THE BUDGET OF POTASSIUM IN THE CRUST-MANTLE DYNAMIC ENVIRONMENT

Potassium is one of the least abundant major element in rock composition. However, it is also one of the most critical element in the formation of cratonic crust on earth. According to the current models on the origin of the earth, the mantle of the earth started with only a trace amount of potassium in percentage, which is represented by the composition of chondritic meteorites. Through various interactive processes between the mantle and the evolving crust, potassium was gradually concentrated to the crust, and is eventually highly condensed to the granitic core of the continent. An critical concept developed in this study is that the movement of potassium from mantle to crust is not a cyclic process, but a one way transportation. The result is the increase of cratonic land mass through time.

This investigation used a STELLA™ model to evaluate the mechanism of potassium storage and transportation processes from the mantle to the crust. Major components involved include: 1. The oceanic crust cyclic process, which goes from the partial melting of asthenosphere at mid-oceanic ridge to the volcanism along subduction zone; 2. Plutonism and high grade metamorphism in the mountain building process; 3. Partial melting and delamination of subcontinental lithosphere; And 4. Mantle plume and hot spot upwelling. In addition to these processes that transport potassium in between the crust and the mantle, this model also considered minor cyclic processes took place in the crustal environment. Which include: 1. Recycling of clastic sediments; 2. Evolution of potassium in seawater.

There are serious limitations to the precision of this preliminary model. Both qualitative and quantitative natures of the said processes are difficult to measure. And the rate of many processes can only be inferred. Nevertheless, a preliminary network of interactions among the involved processes is constructed and the most reasonable values for potassium reservoirs and flows are used. There are plenty rooms left for the refining of the model in future study.