Phosphate in Angra dos Reis: Structure and composition of the Ca3(PO4)2 minerals

Extraterrestrial calcium phosphates (“whitlockites”) have the anhydrous β-Ca₃(PO₄)₂ structure, which is different from that of hydrous terrestrial whitlockite. This has been confirmed by X-ray refinement of the structure of a phosphate from the achondrite Angra dos Reis. In the β-Ca₃(PO₄)₂ structure, there is one crystallographic site, Ca(IIA), which is half-occupied by calcium, and which seems to have an energetically unfavorable configuration; natural phosphates with this configuration (including Angra dos Reis) have composition Ca₁₉(Mg,Fe)₂(PO₄)₁₄. Stability of the structure is probably increased by substitution of Na for Ca in Ca(IIA) giving composition Ca₁₈ (Mg,Fe)₂Na₂(PO₄)₁₄, which occurs in chondrites; by vacancy of Ca(IIA), with rare earths and yttrium substituting for calcium in other sites for charge balance, giving composition Ca₁₆(Y,RE)₂(Mg,Fe)₂(PO₄)₁₄, found in lunar rocks; or by replacing Ca with hydrogen, giving composition Ca₁₈(Mg,Fe)₂H₂(PO₄)₁₄, which is terrestrial whitlockite. Lack of the favorable substitutions of Na, (Y, RE) or H in Angra dos Reis phosphate implies that these elements were relatively scarce in its environment of formation.     

The importance of adsorption in igneous partitioning of trace elements

For accurate mathematical modeling of trace-element partitioning during igneous fractionation, adsorption should be considered. Because of adsorption, the partitioning of elements between liquid and a surface layer of a crystal is often not the same as the partitioning between liquid and the solid crystal at true equilibrium. In some minerals e.g. high-calcium pyroxene, the effect of adsorption during crystal growth may be very important; this is suggested by the frequent occurrence of sector zoning in augite, and the wide range in measured partition coefficients for such elements as rare earths. The ions which are enriched by adsorption are usually those which are favored substituents according to Goldschmidt's rules. In other minerals, uptake of trace elements may be closer to equilibrium partitioning, rather than being determined by kinetic factors. For example, the relative partitioning of REE, U, Th and Pb into feldspars is qualitatively predicted by Pauling's rules for complex ionic crystals, rather than by Goldschmidt's rules.     

Ferroan anorthosite: A widespread and distinctive lunar rock type

Eight of eleven Apollo 16 rake-sample anorthosites are very similar to each other, to hand-specimen Apollo 16 anorthosites, and to Apollo 15 anorthosites. They have feldspar An_96.6, both high- and low-Ca pyroxene with a restricted range of (low-magnesium) composition, minor olivine (～ Fo₆₀), traces of ilmenite and chromite, and originally coarse-grained, but now cataclastic texture. Such ferroan anorthosite is evidently a coherent, distinctive and widespread lunar rock type of cumulate origin which may not necessarily be very closely related genetically to other highland rock types.     

Jet Deflection by a Quasi-Steady-State Side Wind in the Laboratory

We present experimental data on the steady state deflection of a highly supersonic jet by a side-wind in the laboratory. The use of a long interaction region enables internal shocks to fully cross the jet, leading to the development of significantly more structure in the jet than in previous work with a similar setup (Lebedev et al., 2004). The ability to control the length of the interaction region in the laboratory allows the switch between a regime representing a clumpy jet or wind and a regime similar to a slowly varying mass loss rate. The results indicate that multiple internal oblique shocks develop in the jet and the possible formation of a second working surface as the jet attempts to tunnel through the ambient medium.     

The continental record of Ediacaran volcano‐sedimentary successions in southern Brazil and their global implications

The Ediacaran is one of the most important periods on Earth evolution, including the first appearance of soft‐bodied macrofossils, major climatic changes and a supposed rise in free oxygen. In southernmost Brazil, this period is represented by Camaquã Supergroup, including the Bom Jardim Group and the Acampamento Velho Formation, both of which record continental palaeoenvironmental changes in a more than 5000 m thick stratigraphic succession. Age constraints are given by seven Ar‐Ar and U‐Pb determinations on volcanic rocks, which bracket these units between c. 605 and 574 Ma, revealing the best dated and most continuous documented Ediacaran continental succession to date. Depositional systems evolution supports a Phanerozoic‐type glacial context during the last Neoproterozoic glacial event and presents the Picada das Graças Formation (580 ± 3.6 Ma) as the first dated non‐glacial unit coeval to the Gaskiers Formation.     

Composition of hydrated near-Earth object (100085) 1992 UY4

Near-infrared spectra of the near-Earth object (100085) 1992 UY4 are similar to those of P-type asteroids, providing a fitted geometric albedo of 0.052±0.005 and an effective diameter of 1.68±0.08 km. This object, with a likely outer-belt origin, also exhibits a 3-μm absorption feature with a band depth of 3%±1%, corresponding to a regolithic bulk hydrogen-to-silicon ratio of 0.30±0.05. The bulk of this hydrogen seems to be present in H₂O-dominated minerals.     

A synthesis of morphological evolutions and Holocene stratigraphy of a wave-dominated estuary: The Arcachon lagoon,SW France

This work presents the first synthesis of secular to millenary morphological evolutions and stratigraphy of a wave-dominated estuary, the Arcachon lagoon, from a combination of unpublished bathymetric maps (1865 and 2001), core results and high-resolution seismic profiles recorded for the first time in this lagoon. The Arcachon lagoon is located on the Atlantic coast of France, facing the wave-dominated shelf of the Bay of Biscay. It is a mesotidal semi-enclosed environment of about 160 km².The sediment budget of the Arcachon lagoon was computed by subtracting the 1865 bathymetric map from that of 2001. The computed volume difference is low (?9.9±35×10⁶ m³ in 136 yrs) and is the result of the balance between erosion and accretion that occurs within tidal channels and tidal flats, respectively. This morphological evolution pattern is explained by low sediment supply and also by the tidal distortion resulting from the morphology of the lagoon. Deep channels connected to the inlet are dominated by ebb currents inducing erosion. Tidal flats and transverse channels display weak or flood-dominated tidal currents leading to the deposition of silts. The areas of tidal flat siltation locally correlate with the presence of oyster farms, suggesting the influence of Man on the lagoon sediment-fill. Transverse channel-infill is related to weak tidal currents resulting from the hydraulically inefficient orientation of these channels which served as an ancient drainage network.Evidence for tidal channel-infill and channel abandonment are also provided by seismic profiling and cores. The upper stratigraphic succession of the lagoon (about 10 m thick) includes four main stratigraphic units dominated by channel-fills. The two lower units (around 7500–2800 yrs BP) display tabular-shape sandy channels interpreted to be records of the open estuarine phase of the Arcachon lagoon. The two upper units (around 2800 yrs BP to present-day) display U-shaped mixed sand-and-mud channel-fills interpreted to be records of the closure of the lagoon. Given that the basal estuarine units are transgressive and the upper lagoonal units are regressive, the main stratigraphic change at around 2800 yrs BP is interpreted as being the maximum flooding surface (MFS). This late MFS is explained by the low sediment supply. It is proposed that the transition from the estuarine to the lagoonal phase is related to the development of the Cap-Ferret spit in response to an increase in the ratio between wave power to tide power. This change in wave-to-tide ratio may be triggered by wave power increase following the Subboreal/Subatlantic climate instability or a decrease in tide power following a decrease in tidal prism related to the lagoon sediment-fill.Thus, the evolution of the Arcachon lagoon over the last millenaries was mainly controlled by its spit development, leading to a wave-dominated estuary in terms of its geomorphology. Once it was partially closed, extensive mud flats developed in the lagoon which became ebb-dominated.