ALBERT

Description: 40Ar/39Ar apparent age spectra have been measured for unusually retentive potassium feldspars (K-feldspar) from the South Cyclades Shear Zone, Ios, Greece. Our results imply that the Argon Partial Retention Zone (Ar PRZ) for the most retentive domains in potassium K-feldspar can expand into the ductile regime, even when temperatures in excess of about 400-450 {degrees}C apply. In such cases K-feldspar could be used as a geochronometer to estimate the timing and duration of deformation and metamorphism events. Therefore, we have reassessed traditional methods used to analyse Arrhenius plots by simulating the effect of step-heating experiments on argon loss. Fractal multidomain diffusion models were used, with theoretical distributions of diffusion domain size and volume. We discovered a Fundamental Asymmetry Principle that offers objective constraints on slope fitting to allow an analysis to be consistent with the multidomain diffusion hypothesis, and which consistently leads to the estimation of higher activation energies. Reanalysis of existing datasets is encouraged to allow reassessment of the significance of the average values reported. Retentive diffusion parameters for K-feldspar might prove to be commonplace.

Description: We present a new stratigraphy for the Togian Islands, Sulawesi, and interpret the age, character and evolution of Gorontalo Bay. At its western end the bay is underlain by continental crust. The central part is underlain by Eocene to Miocene oceanic and arc rocks, although the area south of the Togian Islands could have continental crust of the Banggai-Sula microcontinent thrust beneath this and the East Arm ophiolite. Gorontalo Bay was not a significant deep bathymetric feature before the Miocene. Field relationships indicate a latest Miocene to Pliocene age for inception of the basin. Medium-K to shoshonitic volcanism in the Togian Islands is not due to subduction but reflects crustal thinning and extension in the Pliocene and Pleistocene, causing the underlying mantle to rise, decompress and melt. Extension is continuing today and is probably the cause of volcanism at Una-Una. Volcanic activity migrated west with time and volcanic products have been offset by dextral strike-slip displacement along the Balantak Fault. Extension and subsidence was driven by rollback of the subduction hinge at the North Sulawesi Trench with a possible contribution due to flow of the lower crust.

Description: Deformation events and episodes of metamorphic mineral growth are usually regarded as relatively local phenomena. It is not expected that specific events and episodes within an orogenic sequence should exactly correlate over large distances. There is no obvious reason, for example, to assume that deformation and/or metamorphic events in the Western European Alps would directly correlate with events taking place in the Aegean continental crust, c. 1000 km distant. Yet linked episodes of deformation and metamorphism appear to take place at the same time over large distances, even in these apparently unrelated segments of the same orogenic belt. This large-scale episodic behaviour appears to be associated with switches in tectonic mode, from compressional orogenesis to extensional tectonism, and may be the result of orgenic surges and/or periods of lithospheric extension following accretion events. The effect of these switches is greatest in back-arc environments, in the over-riding plate above major subduction zones. In these environments, roll-back of the subducting lithospheric slab after individual accretion events ensures that the amount of lithospheric extension after each accretion event is large. As a result this is where coherent high-pressure metamorphic terranes formed in the preceding accretion event are exhumed, and where remnants of newly emplaced ophiolite sheets are stranded by newly formed detachment faults.

Description: Elevated atmospheric [CO 2 ] (e C a ) often decreases stomatal conductance, which may delay the start of drought, as well as alleviate the effect of dry soil on plant water use and carbon uptake. We studied the interaction between drought and e C a in a whole-tree chamber experiment with Eucalyptus saligna . Trees were grown for 18 months in their C a treatments before a 4-month dry-down. Trees grown in e C a were smaller than those grown in ambient C a (a C a ) due to an early growth setback that was maintained throughout the duration of the experiment. Pre-dawn leaf water potentials were not different between C a treatments, but were lower in the drought treatment than the irrigated control. Counter to expectations, the drought treatment caused a larger reduction in canopy-average transpiration rates for trees in the e C a treatment compared with a C a . Total tree transpiration over the dry-down was positively correlated with the decrease in soil water storage, measured in the top 1.5 m, over the drying cycle; however, we could not close the water budget especially for the larger trees, suggesting soil water uptake below 1.5 m depth. Using neutron probe soil water measurements, we estimated fractional water uptake to a depth of 4.5 m and found that larger trees were able to extract more water from deep soil layers. These results highlight the interaction between rooting depth and response of tree water use to drought. The responses of tree water use to e C a involve interactions between tree size, root distribution and soil moisture availability that may override the expected direct effects of e C a . It is essential that these interactions be considered when interpreting experimental results.

Description: Nocturnal water flux has been observed in trees under a variety of environmental conditions and can be a significant contributor to diel canopy water flux. Elevated atmospheric CO 2 (elevated [CO 2 ]) can have an important effect on day-time plant water fluxes, but it is not known whether it also affects nocturnal water fluxes. We examined the effects of elevated [CO 2 ] on nocturnal water flux of field-grown Eucalyptus saligna trees using sap flux through the tree stem expressed on a sapwood area ( J s ) and leaf area ( E t ) basis. After 19 months growth under well-watered conditions, drought was imposed by withholding water for 5 months in the summer, ending with a rain event that restored soil moisture. Reductions in J s and E t were observed during the severe drought period in the dry treatment under elevated [CO 2 ], but not during moderate- and post-drought periods. Elevated [CO 2 ] affected night-time sap flux density which included the stem recharge period, called ‘total night flux’ (19:00 to 05:00, J s,r ), but not during the post-recharge period, which primarily consisted of canopy transpiration (23:00 to 05:00 , J s,c ). Elevated [CO 2 ] wet (EW) trees exhibited higher J s,r than ambient [CO 2 ] wet trees (AW) indicating greater water flux in elevated [CO 2 ] under well-watered conditions. However, under drought conditions, elevated [CO 2 ] dry (ED) trees exhibited significantly lower J s,r than ambient [CO 2 ] dry trees (AD), indicating less water flux during stem recharge under elevated [CO 2 ]. J s,c did not differ between ambient and elevated [CO 2 ]. Vapour pressure deficit ( D ) was clearly the major influence on night-time sap flux. D was positively correlated with J s,r and had its greatest impact on J s,r at high D in ambient [CO 2 ]. Our results suggest that elevated [CO 2 ] may reduce night-time water flux in E. saligna when soil water content is low and D is high. While elevated [CO 2 ] affected J s,r , it did not affect day-time water flux in wet soil, suggesting that the responses of J s,r to environmental factors cannot be directly inferred from day-time patterns. Changes in J s,r are likely to influence pre-dawn leaf water potential, and plant responses to water stress. Nocturnal fluxes are clearly important for predicting effects of climate change on forest physiology and hydrology.

Description: : A geochronological traverse across the Barrovian metamorphic series, Scotland, shows 40 Ar/ 39 Ar apparent age spectra that reflect the influence of progressive metamorphism during the Grampian orogenic episode. The lowest-grade units of the Barrovian metamorphic series retain pre-Grampian detrital ages as components of their white mica 40 Ar/ 39 Ar apparent age spectra. These relict ages are progressively obliterated in the direction of increasing metamorphic grade, with a Grampian-age 40 Ar/ 39 Ar step-heating plateau first occurring in the biotite zone. The microstructure at this point shows only limited recrystallization, suggesting loss of argon mainly by diffusion. Forward modelling of argon diffusion from white mica grains was therefore carried out, for various thermal histories and grain sizes, to match 40 Ar/ 39 Ar step-heating apparent age spectra patterns preserved within the biotite zone of the Barrovian metamorphic series. The results imply a thermal duration of between 1 and 10 Ma for Barrovian metamorphism in the biotite zone. Such short time scales for metamorphism place a limit on length scales for the heat sources responsible. Mid-crustal extensional ductile shear zones that crop out in the NE of the Grampian Terrane once focused narrow, Grampian-age heat sources (e.g. magmas, hot fluids, shear heating) that drove a brief thermal episode, resulting in the Barrovian metamorphism. Supplementary material: 40 Ar/ 39 Ar data tables and plots, including (1) data from analyses on flux monitors and plots used for J -factor determination, and (2) data from analyses on unknowns and 40 Ar/ 39 Ar step-heating spectra and inverse isochron plots for each unknown, are available at http://www.geolsoc.org.uk/SUP18442 .

Description: New 40 Ar/ 39 Ar geochronological data from the Buchan and Barrovian metamorphic sequences on the NE coast of Scotland have been obtained from in vacuo step-heating experiments conducted on white mica extracted from seven samples. The results confirm the synchronous formation of the Barrovian and Buchan metamorphic sequences, with the 40 Ar/ 39 Ar apparent age spectra obtained for samples of andalusite grade and higher recording a range of ages between 470.4 ± 1.8 and 464.6 ± 1.3 Ma. These ages are close to the accepted age for peak metamorphism during the Grampian Orogeny, c. 470 Ma, whereas 40 Ar/ 39 Ar apparent age spectra of white mica from the lower grade cordierite zone near Banff preserve pre-Grampian detrital ages. In these spectra an age of c. 560 Ma is recorded. This indicates that a tectonothermal event occurred at c. 560 Ma and may provide further insight into the timing of sedimentation. These samples also preserve evidence of ages older than 750 Ma. Supplementary material: 40 Ar/ 39 Ar step-heating data tables for all samples, data from analyses of flux monitors, determination of correction factors and mass discrimination used for data reduction and 40 Ar/ 39 Ar step-heating spectra and inverse isochron plots from each sample are available at www.geolsoc.org.uk/SUP18723 .

Description: Episodes of ultrahigh-temperature (UHT, ≥900 °C) granulite metamorphism have been recorded in mountain belts since the Neoarchean. However, evidence for the tectonic mechanisms responsible for the generation of such extreme thermal conditions is rarely preserved. Here we report the discovery of 16 Ma UHT granulites—the youngest identified at the Earth’s surface—from the Kobipoto Mountains of Seram in eastern Indonesia. UHT conditions were produced by a modern tectonic system in which slab rollback–driven lithospheric extension caused core complex–style exhumation of hot subcontinental lithospheric mantle. Overlying continental crust, heated and metamorphosed by exhumed lherzolites, developed spinel + quartz and sapphirine-bearing residual assemblages, shown by phase equilibria modeling to have required temperatures of ~950 °C at ~8 kbar pressure. Seram is therefore a possible modern analogue for ancient orogens that incorporate UHT granulites.

Description: Step-heating experiments in vacuo are routine when conducting 40 Ar/ 39 Ar geochronology, including for white mica. White mica can break down, due to dehydroxylation and delamination, so experiments involving mica are often conducted in relative haste, and not with the care and precision necessary when intending to apply multi-diffusion-domain theory to model the results. Here we show, however, that carefully managed step-heating experiments appear to allow release of argon through solid-state diffusion processes alone. We analysed phengite-muscovite intergrowths in high-pressure metamorphic rocks exhumed in and beneath extensional ductile shear zones during continental extension. Such materials often yield Arrhenius plots in which there is a distinct steepening of slope mid-way through the step-heating sequence. This steepening appears to correspond with steps in which release of argon from phengite components dominate. We analysed the data using a computer program ( eArgon ) and numerically simulated mixing of gas released from multiple diffusion domains. The results suggest that diffusion of 39 Ar in phengitic white mica involves radically different diffusion parameters in comparison with muscovite. If these results extrapolate to nature then 40 Ar/ 39 Ar geochronology may allow direct dating of white mica mineral growth during metamorphism. Supplementary material: Data files A, B and C are available at www.geolsoc.org.uk/SUP18619 . Data file A C++ computer code used to infer data for an Arrhenius plot, assuming different diffusion geometries. These methods are excerpted from the eArgon computer program used to analyse these data. Data file B Analytical methods and procedures used in the laboratory for 40 Ar/ 39 Ar geochronology performed on the samples reported. Data file C XML formatted data tables for the step-heating experiments reported in this study, in a form that can be read by the eArgon computer program.

Description: Nocturnal sap flow ( Q n ) has been found to occur across many taxa, seasons and biomes. There is no general understanding as to how much Q n occurs and whether it is a significant contribution to total daily sap flow ( Q ). A synthesis of the literature and unpublished data was made to determine how significant is Q n , as a proportion of Q (% Q n ), across seasons, biomes, phylogenetic groups and different thermometric sap flow methods. A total of 98 species were analysed to find that % Q n , on average, was 12.03% with the highest average dataset of 69.00%. There was significantly less % Q n in winter than in other temperate seasons, and significantly less % Q n in the wet season than in the dry season. The equatorial and tropical biomes had significantly higher % Q n than the warm temperate and nemoral biomes. The heat ratio method (HRM) and the thermal dissipation (TDP) method had significantly higher % Q n than the heat balance method. Additional analysis between HRM and TDP found HRM to have significantly higher % Q n in winter, wet season and various biomes. In all but one out of 246 cases Q n occurred, demonstrating that Q n is significant and needs to be carefully considered in sap flow and related studies.