Mahesh Anand

Bio: Mahesh Anand is an academic researcher from Open University. The author has contributed to research in topics: Meteorite & Basalt. The author has an hindex of 42, co-authored 211 publications receiving 4639 citations. Previous affiliations of Mahesh Anand include American Museum of Natural History & University of Tennessee.

Magmatic volatiles (H, C, N, F, S, Cl) in the lunar mantle, crust, and regolith: Abundances, distributions, processes, and reservoirs

Abstract: Many studies exist on magmatic volatiles (H, C, N, F, S, Cl) in and on the Moon, within the last several years, that have cast into question the post-Apollo view of lunar formation, the distribution and sources of volatiles in the Earth-Moon system, and the thermal and magmatic evolution of the Moon. However, these recent observations are not the first data on lunar volatiles. When Apollo samples were first returned, substantial efforts were made to understand volatile elements, and a wealth of data regarding volatile elements exists in this older literature. In this review paper, we approach volatiles in and on the Moon using new and old data derived from lunar samples and remote sensing. From combining these data sets, we identified many points of convergence, although numerous questions remain unanswered. The abundances of volatiles in the bulk silicate Moon (BSM), lunar mantle, and urKREEP [last ~1% of the lunar magma ocean (LMO)] were estimated and placed within the context of the LMO model. The lunar mantle is likely heterogeneous with respect to volatiles, and the relative abundances of F, Cl, and H2O in the lunar mantle (H2O > F >> Cl) do not directly reflect those of BSM or urKREEP (Cl > H2O ≈ F). In fact, the abundances of volatiles in the cumulate lunar mantle were likely controlled by partitioning of volatiles between LMO liquid and nominally anhydrous minerals instead of residual liquid trapped in the cumulate pile. An internally consistent model for lunar volatiles in BSM should reproduce the absolute and relative abundances of volatiles in urKREEP, the anorthositic primary crust, and the lunar mantle within the context of processes that occurred during the thermal and magmatic evolution of the Moon. Using this mass-balance constraint, we conducted LMO crystallization calculations with a specific focus on the distributions and abundances of F, Cl, and H2O to determine whether or not estimates of F, Cl, and H2O in urKREEP are consistent with those of the lunar mantle, estimated independently from the analysis of volatiles in mare volcanic materials. Our estimate of volatiles in the bulk lunar mantle are 0.54–4.5 ppm F, 0.15–5.3 ppm H2O, 0.26–2.9 ppm Cl, 0.014–0.57 ppm C, and 78.9 ppm S. Our estimates of H2O are depleted compared to independent estimates of H2O in the lunar mantle, which are largely biased toward the “wettest” samples. Although the lunar mantle is depleted in volatiles relative to Earth, unlike the Earth, the mantle is not the primary host for volatiles. The primary host of the Moon’s incompatible lithophile volatiles (F, Cl, H2O) is urKREEP, which we estimate to have 660 ppm F, 300–1250 ppm H2O, and 1100–1350 ppm Cl. This urKREEP composition implies a BSM with 7.1 ppm F, 3–13 ppm H2O, and 11–14 ppm Cl. An upper bound on the abundances of F, Cl, and H2O in urKREEP and the BSM, based on F abundances in CI carbonaceous chondrites, are reported to be 5500 ppm F, 0.26–1.09 wt% H2O, and 0.98–1.2 wt% Cl and 60 ppm F, 27–114 ppm H2O, and 100–123 ppm Cl, respectively. The role of volatiles in many lunar geologic processes was also determined and discussed. Specifically, analyses of volatiles from lunar glass beads as well as the phase assemblages present in coatings on those beads were used to infer that H2 is likely the primary vapor component responsible for propelling the fire-fountain eruptions that produced the pyroclastic glass beads (as opposed to CO). The textural occurrences of some volatile-bearing minerals are used to identify hydrothermal alteration, which is manifested by sulfide veining and sulfide-replacement textures in silicates. Metasomatic alteration in lunar systems differs substantially from terrestrial alteration due to differences in oxygen fugacity between the two bodies that result in H2O as the primary solvent for alteration fluids on Earth and H2 as the primary solvent for alteration fluids on the Moon (and other reduced planetary bodies). Additionally, volatile abundances in volatile-bearing materials are combined with isotopic data to determine possible secondary processes that have affected the primary magmatic volatile signatures of lunar rocks including degassing, assimilation, and terrestrial contamination; however, these processes prove difficult to untangle within individual data sets. Data from remote sensing and lunar soils are combined to understand the distribution, origin, and abundances of volatiles on the lunar surface, which can be explained largely by solar wind implantation and spallogenic processes, although some of the volatiles in the soils may also be either indigenous to the Moon or terrestrial contamination. We have also provided a complete inventory of volatile-bearing mineral phases indigenous to lunar samples and discuss some of the “unconfirmed” volatile-bearing minerals that have been reported. Finally, a compilation of unanswered questions and future avenues of research on the topic of lunar volatiles are presented, along with a critical analysis of approaches for answering these questions.

Early Proterozoic Melt Generation Processes beneath the Intra-cratonic Cuddapah Basin, Southern India

Abstract: Early Proterozoic tholeiitic lavas and sills were emplaced during the initial phase of extension of the intra-cratonic Cuddapah Basin, southern India. Ar-40-Ar-39 laser-fusion determinations on phlogopite mica,from the Tadpatri Fm mafic-ultramafic sill complex, constrain the age of the initial phase of extension and volcanism in the basin at 1.9 Ga. Despite their Early Proterozoic age, the igneous rocks are unmetamorphosed, undeformed and remarkably fresh. They exhibit a wide range in MgO contents (4-28 wt %) and have undergone varying degrees of accumulation or crystal fractionation. Variable La/Nb ratios (1.2-3.7) and epsilon(Nd) values (1 to -10) suggest that some, but not all, of the mafic rocks have been affected by crustal contamination. This appears to have taken place in magma chambers at similar to9 kbar, i.e. the base of the continental crust. Forward modelling of major and trace elements (Fe and Nd) and inverse modelling of rare earth elements suggest that the primary Cuddapah melts were generated by similar to10-15% partial melting of a lherzolite mantle source. This corresponds to a mantle Potential temperature of similar to1500degreesC. The thickness of the mechanical boundary layer predicted by the geochemical modelling is 70 km with a minimum initial lithospheric thickness of 120 kin. This corresponds to a stretching factor of 1.6-1.8. Richter's (1988) secular cooling model for the Earth predicts that, at 1.9 Ga, the ambient mantle had a potential temperature of similar to1500degreesC (i.e. similar to200degreesC hotter than Phanerozoic mantle). If the cooling model is correct then Proterozoic lithospheric stretching and mantle melting beneath the intra-cratonic Cuddapah Basin could have been caused by passive rather than active rifting.

Diamonds: time capsules from the Siberian Mantle

Abstract: Diamonds are thought to be “time capsules” from the Earth's mantle. However, by themselves, consisting of nearly pure carbon, diamonds provide little geochemical information about their conditions of formation and the nature of their mantle hosts. This obstacle to studying the origin of diamonds and their hosts can be overcome by using two main approaches that focus on studying: (1) the rocks that contain diamonds, i.e., diamondiferous xenoliths; and (2) mineral inclusions within the diamonds, the time capsule's little treasures, if you will. Diamondiferous xenoliths, their diamonds, and mineral inclusions within the diamonds are the subject of this review, focusing on studies of samples from the Yakutian kimberlites in the Siberian Platform. Studies of diamondiferous eclogite xenoliths significantly enhance our understanding of the complex petrogenesis of this important group of rocks and their diamonds. Such studies involve various geochemical and petrological investigations of these eclogites, including major and trace-element, radiogenic as well as stable isotopic analyses of whole rocks and minerals. The results from these studies have clearly established that the Group A–C eclogites originate from subduction of ancient oceanic crust. This theory is probably applicable worldwide. Within the last several years, our research group at Tennessee has undertaken the systematic dissection (pull apart) of diamondiferous eclogites from Siberia, consisting of the following steps: (1) high-resolution computed X-ray tomography of the xenoliths to produce 3D images that relate the minerals of the xenoliths to their diamonds; (2) detailed dissection of the entire xenolith to reveal the diamonds inside, followed by characterization of the setting of the diamonds within their enclosing minerals; and (3) extraction of diamonds from the xenolith for further investigation of the diamonds and their inclusions. In this last step, it is important that the nature and relative positions of the diamond inclusions are carefully noted in order to maximize the number of inclusions that can be exposed simultaneously on one polished surface. In this modus operandi, cathodoluminescence imaging, plus FTIR/N aggregation and C/N isotopic analyses are performed on polished diamond surfaces to reveal their internal growth zones and the spatial relationship of the mineral inclusions to these zones. Knowledge gained by such detailed, albeit work-intensive, studies continues to add immensely to the constantly evolving models of the origin of diamonds and their host rocks in the Earth's mantle, as well as to lithospheric stability models in cratonic areas. Multiple lines of evidence indicate the ultimate crustal origin for the majority of mantle eclogites. Similar pieces of evidence, particularly from δ13C in P-type diamonds and δ18O in peridotitic garnets lead to the suggestion that at least some of the mantle peridotites, including diamondiferous ones, as well as inclusions in P-type diamonds, may have had a crustal protolith as well.

Water on Mars

Abstract: Estimates of the amount of water outgassed from Mars, based on the composition of the atmosphere, range from 6 to 160 m, as compared with 3 km for the Earth. In contrast, large flood features, valley networks, and several indicators of ground ice suggest that at least 500 m of water have outgassed. The two sets of estimates may be reconciled if early in its history, Mars lost part of its atmosphere by impact erosion and hydrodynamic escape.

The Crust of the Moon as Seen by GRAIL

Abstract: High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moon's highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth.

The crust of the Moon as seen by GRAIL

Abstract: The Holy GRAIL? The gravity field of a planet provides a view of its interior and thermal history by revealing areas of different density. GRAIL, a pair of satellites that act as a highly sensitive gravimeter, began mapping the Moon's gravity in early 2012. Three papers highlight some of the results from the primary mission. Zuber et al. (p. 668, published online 6 December) discuss the overall gravity field, which reveals several new tectonic and geologic features of the Moon. Impacts have worked to homogenize the density structure of the Moon's upper crust while fracturing it extensively. Wieczorek et al. (p. 671, published online 6 December) show that the upper crust is 35 to 40 kilometers thick and less dense—and thus more porous—than previously thought. Finally, Andrews-Hanna et al. (p. 675, published online 6 December) show that the crust is cut by widespread magmatic dikes that may reflect a period of expansion early in the Moon's history. The Moon's gravity field shows that the lunar crust is less dense and more porous than was thought. High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moon's highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth.