Geochemistry, PbPb single zircon ages and NdSr isotope composition of Precambrian rocks from southern and eastern Ethiopia: implications for crustal evolution in East Africa

Abstract —We consider the isotope-geochemical features of epithermal fluorite deposits in Transbaikalia, including the REE compositions, Sr isotope ratios, Sm–Nd systems, and isotope compositions of oxygen, carbon, hydrogen, and sulfur. The 87Sr/86Sr ratios in fluorites are within 0.706–0.708, and the εNd values are negative. Oxygen in quartz, the main mineral of the deposits, has a light isotope composition (δ18O = –3.4 to +2.6‰), and the calculated isotope composition of oxygen in the fluid in equilibrium with quartz (δ18O = –9 to –16‰) indicates the presence of meteoric water. The latter is confirmed by analysis of the isotope compositions of oxygen and hydrogen in gas–liquid inclusions in fluorites from three deposits. These isotope compositions are due to recycling caused by the impact of shallow basic plutons. The isotope composition of sulfur indicates its deep source. During ascent, sulfur became enriched in its light isotope (δ34S = –1.8 to –7.7‰). We assess the association of fluorite ores with basaltoids widespread in the study area. The isotope and geochemical parameters suggest their spatial proximity. Probably, the basaltoids were responsible for the recycling of meteoric water. It is shown that the epithermal fluorite deposits formed by the same mechanism as fissure–vein thermal waters in western Transbaikalia.

Download Full-text

Stable isotope composition of tropical high-altitude fresh-waters on Mt. Kenya, Equatorial East Africa

Chemical Geology ◽

10.1016/s0009-2541(99)00233-8 ◽

2000 ◽

Vol 166 (3-4) ◽

pp. 341-350 ◽

Cited By ~ 52

Author(s):

Miri Rietti-Shati ◽

Ruth Yam ◽

Wibjorn Karlen ◽

Aldo Shemesh

Keyword(s):

Stable Isotope ◽

High Altitude ◽

East Africa ◽

Isotope Composition ◽

Stable Isotope Composition ◽

Fresh Waters

Download Full-text

Lower Permian brachiopods from Oman: their potential as climatic proxies

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s1755691008075634 ◽

2007 ◽

Vol 98 (3-4) ◽

pp. 327-344 ◽

Cited By ~ 6

Author(s):

L. Angiolini ◽

D. P. F. Darbyshire ◽

M. H. Stephenson ◽

M. J. Leng ◽

T. S. Brewer ◽

...

Keyword(s):

Isotope Composition ◽

Isotopic Analysis ◽

Lower Permian ◽

Published Data ◽

Sr Isotope ◽

Marine Sedimentation ◽

Ice Accumulation ◽

Secondary Layer ◽

Data Points ◽

Few Data

ABSTRACTThe 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. δ18O 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. δ13C 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.

Download Full-text