Titanium distribution in the lithosphere

Abstract

The regional and global distribution parameters of titanium in various types of igneous, metamorphic and sedimentary rocks are estimated on the basis of available analytical information. The average titanium content in sedimentary rocks is 0.34% and an average variation coefficient 0.35; for the “granitic” layer it is estimated as 0.33% and an average variation coefficient 0.30. Ultrabasic rocks with the exception of kimberlites are depleted in titanium (global average for dunites 0.036%; for peridotites 0.39%; for pyroxenites 0.50%; for kimberlites 1.30% Ti). Three groups of continental basalts can be outlined on the basis of Ti-distribution: (1) regional averages 0.8-1.0% Ti (mainly tholeitic basalts of shield regions and some mobile belts); (2) regional averages 0.6-0.8% Ti (mainly basalts of island arcs); (3) regional averages 1.2-1.4% Ti (mainly continental alkalic olivine basalts related to deep seated tectonic zones). Basaltic rocks of the oceanic crust form two separate groups. Basalts of the mid-oceanic ridges have the same Ti-average as continental basalts of shield regions (0.80% Ti). Tholeitic and especially alkalic basalts related to the volcanoes of central type differ from other basalts by high Ti-averages (1.50-1.90% Ti). Parameters of Ti-distribution in oceanic alkalic basalts are very close to those of lunar basaltic rocks and fines collected during “Apollo 12” and “Luna 16” missions. Global Ti-distribution in continental gabbro resembles that found in basalts of the island arcs. Gabbro series enriched by Ti are situated close to the border zone between continental and oceanic crust or are related to deep-seated tectonic zones. Partial fusion of the upper mantle at considerable depth (80–100 km and more) is considered as a main cause of Ti-concentration in terrestrial basic and ultrabasic rocks. Among metamorphic rocks Ti-averages in amphibolites and eclogites are similar to those in basaltic rocks. Eclogite inclusions from kimberlite pipes are depleted in Ti in comparison to any type of eclogites of mobile belts. The average Ti-content in gneisses and schists is very similar to granodiorites. However, metamorphic rocks as usual have much higher variation coefficients. Granodiorites (0.38% Ti), gneisses (0.35% Ti) and schists (0.36% Ti) represent within the “granitic” layer the average Ti-content of the entire layer. However granites (0.18% Ti) from this point of view are quite different from sedimentary and metamorphic rocks. It leads to the conclusion that the essential removal of about 50% of Ti takes place during granitization.