UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG

FACILITIES

Geo-luminescence Laboratory

Luminescence dating

Optically Stimulated Luminescence (OSL) dating is a dating method for quaternary sediments and archaeological materials. The method utilises the tiny light signal (the luminescence) emitted from mineral grains when they are exposed to light (the optical stimulation). This signal is built up through the absorption of energy from ionising radiation, emitted from radioisotopes that are present in natural sediment. The signal is reset by light, so the method determines the length of time since the sediment was last exposed to sunlight. OSL dating is therefore applicable only to sediments that were exposed to sunlight during their last episode of transport and deposition. This permits the dating of aeolian, fluvial, shoreline and lake sediment, but not, for example, sub-glacial sediment.

Heat can also reset the OSL signal, so in the case of archaeological material (pottery, bricks, ceramics, hearth stones), the event that is dated is the last time the material was heated above 300°C, e.g. the firing of pottery.

OSL dating is usually performed on sand-sized grains of quartz (c.200 μm), although feldspar can also be used. The lower age limit is around 30 years, the upper limit around 100-200 thousand years, depending on the sediment. New advances in OSL methods may push this limit towards ½ million years in the near future. Precision is typically between 5-10 % at 1σ.

Facilities

The luminescence dating laboratory at Wits is the only one of its kind in southern Africa. We have a range of specialist facilities, including:

·A large sample preparation room with subdued red lighting
·A centrifuge and Frantz Magnetic Barrier Laboratory Separator for mineral separation
·Two Riso OSL/TL Readers, both with single-grain attachment
·Gamma-spectrometry, performed at iThemba labs (on campus)

Service

The laboratory is run by Mary Evans and Alastair Cunningham We provide luminescence ages on a collaborative basis (co-authorship on publications using lab results), with a fee of R4600+VAT per sample. Please get in touch before submitting samples. We would like to give advice on sampling strategies before fieldwork is attempted.

Some general points for consideration:

·Target sand-rich sediment beds. If there is little sand, take larger samples. If there is no sand, we can't date it.
·Exposures are easily sampled by driving a metal or opaque plastic tube into the exposure face. Dig out the tubes, pack the ends with bubble-wrap and tape the ends with duct tape; this keeps the sediment firmly in place (it must not be loose), and prevents moisture escaping.
·Target beds that are homogeneous to a distance of 20 cm on all sides of the sample. The grains absorb a radiation dose from this region, and we assume the sample material is representative of it. If this is not possible, collect additional samples from the adjacent beds, make a note of the geometry of the beds, and we will perform the necessary corrections.
·Similarly for archaeological material: be sure to sample the surrounding sediment, as it is needed to determine the natural radiation dose rate.
·Record the co-ordinates, altitude above sea level and the depth of the sample below the surface.

For Further information or submission of samples, please contact Mary Evans

FLUID INCLUSION LABORATORY

In 2010 a new Fluid Inclusions Laboratory was installed. The laboratory is equipped with a Linkam MDSG600 heating/freezing stage with a temperature range of -196°C to 600 °C, capable of heating/cooling rates of 0.01 to 150°C/min, and hold a stability of 0.001°C. The stage is mounted on an Olympus BX51 petrographic microscope, which is fitted with an Olympus SC30 camera, operating on Olympus Stream Essentials software. The microscope is fitted with a regular 5 x objective, and long-working-distance 10x, 20x, and 50x objectives; it is also fitted with a 1.6x magnification changer allowing for up to 80x magnification. It has been used by Honours and Postgraduate students to study fluid inclusions in veins sampled from Witwatersrand gold operations and fluid inclusions in mineralised (Li-Be-Sn-Ta-Nb) pegmatites and uraniferous sheeted leucogranites from the Damara Belt, Namibia.

FOSSIL PREPARATION LABORATORY

GIS TEACHING LABORATORY

30 PC's shared with the School of Geography, on the top floor of the BPI building.

GEOPHYSICS COMPUTER LABORATORY

12 PC's on the top floor of the Geosciences building, for use by Geophysics Honours students only.

UNDERGRADUATE COMPUTER LABORATORY

20 PC's in the basement of the Geosciences building, for use by second year and above Geology students.

Hydrogeology Laboratory

The recently built hydrogeology houses an array of new equipment dedicated to the analysis groundwater. PLease contact Tamiru Abiye for more information

The laboratory equipment in the Hydrogeology Lab includes pH meters,Eh meters, EC meters, Colorimetric spectrophotometer, Bacteriological incubator

TLC meter, DO meter, interface meter, and a River current meter.