liquidus temperature high mg magma

[roman@ipgp.fr]

Hi,
I am trying to simulate isobaric crystallization of a high magnesium basalt at pressure of 10kbar under anhidrous conditions. When I run the Find Liquidus command the Liquidus temperature is found at 1660 degrees, which seems very high compared to the liquidus temperature I've found in the literature for these kind of systems.
Here is the initial composition:
Si02      49.1
Ti02         0.6
Al203    15.17
Cr2O3     0.36
FeO        7.54
MnO       0.14
Mg0       13.11
Ca0       12.28
Na20       1.58
K20         0.08

The initial Fe2+/Fe3+ ratio is calculated by constraining with the FMQ buffer, but the choice of the buffer seems quite irrelevant to the liquidus temperature found by Melts.
Am I missing something?
Thank you very much!
alberto

[asimow]

Hi Alberto,

The result for find liquidus that you are getting is controlled by the appearance of a tiny amount of high-Cr spinel. After doing a find liquidus you can see what is the liquidus phase by scrolling down the list of phases at the bottom of the MELTS interface window until you see which one has an affinity of just a Joule or so.

So, to some extent this is an artifact of the poor constraint on the behavior of Cr at high pressure and temperature, and to some extent even if it is right it may not be the liquidus you care about. Note that you can come all the way down to 1424 °C before a second phase enters, orthopyroxene (for a small temperature interval, quickly replaced by clinopyroxene). Over this 240 °C range, only 1 gram of spinel crystallizes! This shows how sensitive the liquidus T calculation can be to small shifts in the behavior of compatible components. Another way to see this is to remove the Cr from the calculation, in which case Find Liquidus gives you orthopyroxene on the liquidus at 1424 °C (that is, the other components are hardly affected by the crystallization of 1% Cr spinel in this high-T range).

So, the lessons are:
• Cr is a challenging component for MELTS, and generally makes spinel too stable
• The Find Liquidus command can be very sensitive to the appearance of small amounts of spinel; you probably want to look instead at the first appearance of olivine or pyroxene
• After a Find Liquidus calculation, do a batch isobaric crystallisation sequence to see the masses and compositions of the phases. If a tiny amount of something crystallizes over a wide temperature interval then you know the Find Liquidus result is probably not meaningful.

-- P

[roman@ipgp.fr]

thank you very much for the prompt answer!