Geochemistry, and carbon, oxygen and strontium isotope composition of brachiopods from the Khuff Formation of Oman and Saudi Arabia GeoArabia

Brachiopods are abundant in the Oman Khuff Formation and similar brachiopod faunas are present at a few horizons in the same formation in Central Saudi Arabia. Following extensive systematic and biostratigraphic studies of these faunas, specimens from the base of the Midhnab Member of the Khuff Formation of Saudi Arabia (Buraydah Quadrangle), and from Member 3 of the Khuff Formation of the Huqf outcrop of Oman were assessed for isotope geochemistry (Sr, O and C). Dating using 87Sr/86Sr alone is not conclusive. Five pristine Oman brachiopods from biostratigraphically well-constrained lower Wordian horizons record a range of 87Sr/86Sr values that form a separate cluster offset from the current Sr isotope seawater curve, which defines the Early Permian and earliest Mid-Permian. The 87Sr/86Sr of the pristine Saudi Arabian brachiopod sits in an area which corresponds to a wide scatter of 87Sr/86Sr in the seawater curve data. However, the Saudi Arabian data does indicate that the Midhnab Member is likely younger than Member 3 of the Khuff Formation of the Huqf outcrop. The well-preserved brachiopod carbonate allows deductions to be made about the palaeotemperature of the Oman Khuff Formation Member 3 seawater using its oxygen isotope composition (\u3b418O). Assuming \u3b418O of seawater < \u20130.5\u2030, then palaeotemperature derived from brachiopods in the Oman horizons would be +25\ub0C, +22\ub0C and +17\ub0C respectively. This is consistent with the trend of shallowing within Member 3, suggested by facie

Lower Permian brachiopods from Oman : their potential as climatic proxies

The Lower Permian of the Haushi basin, Interior Oman (Al Khlata Formation to Saiwan Formation/lower Gharif member) records climate change from glaciation, through marine sedimentation in the Haushi sea, to subtropical desert. To investigate the palaeoclimatic evolution of the Haushi sea we used O, C, and Sr isotopes from 31 brachiopod shells of eight species collected bed by bed within the type-section of the Saiwan Formation. We assessed diagenesis by scanning electron microscopy of ultrastructure, cathodoluminescence, and geochemistry, and rejected fifteen shells not meeting specific preservation criteria. Spiriferids and spiriferinids show better preservation of the fibrous secondary layer than do orthotetids and productids and are therefore more suitable for isotopic analysis. 18O of –3.7 to –3.1‰ from brachiopods at the base of the Saiwan Formation are probably related to glacial meltwater. Above this, an increase in δ18O may indicate ice accumulation elsewhere in Gondwana or more probably that the Haushi sea was an evaporating embayment of the Neotethys Ocean. 13C varies little and is within the range of published data: its trend towards heavier values is consistent with increasing aridity and oligotrophy. Saiwan Sr isotope signatures are less radiogenic than those of the Sakmarian LOWESS seawater curve, which is based on extrapolation between few data points. In the scenario of evaporation in a restricted Haushi basin, the variation in Sr isotope composition may reflect a fluvial component

Active geothermal systems with entrained seawater as modern analogs for transitional volcanic-hosted massive sulfide and continental magmato-hydrothermal mineralization: The example of Milos Island, Greece

Low-sulfidation epithermal mineralization on Milos (Aegean arc) records high paleo-fluid salinities that cannot be explained by a Broadlands-type low-salinity geothermal system. The δD and δ18O data do not document 18O-shifted meteoric waters, one of the characteristic features in terrestrial geothermal systems. Nor is a submarine origin indicated - stable isotope data show mixing of meteoric, seawater, and volcanic-arc gases. Strontium isotope data are comparable to those of a nearby active seawater-entrained geothermal system. These are features seen in hydrothermal systems associated with emergent volcanoes. The similarities between ancient and active systems on Milos in terms of salinity, δD vs. δ18O, and strontium isotope systematics strongly suggest that seawater is the main source for Na and Cl. We suggest that geothermal systems containing seawater associated with emergent volcanoes are an additional analog for intrusion-centered ore-deposit models. Furthermore, such systems bridge the gap between submarine and terrestrial geothermal systems - the modern analogs for volcanic-hosted massive sulfide and epithermal mineralization in the scheme of intrusion-centered hydrothermal mineralization. © 2005 Geological Society of America

Palaeohydrological significance of Late Quaternary strontium isotope ratios in a tropical lake

Ostracods preserved in late Quaternary sediments of Wallywash Great Pond, a fresh coastal lake in SW Jamaica, record temporal variations in the strontium-isotope composition of lake water. Oxygen-isotope and Sr/Ca ratios in ostracods reveal temporal variations in the lake's hydrology, related to effective precipitation, and in its salinity related to varying marine-saline groundwater input from changes in relative sea level. Evaluation of isotopic and trace-element data indicates that the stratigraphic variations in 87Sr/86Sr ratios during the late Quaternary are best explained by climatically-controlled hydrological changes. During wetter periods, the lake's Sr budget was dominated by springwater input with relatively low 87Sr/86Sr ratio, whereas during drier times reduced springflow, possibly coupled with input of more-radiogenic Sr from other sources, such as sea-spray aerosols and perhaps Saharan dust, led to an increase in the Sr-isotope ratio of the lake water. Despite proximity of the lake to the sea and evidence for slight intrusion of marine saline groundwater in the past, however, the extent of marine input appears to have had limited influence on the lake's Sr-isotope ratios. Whereas the 87Sr/86Sr ratios cannot be used as a palaeosalinity proxy in this particular lake, they do provide valuable information about the mechanisms underlying hydrological change

The Tambien Group, Ethiopia : an early Cryogenian (ca. 800-735 Ma) Neoprotozoic sequence in the Arabian-Nubian Shield

The Tambien Group in Tigrai, Ethiopia, comprises a number of inliers each containing 2–3 km thickness of interbedded carbonate and clastic sediments, capped in one inlier by a glaciogenic diamictite. A range of geochemical indices suggest near-pristine C- and Sr-isotope values are preserved and these, together with lithological variations, allow local correlation between these inliers and correlation with the global Neoproterozoic isotope stratigraphy. A composite section of the Tambien Group shows δ13C in carbonate of +6‰ at its base, decreasing upwards to twin lows of –4‰ separated by a brief excursion back to positive values, then rises again to a plateau of +6‰ before finally decreasing sharply to −2‰ beneath the Negash diamictite at its top. No glaciogenic sediments are observed associated with the lower twinned negative anomalies. The 87Sr/86Sr values in carbonates change from 0.7063 in the lower units to 0.7067 in the upper units. The δ13C of organic matter changes little through the sequence (−24.2 ± 1.3‰), with the result that the C-isotope fractionation between carbonate and organic matter decreases from +26 to +21‰ up through the second low in carbonate δ13C before increasing to +29.5‰ in the rest of the sequence. Together with existing radiometric age constraints, the Sr-isotope data indicate that the Negash diamictite is Sturtian in age, and the lower negative C-isotope anomalies appear to correlate with the non-glaciogenic Bitter Springs Stage recognised in Australia and Svalbard. According to this correlation the Tambien Group was deposited in the interval ca. 800–735 Ma. Our data reinforce the emerging view that not all Neoproterozoic negative C-isotope anomalies are associated with glaciation. The variations in C-isotope fractionations within our sequence contrast with those from Australia and are attributed to differences in local environmental variables, probably temperature. These differences are consistent with the recent proposal that the Bitter Springs Stage anomaly is the result of a pair of Inertial Interchange True Polar Wander events [Halverson, G.P., Maloof, A., Schrag, D., Dudas, F., Hurtgen, M., in press. Stratigraphy and geochemistry of a ca 800 Ma negative carbon isotope stage in northeastern Svalbard. Chem. Geol.] [Maloof, A.C., Halverson, G.P., Kirschvink, J.L., Schrag, D.P., Weiss, B.P., Hoffman, P.F., in press. Combined paleomagnetic, isotopic and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard. GSA Bull.], and further high-resolution dating and palaeomagnetic studies of the Tambien Group should allow testing of this hypothesis

Sources of post-orogenic calcalkaline magmas : the Arrochar and Garabal Hill-Glen Fyne complexes, Scotland

The 425 Ma Arrochar and Garabal Hill–Glen Fyne complexes of highland Scotland are examples of post-orogenic magmatism accompanying extensional collapse of an orogen, in this case the Caledonian. The rocks are dominantly high-K series, but range from medium-K to shoshonitic. Mantle upwelling, melting and the intrusion of large volumes of mafic magma into the crust are inferred to have accompanied lithospheric thinning, and to have provided the heat source for melting of young arc crust accreted during the preceding subduction epoch. Fluids evolved from the subducting slab are inferred to have caused high degrees of enrichment in the overlying mantle wedge. Deep in the crust, the mantle-derived, K-rich mafic to intermediate magmas mixed with felsic crustal melts to form the spectrum of magmas intruded in the two complexes. Microgranular enclaves in the granitic rocks represent mafic magmas derived from the enriched mantle and hybridised by reaction, diffusion and mechanical mixing with their host felsic magmas, but they do not form part of the evolutionary series that produced the host magmas. Rather than inheriting its LILE-enriched character directly from crustal melts, or from crustal assimilation by mafic magmas, the high-K series may commonly owe at least part of its potassic character to the involvement of mantle (highly metasomatised by slab-derived fluids) as a major magma source. Enclave suites, though prominent in some granitic rocks should not be assumed to represent magmas that played a significant role in the production of the chemical variations in their host magmas