Evidence of Evolution and Geochemical diversity of Rhyolitic Magma From the Taupo Volcanic Zone, New Zealand Essay

Evidence of Evolution and Geochemical diversity of Rhyolitic Magma From the Taupo Volcanic Zone, New Zealand
By Mary K. Legawiec
ABSTRACT
Between 550 ka to present day, two magmatic cycles showed a geochemical evolution that scientists were able to track throughout the central region of Taupo Volcanic Zone (TVZ), New Zealand (Deering et al, 2009.) Through the analysis of Ba/La ratio, Volatiles, and water content scientists were able to determine the quantity of the contribution of fluids in the variations to mantle melting processes. The magmas geochemistry varied depending on the amounts of clinopyroxene, orthopyroxene, and amphibole present and in balance with the magma(Deering et al, 2009). Data studies found that the first magmatic cycles composition (R1) was cold-wet-oxidizing magma that contained amphibole +- biotite high Sr, low Zr and FeO*/MgO, depleted middle rare-earth elements (MREE) while the second magmatic cycle (R2) was dominantly anhydrous ferromagnesian minerals (orthopyroxene +- clinopyroxene +- hornblende) and is crystal poor, with high FeO*/MgO (tholeiitic series), enriched Zr, Y, MREE and depleted Sr (Deering et al, 2009).
INTRODUCTION
The Taupo volcanic zone (TVZ) is located on the North island of New Zealand and has been active for over 1.6 Ma. The TVZ magma is composed of 90% rhyolites. From about 550 ka to present, shows that the variation of rhyolitic magma formed as a result of the convergence of the two plates. The Australian and pacific plates converging allowed for subduction to occur. Due to subduction of a slab, volatiles become added to the system. Subducted slabs in the TVZ also contain high water content which form in a hydrous melt.
In 1993, two scientists, Sisson and Layne, discovered that mafic glass inclusions that are in subduction related volcano lavas have high water content. A gradual drying within the lower- to mid-crustal 'hot-zone', where rhyolitic magmas are proposed to be generated, produces this continuum of compositions that reflects the change in fluid input. The change in the geochemistry of the magma has been linked to the fluctuation in the release of fluid from the subducting slab at TVZ.
GEOLOGICAL STRUCTURE
The Caldera structural boundaries and regional tectonic fault structures define the central TVZ as a ca. 120 km long and up to 60 km wide envelope (Houghton and Wilson ,1995). These settings formed by the caldera and fault structure help limit the size of the TVZ. The limitation of the size of the TVZ helps researchers and scientists have better understanding of the chemistry of the magma and allows for more accurate data to be collected.
Fig. 2 illustrates the differences between R1 and R2 magmas.a and b illustrate the disparity between R1 and R2 orthopyroxene (opx) compositions, c Biotite display a wide range of compositions, but are only found in R1 magma types. d Hornblende (hb) compositions (Deering et al, 2009)
OBSERVATIONS
Geochemical Data total alkali vs. SiO2 data was collected at the TVZ. The graph shows that there was a high concentration of rhyolite within the magma. The high weight of SiO2