The origin of high δ<Superscript>18</Superscript>O zircons: marbles,megacrysts, and metamorphism

The oxygen isotope ratios (δ¹⁸O) of most igneous zircons range from 5 to 8‰, with 99% of published values from 1345 rocks below 10‰. Metamorphic zircons from quartzite, metapelite, metabasite, and eclogite record δ¹⁸O values from 5 to 17‰, with 99% below 15‰. However, zircons with anomalously high δ¹⁸O, up to 23‰, have been reported in detrital suites; source rocks for these unusual zircons have not been identified. We report data for zircons from Sri Lanka and Myanmar that constrain a metamorphic petrogenesis for anomalously high δ¹⁸O in zircon. A suite of 28 large detrital zircon megacrysts from Mogok (Myanmar) analyzed by laser fluorination yields δ¹⁸O from 9.4 to 25.5‰. The U–Pb standard, CZ3, a large detrital zircon megacryst from Sri Lanka, yields δ¹⁸O = 15.4 ± 0.1‰ (2 SE) by ion microprobe. A euhedral unzoned zircon in a thin section of Sri Lanka granulite facies calcite marble yields δ¹⁸O = 19.4‰ by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble. Small oxygen isotope fractionations between zircon and most minerals require a high δ¹⁸O source for the high δ¹⁸O zircons. Predicted equilibrium values of Δ¹⁸O(calcite-zircon) = 2–3‰ from 800 to 600°C show that metamorphic zircon crystallizing in a high δ¹⁸O marble will have high δ¹⁸O. The high δ¹⁸O zircons (>15‰) from both Sri Lanka and Mogok overlap the values of primary marine carbonates, and marbles are known detrital gemstone sources in both localities. The high δ¹⁸O zircons are thus metamorphic; the 15–25‰ zircon values are consistent with a marble origin in a rock-dominated system (i.e., low fluid_(external)/rock); the lower δ¹⁸O zircon values (9–15‰) are consistent with an origin in an external fluid-dominated system, such as skarn derived from marble, although many non-metasomatized marbles also fall in this range of δ¹⁸O. High δ¹⁸O (>15‰) and the absence of zoning can thus be used as a tracer to identify a marble source for high δ¹⁸O detrital zircons; this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks. Metamorphic zircon megacrysts have not been reported previously and appear to be associated with high-grade marble. Identification of high δ¹⁸O zircons can also aid geochronology studies that seek to date high-grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble.     

Precise orbit determination for the GRACE mission using only GPS data

The GRACE (gravity recovery and climate experiment) satellites, launched in March 2002, are each equipped with a BlackJack GPS onboard receiver for precise orbit determination and gravity field recovery. Since launch, there have been significant improvements in the background force models used for satellite orbit determination, most notably the model for the geopotential. This has resulted in significant improvements to orbit accuracy for very low altitude satellites. The purpose of this paper is to investigate how well the orbits of the GRACE satellites (about 470 km in altitude) can currently be determined using only GPS data and based on the current models and methods. The orbit accuracy is assessed using a number of tests, which include analysis of orbit fits, orbit overlaps, orbit connecting points, satellite Laser ranging residuals and K-band ranging (KBR) residuals. We show that 1-cm radial orbit accuracy for the GRACE satellites has probably been achieved. These precise GRACE orbits can be used for such purposes as improving gravity recovery from the GRACE KBR data and for atmospheric profiling, and they demonstrate the quality of the background force models being used.     

Statistical downscaling of reference evapotranspiration in Haihe River Basin: applicability assessment and application to future projection

Future changes in reference evapotranspiration (ET₀) are of increasing importance in assessing the potential impacts on hydrology and water resources systems of more pronounced climate change. This study assesses the applicability of the Statistical Downscaling Model (SDSM) in projecting ET₀, and investigates the seasonal and spatial patterns of future ET₀ based on general circulation models (GCMs) across the Haihe River Basin. The results indicate that SDSM can downscale ET₀ well in term of different basin-averaged measures for the HadCM3 and CGCM3 GCMs. HadCM3 has a much superior capability in capturing inter-annual variability compared to CGCM3 and thus is chosen as the sole model to assess the changes in future ET₀. There are three homogeneous sub-regions of the Haihe River Basin: Northwest, Northeast and Southeast. Change points are detected at around 2050 and 2080 under the A2 and B2 scenarios, respectively. The Northwest is revealed to have a slight to strong increase in ET₀, while the Northeast and the Southeast tend to experience a pattern change from decrease to increase in ET₀.

EDITOR M.C. Acreman

ASSOCIATE EDITOR J. Thompson     

A spatial perspective for predicting enrollment in a regional pharmacy school

Having the ability to predict enrollment is an important task for any school’s recruiting team. The purpose of this study was to identify significant factors that can be used to predict the spatial distribution of enrollments. As a case study, we used East Tennessee State University (ETSU) pharmacy school, a regional pharmacy school located in the Appalachian Mountains. Through the application of a negative binomial regression model, we found that the most important indicators of enrollment volume for the ETSU pharmacy school were Euclidean distance, probability (based on competing pharmacy schools’ prestige, driving distance between schools and home and tuition costs), and the natural barrier of the Appalachian Mountains. Using these factors, together with other control variables, we successfully predicted the spatial distribution of enrollments for ETSU pharmacy school. Interestingly, gender also surfaced as a variable for predicting the pharmacy school’s enrollment. We found female students are more sensitive to the geographic proximity of home to school.     

Middle Campanian–lowermost Maastrichtian nannofossil and foraminiferal biostratigraphy of the northwestern Australian margin

The middle–late Campanian was marked by an increase in the bioprovinciality of calcareous microfossil assemblages into distinct Tethyan, Transitional, and Austral Provinces that persisted to the end of the Maastrichtian. The northwestern Australian margin belonged to the Transitional Province and the absence of key Tethyan marker species such as Radotruncana calcarata and Gansserina gansseri has led petroleum companies operating in the area to use the locally developed KCCM integrated calcareous microfossil zonation scheme. The KCCM zonation is a composite scheme comprising calcareous nannofossil (KCN), planktonic foraminiferal (KPF) and benthonic foraminiferal (KBF) zones. This paper presents the definitions and revisions of Zones KCCM8–19, from the highest occurrence (HO) of Aspidolithus parcus constrictus to the lowest occurrence (LO) of Ceratolithoides aculeus, and builds on our previous early–late Maastrichtian study. The presence of a middle–upper Campanian disconformity is confirmed by microfossil evidence from the Vulcan Sub-basin, Exmouth and Wombat plateaus, and the Southern Carnarvon Platform. In the Vulcan Sub-basin and on the Exmouth Plateau (ODP Hole 762C) the hiatus extends from slightly above the LO of common Rugoglobigerina rugosa to above the LO of Quadrum gothicum. On the Wombat Plateau (ODP Hole 761B) it spans from above the LO of Heterohelix semicostata to above the LO of Quadrum gothicum; and in the Southern Carnarvon Platform the disconformity has its longest duration from above the HO of Heterohelix semicostata to above the LO of Quadrum sissinghii. A significant revision of the events which define Zones KCCM18 and 19 was necessary owing to the observation that the LO of Ceratolithoides aculeus occurs below the HOs of Archaeoglobigerina cretacea and Stensioeina granulata incondita and the LO of common Rugoglobigerina rugosa. In the original zonation these events were considered to be coincident.     

Josephinite: Crystal structures and phase relations of the metals

Josephinite, a complex, metal-bearing rock from the region of the Josephine Peridotite in southwest Oregon, contains FeNiCo metal alloy phases having exsolution textures. Scanning and transmission electron microscopy have revealed Widmanstaetten patterns in which the lamellae are polysynthetically twinned, ordered, face-centered-cubic FeNi₃ surrounded by untwinned, ordered, face-centered-cubic FeNi₃ and body-centered-cubic FeCo. These exsolution textures require a temperature in excess of 500°C for their formation. This is consistent with a mantle derivation of josephinite prior to obduction of the peridotite in one of the Klamath ophiolites.     

Are carbonate barrier islands mobile? Insights from a mid to late-Holocene system,Al Ruwais,northern Qatar

Barrier islands are important landforms in many coastal systems around the globe. Studies of modern barrier island systems are mostly limited to those of siliciclastic realms, where the islands are recognized as mobile features that form on transgressive coastlines and migrate landward as sea-level rises. Barrier islands of the ‘Great Pearl Bank’ along the United Arab Emirates coast are the best-known carbonate examples. These Holocene islands, however, are interpreted to be anchored by older deposits and immobile. The mid-Holocene to late-Holocene depositional system at Al Ruwais, northern Qatar, provides an example of a mobile carbonate barrier island system, perhaps more similar to siliciclastic equivalents. Sedimentological and petrographic analyses, as well as ¹⁴C-dating of shells and biogenic remains from vibracored sediments and surface deposits, show that after 7000 years ago a barrier system with a narrow back-barrier lagoon formed along what is now an exposed coastal zone, while, contemporaneously, a laterally-extensive coral reef was forming immediately offshore. After 1400 years ago the barrier system was forced to step ca 3 km seaward in response to a sea-level fall of less than 2 m, where it re-established itself directly on the mid-Holocene reef. Since that time, the barrier has retreated landward as much as 1000 m to its current position, exposing previously-deposited back-barrier lagoonal sediment at the open-coast shoreline. In modern neritic warm-water carbonate settings mobile barrier island systems are rare. Their construction and migration may be inhibited by reef formation, early cementation, and the relative inefficiency of sourcing beach sediments from open carbonate shelves. Carbonate barrier island systems likely formed more commonly during geological periods when ramps and unrimmed shelves predominated and in calcite seas, when meteoric cementation was minimized as a result of initial calcitic allochem mineralogy. As with their siliciclastic analogues, however, recognition of the influence of these transient landforms in the rock record is challenging.     

Quantification of mineral matter in the Argonne Premium Coals using interactive Rietveld-based X-ray diffraction

The mineral matter in the eight reference North American coal samples of the Argonne Premium Coal series has been investigated on a quantitative basis using X-ray diffraction (XRD) techniques. X-ray diffraction data obtained from electronic low-temperature (oxygen–plasma) ash (LTA) residues, from ashes produced by heating the coals in air at 370°C, and also from the raw coals themselves, were evaluated using an interactive data processing system ( ™) based on Rietveld interpretation methods. The results from the three types of material (LTA, 370°C ash and raw coal) were compared for each sample. This allowed the components present in the raw coals in crystalline form to be recognised separately from mineral artifacts produced, particularly in the low-rank coals, from interaction of organically associated elements (Ca, S, etc.) during the two ashing processes.After the allowance for the production of any artifacts, the quantitative mineral assemblages identified from XRD of the raw coals were found to be consistent, even for coals having a relatively low ash percentage (around 5%), with the results obtained from the respective mineral concentrates prepared by the ashing methods. The effects of heating the coal to 370°C could also be distinguished, relative to the raw coal or the LTA, through changes in components such as pyrite and the clay minerals.Although some areas of uncertainty exist, particularly with magnesium in the low-rank coals, the calculated chemical compositions of the coal ash derived from the mineral mixtures identified for each coal were also found to be consistent with the results of direct chemical analysis of the respective coal ash materials.