in Fig. 1b. Sayh al Uhaymir (SaU) 169. Another distinctive but much less common composition is represented by relatively mafic impact-melt breccia that is rich in incompatible elements known as KREEP. These meteorites can be related to the western nearside Procellarum KREEP Terrane, especially through a combination of Fe and Th contents. Among the most enriched is SaU 169, which has been related to high- Th impact-melt breccia found at the Apollo 12 site [7]. Through detailed EPMA and ion microprobe analysis we have shown that these two rock types are related in age and origin.

Using Electron-Probe Microanalysis and Quantitative Compositional Mapping to Study Lithic Clasts in Lunar Meteorites NWA 2727 and NWA 3170

Northwest Africa (NWA) 2727 and NWA 3170, two different stones of the same lunar meteorite, are mineralogically and chemically similar [1,2]. Both are breccias that contain an apparently related suite of lithic and mineral clasts. To investigate petrogenetic relationships, we have determined mineral compositional zoning characteristics with Electron Probe Microanalysis (EPMA). The mineralogy includes alkali feldspar (K, Ba, Na), plagioclase, pyroxene (zoned Mg-Fe-Ca), olivine (Fe-rich), and minor amounts of Fe-Ti oxides, phosphates, and troilite. Alteration (Ca-carbonate fracture fillings) occurs in both meteorites, resulting from hot-desert weathering. In the lithic clasts, mineral-chemical zoning trends, e.g., in pyroxene, are especially useful for comparing lithic clasts and constraining their origins. We used quantitative compositional mapping to study compositional variations, especially for pyroxenes as diagnostic of petrogenetic relationships. We find that quantitative compositional maps extend the characterization of mineral-chemical trends beyond that obtained by EPMA spot analyses.

Trace-element Analyses in Lunar Meteorite Sayh al Uhaymir 169

We acquired high precision measurements of Ti in zircon, Sr in plagioclase, and Y, REEs (rare earth elements), and Th in phosphates in Sayh al Uhaymir (SaU) 169, a lunar meteorite collected in Oman that is exceptionally rich in incompatible elements. SaU 169 is dilithologic, consisting of an impact-melt breccia (IMB) lithology and a regolith-breccia lithology [1]; all analyses were done on the former. Measurements were done at Washington University on a five-spectrometer JEOL 8200 Superprobe using Probe for Windows software.

Shergottite Northwest Africa 6963: A Pyroxene‐Cumulate Martian Gabbro

Northwest Africa (NWA) 6963 was found in Guelmim-Es-Semara, Morocco, and based on its bulk chemistry and oxygen isotopes, it was classified as a Martian meteorite. On the basis of a preliminary study of the textures and crystal sizes, it was resubclassified as a gabbroic shergottite because of the similarity with terrestrial and lunar gabbros. However, the previous work was not a quantitative investigation of NWA 6963; to supplement the original resubclassification and enable full comparison between this and other Martian samples; here we investigate the mineralogy, petrology, geochemistry, quantitative textural analyses, and spectral properties of gabbroic shergottite NWA 6963 to constrain its petrogenesis, including the depth of emplacement (i.e., base of a flow versus crustal intrusion). NWA 6963 is an enriched shergottite with similar mineralogy to the basaltic shergottites but importantly does not contain any fine-grained mesostasis. Consistent with the mineralogy, the reflectance (visible/near-infrared and thermal infrared) spectrum of powdered NWA 6963 is similar to other shergottites because they are all dominated by pyroxene, but its reflectance is distinct in terms of albedo and spectral contrast due to its gabbroic texture. NWA 6963 represents a partial cumulate gabbro that is associated with the basaltic shergottites. Therefore, NWA 6963 could represent a hypabyssal intrusive feeder dike system for the basaltic shergottites that erupted on the surface. Plain Language Summary This study investigates a new meteorite from Mars, which has different properties than previous Martian meteorites. Specifically, this rock has large crystals that likely formed as the magma ponded in the crust instead of erupting as a lava flow. On Earth, 10 times more magma gets stuck in the crust than erupts on the surface; therefore, we would expect something similar on Mars—yet this rock is the first example of an intrusive magma on Mars. This work shows that this meteorite possibly represents the feeder dike system that fed the lava flow represented by the other shergottite meteorites.

Very Large Area Phase Mapping of a Petrographic Thick Section using Multivariate Statistical Analysis of EDS Spectral Images

There are many applications that require knowing the correct distribution of phases in a sample, such as determining the quality of steel or determining the bulk composition of a sample by modal recombination. Modal recombination is a calculation of a sample’s bulk chemistry based on the assumption that the distribution of phases in a cross section of a sample is a good representation of the distribution of phases in the three dimensional volume of the sample. If the abundance of each phase and the composition of each phase are known, the bulk composition of the sample can be calculated. The greater the area of the section from which the distribution of phases is determined, the more accurate the bulk composition calculated from a modal recombination can be. Large area electron imaging, elemental X-ray mapping, and quantitative elemental X-ray mapping have existed for some time. Historically, most modal recombination calculations have been done relying on the phase abundances determined from the analysis of contrast in backscattered electron image which discriminates phases based on average atomic number. However, many phases that commonly occur in the same sample have similar average atomic numbers (e.g., pyroxene and apatite) and may be impossible to successfully discriminate based on electron image contrast. Here, we demonstrate that modern X-ray microanalytical software, if it includes multivariate statistical analysis algorithms, enables phase mapping based on X-ray microanalysis of very large areas (including petrographic thin or thick sections) in practical amounts of time.

Calibrated Procedure for Setting Pulse-Height Parameters in Wavelength-Dispersive Spectrometry

Electron-probe microanalysis (EPMA) routinely makes use of wavelength-dispersive spectrometry (WDS) to measure characteristic X-ray photons over a wide range of concentration, so the WDS pulse processing system should separate counts from electronic noise over a wide range of count rate. WDS pulse processing hardware is simple compared to the sophisticated pulse processing hardware used in energy-dispersive spectrometry (EDS), as there is no monitoring of deadtime or active referencing to a zero strobe. WDS uses X-ray diffraction to select the desired X-ray wavelength, and all measured pulses are presumed to be for the element of interest. The pulse height analysis (PHA) system is set to discriminate between baseline noise and the measured X-ray pulses.