USING THE WATER/ROCK RATIO EQUATION FOR MODELING THE MIXING CURVES OF CONTRASTED SUITES OF ROCKS

The Water /Rock ratio equation reported by McCulloch et al. (1980 & 1981) and based on changes of the Sr & Nd isotope ratios in rocks, is a useful application in the assessment of the isotopic exchange between seawater and rocks. This equation can be written as:

W/R=[(E^fr- Eⁱr) / (Eⁱw - E^fr)] [Xr/Xw]

The final ratio of an isotopic element was used to modulate the mixing curves between seawater and rocks. Several suites of rocks of various mineral and isotopic compositions were used to test the alteration effect on these rock types, in terms of isotopic compositions, with a systematic change in the W/R ratio, assuming a closed system for isotopic elements.

Sr composition of rocks immediately alters by seawater, while Nd composition keeps unchanged until large volumes of water (»10⁴Rock, for all calculated rocks except ultramafic »10²Rock) are added to the rocks. This can be attributed to the very low Nd concentration in seawater (2.6*10^-6). However, the large volumes of water required for the Nd-exchange indicate that: 1) Nd-exchange may only take place under seawater, possibly through hydrothermal alteration by circulated seawater, 2) Nd-concentration of not highly altered crustal rocks are presumably primary.

On the contrary with igneous rocks, rocks of metamorphic and sedimentary signatures such as carbonates and Precambrian basement rocks show a similar but inverse half-square mixing line. This inverse progression is due to their higher Sr and lower Nd initial ratios than seawater.

The volume of water required to cause isotopic alteration in rocks depends on isotopic compositions and rock mineralogy. Crustal rocks require higher volumes of water (³10⁴Rock) for alteration than what mantle ultramafic rocks require (»10²Rock). Despite the mineral diversity of the crustal rocks, they all contain relatively low temperature minerals that are less susceptible to alteration, and lack of high temperature minerals such as olivine and orthopyroxene. On the other hand, residual olivine minerals are the dominant minerals in the mantle peridotites, which are highly susceptible to alteration and lack of Sr and Nd compositions. This is consistent with the fact that large portions of the mantle sequence of ophiolite complexes are serpentinized.