Trace element characteristics of lavas from destructive plate boundaries. Shane Turner

Trace Elements and Subduction Zones A trace element is a chemical element which has a concentration of less than 1000 parts per million (ppm) or in other words, makes up 0.1 % of a rock's composition. Subduction zones are zones where cool lithospheric plates are dragged down back into the mantle.

Mid Ocean Ridge & Subduction Zone System

Destructive Plate Boundaries http://www.earthquakes.bgs.ac.uk/latest/plate_boundaries.jpg www.platetectonics.com/.../Subduction1.gif

Magma Composition: Have a basaltic composition and form the ‘Volcanic Arc Basalts’. They can be sub-divided into Island Arc Tholeiites, Calc-Alkaline Basalts and Shoshonites according to the K2O-SiO2 discriminant of Taylor and Peccerillo (1976).

Magma Composition The most convenient way of comparing the analyzed data for these various magma types is to plot the data as geochronological patterns.

Figures 1(a) & 1(b). 1(a): Mid Ocean Ridge Basalt variations 1(b): Within Plate variations

Figures 1(a) & 1(b). 1(a): Mid Ocean Ridge Basalt variations 1(b): Within Plate variations

Volcanic Arc Basalt variations

Discrimination and Modeling. Discriminant diagrams based on Ti (immobile incompatible element) and Cr (fractionation index), are used to successfully discriminate between IAT and MORB magma types. Y is often used instead of Ti.

Discrimination diagram for characterizing island-arc tholeiites

Figure 3: Possible explanations for the lower Y contents of IA Tholeiites relative to MOR Tholeiites.

Fig 3a. (Degree of partial melting).

Fig 3b. (Presence of residual minor phases)

Fig 3c. (The source composition).

Fig 3d. (Mixing with melt derived from the subducted oceanic crust)

Mobile - immobile element co-variations It is also necessary to filter out the effects of other types of enrichment processes. Easiest Way: Choose a pair of elements which are enriched to a similar extent in within plate and MOR settings, yet behave differently in the volcanic arc environment, because of their different mobility’s in aqueous fluids. Examples of such element pairs (fig 1a and 1b), are Sr, Ce and Nb or Ta and Ba.

Fig 5a. (Discrimination diagram on the basis of Ce/Sr ratios)

Final words: By using a careful selection of elements, discrimination diagrams can be plotted which highlight and indicate the various characteristics of lava’s. These diagrams can therefore enable us to recognize volcanic arc basalts, where geological evidence is insufficient, due to the factors of deformation, erosion, metamorphism etc. These plots can be modeled in terms of vectors, which represent different petrogenetic processes. A convenient way of comparing analyses for these various magma types is to plot the data as geochronological patterns. Many geologists rely heavily on these volcanic rocks for providing the evidence by which past destructive plate boundaries can be located. It is therefore important to be able to detect whether or not a lava sequence was interrupted in a volcanic arc. This is particularly true in many orogenic belts, where allochtonous fragments of island arc and back-arc basins are thought to exist.

References: Armstrong, R.L. (1971). Isotopic and chemical constraints on models of magma genesis in volcanic arcs, 137 – 142. Jakes, P. and Gill, J. (1970). Rare earth elements and the island arc tholeiite series, 17 – 28. Jakes, P. and White A.J.R (1972). Major trace element abundances in volcanic rocks of orogenic areas, 29 – 40. Pearce, J.A (1982). Trace element characteristics of lava’s from subduction plate boundaries, 525 – 548. Skinner, B.J. Porter, S.C. and Botkin, D.B (1999). The Blue Planet (2nd edition). Page 152.