ALBERT

Description: Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered ( R ), but transforms with increasing temperature into a disordered higher symmetry structure ( R c ). To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300–1350 K and 2.6–4.2 GPa. The long-range order parameter s , quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20–30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and X Fe . Complete disorder is attained in dolomite at ~1240 K, 100–220 K lower than previously thought, and in an ankeritic-dolomite s.s. with an X Fe of 0.43 at temperatures as low as ~900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ~470 K. The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ~800 K favoring an almost straight Clapeyron-slope for this disputed reaction.

Description: Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface...

Description: Phase assemblages, melting relations and melt compositions of a dry carbonated pelite (DG2) and a carbonated pelite with 1·1 wt % H 2 O (AM) have been experimentally investigated at 5·5–23·5 GPa and 1070–1550°C. The subsolidus mineralogies to 16 GPa contain garnet, clinopyroxene, coesite or stishovite, kyanite or corundum, phengite or potassium feldspar (≤8 GPa with and without H 2 O, respectively), and then K-hollandite, a Ti phase and ferroan dolomite/Mg-calcite or aragonite + ferroan magnesite at higher pressures. The breakdown of clinopyroxene at 〉16 GPa causes Na-rich Ca-carbonate containing up to 11 wt % Na 2 O to replace aragonite and leads to the formation of an Na-rich CO 2 fluid. Further pressure increase leads to typical Transition Zone minerals such as the CAS phase and one or two perovskites, which completely substitute garnet at the highest investigated pressure (23·5 GPa). Melting at 5·5–23·5 GPa yields alkali-rich magnesio-dolomitic (DG2) to ferro-dolomitic (AM) carbonate melts at temperatures 200–350°C below the mantle geotherm, lower than for any other studied natural composition. Melting reactions are controlled by carbonates and alkali-hosting phases: to 16 GPa clinopyroxene remains residual, Na is compatible and the magnesio- to ferro-dolomitic carbonate melts have extremely high K 2 O/Na 2 O ratios. K 2 O/Na 2 O weight ratios decrease from 26–41 at 8 GPa to 1·2 at 16 GPa when K-hollandite expands its stability field with increasing pressure. At 〉16 GPa, Na is repartitioned between several phases, and again becomes incompatible as at 〈3 GPa, leading to Na-rich carbonate melts with K 2 O/Na 2 O ratios 1. This leaves the pressure interval of c . 4–15 GPa for ultrapotassic metasomatism. Comparison of the solidus with typical subducting slab-surface temperatures yields two distinct depths of probable carbonated pelite melting: at 6–9 GPa where the solidus has a negative Clapeyron slope between the intersection of the silicate and carbonate melting reactions at ~5 GPa, and the phengite or potassium feldspar stability limit at ~9 GPa. The second opportunity is related to possible slab deflection along the 660 km discontinuity, leading to thermal relaxation and partial melting of the fertile carbonated pelites, thus recycling sedimentary CO 2 , alkalis and other lithophile and strongly incompatible elements back into the mantle.

Description: Carbonatite and silicate rocks occurring within a single magmatic complex may originate through liquid immiscibility. We thus experimentally determined carbonatite/silicate melt partition coefficients ( D carbonate melt/silicate melt , hereafter D ) for 45 elements to understand their systematics as a function of melt composition and to provide a tool for identifying the possible conjugate nature of silicate and carbonatite magmas. Static and, when necessary, centrifuging piston cylinder experiments were performed at 1–3 GPa, 1150–1260°C such that two well-separated melts resulted. Bulk compositions had Na K, Na ~ K, and Na K; for the latter we also varied bulk H 2 O (0–4 wt %) and SiO 2 contents. Oxygen fugacities were between iron–wüstite and slightly below hematite–magnetite and were not found to exert significant control on partitioning. Under dry conditions alkali and alkaline earth elements partition into the carbonatite melt, as did Mo and P ( D Mo 〉8, D P = 1·6–3·3). High field strength elements (HFSE) prefer the silicate melt, most strongly Hf ( D Hf = 0·04). The REE have partition coefficients around unity with D La/Lu = 1·6–2·3. Transition metals have D 〈 1 except for Cu and V ( D Cu ~ 1·3, D V = 0·95–2). The small variability of the partition coefficients in all dry experiments can be explained by a comparable width of the miscibility gap, which appears to be flat-topped in our dry bulk compositions. For all carbonatite and silicate melts, Nb/Ta and Zr/Hf fractionate by factors of 1·3–3·0, in most cases much more strongly than in silicate–oxide systems. With the exception of the alkalis, partition coefficients for the H 2 O-bearing systems are similar to those for the anhydrous ones, but are shifted in favour of the carbonatite melt by up to an order of magnitude. An increase of bulk silica and thus SiO 2 in the silicate melt (from 35 to 69 wt %) has a similar effect. Two types of trace element partitioning with changing melt composition can be observed. The magnitude of the partition coefficients increases for the alkalis and alkaline earths with the width of the miscibility gap, whereas partition coefficients for the REE shift by almost two orders of magnitude from partitioning into the silicate melt ( D La = 0·47) to strongly partitioning into the carbonatite melt ( D La = 38), whereas D La / D Lu varies by only a factor of three. The partitioning behavior can be rationalized as a function of ionic potential ( Z / r ). Alkali and alkaline earth elements follow a trend, the slope of which depends on the K/Na ratio and H 2 O content. Contrasting the sodic and potassic systems, alkalis have a positive correlation in D vs Z / r space in the potassic case and Cs to K partition into the silicate melt in the presence of H 2 O. For the divalent third row transition metals on the one hand and for the tri- and tetravalent REE and HFSE on the other, two trends of negative correlation of D vs Z / r can be defined. Nevertheless, the highest ionic strength network-modifying cations (V, Nb, Ta, Ti and Mo) do not follow any trend; understanding their behavior would require knowledge of their bonding environment in the carbonatite melt. Strong partitioning of REE into the carbonatite melt ( D REE = 5·8–38·0) occurs only in H 2 O-rich compositions for which carbonatites unmix from evolved alkaline melts with the conjugate silicate melt being siliceous. We thus speculate that upon hydrous carbonatite crystallization, the consequent saturation in fluids may lead to hydrothermal systems concentrating REE in secondary deposits.