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A Glossary of Terms and Abbreviations GEOKEM was begun as a textbook and geochemical reference summary for graduates and research workers and it was assumed that a working knowledge of common geological terms would have already been acquired. Abbreviations AA: Activation analysis. Samples are irradiated in a nuclear pile along with a standard. At a fixed time the gamma radiation for one of the less quickly decaying products is counted along with the standard and the count rates ratioed against element concentration in the standard. A decay product with a half life of minutes is no good as there would not be time to count both sample and standard before the countrate diminished. One with a half life of years might take unacceptable times to count. AA used to be a favoured way of determining the REE but was pretty poor for La especially. AOB,s: Alkaline olivine basalts, ie, basaltic rocks with a normative labradorite feldspar and normative nepheline. CAB: Calc-alkaline basalt. Basalts found in Orogenic Andesite series, showing the characteristic continental fingerprint with anomalously low Nb-Ta and Ti with (at least in continental and matures arc series) relatively high Cs, Rb, Ba, Th, U, K. CAB's are not that common in continental arcs, one can work for years without seeing one. CFB: Continental Flood Basalts. Basalts and ferro-basalts (with minor granophyres and orthopyroxene or even olivine cumulates), produced in continental regions in enormous volumes, mainly at the times of the break-up of Gondwana (165 myr), though some such as the CRB’s (Columbia River Basalts) are younger, (15myr) and the closing members of the Snake River Plains are only a few thousand years old.. The silica range is from 49-50 to as much as 58%, with a composition ranging from about EMORB to the most LILE, Nb and Zr-enriched, silica-saturated basalts known. All the plateau or shield-building tholeiites have the anomalously low Nb and Ta as seen in all continental rocks. They include the Antarctic Ferrar Dolerites, the Tasmanian Dolerites, the Karoo Series, the Deccan Traps, the Parana Basalts, the British Tertiary, the Siberian Traps etc. Because of the continental environment and the lighter weight of sediments, sills are very common in the CFB’s, volumes of sill in the Ferrar Dolerites for example exceed 200,000 cubic km. Most layered intrusions if not all are CFB’s. EARS: The East African Rift System extending from the Afar Triangle on the Red Sea north of Abyssinia to the south at Lake Rudolf (Turkana) where it splits in two, the East or Gregory or Kenyan Rift passing by Nairobi and Lake Natron, west of Kilimanjaro. The Western Rift passes through Lake Albert, Lake Edward, L. Kivu, L. Tanganyika, L Rukwa and Nyasa almost reaching the Indian Ocean near the mouth of the Zambesi. Basalts - trachytes - commendites of highly variable composition both sodic and potassic are scattered along it's length. EMORB: Enriched Mid-Oceanic Ridge Basalt. (see MORB, below). EPR: The East Pacific Rise, the spreading centre sector between Lat 23ºN and 23ºS. Fractionation: The process resulting from the fact that the first formed crystals in a cooling magma always have a higher ratio of Mg to Fe, of Ni to Mg, of Ca to Na, and of HFSE to LILE elements in general than the magma has. The result is a steady decrease in HFSE elements and increase in LILE as the magma cools. The amount of LILE in a magma can treble before cooling is complete. Some elements, eg, Rb, La, Nb may increase by factors of 30 - 50 FeOT (and Fe2O3T). Total iron expressed as the calculated ferrous or ferric amount. The iron in a rock is usually mainly in the divalent ferrous state but about 25% is in the trivalent state. Ferrous and ferric iron behave like two different elements, ferric iron is found in titanomagnetite etc, ferrous iron in olivines and pyroxenes. With the slightest degree of weathering of these minerals, their iron oxidises to the ferric state which upsets NORM calculations. Volcanic submarine glasses are usually analysed by electron microprobe which cannot differentiate between the two; iron is iron! Glasses are seldom determined for ferric iron which has to be done by wet chemistry. Or iron may be reported as total Ferric or as total ferrous or as both. Sometimes in databases, what is obviously total ferric iron is simply reported as Fe2O3. This may cause large errors in some calculations. Remember that ferric is usually at most 1/3 of the total amount of iron present, in basaltic rocks, about 20-25%. However, there are exceptions. Some years ago Haraldur Sigurdsson analysed a basalt from Kolbeinsey Id, north of Iceland, which was completely oxidised with only Fe+++. The olivine was a pure forsterite, Fo 99-100! (Trivalent ferric iron cannot enter the olivine lattice so the olivine was completely magnesian). GR: The Galapagos Rise, a short sector of spreading centre projecting in towards Ecuador from the EPR passing north of the Galapagos Is. near 0 deg Lat.. HAB: High alumina basalt, ie with more than 16% Al2O3. Some MORBS have high alumina, but more usually to be found in Calc-alkaline series. HFSE: High field strength elements, ie, those with high charge, small ionic radius etc which are the first to form crystals in a cooling magma. Including, Cr, Ni, Mg, the heavy REE, Y, Ca, etc IAB: Island Arc Basalt. In immature, primitive arcs, eg Tonga- Kermadec, Scotia, IBM arcs, characteristically very depleted in LILE elements, sometimes more so that the Oceanic Ridge basalts, but still showing the anomalously low Nb-Ta. Shows stronger iron enrichment trends than do continental andesites. IBM: This does not stand for a once dominant but now rarely heard-of computer company, but for the Izu Bonin-Marianas Island arc in the Western Pacific. ICPMS: Instrumental Plasma Mass Spectrometry: A sample is vapourised into a plasma gas and ions are accelerated down a flight tube of a mass spectrometer between magnets. Elements are separated by their difference in mass. Can analyse a wide range of elements to as low as 0.01 ppm, but the standard is highly variable, some operators producing seemingly very accurate data, others disagree with reliable XRF or ID (Isotope Dilution) by 20 – 100%. Pre-2000 data to be used with caution. ICP-La Laser-Ablated ICPMS, sometimes used for trace determinations on minerals. ID: Isotope Dilution. A means of analysing very low concentrations of an element using a mass spectrometer. The sample is spiked with a known amount of a rare isotope and the ions, separated from each other by their mass difference when bent through an angle by powerful magnets are ratioed against each other several times (intensity of sample/sample plus spike) IMHO: In my humble opinion. Cynical geeks say there is no such thing as a humble opinion! INAA: Instrumental Neutron Activation Analysis. IOR : Mid Indian Ocean Rise. JDF: The Juan de Fuca Rise, the Pacific spreading centre sector lying west of Juan de Fuca Strait, off British Columbia. LILE: Light Element, Lithophile elements, (Cs, Rb, Ba, Th, U, K, Nb, Ta and the light REE.) What I once termed the "Residual Elements", those that are the last to form crystals in a cooling magma. MAR: The Mid Atlantic Ridge. The sector between Lat.55ºS and 52ºN (At 52N occurs the Gibbs Fracture zone) ME: The major elements, ie those silicates measured in percent oxide in both rocks and minerals. Usually SiO2, Al2O3, TiO2 ,Fe2O3, FeO, MnO, MgO, CaO, Na2O, K2O, P2O5, H2O etc. Sometimes Cr2O3 and NiO are reported as major elements even though they are usually sub 1%. MORB: Mid-Oceanic Ridge Basalt. A general term for those low K, tholeiitic basalts erupted along the oceanic ridge spreading centres. ( see NMORB and EMORB) NMORB: The "normal" or more common type of Oceanic Ridge basalt, (see above), strongly depleted in the LILE elements Cs, Rb, Ba, Th, U and formed by a relatively high degree of melt of the uprising mantle. Not always associated with the faster spreading centres, Kolbeinsey and Reykjanes Ridge spread at only 4cm/yr but are VERY depleted, eg Nd=>Ce. Just how constant over short periods of time ARE spreading rates?? EMORBS are the more enriched (50-100 ppm La) MORB variants, compared to the 1-5 ppm seen in the most depleted NMORBS. Found usually in very slow-spreading centres (< 1 cm per year) or at some distance from the median trench in the faster spreading centres usually in off-lying seamounts. ODP: the Ocean Drilling Program. A research organisation located at College Station, Texas, and funded by the National Science Foundation (NSF) to drill and explore the floors of the ocean basins which make up 60% of the Earth’s surface. Various drill ships have been used, including the Glomar Challenger, the Glomar Explorer and the JOIDES Resolution. There have been various name changes, the ODP once being JOIDES, the Joint Organisation for the Investigation of Deep something or other. Keeping drill ships at sea for 30 years has cost a fortune or two but the specialised work that has followed by small oceanographic ships and submersibles has resulted in better information. It is now hard to believe that 50 years ago that we had no idea of what the ocean basins consisted off but we mainly assumed they were pretty much like the continents but just happened to be below sea-level. OIB: Oceanic Island Basalts. Basalts found on oceanic islands or seamounts always more enriched than Ridge Basalts but can range from tholeiitic basalts to basanite, nephelinite-phonolite. A few really depleted basalts-picrites from northern Iceland Theistareykir are the only depleted OIB's known but Iceland should really be included in the Oceanic Ridge rocks as they are chemically identical.. OS: Optical Spectrography. Once the only means of analysing trace-elements but each element emits such an array of spectra in the visible light range, that interfences and overlap are not only common but almost universal. REE: The "Rare Earth" elements, mainly trivalent and of common properties. Includes La, Ce, Nd, Pr, Sm, Eu etc down to Lutecium. Eu is divalent and goes into feldspar, the others do not. Strong feldspar fractionation creates a "Europium anomaly" as seen in commendites. Other REE, eg Ce, form trichlorides and seem to have variable valency. SEIR: The South East Indian Ocean Rise. SEM: The Scanning Electron Microprobe. An expensive instrument which using magnetic lenses can focus a fine beam of electrons on a few microns of mineral or glass. The elements in the mineral will fluoresce their characteristic X-radiation of a series of wavelengths which seldom overlap. A cluster of 3-5 or more crystal monochromators set at the right angle can each separate a single wavelength which is passed onto a counter. The concentration of the element is determined by ratioing against usually a standard analysed mineral (or artificial fused glass). The detection limit for common elements is usually a few tenths or hundredths of a percent. SWIOR: The South West Indian Ocean Rise. TE: Trace elements, eg those measured in parts per million, usually including Cs, Rb, Ba, U, Th, Nb, the REE, Cr, Ni, Co, Cu, Zn etc. XRD: X-ray Diffraction. A beam of X-rays is foccussed on a powdered sample where crystals have varied and random orientation. Rotating the sample through about 120 degrees and using a graphite monochromator to give an incident beam of only the K alpha Cr line, x-rays will be reflected (not fluoresced) at the appropriate Bragg angle. The 2D spacings between the denser layers in the crystal lattice can be measured. XRD can be used to identify minerals, but also to analyse one element in one mineral by selecting a known 2D spacing and counting the intensity. (This OUGHT to make sense to those who know a little about XRD) XRF: The X-ray Fluorescence spectrometer or spectrometry. Like the probe, the XRF measures the intensity of emitted radiation, in this case from a fused glass or pressed powder sample an inch or more across, but the excitation energy is provided, not by an electron beam, but by a primary X-Ray source of very high energy produced by focussing an electon beam inside a sealed tube onto a metallic target which may be Cr, W, Rh, etc. These primary tubes operate at 2 – 3 kilo-watts and a 100 KV. XRF is still one of the best analytical tools for determination of major elements as well as the TE Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Zr, Nb, Th, Ba in bulk samples. Detection limits can be as good as 0.2 ppm, more commonly about 1 – 3 ppm. Definitions of Common Igneous Rocks Andesite The most common volcanic rock found in island arcs and sectors of continental margins above an active or now defunct subduction zone. Originally defined as having an andesine feldspar, (An 30-50), but as the feldspars are often oscillatory zoned, or may not have even formed in very fine grained rocks, it is more common to use the Normative feldspar composition as calculate by a NORM program from the chemical analysis. Andesites (and all continental rocks) are characterised by their anomalously low Nb-Ta content, and low Ti ,(see fingerprint diagrams diagrams in "Andesite Series"). Sometimes in a calc-alkaline series ( an old term defined by their Calcium-soda ratio at specific silica) andesites may be defined on their silica content only, usually 58 – 63 %., those of 53 – 57% being "Basaltic Andesites". Above 63% the rocks are "Dacites" with rhyolites above 70%. A rock with an andesine feldspar and 52% silica would probably be called an "andesine basalt". The plutonic intrusive equivalents are granodiorites. Basalt The most common and most abused rock name in petrology. A dark mafic rock of 5 – 10% MgO with a labradorite modal or normative feldspar. Always with associated clinopyroxene as well as plagioclase, possibly with <10% olivine, orthopyroxene or pigeonite, in which case are "olivine basalts", or "hypersthene basalts" or "pigeonite basalts". Basaltic rocks with more than 10% olivine may be termed "picritic basalts" those with more (and MgO from 12 to as much as 35%) are "Picrites". With fractionation and increasing iron and titanium, basalts with more than 14% FeOT are termed "Ferrobasalts" (More to be added) |