I'm currently studying thermobarometry calculated from clinopyroxene of plutonic and volcanic rocks. I would like to understand why, invariably, jadeite compound produces higher values of P and T in respect to Ca-Tschermak compound. Could jadeite represent the P-T condition of the source rock? What are your ideas?
My experience is represented by Jd rich compositions coexisting with Quad / Di rich compositions in adjacents crystals of pristine Plagioclase and Diopside in metamorphosed gabbroic rocks from stacked high pressure tectonic units of Ligurian Alps. The coexisting compositions draw the miscibility gap of Na-Ca clinopyroxenes at different pressures.
In igneous systems I would rather consider fractionation or mixing mechanisms. What is your bulk rock composition? etc
You should look for other trivalent cations. Eg. Fe 3+ maybe you will get positive corelation. ..
Dear Laura i'm observing cpx compositions in andesitoid-dioritoid rocks. I observe the difference in P-T values both in phenocrysts and matrix-microcryst.
in the andesitoid sample i've chosen, to use-study putirka thermometry, there are no evidences of mixing (from mineral phases and WR geochemistry) but there are many clues of fractionation (i.e. diffferent steps od amphibole and spinel growth with normal evolution).
I start to believe that clinopyroxene in mafic-intermediate magma could retain "P-T memory" of its mafic source, since P-T from Jadeite are higher than expected ones for an andesitoid melt.
Dear Zoran, I've tried many different calculation of cpx-compounds following classical methods and also spreadsheet as calcmin, but the Fe3+ values in cpx is trascurable (as expected for calcalkaline andesitoid melts)
Lucci:
I'm one of Dr. Putirka's students. I'm mainly working with his plag-liq-melt equilibria temps, but I double-check them using his Cpx-thermometer and Ol-thermometer. Make sure that you're only including Cpx at equilibrium, and double check your temps using his Ol-thermometer. That will constrain your temperatures.
Putirka is very friendly. I suggest emailing him.
Melissa,
thank You very Much
I've used many thermobarometers (Al-in Hbl, Hbl-Plag, Zircon saturation, Monazite saturation, rutile thermometer, Ti in Zircon thermometer) and i've obtained a good P-T concordance.
When i use P-T Putirka thermometers on clpx they produce higher temperatures suggesting that clpx is crystallized before other phases.
I've got no olivine in my assemblage and this is a great problem (since i cannot check cpx-thermometers results).
I think that your solution is the best...
Maybe Dr. Putirka could give me some lights on this problem!
Ciao
F
I calibrated eclogite barometer (320 experimental runs) (Ashchepkov et al. 2010) and Cr- di - - augite barometer using the experimental data sets (535 runs) and found that for the equation for the calculations are similar but the free member is higher (7-8 kbar) for the omphacites so I introduced a bit different equations for common Cr- diopsides , for augites, omphacites. It is necessary to work a bit more to make more smooth calibrations without the high difference. May be the reason in the crystal structure augites are C2/c but omphacites may be also P2/n or P2. My PT program is analyzing the compositions of the pyroxenes and it sent it to the appropriate equation automatically
Dear Dr. Igor Ashchepkov
reading from your indication, i found confirmation about one of my "check-point".
Calculation of Activity Compounds in pyroxenes always produces a rest (something that is not allocated nor in Di, nor in Hd, nor in Ca-Tschk, nor in Jd, nor in Opx).
Probably is this not allocated rest that produces discordance between tschermak and jadeite thermobarometers?
ciao
F
Federico
there are several possible reasons why two thermobarometers may give inconsistent estimates when applied to specific rocks. Some more information would be helpful, e.g.:
- how different your jadeite- and CaTs-based PT estimates are? (i.e., have you considered the intrinsic uncertainties of the thermobarometers?);
- how did you set the composition of the liquid that is supposed to be in equilibrium with your cpx's? (i.e., from your brief description your rocks are not aphyric, so it may be tricky to define the chemistry of the melts parent to your cpx's);
- do your cpx's have "normal" compositions? (i.e., do they fall within the range that was covered by the calibration of the thermobarometers?).
Dear Paolo
The differences between Jadeite and Ca-Ts are consistent: 4-5 kbar and 200°C
I know that defining the parental melt composition could be hard.
But i've tried to use the whole rock geochemistry in MELTS software: i've obtained the same mineral assemblage i've observed in the andesitoid sample.
Moreover, MELTS produced cpx at P-T values in accordance with CaTs thermobarometer (lower than Jadeite).
i start to believe that the jadeite thermobarometers is suffering too much the uncertainties related to jadeite compound calculation... (which is not so intuitive as Ca-Tsch, Di and Hd)
The chemical composition of Ca-Tschermak is very complex compared of jadeite, NaAlSi2O6. This perhaps would explain the absence of concordance between these two pyroxene in clinopyroxene thermometry
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