|Element Concentration Processes
Processes leading to Element Concentration in Igneous Rocks
Similarity of many inter-element patterns makes it appear that the earth was originally formed of a mass of quite homogenous chondrite material consisting mainly of magnesium, silicon and iron. With the heat of compaction the native nickel-iron melted and worked its way down to form a metallic core. The remainder began a process still commonly seen of partially melting small quantities of basalt containing a much higher proportion of the elements which are not to any degree absorbed into olivine, orthopyroxene, clinpyroxene and plagioclase. Removal of these minerals from slowly cooling magma leads to higher concentration of these same element, the LILE, (Lithophile light elements) in the magmatic residua. So we now may find igneous rocks with up to 500 times the original chondrite levels of Cs, Rb, Ba, Th, U, Nb, K etc. Subaerial weathering extends these processes so than banded iron formation may contain 60% of Fe2O3, while other rocks may be pure calcium carbonate or pure quartzite (SiO2). Sulphur has the ability to combine with many metals including Cu, Ni, Zn, Co, Pb, Cd. Segregations of sulphides in igneous rock may have an average of several percent metal, making it much more available than the parts per million found in the parental chondrite.
Our whole civilization depends on finding and using these concentrations whether occurring as oxides, silicates, or sulphides and the processes must be understood; a great deal of research has gone into the origin of sulphides, less into silicates which are more refractory and more difficult to use as a source of metals. Even the nickel industry which obtains Ni from sources in peridotites and komatiites, relies on secondary Ni sulphides, not on the original Ni silicates. However as sulphide ore bodies become worked out, sources within silicates will become more important. In addition an understanding of the compositional envelope for the Oceanic Basalts which make up 60% of the Earths surface is dependent on an understanding of these differentiation processes.
Three of the more important elements used in differentiating between volcanic magmas are Zr, Nb and Y and can be used to illustrate processes. In the range of ORB and OIB rocks, Nb may vary in amount from 1 600 ppm, Zr from 38 2200 ppm and Y from 18 to 220 ppm. Even higher amounts especially of Nb are found in special rocks such as carbonatites. At present zirconium for example is recovered from zircon minerals in beach sands, Should these supplies run out, it is better we know that commendites have perhaps 20-30 times the amount of Zr than have some basalts. Other elements especially Cs, Rb, Ba, U, REE etc also show the manner in which elements may be concentrated and can be studied in the same manner as we shown here
Once formed, the parental basalt rarely reaches the surface unmodified. In the case of the depleted NMORBs, plagioclase and olivine form and may be separated from the magma which will have been depleted in Al, Sr, Mg, Ni and enriched in Na, Fe and the residual LILE including the REE and Nb. Neither plag nor ol contain more than infinitesimally small amounts of Zr, Y, Nb.
Differentiation in OIB alkaline rocks
Here we are on less sure ground as we have few analysed glasses nor has anyone found a parental series as we have for the ORBs. However, they cannot be too different from the OIBs, being probably more magnesian and lower in silica but all are enriched in the LILE elements.
In all ORB OIB lavas, Ba, Rb, Th U K, LREE, are progressively elevated relative to Zr in the same manner Nb is.
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