Igneous rocks, basaltic

Gabbro An intrusive mafic igneous rock. Basalts are extrusive equivalents of gabbros. 

We start with another set of isotope signatures. The rate of erosion in the distant past can be estimated by measuring the ratio of strontium isotopes in marine carbonates. Two stable isotopes of strontium — strontium-86 and strontium-87 — differ in their distribution between the Earth s crust and the mantle underneath it. The mantle is rich in strontium-86, whereas the crust is more richly endowed with strontium-87. The major source of strontium-86 in the oceans is the igneous rock basalt. This rock is extruded continuously from the mantle at the mid-ocean ridges, from where it spreads slowly across the ocean floor before diving back into the mantle beneath the ocean trenches. A little strontium dissolves from the basalt into seawater. The speed of dissolution is more or less constant. The gradual build-up of dissolved strontium-86 in the oceans is balanced by a steady uptake of strontium by marine carbonates, such as limestone (calcium carbonate). This is because strontium can displace its sister element, calcium, in the crystalline structure of limestone. As each of these processes takes place at a steady rate, we would not expect the relative amount of strontium-86 in limestone to fluctuate a great deal. In fact it varies quite a lot. Strontium-87 is to blame. 

The Sm-Nd method can be used to date mafic igneous rocks (basalt and gabbro) which are not suitable for dating by the Rb-Sr method. Magma of basaltic chemical composition originates by decompression melting in the mantle of the Earth which contains radiogenic Nd that has formed by decay of Sm. The isotopic evolution of Nd in the mantle of the Earth is represented by a model in Fig. 3.19 that is based on the isotope composition of neodymium in chondrite meteorites and which is therefore known as the Chondritic Uniform Reservoir (CHUR). The present value of the Nd/ Nd ratio in CHUR is ... 

Igneous rocks Basalt 5,2-6.4 Sedimentary rocks Gypsum 2.0-3.5... 

The earliest record of human usage of iron dates to ca 2000 BC (5) in Egypt, Asia Minor, Assyria, China, and India. It is almost certain, however, that the first iron to be used was not processed but was obtained from meteorites (1). One of the few places where native iron is found is in Greenland, where it occurs as very small grains or nodules in basalt (an iron-bearing igneous rock) that empted through beds of coal. 

Rouxel O, Galy A, Elderfield H (2006) Germanium isotope variations in igneous rocks and marine sediments. Geochim Cosmochim Acta 70 3387-3400 Rouxel O, Ono S, Alt J, Rumble D, Ludden J (2008) Sulfur isotope evidence for microbial sulfate reduction in altered oceanic basalts at ODP Site 801. Earth Planet Sd Lett 268 110-123 Rozanski K, Sonntag C (1982) Vertical distribution of deuterium in atmospheric water vapour. Tellus 34 135-141... 

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