Ternary feldspars: Kinetics and possible equilibria at 800° C

The ternary feldspar system KAlSi₃O₈ - NaAlSi₃O₈ - CaAl₂Si₂O₈ was reinvestigated at 650 ° C and 800 ° C (P _H2O = 1 kb) using mixtures of crystalline plagioclases and alkali feldspars as starting materials. The compositions of plagioclases and alkali feldspars of the run products were determined by X-ray means. The Or-content of the feldspar phases was determined by measuring the position of the (201) X-ray peak of the unexchanged feldspars, whereas the An-content was determined by measuring the same X-ray peak of the K-exchanged feldspars. The reaction rate of a reaction leading to a more An-rich plagioclase (type II reaction) is much faster than a reaction producing a more Ab-rich plagioclase (type I). In a type II reaction run times of approximately 20 days are needed to reach new constant plagioclase and alkali feldspar compositions at 650 ° C, and 10 days are needed to reach constant compositions at 800 ° C. In a reaction of type I only the outer zone of the plagioclases reacts to more Abrich compositions. A diffuse zone with a wide range of compositions was observed in 650 ° C runs. Equilibrium could not be reached in these experiments within 45 days. At 800 ° C a new zone having a specific composition develops in 42 days. This new zone is believed to be in equilibrium with the coexisting alkali feldspar. The depth of reaction is calculated as 0.03 μm after 42 days (800 ° C, P _f= 1 kb). The reaction between the two feldspar phases could be reversed at 800 ° C. The following compositions are considered to represent equilibrium data at 800 ° C and P _t = 1 kb:

An 43 Ab 51 Or 6 coexisting with Or 79 Ab 20 An 1, and

An 40 Ab 54 Or 6 coexisting with Or 75 Ab 24 An 1.

Recent data obtained with gels of ternary feldspar composition as starting materials do not agree with the results presented in this paper. Gels obviously crystallize spontaneously forming coexisting feldspars of non - equilibrium composition - alkali feldspars too rich in Ab and plagioclases too rich in An.     

Nitrogen isotopes in the solar system

Measurements of the isotopic composition of nitrogen in the solar system are summarized. We show that the 30% change, during the last 3 to 4 billion years, of ${}^{15}\text{N}{}^{14}\text{N}$ in solar-wind-bearing lunar soils and breccias probably does not reflect changes in this ratio at the solar surface. Such changes, whether by spallation or thermonuclear reactions are ruled out by comparing the yields of ¹⁵N with those of other rare isotopes such as ⁹Be, ¹¹B, ³He or ¹³C, even if an arbitrary degree of solar mixing is introduced. Moreover, we calculate that the solar activity required for producing significant amounts of ¹⁵N by spallation at the solar surface should have resulted in a particle bombardment of the Moon of an intensity that would have produced amounts of spallation isotopes (e.g.¹⁵N, ²¹Ne, ³⁸Ar, ¹³¹Xe) several orders of magnitude in excess of what is actually found in the whole regolith.We argue that accretion of interstellar matter also does not work as a cause for a significant change of the photospheric ${}^{15}\text{N}{}^{14}\text{N}$ ratio. Evidence is presented that the mixing depth at the solar surface on a time scale of ?10⁹ years is (10^?2 ?10^?1) M_⊙ Mixing to this depth renders accretion of interstellar matter as a source of compositional changes at the solar surface inefficient, even if allowance is made for the expected large difference in the accretion rates of condensed and gaseous matter. A quantitative treatment of several alternatives of solar accretion leads to serious contradictions (e.g. with the low Ne abundances in planetary atmospheres or with the amounts of nitrogen that should have been directly accreted by the Moon), and we conclude that accretion during the main sequence life of the Sun is an unlikely source of changes in ${}^{15}\text{N}{}^{14}\text{N}$ at the solar surface.A ratio of ${}^{15}\text{N}{}^{14}\text{N}\text{= (4.0 ± 0.3) × 10}{}^{\mathrm{?3}}$ is our best estimate for average solar system material and for the Sun. We propose that a rare, very light nitrogen component (called LPN) is admixed in varying amounts to planetary matter. Undiluted LPN has not been found in meteorites or planetary atmospheres, but we show that the combined effects of LPN admixture and isotope fractionation can in principle account for the variability of ${}^{15}\text{N}{}^{14}\text{N}$ observed in the planetary system. Determination of the Jovian ${}^{15}\text{N}{}^{14}\text{N}$ ratio with an accuracy of ~10% would crucially test our interpretation of the nitrogen isotope observations.     

Multi‐scale stratigraphic forward modelling of the Surat Basin for geological storage of CO2

Underground geological storage of CO₂ (GSC) requires a high level of subsurface understanding that is often hindered by a lack of data. This study demonstrates the use of stratigraphic forward modelling (SFM) in generating and characterising a static reservoir model using limited well data, with multiple potential applications within the GSC workflow. Sedsim SFM software was used to create a static model of the Surat Basin, including a high‐resolution nested model of the EPQ‐7 GSC tenement within the basin. Deposition and burial of the Jurassic Precipice Sandstone, Evergreen Formation and Hutton Sandstone were simulated. Modelling results show a close match with gamma‐ray well logs in the tenement area, and the model can be considered a credible model of the subsurface. The Sedsim‐predicted formation thicknesses and porosity and permeability distributions meet criteria set for GSC, suggesting that the EPQ‐7 tenement may be a prospective GSC location.     

Equilibrium-disequilibrium relations in the Monte Rosa Granite,Western Alps: Petrological,Rb-Sr and stable isotope data

Nine samples from the Monte Rosa Granite have been investigated by microscopic, X-ray, wet chemical, electron microprobe, stable isotope and Rb-Sr and K-Ar methods. Two mineral assemblages have been distinguished by optical methods and dated as Permian and mid-Tertiary by means of Rb-Sr age determinations. The Permian assemblage comprises quartz, orthoclase, oligoclase, biotite, and muscovite whereas the Alpine assemblage comprises quartz, microcline, albite+epidote or oligoclase, biotite, and phengite. Disequilibrium between the Permian and Alpine mineral assemblages is documented by the following facts: (i) Two texturally distinguishable generations of white K-mica are 2 M muscovite (Si=3.1–3.2) and 2 M or 3 T phengite (Si=3.3–3.4). Five muscovites show Permian Rb-Sr ages and oxygen isotope fractionations indicating temperatures between 520 and 560 ° C; however, K-Ar ages are mixed or rejuvenated. Phengite always shows mid-Tertiary Rb-Sr ages, (ii) Two biotite generations can be recognized, although textural evidence is often ambiguous. Three out of four texturally old biotites show mid-Tertiary Rb-Sr cooling ages while the oxygen isotopic fractionations point to Permian, mixed or Alpine temperatures, (iii) Comparison of radiogenic and stable isotope relations indicates that the radiogenic isotopes in the interlayer positions of the micas were mobilized during Alpine time without recrystallization, that is, without breaking Al-O or Si-O bonds. High Ti contents in young muscovites and biotites also indicate that the octahedral (and tetrahedral) sites remained undisturbed during rejuvenation. (iv) Isotopic reversals in the order of O¹⁸ enrichment between K-feldspar and albite exist. Arguments for equilibrium during Permian time are meagre because of Alpine overprinting effects. Texturally old muscovites show high temperatures and Permian Rb-Sr ages in concordancy with Rb-Sr whole rock ages. For the tectonically least affected samples, excellent concordance between quartz-muscovite and quartz-biotite Permian temperatures implies oxygen isotope equilibrium in Permian time which was undisturbed during Alpine metamorphism. Arguments for equilibrium during the mid-Tertiary metamorphism are as follows: (i) Mid-Tertiary Rb-Sr mineral isochrons of up to six minerals exist, (ii) Oxygen isotope temperatures of coexisting Alpine phengites and biotites are concordant.The major factor for the adjustment of the Permian assemblages to Alpine conditions was the degree of Alpine tectonic overprinting rather than the maximum temperatures reached during the mid-Tertiary Alpine metamorphism. The lack of exchange with externally introduced fluid phases in the samples least affected by tectonism indicates that the Monte Rosa Granite stewed in its own juices. This seems to be the major cause for the persistence of Permian ages and corresponding temperatures.     

An Eocene/Oligocene blueschist‐/greenschist facies P–T loop from the Cycladic Blueschist Unit on Naxos Island,Greece: Deformation‐related re‐equilibration vs. thermal relaxation

Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure–temperature (P–T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 ± 16 to 619 ± 32°C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist facies overprint at 384 ± 30°C and 3.8 ± 1.1 kbar. Multi‐mineral Rb–Sr isochron dating yields crystallization ages for near peak‐pressure blueschist facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high‐P rocks from south Naxos were exhumed to upper mid‐crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km/Ma, completing a full blueschist‐/greenschist facies metamorphic cycle soon after subduction within c. 8 Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid‐controlled metamorphic re‐equilibration, and is unrelated to the strong high‐T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.     

Black carbon decomposition under varying water regimes

The stability of biomass-derived black carbon (BC) or biochar as a slow cycling pool in the global C cycle is an important property and is likely governed by environmental conditions. This study investigated the effects of water regimes (saturated, unsaturated and alternating saturated–unsaturated conditions) and differences in BC materials, produced by carbonizing corn residues and oak wood at two temperatures (350 °C and 600 °C) on BC degradation at 30 °C over 1 year in a full factorial experiment. Effects of water regime on C loss and potential cation exchange capacity (CECp at pH 7) significantly depended on biomass type. Corn BC was both mineralized (16% C loss for the first year) and was oxidized [1000 mmole(+) kg^?1 C] significantly faster under unsaturated conditions than under other water regimes, whereas oak BC mineralized most rapidly (12%) under alternating saturated–unsaturated conditions with similar oxidation, irrespective of water regime. Over 1 year of saturated incubation, the O/C ratio values did not significantly (P > 0.05) increase even though BC was mineralized by 9% and CECp increased by 170 mmole(+) kg^?1 C, in contrast to unsaturated and alternating saturated–unsaturated conditions. While mineralization and oxidation significantly decreased at higher charring temperature for corn, no difference was observed for oak (P > 0.05). Unsaturated and alternating conditions increased carboxylic and OH functional groups, while they decreased aliphatic groups. The pH increased by about one unit for corn BC, but decreased by 0.2 units for oak BC, indicating strong mineral dissolution of corn BC. Carbon loss strongly correlated with changes in O/C values of both corn BC and oak BC, indicating that oxidation of BC was most likely the major mechanism controlling its stability. However, under saturated conditions, additional mechanisms may govern BC degradation and require further investigation.     

Analysis of coprolites from the extinct mountain goat Myotragus balearicus

Humans colonized the Balearic Islands 5–4 ka ago. They arrived in a uniquely adapted ecosystem with the Balearic mountain goat Myotragus balearicus (Bovidae, Antilopinae, Caprini) as the only large mammal. This mammal went extinct rapidly after human arrival. Several hypotheses have been proposed to explain the extinction of M. balearicus. For the present study ancient DNA analysis (Sanger sequencing, Roche-454, Ion Torrent), and pollen and macrofossil analyses were performed on preserved coprolites from M. balearicus, providing information on its diet and paleo-environment. The information retrieved shows that M. balearicus was heavily dependent on the Balearic box species Buxus balearica during at least part of the year, and that it was most probably a browser. Hindcast ecological niche modelling of B. balearica shows that local distribution of this plant species was affected by climate changes. This suggests that the extinction of M. balearicus can be related to the decline and regional extinction of a plant species that formed a major component of its diet. The vegetation change is thought to be caused by increased aridity occurring throughout the Mediterranean. Previous hypotheses relating the extinction of M. balearicus directly to the arrival of humans on the islands must therefore be adjusted.