GLOBAL DISTRIBUTION OF ELEMENTS
Elements covered include:
The figures relating to the consumption of carbon are of course almost trivial when compared to the amounts of oxygen consumed in the conversion of C to CO2. The proportion of atmospheric oxygen was 23.15% by weight in mid century reduced to 22.87% today. About 280 billion tons of O2 is consumed in all processes compared to the 200 - 230 Bt returned to the atmosphere from all plants, forests, and algae.
Panting hordes of people use on average 566 - 591 kg of O2/ year with domestic animals using another 5.4 bt in total.
Respiration by the human population, each one of which is a mobile slow-burning fire, accounted for 2306.4 million tons of O2 in 1975 rising to 3378.7 million tons in 1995.
It is a paradox that oxygen is our most abundant element, the whole world has been described as a mass of oxygen ions bonded together by silicon and metals. It makes up 88.9% of the Earth and 63% of human beings by weight, yet the amount of free O2 in the atmosphere is trivial by comparison.
Projections fifty years into the future at the same rates of increasing population, declining forest areas, and increasing desertification and burning of fossil fuels all point to an inescapable total disaster, which politicians world-wide decline to acknowledge. From where I sit I can see whole mountain ranges devoid of vegetation except for the odd "Wilding" lodgepole pine or douglas fir. The Department of Conservation periodically sends out gangs to fell these "weeds".
The amount of wealth being spent on the Iraqi war, if spent on alternative fuel research and on tree planting, could have saved the human race from disaster. Ironic, aint it?
Th (along with U, Rb, Cs, Tl and Ba) are usually put together as the
"K-group" elements. While Ba is divalent it can to some degree
replace K+ in potassium feldspars. However K/Th does not seem to follow
any logical sequence. Th can be very low in rocks of the oceanic crust
(sub 0.5 ppm) but U is even lower and Th/U follows a much more logical
distribution. Parental melts of basalts have a maximum of 7 ppm Th and
What intrigued me was the fact that Rb, Ba and Th showed steeper trends relative to K, while Cr, Ni, showed steeper trends than Mg, Co less steep, while Zn and Mn were invariant. No one had seen element pairs with correlation coefficients of better than 0.99 before and there were bitter remarks at AGU meetings, some people saying loudly they did not believe it. But others did and now none of the work of that era would raise an eyebrow. Why did it never occur to me to put all the "residual elements" as I called them on the same diagram? Peter Baker began doing than in 1985, 20 years later and not till 1989 did "Sun Shine Soon" and Steve McDonnough make a workable system of normalising to mantle compositions though normalising is only a small extension to the chondrite normalised diagrams for the REE which had been used since pre-1960.
Production and Consumption
Though Rb is the most electronegative element after Cs, its consumption is only a few thousand Kgm pa, (Handbook of Chem. & Physics, 78th ed). The main sources are the minerals leucite, pollucite and the mica lepidolite. Rb is always found in any potassium mineral and does not form it's own. In quantity, Rb only costs $20/g.
K/Rb in Simatic Rocks
I have analysed and seen others done by ID of single MORB samples with about 0.01 ppm Rb and 0.02 max K2O with a K/Rb of over 1000 very frequently in very depleted NMORBs, but here we will only show coherent fractionated series, which are almost unkown in the very depleted NMORBs..
Parental Basalts have a range of Rb from <1 ppm in DMORB to 60ppm in extreme EMORB.
Cont. Flood Basalts
Orogenic, IAB's & Plutonic
The Oceanic rocks are reasonably predictable, the K/Rbs being over 600 for NMORBs, 400-600 for tholeiitic OIBs and mainly 3-400 for alkaline OIBs, but, rather oddly, even lower in the most enriched EMORBs of Macquarie with a range of about 330- 210 The southern Indian Ocean including the Kerguelen Plateau is however a region where potassic rocks are common, though Macquarie actually shows a negative K anomaly! (Kamenetsky et al, 2000).
Distribution of Rb in OIBs
Phosphorus is invariably recorded as the phosphate, P2O5. While P has been recorded since the very earliest days of geochemistry, it seems to seldom attract much attention, probably because early colorimetric methods were not too precise. P, like Sr however is a sensitive indicator of parental magma variations and is well worth looking at. Like Sr is reaches a peak at about 4% MgO when apatite appears on the solidus and then rapidly declines to almost vanishing point in trachytes-rhyolites.
Uses of Phosphate
P2O5 is of course one of our main fertilizers in agriculture. It is also has a heavy but declining use in detergents. World annual production of phosphate concentrate is currently about 35 million tons.
P in MORB, EMORB, OIB and Sialic rocks.
Andesite series show the same scatter with a maximum of 0.2% in the Valley of Mexico, but with higher values in younger, more alkaline rocks. We see 0.2 - 0. 4 for Azufre 1, and Sangay. Nevado del Ruiz has rather less with 0.15- 0.25, Calbuco 0.2, Puyehue 0.4 - 0.6, Parinacota rather more with a variable 0.75%' Vesuvius is one of the few to show better defined trends with 0.9% at 4.5% MgO, (Belkin & Gunn, unpub data).
The platinum group elements include platinum, ruthenium, rhodium, palladium, rhenium, osmium and irididium. These may occur native or as mixed alloys such as osmiridium (osmium-iridium) in placers in outwash from gabbroic to ultrabasic intrusions. More often they occur in the early-formed sulfides of basic intrusions. Long used for jewelry, the PGE are also noted for their ability to act as catalysts to a range of chemical reactions. Now widely used to reduce atmospheric pollution from car emissions by catalytic conversion, platinum is being universally sought for. As yet the distributions do not make much sense, being mainly in the ppb range. The Merensky Reef in the Bushveldt Intrusion of South Africa is still the world's biggest supply source.
Rhodium is the rarest of all metals but about 3 t /yr is produced valued at $13,000/100gm or $1000/troy oz (prices change rapidly.)
Ruthenium (At.No.44) has a mp of 2250º C and is found in Ni ores, its value being variously given as $1400/100gm and as $30/gm. It has some curious properties, one being that it increases the resistance of Ti to corrosion a hundred-fold when the Ti alloyed with 0.1% Ru. It also is used as a hardener for Pt as is iridium.
Pb is one of the metals that has been longest in use being easily worked and easily smelted from the main ore galena (PbS). The Romans used lead extensively in piping water to houses so that we still call a man who works on water pipes a "plumber" (Plumbum = "Lead"). Lead weights have also long been used in "Plumbobs". Lead is no longer used in water pipes as, while resistant to corrosion, enough dissolves especially in acid solutions, to cause lead poisonng.
World use is currently about six million short tons per tyear, or 5.4 mil. metric tonnes. About 3 mt is mined directly but there is a high recovery rate of about 50%+ eg from storage batteries which is the main product. Current bulk price is $520/t.
The classic lead mine was Broken Hill, out in the desert NE of Adelaide. The ore body consisted of galena, sphalerite (ZbS) and rhodonite (MnSiO3) with many minor Mn minerals, eg, the Mn olivine, and the Mn clinopyroxene, bustamite. On the oxidised cap of the ore body was a thick layer of coronadite, the Mn-lead hydroxoxide. The galena contained about 10 oz of silver per ton which paid for recovery, the Pb, Zn, being sheer profit.
Lead in Oceanic Rocks
Pb appears to have a similar distribution to K being extremely low in MORB rocks and increasing quite steeply with fractionation. Only in the last decade has good Pb data been produced and there is no data for many important petrological provinces.
Pb in Sialic Rocks
In all andesitic series, lead appears in greater amount than Th. In the Scotia Arc Pb is high but variable, but in Puyehue while Th is in the 0.5 - 8 ppm range, Pb is 2 -20 ppm, Th/Pb = 0.32. Parincota has also > 20 ppm and Vesuvius >45 with a Th/Pb of 0.6 which seems about average.
The proportion of radiogenic lead included with common lead will depend on the amount of U-Th present and on the age. When time permits we will try to assemble some figures.
Lanthanides or Rare Earth Elements
The Rare Earths are usually taken to include the 15 Lanthanide elements plus Sc and Y (which are dealt with separately)
Characteristics of the REE
In depleted rocks, the light REE (La, Ce, Nd) are present in very low amount relative to the heavy REE Er, Tm, Yb, Lu. Both with fractionation and with evolution from tholeiitic to alkaline rocks, the ratio of LREE to HREE increases rapidly so that the ratio of, say La/Lu which may be about 10 in NMORBS may rise to about 150 in alkaline rocks and even higher in phonolitic members. Both garnet and clinopyroxene have much higher partition coefficients for the HREE, in both in the subcrustal mantle, and pyroxene in the fractionation of basalt melts where garnet does not usually occur.
REE in MORB & OIBs
If we include harzburgites in the EPR and in the SEIR and also fractionated rocks, we see a range of 0.16 - 61 ppm Ce in the tholeiites. Common little fractionated MORBs however range from 1-6 ppm Ce. Ratios such as Ce/La, La/Sm, even La/Lu tend to stay constant in fractionated series but to curves towards high La in more alkaline rocks. When a curved relationship was seen, the base of the curve was taken for the ratio given.
In the upper members of the more highly alkaline rocks, compositions become increasingly erratic. Carbonatites have extremely coarse crystals of calcite and homogenous samples are difficult to aquire. Averages of the Oka, Algerian, Turiy Peninsula and Fen carbonatites have 2000, 2500 ppm Ce with Ce/La of 1.96-2.1.
Range of REE in Sialic rocks
REE of OIBs
Sc in no way emulates Y or the REE. The liking of the REE for clinopyroxenes is carried to extremes, and in crystallising basalts, with the advent of Cpx on the solidus, Sc quickly vanishes, declining with MgO and CaO. In the "basalt triangle at 7.5% MgO, Sc is at 45 ppm levels in the EPR declining to 25 at 3% MgO, in Mauna Loa it is 60 ppm, and 30 ppm in the MK HSDP drill core.
Se is also recovered in electrolysis of Cu ores, 200 tons of ore yielding l lb.
There seems to be a strange logic about the construction of both earth and Universe which is often overlooked. Obviously the basic home of an intelligent life form must be mainly built of highly inert materials, we would not last long if the whole earth caught fire if we dropped a march or let off a hydrogen bomb or two.
All ionic elements form complex compounds with grids of silicon tetrahedra and there they remain, inert and almost ungetatable. Only the few chalcophile elements such as Cu, Zn, Pb, Bi which form compounds with S (or As) are fairly easily roasted and reduced to metal.
Silicon is the substrate on which the entire silicon chip computer technology is based. The extremely large glass container and window industry is also based on silica, which alone amongst inorganic compounds is transparent to light due to it's ability, if chilled fairly quickly, to form a clear, transparent, non-crystalline glass.
Native quartz occurs in small amount in granites, in much greater amount as bands in medium-grade metamorphic schists, but due to it's resistance to abrasion, in even larger amount and often in a highly pure state in sedimentary quartzites and sandstones.
In the form of non-crystalline opal or chalcedony or agate, quartz was formed by Early Man into hand axes by peeling off slivers to form a pointed hand axe. Ian MacDougall who ran the K/Ar dating laboratory in Canberra once showed me a beautifully shaped one 500,000 years old. Acheulian man peeled chalcedonic quartz into arrow and spear points that decimated the post-glacial animal herds and probably exterminated the mammoth. Glassware is known from 4500 BC.
Optical fibres, lenses, and computer chips are a few of modern uses, quartz fume, the residue left from heating quartz is added to concrete as a filler or pozzalana resulting in compressive strengths of up to 15,000 psi.
Carbonatites are pretty well the only forms of igneous rock which do not contain large amounts of silica, in the range 30 - 80%. Our tens of thousands of analyses of MORBs show that 99% lie in the very narrow range of 59 to 51% except for the few fractionated examples, which by removal of the HFSE elements may be enriched up to 70% silica.
Alkaline rocks have less, the extreme nephelinites formed presumably under very high pressure may have as little as 30%, while the fractionated trachytes-phonolites may have between 65 and 55%.
In the island arcs, basalts with 50 - 54% SiO2 are quite rare, andesites with 58 - 62% are common, dacites of 63-69% are not very common and obsidians of > 70% quite rare. There are huge amounts of ignimbrite of 68 - 80% SiO2 but these are formed as partial melts of the continental crustal keels. Continental rhyolites may contain 80% silica.
Normal rocks may, very slowly, disintegrate and reform into clay minerals and form soils, at the rate of about 1 inch per thousand years. Any native quartz present is usually unaffected, at least in temperate zones, but silca is selectively leached in tropical soils subject to heavy, warm rainfall a process probably accelerated by acidic vegetation.
Sr is an unsatisfactory element from the geochemists point of view. It is always present in quite adequate abundance, commonly 100-1400 ppm, it has been easily determined to great accuracy by XRF for the last 40 years and it tends to occur in markedly more abundance in alkaline rocks. However, it is neither a high field strength element nor is it a residual or LILE element. With fractionation it rises to a peak in the 6% - 2% MgO range and then tends to begin to diminish as soon as plagioclase appears on the solidus and even more rapidly with sodic and potassic feldspars. Consequently it seems to be rarely mentioned in publications in latter years.
Characteristics and source of Sr
Atomic No 38, Sr has 4 stable isotopes and no less than 16 unstable ones. One of the best known is Sr90 which is produced in atomic bomb fallout, which with a half life of 29 years is a vigorous emitter of gamma rays and is used as a light-weight energy source... there are few thorns without a rose.
Sr in MORBs and Tholeiitic Basalt Series
In the primary parental tholeiitic melt series, Sr ranges from as low as 60 ppm (site 504), or 70-80 ppm in Kolbeinsey Ridge but rises very steeply to 700 ppm in the most enriched Macquarie primary melts. Fractionated series branch off the primary stem at very flat slopes in the most tholeiitic groups, almost certainly due to the presence of plagioclase on the solidus.
For Alcedo volcano in Galapagos and for Iceland, when fractionation passes the 3% MgO point there is a sharp decline towards rhyodacites, the maximum being 300 ppm. Iceland has a wide range in Sr contents, with the Snaefellsnes alkaline rocks peaking at 420 ppm, Westmanneyjar and Hekla being slightly less at 400. The Krafla centre has the least Sr with only 150ppm while the NVZ and the tertiary centres both east and west have a spread between 200-300, (Wood, D.A, 1977; Gunn & Siggurdsson, unpub)
Sr in Alkali Basalt Series
Perhaps better than other elements , the behaviour of Sr illustrates the difference between tholeiitic and alkaline series. Because there is no feldspar initially on the solidus, Sr increases steeply with fractionation. Not only that, the trends originate well on the Mg side of the tholeiitic plane. We have no alkaline series based on glasses only so we cannot define where the alkaline primary melts lie, but it must be around 12 - 14% MgO. At about 3% MgO the Sr trend curves over and begins to descend. The maximum reached is 1200 ppm in Heard Id, 1060 in Mt Ross, 800ppm in St Helena and Gough, at 500ppm for Ascension, 750 ppm for Westmanneyjar. The 1300 ppm already mentioned for the summit hawaiites of Mauna Kea is therefore quite high.
Sr in Orogenic Andesites
For perhaps the first time, we can say there are some similarities in element behaviour in both simatic and sialic rocks. As in alkaline series, Sr reaches a maximum at about 3% MgO and rises in abundance steadily from IAB's to the most potassic centres.
Sr in Continental Flood Basalts
Our knowledge here is very imprecise as CFB's, while often poured out
in enormous volume, are very little fractionated and different parental
magmas have quite variable MgO , usually between 7 and 4% MgO. Ferrar
Dolerites have a low undifferentiated 110-120 ppm Sr and not rising
above 140 ppm in the type differentiated sills. Coates Land has a variable
180 ppm and the Kirwan Basalts 160-180. Tasmanian Dolerites undifferentiated
have a range of 120-200; the tholeiitic Karroo approx 200; Parana, 200,
while the Columbia River Basalts as usual are erratic. Prineville has
an average of 275 pm, Grande Ronde 325, Picture Gorge 250, Rosa 320
and Umatilla 375. The Palisades Sill has less than 200 ppm. In general
the Sr levels in CFB's seem to be much lower than for orogenic basalts.
Sulfur in the form of SO2, H2S and minor amounts of S2, S2O, H2S2, SO, OCS, AsS, PbS, SbS, and BiS is emitted from fumaroles and from active lava flows associated with active and recently active volcanic centres world wide. The pungent smell of H2S and SO2 is familiar to all who have visited such centres. Reaction between these gases leads to the formation of native sulfur around fumaroles. (SO2 + 2H2S = 2H2O + 2S)
Using satellite scanners and UV interferometers the GEIA (Global emissions Inventory Activity) group have made estimates of the amounts of S emitted from an average of 56 volcanoes erupting per year between 1978 and 1988, but covering the period 1965 to present. Short lived eruptions can contribute large volumes so that atmospheric S can range from 10 - 30 Tg/a. A Teragram is 10^12 gm, so if 1,000,000 gm = 1 ton, we are talking about 10 - 30 million tons, if I have not slipped a zero some place. The average is estimated at 13.4 million tons per annum. Remember man-made emission are treble this. Try calculating the height of an equilateral cone of bright yellow S formed from the total world output for a year!
S in Silicate Rocks
I once had a brilliant idea. We would measure the S content of andesitic rocks and this would indicate the centres most likely to form sulfides. The results were extremely variable and correlated more with age. Young bits of flow still had native sulfur in vesicles, old rocks had lost all except that remaining in blebs of pyrite, chalcopyrite, marcasite etc in the rock. A normal figure for S in silicate rocks is about 0.05%.
These two radioactive elements are present in all crustal rocks, with a range for Th of 1.5 to 100ppm and a Th/U ratio varying from 2.5 in the most basic MORBs to about 5 in highly fractionated potassic phonolites.
We currently consume 72,000 tons of U3O8 yellow cake uranium oxide pa, mainly in power stations, the major nations producing 30% to 50% of their electricity from atomic power. Australia has 27% of the world's known reserves and exports 10,000 tons annually. Current price is about $10US/lb. U3O8 is quite soluble and precipitates in porous sandstones in desert country. It is not conceivable that silicate rock with 1-10 ppm should be used as a primary ore at this time.
U/Th in Simatic Crust
Considering the importance of U-Th in maintaining heatflow, and, it is claimed, volcanism, in the Earth, the amount of accurate U, Th data available is minimal. A great deal of basic work was done by gamma emission spectrometry in the "Nuclear Arms Race" era but the accuracy of determination is no longer acceptable. Amounts present in MORB rocks is so low that only the advent of ICPMS techniques have allowed good determinations to be made.
The data is not too precise in both Site 504 and for the Galapagos Rift at these low levels but for the EPR the regression line has a correlation coefficient of 1.0 while 25 random samples from the MAR also shows a coefficient of 1.0 as has the data for the NMORBs of the N. Chilean Ridge so we may take the best approximation for Th/U in an N-MORB series to be 2.5 ranging up to 5.0 in commendites. It is a great pity U data is lacking for Pantellaria, also for Heard, Tristan da Cunha and Gough. There is very detailed data for Macquarie Id which is a series of EMORB basalts of a range of composition but little fractionation, and in 55 samples the Th/U stays close to 3.65 throughout.
Note that the fractionated Hawaiian and EPR series only diverge very
slightly from the partial melt plane.ie, a very low degree partial melt
will not differ much from a higher degree of melt. This explains the
small range in Th/U ratio found.
Th/U in Arc Andesites
While in general similar to OIB's and MORB's, the ratio Th/U tends to be much more variable in sialic (andesitic) rocks. In 2000 Andean samples for example of which perhaps 3-400 have been determined for Th-U, there is a concentration at a rather high level of about 4. This is reflected in a rather high Pb208/204 level as well. However, the range is large with samples ranging from 1.5 to 10. This might indicate some contamination of primary andesite with oceanic sediments, as the Sr/Rb isotopes are equally variable, as are many other elements.
In the more primitive oceanic arcs, Th/U is much lower being commonly in the range 1 - 1.5 with a marked negative Th anomaly being seen in the fingerprint normalised diagrams. Thus proto-arc centres such as Kao, Tafahi, Late, Curtis, Macauley plus the Leg 135 Tongan Back Arc samples all show a negative Th anomaly, but not the more enriched islands of Raoul and Esperance. This also seen in proto-arc IBM, Scotia Arc, Merelava in the New Hebrides, and in Statia - St Kitts in the Lesser Antilles. Pb/ Th may be quite high, in the region of 10.
In mature arcs we find a dominant level of 4 in the Andes, 3.3 in Mexico, 2.7 in the Cascades, 2.4 in the Aleutians, 2.3 in Kamchatka, about 2 in Honshu, 1 - 3 in Tonga, 3.3 in the Scotia arc but 4.2 in Indonesia.(Sunda Arc). All however show considerable variability.
Kluyuchevskoy Volcano in Kamchatka, while a massive cone, is of general Early Arc composition and has a very low level of U-Th with a Th/U little greater than 1 (Kersting & Arculus, 1995, EPSL 136; Dorendorf et al, 2000, EPSL 175)
Te (At No. 52) is associated with sulfur but may form separate telluride minerals especially in sylvanite and calaverite. It is recovered in the anode mud during the electrolysis of copper sulfides, 500 tons of ore yielding about 1 lb of Te. Te has a total of 30 isotopes ranging from atomic weight 108 - 137. The ore in the famous Cripple Creek mine in Colorado consisted of gold telluride in fluorite.
Tl (At. No. 81) About 15 tons of Tl are produced yearly, from flue dust from smelting Zn, Cu and Pb, with a value of $600/lb. It is used in high temp superconductors, in alloys and in glass. It is poisonous and an estimated 1000 tons Tl released into the air from metal refining and cement manufacture in VietNam is reputedly causing severe health problems. (Note: This sounds unreasonably high, we will try to check the source.)
Ti Uses and Consumption
Titanium is mainly produced from ilmenite (FeTiO3) in beach sands, the concentrate costing only $87/ton in Australia. The oxide rutile (TiO2) is rather more expensive at $563 per ton. World consumption is now 4,100,000 tons of ilmenite concentrate per year and 370,000 tons of rutile, about a quarter of which total is mined in Australia. Ti metal is as strong as steel and half the weight and is increasingly used in aircraft and the space industry. The latest laptop also has a titanium cabinet. Being non-magnetic titanium is used in building atomic submarines, but 90% of all production goes into the paint industry as a pigment, unlike lead it is non-toxic. It is the only metal known to burn in nitrogen!
Titania Fractionation Pattern
Ti climbs very steeply relative to MgO during early fractionation, in some cases more steeply than the primary melt series. It reaches a maximum at about 4.25% MgO when titanomagnetite forms. This seems to be temperature controlled rather than compositionally as the Ti may peak at 2 - 3 - 4 - 5% but almost never higher, while iron may reach 10 - 16% at the same point. Closely related rocks such as in the EPR may show several sub-parallel paths peaking at different levels, it may be PO2 plays a part in regulating the amount of FeO present.
Ti in NMORB - EMORB
The most depleted, highest degree melts may have as low as 0.5% TiO2, eg in Kolbeinsey Ridge. The range in the Macquarie primary melts is only 1 - 2%. The glass data of Melson and O'hearn show that fractionated groups along the MAR may have 1-2% at 22N, and 0.6 - 3.5% in the South Atlantic. The EPR data of Regelous et al, 1999, show a peak of 3.75% as does Volcan Alcedo in the Galapagos.
Ti in OIBs
Iceland is variable with Krafla peaking at only 2.5% while the Austerhorn and East Iceland have up to 4%. The Hekla - Katla group show variable data according to the author, but in the range 4 - 4.7% with the alkaline Westmanneyjar at slightly over 3%
Ti in Orogenic Andesites
There seems to be no compositional pattern here. About 1% is the common maximum found in centres as different as the South Sandwich IABs, in Sangay, Calbuco and Mt Vesuvius. Planchon Petaroa and Azufre 1 are a little higher at 1.2% while Deception Island and Puyehue have 1.75%. There seems to be considerable shift in the MgO point at which the TiMt IN point occurs, more work needs to be done on this as it is not fully understood.
Ti in Continental Flood Basalts
V behaves like Ti, building up in the early stages of fractionation in silicate rocks to the 250 - 500 ppm levels and then precipitating in titanomagnetite in which it may have a content of about 2-4%. There is however, proportionately less variation than in the Ti in high and low Ti series, as seen in Flood basalts for example.
Uses of V
V is used in alloys, especially of rust resistant steels. Time was when no spanner would have sold if it did not have "Chrome Vanadium" stamped on it. The alloy, while less resistant than say Type 504 Ni-Cr, is somewhat harder. V is also used in ceramics and V-Ga alloy is used in superconducting magnets.
This was some 35,000 tons in 1998, half coming from South Africa, the price being only $4.00/lb. Reserves are estimated at 63 mt. V is found, not only in TiMt but in phosphates, uraniferous sandstones, bauxite, coal, crude oil and tar sands.
V in Ridge basalts and OIBs
The Macquarie primary melt curve shows a slight negative slope at an average of 200 ppm from which we can guess that there will not be a great difference between tholeiitic and alkaline rocks. However with fractionation, V in tholeiitic rocks rises steeply, to a maximum of 400 ppm for the EPR, to then declines sharply at the TiMt point.
Alkaline rocks, below the TiMt point have a flat trend extending towards ankaramites, so we might guess that Cpx includes several hundred ppm V. Marc Norman (1997) has shown in a laser ablation study of clinopyroxenes from 1955 Kilauea, that the V range is from 200 to 400 ppm. However, the Ol+Cpx+Pl of tholeiites also includes about 35% cpx, so why is the trend for tholeiites so steep compared to the flat ankaramite trend? An olivine trend only ought to be steep. Interestingly, Puu Oo shows a flattish trend at 280-290 ppm in the magnesian endmembers, so just what is the V content of olivines? Sure enough, Rhodes et al, (1995) show a V trend in the picrites of Mauna Loa to decline steeply from 300 ppm in undifferentiated basalt to about zero in pure olivine. Puu Oo has about 40 ppm higher V and appears to have a flatter trend towards higher Mg but the range in MgO is not great enough to be positive. Rocks like the Ferrar Dolerites seem to suggest there is a fair amount of V in orthopyroxene-pigeonite also, but so far no data has been found.
Of all comodities, water is likely to be the one soonest and most urgently in short supply. A consultantcy group, Lenntech, have written us a URL explaining the situation, and detailing alternatives.
Information about water quantities from a water purification company.
Yb is usually taken to be geochemically associated with the lanthanides, it being element 39 and being in the same column in the periodic table as Lu, (el.No. 71). As with the REE the main source of Y is in monazite beach sands and carbonatites. The current cost of Y is $75/oz.
Distribution of Y in MORBs and OIBs
The distribution of Y is to say the least, unexpected. All basalts of whatever composition appear to have close to 25ppm of Y at 7.5% MgO but the fractionation paths differ widely being steep and linear in tholeiites but flat in alkali basalts but sweeping up very steeply at sub 0.5% MgO compositions.
|__| Mg/Y for Site 332b.
Y in Sialic Rocks
Early arc rocks such as the Scotia or Tonga-Kermadec arc have much flatter trends than do MORBs. Y rises relative to SiO2 rather steeply from 7-8 ppm in IABs to 30 in basaltic andesites and then flattens with rhyodacites having only 35. Deception is similar, this being about 1/4 the amount seen in the EPR. This rather puts paid to the theory that arc andesites = MORB plus a small amount of sediment, unless a very different kind of fractionation pertains.
The very potassic Vesuvius has a flat distribution with 25-30ppm Y but the more fractionated Campi Flegri ignimbrites sweep up steeply, to a maximum of 50ppm but the scatter is bad.
Zn has a number of very different interests, without it as an anti-corrosive coating on steel cladding whole sections of the construction industry would fail and without sacrificial zincs on ships hulls a steel ship would not last two years and life would be extremely difficult. Zn deficiency can kill off whole forests and it is fundamental to brass metallurgy.
Zn is produced usually along with Pb and Ag being mined as sphalerite (ZnS) and galena,(PbS). The galena usually (eg at Broken Hill) contains a few oz of silver (about 10) to the ton. Zn is the fourth most important metal, total world production in year ending Oct 2001 being 8,177,000 tons. This is very hard to break down to a by country basis as the big metal companies are international and have a finger in every pie, eg Con-Zinc Rio Tinto are supposed to own Mt Isa but many other companies have a share in Con-Zinc. Con Zinc in turn owns a chunk of Port Hedland and many other mining ventures. Canadian miners Cominco have a major share in the very large "Red Dog" Zn mine in Alaska.
Zn in MORBs and OIBs
Again many key islands and centres have no Zn data. The minimum in unfractionated NMORBs appears to be 60 ppm rising with fractionation to 200. EMORBs may have slightly more, perhaps 80 but such data as is are not good.
Zn in Andesites
This can only be termed "low and erratic". In Deception Id and Puyehue Zn increases somewhat with fractionation from 65 -110 and 90 -120. In Planchon Petaroa, Zn is fairly constant at 80-90 ppm but in Parinacota it decreases from 150ppm at 6% MgO to under 45 ppm in dacites. A similar decline is seen in the Valley Of Mexico where basaltic andesites of 75-90 ppm decline to ~40 ppm in rhyolite.
As fingerprint diagrams in these pages show, the HFSE elements, including Zr, Ti, Eu, Gd, Y, Yb, Lu while showing less variation than extreme LILE elements such as Cs, Rb, Ba, Th, U, Nb, K, can nevertheless still vary a good deal. This is usually between about x0.8 and x10 EMORB in common rocks, occasionally up to x 30. How much of this is due to differences in parental magma type and how much to fractionation we shall see.
Consumption & Sources
If asked to name the rock with the highest content of the mineral zircon
(ZrSiO4) most people of geological background would probably name granite
or rhyolite, probably because those little euhedral zircons are so prominent
under the microscope in granites. As we will see, this is not the case,
as commendites-trachytes-phonolites contain much more. World
production of zircon as a source of ZrO2 is now almost a million tons,
almost all recovered from beach-sands as a by product in the recovery
of ilmenite-rutile for titanium. Zircon is a tough little mineral and
resists abrasion much better than feldspar does. 57% of zircon concentrate
is produced in Australia, 45% in W.A. The main usage is in ceramic glazes
and tiles, and as a refractory. It is also used as a radiation screen
in TV sets.
Zr in the Simatic (Oceanic) crust
Zr is one of the more interesting elements to the geochemist. It is present
in amounts ranging from <40 to 2500 ppm in common igneous rocks and
varies widely with fractionation by factors of up to 6-7 from basic
to felsic rocks; in the primary melt series it may be as low as 40 ppm
in depleted NMORB of high degree melt, but up to 160ppm in the smallest
degree of melt rocks of Macquarie (Kamenetsky et al, 2000), a relatively
small range which is important in understanding it's distribution. Zr
increases somewhat as we progress from tholeiitic to alkali basalt to
basanite but as we do not have glass data for the latter it is impossible
to identify exactly. About 150 ppm for larger degree melt alkali basalts
and up to about 250 ppm in basanites is a best guess. As it is unlikely
a "high degree melt" can exist for basanites the comparisons
are not simple. The variation in the range tholeiite - basanite is much
less than we see in Nb however but it is there.
In the MgO/Zr diagram we see a wide range of rocks from tholeiites to basanites following almost similar paths with a logarithmic increase as MgO decreases. Why?
Zr in OIB's other than tholeiites
Trachytes and phonolites of the oceanic islands, being richer in alkalis show more extended fractionation effects than do tholeiites of the ocean basins and ridges and big increases in Zr may be seen when the MgO content is below 0.5%. Trachytes from Kerguelen reach >1200ppm (Weiss & Frey et al, 1993, EPSL 18); while Heard Island also in the southern Indian Ocean (Barling, et al, 1994, J.Pet.35) has 600 ppm at 4% MgO and reaches 1600 in trachytes with the minimum in ankaramites being ~150ppm.
Zr in Sialic Crust
The very depleted proto-arc basalts of Candlemas in the South Sandwich Id and those of the Tonga-Kermadec Arc and those of the IBM Arc (see Orogenic Andesite pages for references) have a common, quite flat trend rising at about 30 ppm at 6-7% MgO with a maximum in rhyodacites of only 100 ppm. This contrasts with Deception Id also in the South Sandwich group where the range is 60 - 550 ppm. Unfortunately there is a problem as of four sets of determinations for Zr from Deception Island, the interlaboratory differences span some 30%. Deception appears to be the most Zr enriched of all andesite series and no equivilent has been found elsewhere, as yet. However, all andesites series appear to have less than half the Zr found in EPR series.
North Chilean ignimbrites actually show Zr sloping down to ~100ppm with increasing silica but ignimbrites are always subject to the suspicion of mechanical sorting. The Topopah Tuff of western USA also shows Zr declining to almost zero at very low MgO levels. We cannot tell is this is a fractionation effect or a mechanical effect.
Continental Flood Basalts
These follow much the same pattern as the more tholeiitic of the simatic basalts. Basalts of 6-7% MgO in the Ferrar Dolerites, Antarctica, have 80 ppm but one series in the Coates Land dolerites, (Brewer, et al, 1992), the so-called high Ti series, have elevated HFSE including Zr with about 150 ppm. Tasmania dolerites are similar to the FD (Hergt, pers comm) and unfortunately in the Parana rocks where they are labelled high and low Ti series, the latest data includes no major elements.
The winner appears to be the island of Pantellaria. If zirconium is desperately needed try the Pantellaria beaches or some late-stage CFB's.. The Garda area has been mined out for cryolite but we may see a rejuvenation with Zr prospecting when the beaches run out but separation from syenitic pegmatites is not easy. However any highly fractionated trachyte or phonolite should be a likely source with on average five times as much as in rhyolites! It would be interesting indeed to find a rhyodacite derived from the Umatilla CRB's but perhaps it would be in the same range as the late-stage Snake River rocks with 2400, 2500 ppm.
Copyright © Dr B.M.Gunn 1998-2003