There are immense problems associated with putting a rock name on a fine grained or glassy sample. Thus one sees in the Oceanic basalts the vast majority, from 10 to 2% MgO and from extreme NMORB to extreme EMORB are merely named "basalt" which is not exactly helpful. At least a third of the samples in GEOROC are misnamed and while NAVDAT claim to have on file over 30,000 analysed rocks none are given a name as yet.
The best part of a century ago, Cross, Iddings, Pearson and Washington devised a method of calculation simplified minerals from a chemical analysis. If a rock possessed normative excess silica, and has a normative feldspar in the range An30 to 40 it became an "andesite". If the normative feldspar was labradorite, it was a "basalt".
In the early 1960s we had a Fortran-based computer program which would read in boxes of cards (at about 1200 per box) of chemical data, print out the analysis and Norm in columns of ten until it hit a card saying "end". It then printed out some ternary diagrams of qz hy ol - ne etc and was very useful. It also gave a rock name which was usually correct.
Since the advent of the 16, 32 and 64 bit PC simple languages like Fortran have almost vanished and so have programs that compute Norms. There are some based on EXCEL but can only do one hand-entered set of data at a time.
We really need a program that can compute 10 or 20,000 at a time, preferably within a few seconds. Without a reasonably accurate rock name a database is relatively useless. We recently decided to define the compositional envelope of "Alkali Basalts" to find samples included under this name had from 40 to 70% SiO2! Most AOBs in fact have about 45%.
Preliminary results show that the JAVA version can compute NORMS for the entire PETDB glasses file of 15,000 lines of which only about 10,000 are complete and write the results to an EXCEL-compatible .csv file in about 3 sec.
A number of decisions have to be made. Any rock sample, even a glass has some of the iron oxidized. For the present we shall assume 80% of the iron is FeO and 20% is oxidized to Fe203. This may be not be constant, EMORBS have a higher water content than do NMORBs.
The Oceanic basalts have up to 12% normative orthoclase and 2% apatite. High silica commendites have up to 32% normative silica which is higher than might be expected.
We have been able to confirm that the "Trachytes" of the Revillagigedo Is (esp Socorro) are not even alkaline, the "Trachytes " having about 10% normative Qz and being commendites. As we suspected from the Zr-Nb-Y data they are median EMORBS, slightly enriched above the average, but much more fractionated than is seen in the small submerged off-ridge cones along oceanic ridges. The Galapagos Islands are also confirmed as being slightly below median EMORB on composition and as we have said for some time, can be matched with Icelandic centres.
We will later present here a page of analysed glasses, NORMS and rock names. EXCEL can only take 256 samples laterally compared with 64 K vertically. This wishing to publish in the traditional format will have to restrict the numbers presented.
The interface is not yet complete and other implications have to be considered.
Copyright © 1998-2006 Dr B.M.Gunn