http://jersey.uoregon.edu/~mstrick/AskGeoMan/AskGeoImages/Bowen's.gif

I don't quite get this diagram.

1) All magma will eventually cool to below 800 degrees C, so why are there still olivine and proxene when the magma cooled? won't they all become quartz?

2) I don't get the realtionship between temperature and type of magma. Is it saying that when magma continue cooling, all mafic magma will become intermediate and then felsic magma, so no rocks are mafic?

Can someone explain? Thank you! ;)

The diagram shows how the minerals crystallising out of a melt change as the intrusion cools.

For the sake of simplicity, imagine a large mafic intrusion. Being mafic, it is rich in iron and magnesium, but relatively poor in silica, sodium, aluminum, etc. After emplacement, the pluton begins to cool slowly. As the temperature falls it passes through the melting points of various minerals - the highest of which is olivine. This means that the olivine crystallises out of the magma and settles on the floor (as well as plating the ceiling and walls) of the magma chamber. The residual melt is now relatively enriched in the more felsic minerals (through depletion of some mafic ones). This continues to happen for different minerals as the temperature falls.

If, for some reason, the remaining magma continues upwards through the host rock it will produce more intermediate or even felsic intrusions, or eruptions if it breaches the surface. It is partly for this reason that igneous rocks formed in a continental setting are generally more evolved than their oceanic conterparts - they've just had more opportunities to pool and fractionate on the way up.

The reason for the two branches (continuous and discrete) is that feldspar composition spans a single continuum with calcium-rich, sodium-rich and potassium-rich as the end members. The minerals olivine, pyroxene, amphibole, etc., are qualitatively different minerals, however, which is why they are placed on a discontinuous series.

Incidentally, as the melt becomes increasingly acidic (as opposed to mafic rocks, which are basic), large elements such as uranium also become concentrated. This is because they are not easily compatible with any of the previously crystallised minerals so (like being picked last for a sports team) they end up solidifying with the dregs.

Hope this helps.

Hello,

1) But all magma will keep cooling to low (normal) temperatures, then how can minerals like olivine be found? Wouldn't all olivine be gone? (because all olivine are changed to other minerals)

2) Is it true that when magma cools, it is becoming increasingly felsic and less mafic? If so, why are there mafic rocks/minerals/lava existing? Wouldn't all minerals be felsic because they will all be cooled to normal temperatures?

1) It's not the olivine that changes. The magma changes when these minerals precipitate (non-technically) out of the magma. What you're talking about is metasomatism, which doesn't really occur significantly in this case.

2) Read Laika's post more carefully. The mafic minerals crystallize out first (hence, they exist). Otherwise I don't know really know what you're asking.

Hello ;)

1) "At a certain temperature a magma might produce olivine, but if that same magma was allowed to cool further, the olivine would "react" with the residual magma, and change to the next mineral on the series (in this case pyroxene). Continue cooling and the pyroxene would convert to amphibole, and then to biotite."
http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry32.html

I have read the paragraph in the web site again. According to the quote, wouldn't all the olvine be gone, since all olivine is converted into pyroxene, and so on, and so on. The final product is quartz and should be the only mineral that exists because it used up all the minerals above, to convert to quartz, eventually.

2) As magma cools, is it true that the composition of the magma become increasingly felsic? Is that one of the things that Bowen's Reaction Series is saying? That's what I get from the diagram, because the word "mafic" (and 1400 oC) is on the top and "felsic" (and 800 oC) on the bottom. If I am wrong, please correct me!

1) "At a certain temperature a magma might produce olivine, but if that same magma was allowed to cool further, the olivine would "react" with the residual magma, and change to the next mineral on the series (in this case pyroxene). Continue cooling and the pyroxene would convert to amphibole, and then to biotite."
http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry32.html

I have read the paragraph in the web site again. According to the quote, wouldn't all the olvine be gone, since all olivine is converted into pyroxene, and so on, and so on. The final product is quartz and should be the only mineral that exists because it used up all the minerals above, to convert to quartz, eventually.

Only if the minerals in question are left in contact with the magma and the magma is cooling slowly enough for the reactions to occur - neither happens often. As an aside, the process you are reffering to causes the "zoning" seen in olivine and plagiclase - the changing composition of the magma causes a slightly different composition of the mineral to precipitate out on top of the existing mineral. The mafic minerals often settle to the bottom of the chamber (forming a cumulative layer), effectively removing them from the magma chamber.


2) As magma cools, is it true that the composition of the magma become increasingly felsic? Is that one of the things that Bowen's Reaction Series is saying? That's what I get from the diagram, because the word "mafic" (and 1400 oC) is on the top and "felsic" (and 800 oC) on the bottom. If I am wrong, please correct me!

True - the magma becomes increasingly felsic as it cools and mafic minerals are removed (in a simple case, at any rate - it also depends on changes in silica saturation). This process is differentiation. However, due to the effect in point 1), mafic rocks can still exist. Also, remember magma is mobile and the magma chamber may be "re-charged" with "fresh" mafic magma at regular times. For more info, try googling the Ilimaussaq intrusion in Greenland - an excellent example of differentiation with many cummulative layers!

Jon