FOCUS_FACTOR

[tm3]

Hi Paula and the MELTS community,

I'm trying to model the impact of porous flow on melting residues using the ALPHAMELTS_FOCUS_FACTOR environmental variable. I want to do so at a mantle potential temperature of 1550 C by polybaric isentropic near fractional melting of primitive mantle between 4 and 0.2 GPa. In order to get a dunitic residue which I'm looking for, I need to use a focus factor of 52. I notice this gives a batch focus factor that compounds during the calculation to an infinite value, which set off some warning bells for me. Unfortunately, I don't really understand the focus factor from Paula's 1999 paper and was wondering if anybody could explain it and tell me if the focus factor I'm using is realistic? Maybe it's too high and just not possible to produce a dunite realistically under these conditions?

Below are my environmental variables and I used the Primitive Mantle of Sun & McDonough '89. I set the entropy at 1 bar and 1550 C with the liquid off. I added 150 pm water. Turned on the liquid, set the pressure to 4 GPa, re-equilibrated, and let it run with option 4.

Thanks!
Tim

ALPHAMELTS_VERSION pHMELTS
ALPHAMELTS_MODE isentropic
ALPHAMELTS_DELTAP -100
ALPHAMELTS_DELTAT 0
ALPHAMELTS_MAXP 40000
ALPHAMELTS_MINP 0
ALPHAMELTS_MAXT 2000
ALPHAMELTS_MINT 0
ALPHAMELTS_CONTINUOUS_MELTING true
ALPHAMELTS_MINF 0.005
ALPHAMELTS_DO_TRACE true
ALPHAMELTS_DO_TRACE_H2O true
ALPHAMELTS_FOCUS true
ALPHAMELTS_FOCUS_FACTOR 52

[asimow]

Hi Tim. Good questions! The value of ALPHAMELTS_FOCUS_FACTOR you are using is too big. The environment variable sets the factor by which the melt flux is increased *at each step of the calculation*, i.e. every 100 bars of decompression. It corresponds to the intensity of a single focusing event, Omega_D as shown in Figures 1 and 2 of Asimow & Stolper 1999. So, indeed, as you say, it compounds during the calculation to a (near) infinite value of the Integrated Focusing Factor Omega_I, something like 52^400. The focusing factors shown in Figures 4, 5, and 6 are these integrated focusing factors. So, with 400 steps of 100 bars each, to match an integrated focusing factor of, say, 100 (which ought to give you a dunite residue by 0.6 GPa or so), you want to set ALPHAMELTS_FOCUS_FACTOR such that Omega_I^400 = 100, which yields 1.01158. Try that, and make small adjustments to that just-slightly-bigger-than-unity value if you need to fine-tune the opx-out pressure!

-- Paul

[tm3]

Hi Paul,

Thanks for the reply! I apologize for confusing you and Paula...

So, I've tried your suggestion and ran alphamelts with the focus factor set to 1.01158 and got the same results as using no focus factor. For some reason I don't seem to be getting different results until I use those very high values I mentioned, i.e., focus factor >~50. Thinking this problem was the version I was using, I've attempted this with alphamelts 1-9 in both the wsl and VM formats and using alphamelts 1-8 in VM (I haven't figured out alphamelts 2.0 yet). Is there maybe a step I could be doing wrong?

To elaborate on my setup using the environment variables in my initial post, the steps I use are:
option 1 read melts file - Sun_McDonough_PM.melts
option 9 suppress liquid
option 2 set T to 1550 and p to 1 bar
option 3 subsolidus initial guess (lately I've been lazy here and just adding olivine and re-doing option 3 until melts figures it out)
option 7 set entropy (for this configuration it ends up being 272.19)
option 6 set h2o to 150 ppm
option 9 turn on liquid
option 2 leave T and set pressure to 20000 bar (I need to scale up in 2-steps or I crash the system)
option 3
option 2 leave T and set pressure to 40000 bar
option 3
option 4

Cheers,
Tim

[tm3]

Also thanks for the focus factor description! It makes more sense now.