Origin of sapphirine-bearing garnet-orthopyroxene granulites: possible hydrothermally altered ocean floor

Sapphirine is a metamorphic mineral that forms in the deep crust in rocks with distinctive bulk compositions, in particular high concentrations of Mg and Al. This study investigates a thin, discontinuous layer of sapphirine-bearing garnet-orthopyroxene (SGO) granulite within a Palaeoproterozoic mafic granulite together with metamorphosed mafic and ultramafic rocks, micaceous rocks rich in magnetite and pyrite, and marble. All of these rocks occur within a low-strain zone in the Lewisian complex of South Harris, NW Scotland. Data on mineral compositions and major, trace, and rare earth element (REE) patterns provide evidence for the origin of the precursor of the SGO granulite. The host mafic granulite has a trace element signature and REE pattern comparable with that of E-type MORB. The chemical abundances of Nb, Ta, La, Ce, Nd, Zr, Hf, Ti, and V in the SGO granulite, except for one sample that records total element loss, are similar to those of the host mafic granulite; however, in terms of whole-rock element abundances, the former is relatively enriched in MgO and depleted in CaO, Na₂O, MnO, Sr, and Eu. Elements within the SGO granulite that were apparently unaffected by hydrothermal alteration—V, Y, Zr, and Cr—are within the range of values observed in the host mafic granulite. Sm–Nd whole-rock isotope systematics suggest that both the host mafic granulite and SGO granulite were metamorphosed at 1.9 Ga, and the Nd initial ratio is consistent with a MORB source at that time. There is no significant difference in the Nd values of the two rock types, suggesting that they originally belonged to the same protolith. Chemical trends within a narrow zone between the SGO and host rock granulite suggest that the former was derived from a basaltic precursor of the latter by a process of infiltration metasomatism, comparable with the chemical exchange that takes place when hydrothermal fluids in present-day oceanic crust pass through vents in a “recharge zone.” The geological and chemical relations observed in the South Harris rocks are consistent with the following model: hydrothermally altered oceanic basaltic crust was trapped in an accretionary wedge and subducted, followed by granulite-facies metamorphism in a deep continental environment during arc–continent collision. This is the first report of hydrothermally altered oceanic crust that was converted to sapphirine-bearing granulite deep in the continental crust.     

Quaternary features beneath Lake Simcoe,Ontario, Canada: drumlins,tunnel channels,and records of proglacial to postglacial closed and overflowing lakes

Lake Simcoe is a large lake 45 km across and in places over 30 m deep, located between Lake Huron and Lake Ontario, in the glaciated terrain of southern Ontario, Canada. Seismostratigraphic analysis of high-resolution seismic reflection profiles, together with lakebed sediment sampling and pollen study, revealed distinctive sequences in the sediments beneath Lake Simcoe, Ontario. A surface unit (Blue Sequence) of soft Holocene mud (low-amplitude surface reflection, discontinuous parallel internal reflections) lies in the deeper basins of the lake. The underlying unit (Green Sequence) is characterized by high-amplitude parallel internal reflections; basal sediments of this sequence consist of clay rhythmites with dropstones. The Green Sequence was deposited by lacustrine sedimentation in proglacial Lake Algonquin; sedimentation persisted until the basin was isolated from other glacial lakes at about 10 ¹⁴C ka at the Penetang post-Algonquin phase. Subsequent erosion of the uppermost portion of the Green Sequence is attributed to wave action in a low-level early Holocene lake, possibly closed hydrologically and coeval with closed lowstands in the Huron and Georgian Bay basins. Two sequences with high-amplitude surface reflections and chaotic internal reflections (Purple and Red Sequences) lie below the Green Sequence. Northeast-southwest trending ridges, tens of metres in height, on the Red Sequence (the lowermost of these two units) are interpreted to be drumlins. An erosion surface descends into narrow valleys 50–80 m deep beneath the lake in bays to the west and south of the main lake basin. These depressions are interpreted as subglacial tunnel channels cut by rapid flows of meltwater. The sediments of Purple Sequence are interpreted as channel-fill sediments rapidly deposited during waning stages of the meltwater drainage. The Red Sequence is correlated with the Newmarket Till of the last glacial maximum identified beneath the Oak Ridges Moraine to the south.     

X-ray irradiation in XTE J1817−330 and the inner radius of the truncated disc in the hard state

The key aspect of the very successful truncated disc model for the low/hard X-ray spectral state in black hole binaries is that the geometrically thin disc recedes from the last stable orbit at the transition to this state. This has recently been challenged by direct observations of the low/hard state disc from CCD data. We reanalyse the Swift and RXTE campaign covering the 2006 outburst of XTE J1817−330, and show that these data actually strongly support the truncated disc model as the transition spectra unambiguously show that the disc begins to recede as the source leaves the disc-dominated soft state. The disc radius inferred for the proper low/hard state is less clear-cut, but we show that the effect of irradiation from the energetically dominant hot plasma leads to an underestimate of the disc radius by a factor of 2–3 in this state. This may also produce the soft excess reported in some hard-state spectra. The inferred radius becomes still larger when the potential difference in stress at the inner boundary, increased colour temperature correction from incomplete thermalization of the irradiation, and loss of observable disc photons from Comptonization in the hot plasma is taken into account. We conclude that the inner disc radius in XTE J1817−330 in the low/hard spectral state is at least six to eight times that seen in the disc-dominated high/soft state, and that recession of the inner disc is the trigger for the soft-hard-state transition, as predicted by the truncated disc models.     

The DEEP2 Galaxy Redshift Survey: the role of galaxy environment in the cosmic star formation history

Using galaxy samples drawn from the Sloan Digital Sky Survey and the DEEP2 Galaxy Redshift Survey, we study the relationship between star formation and environment at   z ∼ 0.1  and 1. We estimate the total star formation rate (SFR) and specific star formation rate (sSFR) for each galaxy according to the measured [O  ii ]λ 3727 Å nebular line luminosity, corrected using empirical calibrations to match more robust SFR indicators. Echoing previous results, we find that in the local Universe star formation depends on environment such that galaxies in regions of higher overdensity, on average, have lower SFRs and longer star formation time-scales than their counterparts in lower density regions. At   z ∼ 1  , we show that the relationship between sSFR and environment mirrors that found locally. However, we discover that the relationship between total SFR and overdensity at   z ∼ 1  is inverted relative to the local relation. This observed evolution in the SFR–density relation is driven, in part, by a population of bright, blue galaxies in dense environments at   z ∼ 1  . This population, which lacks a counterpart at   z ∼ 0  , is thought to evolve into members of the red sequence from   z ∼ 1  to ∼0. Finally, we conclude that environment does not play a dominant role in the cosmic star formation history at   z < 1  : the dependence of the mean galaxy SFR on local galaxy density at constant redshift is small compared to the decline in the global SFR space density over the last 7 Gyr.     

Regime shifts between clear and turbid water in New Zealand lakes: Environmental correlates and implications for management and restoration

We reviewed lakes in New Zealand reported to have undergone regime shifts between macrophyte‐dominated clear water states and de‐vegetated, turbid states. Regime‐shifting lakes (RSLs) occurred along a wide latitudinal gradient. We obtained catchment land‐use data as well as data on the occurrences of introduced (non‐indigenous) macrophytes and herbivorous and benthivorous fish for the 37 RSLs and for 58 lakes with similar maximum depths and climates, but which had not been reported to have undergone regime shifts. All RSLs had a maximum depth <20 m and mean annual surface air temperature between 9 and 16°G Regime shifts were positively related to the percentage of the catchment in pasture and negatively related to the percentage of the catchment in forest. The occurrences of the introduced macrophyte Egeria densa and the introduced fish, Ameiurus nebulosus (catfish), Carassius auratus (goldfish), Scardinius erythrophthalmus (rudd), Cyprinus carpio (koi carp), and Tinca tinca (tench), were significantly correlated to regime shifts in lakes. Although the presence of other introduced aquatic macrophytes was not significantly correlated with RSLs, the number of exotic fish taxa present in lakes was strongly positively correlated with increasing prevalence of regime shifts. The strength of the correlations between land use and introduced species versus regime shifts illustrates a number of factors which could be managed to reduce the susceptibility of lakes to regime shifts and to restore lakes that have become de‐vegetated. Our findings also suggests that regime shifts in lakes were unlikely to have been common in New Zealand lakes before anthropogenic deforestation and introductions of introduced aquatic taxa.     

Laboratory Generated M -6 Earthquakes

We consider whether mm-scale earthquake-like seismic events generated in laboratory experiments are consistent with our understanding of the physics of larger earthquakes. This work focuses on a population of 48 very small shocks that are foreshocks and aftershocks of stick–slip events occurring on a 2.0 m by 0.4 m simulated strike-slip fault cut through a large granite sample. Unlike the larger stick–slip events that rupture the entirety of the simulated fault, the small foreshocks and aftershocks are contained events whose properties are controlled by the rigidity of the surrounding granite blocks rather than characteristics of the experimental apparatus. The large size of the experimental apparatus, high fidelity sensors, rigorous treatment of wave propagation effects, and in situ system calibration separates this study from traditional acoustic emission analyses and allows these sources to be studied with as much rigor as larger natural earthquakes. The tiny events have short (3–6 μs) rise times and are well modeled by simple double couple focal mechanisms that are consistent with left-lateral slip occurring on a mm-scale patch of the precut fault surface. The repeatability of the experiments indicates that they are the result of frictional processes on the simulated fault surface rather than grain crushing or fracture of fresh rock. Our waveform analysis shows no significant differences (other than size) between the M -7 to M -5.5 earthquakes reported here and larger natural earthquakes. Their source characteristics such as stress drop (1–10 MPa) appear to be entirely consistent with earthquake scaling laws derived for larger earthquakes.