ALBERT

Description: To evaluate compaction and interstitial melt expulsion during cumulate formation, a 20 m cumulate section including the UG2 and UG3 chromitites from a 264 m drill core through the Upper Critical Zone of the Bushveld Complex (South Africa) has been studied. The cumulates in the studied section are as follows: 3 m plagioclase pyroxenite to pyroxenite, pegmatoid footwall pyroxenite at the lower contact to UG2, 0·7 m UG2 chromitite, 6·8 m pyroxenite, 0·24 m UG3 chromitite, 2·0 m plagioclase-rich pyroxenite changing locally to norite, the two 5 cm leader stringers UG3a and UG3b, and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites. All pyroxenites are dominated by orthopyroxene (opx) and the cumulate sequence is topped by mottled anorthosite grading into norite. Stratigraphic concentrations of major and trace elements of 52 bulk-rock samples were determined. Bulk-rock Mg-numbers are 0·79–0·81 throughout the silicate cumulate units, and 0·40–0·46 in the chromitite layers. The stratigraphic distribution of six incompatible trace elements (K, Rb, Ba, Cs, Zr and Th) has been used to determine the amount of trapped liquid ( F TL ) or paleo-porosity in the cumulate rocks. Final porosities (volume fractions), based on averages from the six trace elements, are 0·06–0·33 in the pyroxenites. In chromitite layers, trapped melt fractions of 0·12–0·36 are calculated from incompatible trace element concentrations, but bulk SiO 2 concentrations and X-ray tomography yield 0·04–0·17 higher porosities. Hence, the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction, as poikilitic opx was crystallizing while the silicate melt still equilibrated. Using a previously derived experiment-based model for compaction time scales, gravitationally driven chemical compaction in the UG2–UG3–pyroxenite section is calculated to occur within 1–10 years. This time frame corresponds to the times necessary to cool a 20 m layer by 10–50°C, the temperature interval argued to encompass the liquidus and almost complete solidification. Compaction within a decade can in fact easily develop the paleo-porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid. Contrary to previous assertions, melt expulsion from the cumulate pile does not hinder compaction; calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate. The pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites. Their entrapment below the chromitite layers suggests that these act as permeability barriers. This is in part due to their finer grain size compared with the pyroxenites, but is mainly due to the crystallization of large poikilitic opx during compaction.

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: Although alkali-alkali earth carbonates have not been reported from mantle-derived xenoliths, these carbonates may have a substantial role in mantle metasomatic processes through lowering melting temperatures. On the Na 2 Mg(CO 3 ) 2 –K 2 Mg(CO 3 ) 2 join only the Na-end-member eitelite ( R space group), was reported in nature. The K-end-member ( R m ) readily hydrates even at low temperatures, therefore, only baylissite, K 2 Mg(CO 3 ) 2 ·4H 2 O, has been observed. Because of the role of (K,Na)Mg-double carbonates in mantle metasomatism, we performed high P-T experiments on K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 . Structure refinements were done upon compression of single crystals from 0 to 9 GPa at ambient temperature employing synchrotron radiation. Fitting the compression data to the second-order Birch-Murnaghan EoS resulted in V 0 = 396.2(4), 381.2(5), and 347.1(3) Å 3 and K 0 = 57.0(10), 54.9(13), and 68.6(13) GPa for K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 , respectively. These compressibilities are lower than those of magnesite and dolomite. The KMg-double carbonate transforms into a monoclinic polymorph at 8.05 GPa; the high- P phase is 1% denser than the low- P polymorph. The NaMg-double carbonate has a phase transition at ~14 GPa, but poor recrystallization has prevented structure refinement. The parameters for a V-T EoS were collected at 25–600 °C and ambient pressure and are α 0 = 14.31(5) x 10 –5 K –1 and 16.73(11) x 10 –5 K –1 for K 2 Mg(CO 3 ) 2 and Na 2 Mg(CO 3 ) 2 , respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a - and c -axis: α 0 ( a ) = 2.84(6) x 10 –5 and 4.78(5) x 10 –5 K –1 and α 0 ( c ) = 10.47(11) x 10 –5 and 8.72(5) x 10 –5 K –1 for K 2 Mg(CO 3 ) 2 and Na 2 Mg(CO 3 ) 2 , respectively.

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.