Recent Posts

1

Scientific discussion / Compositional gap when spinel crystallizes

« Last post by AdrienBoucher on March 25, 2019, 06:26:06 pm »

Hello, I am trying to compute the differenciation of a basaltic-andesitic melt and get some results that I am not sure to understand.

First of all, the composition I am using is that of a late archean feldspar macrocrysts bearing flow from which I removed the macrocrysts prior to analysis (when runing calculations with a flow still containing those macrocrysts I get feldspar crystallizing first, leading to an initial increase of MgO in the residual melt, the results are much more consistent with the macrocrysts removed). The sample is relatively fresh (according to mass balance calculations using Trépanier et al. 2016 routine).

As most of the other samples I collected were altered I am specifically looking at elements such as Cr, Ni or at ratios such as Zr/TiO2 in order to infer the differenciation level of each flow in the sequence I am studying.

The initial parameters I am using are those :

ALPHAMELTS_DELTAT -1
ALPHAMELTS_MAXT 2400
ALPHAMELTS_MINP 1
ALPHAMELTS_FRACTIONATE_SOLIDS true
ALPHAMELTS_MINT 500
ALPHAMELTS_MODE isobaric
ALPHAMELTS_DELTAP 0
ALPHAMELTS_MAXP 30000
ALPHAMELTS_CELSIUS_OUTPUT true
ALPHAMELTS_VERSION MELTS
ALPHAMELTS_DO_TRACE true


The input file is :

Title: UCB0005CG02
Initial Composition: SiO2 48.9
Initial Composition: TiO2 0.91
Initial Composition: Al2O3 15.65
Initial Composition: Fe2O3 12.05
Initial Composition: FeO 0.00
Initial Composition: K2O 0.02
Initial Composition: MnO 0.18
Initial Composition: MgO 8.26
Initial Composition: CaO 10.05
Initial Composition: Na2O 1.72
Initial Composition: P2O5 0.06
Initial Composition: H2O 0.2
Initial Trace: Cr 300.0
Initial Trace: Ni 162.0
Initial Trace: Th 0.19
Initial Trace: Nb 2.2
Initial Trace: Ta 0.4
Initial Trace: Zr 50.0
Initial Trace: Hf 1.4
Initial Trace: La 2.7
Initial Trace: Ce 7.4
Initial Trace: Pr 1.21
Initial Trace: Sr 103.5
Initial Trace: Nd 6.4
Initial Trace: Sm 2.15
Initial Trace: Eu 0.78
Initial Trace: Ga 17.6
Initial Trace: Gd 2.92
Initial Trace: Tb 0.5
Initial Trace: Dy 3.47
Initial Trace: Y 20.5
Initial Trace: Er 2.38
Initial Trace: Tm 0.34
Initial Trace: Yb 2.53
Initial Trace: Lu 0.34
Initial Temperature: 1500.00
Final Temperature: 1000.00
Increment Temperature: 3.00
Initial Pressure: 500.00
Final Pressure: 500.00
Increment Pressure: 0.00
dp/dt: 0.00
log fo2 Path: FMQ
Log fO2 Delta: 0.00
Mode: Fractionate Solids

When looking at the results of the calculations everything seems fine (except for a slight increase of Cr during the first steps of calculation as olivine, which is first to crystallize, takes much less Cr than clinopyroxene), but when spinel starts to crystallize, a compositional gap in several elements such as SiO2, TiO2, Zr, Ni or Cr, leading to discontinuity in the model.

I runned the calculations again with spinel supressed and got smoother results, but Cr remains pretty high for intermediate/felsic compositions and I get a rhm-oxide crystallizing instead of spinel. I am not sure to grasp all the implications of this.

Could anyone enlighten me?

2

Scientific discussion / OPX lost at ~1GPa, 1250K-1800K?

« Last post by matrix on March 18, 2019, 07:05:56 am »

Hi,
Recently, I'm working on partial melting of primitive mantle via pMELTS. The initial condition is set as: primitive Earth mantle composition (Sun and McDonough, 1989), isentropic, continuous melting with minimum melt fraction 0.2%, and starts at 4GPa, 1723K. As pMLETS yielded, OPX is stable within the following area P=0.9-1.2GPa and T>=1250K (of course, OPX is stable at other T and P condition). However, one paper ("Evolution of young oceanic lithosphere and the meaning of seafloor subsidence rate", Korenaga, 2016) shows that there is no OPX within 0.9-1.2 GPa and T>1250K, while CPX is much higher than pMELTS generally predicted in the same area (in their Fig.2b and 2c). What's more insteresting is, e.g., when P=1.0 GPa, T=1622K (which is normal P-T path within Earth upper mantle), I got identical bulk chemical composition to their results (their Table 2, row 1 GPa). (We have the same initial condition). So what is the problem with missing OPX while having the same bulk chemical composition? Possibly it's related to phase transitions. Somebody help me!


Thanks,
Mingming

3

MELTS, pMELTS and pHMELTS algorithms / Re: How is the Density of Fluid Phase Computed?

« Last post by zihanwei on February 06, 2019, 08:52:39 am »

Thanks a lot!

4

Tips, tools & add-ons / Re: Batch script for multiple runnings with Alphamelts

« Last post by Paula on February 04, 2019, 01:57:01 pm »

It has been pointed out that the attachment on this post is no longer available. In fact, I disabled attachments when moving the forum to a new server a couple of years ago. However, I have backups of all the previously attached files. Although it may be a bit out of date for alphaMELTS 1.9 / 2.0, if you would like a copy of this batch script just post here, email me, or send me a message via the forum and I'll send it to you.

That goes for other attachments that you can't access also - just let me know and I'll email is as attachment, or post the contents as plain text or upload and link to images.

Thanks,
Paula

5

MELTS, pMELTS and pHMELTS algorithms / Re: How is the Density of Fluid Phase Computed?

« Last post by Paula on February 02, 2019, 10:03:55 am »

Taken from Understanding rhyolite-MELTS versions on the MELTS website:

rhyolite-MELTS + H2O-CO2 fluid (v 1.2.x)
This version is a replacement and upgrade from rhyolite-MELTS 1.0.x. It is the rhyolite-MELTS model with water properties completely replaced with the H2O-CO2 fluid saturation model of Ghiorso and Gualda (2015). Thermodynamic properties of the fluid phase are calculated from the model of Duan and Zhang (2006). Use this version to model crystallization in bulk compositions other than those that saturate in quartz and crystallize to the two-feldspar-, quartz-, fluid-saturated ternary minimum.
Cite as Gualda et al., 2012, and as Ghiorso and Gualda, 2015.

rhyolite-MELTS + H2O-CO2 fluid (v 1.1.x)
Use this version ONLY to model crystallization in bulk compositions that saturate in quartz and crystallize to the two-feldspar-, quartz-, fluid-saturated ternary minimum. This version is the rhyolite-MELTS model with CO2 melt properties and mixed-fluid energetic terms from Ghiorso and Gualda (2015), utilizing H2O melt properties and H2O-anhydrous component energetic mixing terms from MELTS (which are the same as in rhyolite-MELTS v 1.0.x). Thermodynamic properties of the fluid phase are calculated from the model of Duan and Zhang (2006).
Cite as Gualda et al., 2012, and as Ghiorso and Gualda, 2015 (water model from Ghiorso and Sack, 1995).

rhyolite-MELTS (original, v 1.0.x)
This version is the MELTS model (Ghiorso and Sack, 1995) with corrections to the free energies of quartz and sanidine so that the eutectic-like crystallization at the ternary minimum in the quartz-two-feldspar+fluid system is recovered. (This version replaces MELTS, which should no longer be used to calculate phase relations in magmatic systems.) Use this version to model crystallization in natural composition liquids at pressures below 2 GPa. The fluid phase is modeled as pure H2O. PhasePlot implements the rhyolite-MELTS model.
Cite as Gualda et al., 2012.

• Gualda G.A.R., Ghiorso M.S., Lemons R.V., Carley T.L. (2012) Rhyolite-MELTS: A modified calibration of MELTS optimized for silica-rich, fluid-bearing magmatic systems. Journal of Petrology, 53, 875-890.
• Ghiorso M.S. and Gualda, G.A.R. (2015) An H2O-CO2 mixed fluid saturation model compatible with rhyolite-MELTS. Contributions to Mineralogy and Petrology 2015, in press.
• Duan Z. and Zhang Z. (2006) Equation of state of the H2O, CO2, and H2O–CO2 systems up to 10 GPa and 2573.15 K: Molecular dynamics simulations with ab initio potential surface. Geochimica et Cosmochimica Acta, 70, 2311–2324.

6

MELTS, pMELTS and pHMELTS algorithms / How is the Density of Fluid Phase Computed?

« Last post by zihanwei on February 01, 2019, 01:00:03 pm »

In the output of rhyolite-MELTS, the density of the fluid phase is given. I'm wondering how the density of H2O and CO2 under magmatic temperature and pressure is computed in the MELTS algorithm. Thanks!

7

Software tools, support and development / Re: MELTS on Mojave

« Last post by Paula on December 15, 2018, 05:11:00 pm »

Yes, it can work on Mojave. What version of Xquartz do you have?

Paula

8

Software tools, support and development / MELTS on Mojave

« Last post by esmrln on December 11, 2018, 04:04:50 pm »

Is MELTS able to run on the new Mac OS Mojave yet? I can load the exec but I get an error in terminal "Error: Can't open display   :0" and then it logs out.

Any help would be much appreciated. 

Thanks!

-Erin

9

alphaMELTS / Partial melting

« Last post by HaranHennig on December 10, 2018, 07:42:06 am »

Hi Paula/Paul,
 I have a chemical analysis of Spinel-Hertzburgite Xenolite which i would like to partial-melt at ~9 Kbar and approx. 1200 c  , to see which % melting could be my parental melt for the field assemblage.
the P/T was determined previously using melt inclusions.
The problem:
at those conditions, if i just enter the *.melts file and using the manu "option 3" the program gives a ~3.5% melt which is ultra-low on silica(22%).
I would expect to form at least 40% silica in a 3.5% partial melt liquid or even higher than that, without getting to 20+% partial melting which is unreasonable for me.

file used:
Title:Xenolite smp. BH-16-G Hertzburgite
Initial Composition: SiO2 44.06
Initial Composition: TiO2 0.055
Initial Composition: Al2O3 1.15
Initial Composition: Fe2O3 1.49
Initial Composition: FeO 9.18
Initial Composition: MnO 0.165
Initial Composition: MgO 42.72
Initial Composition: CaO 1.48
Initial Composition: Na2O 0.19
Initial Composition: K2O 0.04
Initial Composition: P2O5 0.03
Initial Temperature: 1200.00
Initial Pressure: 9000.00
Log fO2 Path: none

and the results:
Initial alphaMELTS calculation at: P 9000.000000, T 1200.000000
liquid: 3.410 g 22.07 0.28 0.40 33.31 35.82 0.01 4.19 0.22 3.63 0.01 0.06
Activity of H2O = 0  Melt fraction = 0.0339143
olivine: 70.503555 g, composition (Ca0.00Mg0.90Fe''0.10Mn0.00Co0.00Ni0.00)2SiO4
orthopyroxene: 21.148919 g, composition opx Na0.01Ca0.05Fe''0.17Mg1.68Fe'''0.03Ti0.00Al0.15Si1.91O6
clinopyroxene: 5.241394 g, composition cpx Na0.06Ca0.72Fe''0.10Mg0.93Fe'''0.06Ti0.01Al0.26Si1.86O6
leucite: 0.194745 g, composition K0.94Na0.06AlSi2O6.00(OH)0.00
whitlockite: 0.061 g, composition Ca3(PO4)2


AM i doing it right? or is it another way of approaching this kind of calculation?
thank you,
Haran.

10

Scientific discussion / Trondhjemite petrogenesis modeling by using RhyoliteMelts

« Last post by yanweiyao on November 28, 2018, 08:56:09 am »

Hi,
Now I am using RhyoliteMelts program to simulate the petrogenesis of plagiogranite, particularly of trondhjemite. However, I never obtained the exact trondhjemitic melts composition (SiO2 > 63% while K2O <1%) and the phases crystallized do not match our field data.

For my modeling, I used basaltic melts (low K, 0.1 to 0.2 wt.%) as starting materials and set up changing temperature (1300℃ to 600℃) but constant pressure (I tried both 2 kb and 0.7 kb) with various water content (0 to 3 wt.%) and oxygen fugacity (QFM -1 to 3).

For the modeling results which should mimic the real data, we predict that we should have quartz as the cumulate phase at relatively late stage (e.g. SiO2 reaches around 60 wt.% and after), since the parent basalt should evolve from diorite, quartz diorite to trondhjemite. However, the problem is we never have any quartz until all the remaining liquid disappeared, which means that for the evolving process, no intermediate composition, such as quartz diorite, is obtained. Also, in our modeling, the K keeps increasing to extremely high value (~6 wt.%) while in plagiogranite, it should be less than 0.7 wt.%.

So we are wondering if there is anything wrong with our modeling? Or can this program be used for simulating the plagiogranite petrogenesis and include plagiogranite data in your database? We will really appreciate if you can give us some suggestions.

Thanks,

Weiyao