The Al in Olivine thermometry has been used on mantle peridotites, lherzolites and Hurzburgites. But can the same Process be used for dolerite and gabbro samples? if yes, is their any published work of Al in Olivine thermometry of dolerite/gabbro rocks? Moreover will the Mg# of the Olivines be high enough in the mantle derivative rocks, to be used in the formula suggested by De Hoog ( 2010)?
The dolerite is the extrusive equivalent for gabbro and have same mineral composition
Dear Srinjoy Datta, this is just a suggestion I give you: Since peridotites (harzburgites and lherzolites) are usually cumulative rocks, I would think that applying de Al in olivine geothermometer to cumulate olivine in dolerite could be a valid technique. And quite often olivine in dolerites or diabases has a cumulative origin. Dolerite or diabase are not the extrusive equivalent of gabbro, they are intrusive rocks emplaced at somewhat shallower levels, sometimes called hypabyssal, but they are holocrystalline and phaneritic, and don't have any glass! I've studied plenty of Paleoproterozoic huge dolerite sills and dike swarms in the Guiana Shield, in southern Venezuela. One of the dike swarm is very rich in olivine, and it shows an orthopyroxene corona texture surrounded by a symplectite. Regards, Sebastian.
Thankyou for your suggestions @Sebastian Grande. It will be really helpful. I was just worried about the Mg#, because mine were coming little lesser than the ones studied by De hoog 2010 and Bussweilier 2013. Now that you say that it can be done.. i can go ahead with it.
The temperatures derived via De Hoog geothermometer depend on Pressure, and the abstract states that the formulation was derived from the analysis of mantle lherzolites, although it does apparently perform well for clinopyroxene-free harzburgites and also spinel peridotites. The abstract even suggests that a derivation of the formula may be used to study olivine inclusions in diamond. If you are looking at hypabyssal dolerites the magmas may very well have undergone variable degrees of fractional crystallization at low pressure prior to the formation of your olivine crystals. I would suggest caution in using a geothermometer calibrated for high pressure mantle rocks, to interpret the crystallization temperatures of low pressure basaltic magmas.
There is the old work by Roeder et al (1979) that looked at Olivine-Spinel, but that looked at Mg/Fe partitioning if I remember correctly. Work by Wan et al in 2008 looks at Al partitioning between olivine and spinel, and they suggest that it is independent of pressure, which is ideal for your application. There may very be others of course. However, both olivine and spinel are very susceptible to sub-solidus re-equilibration, so if your temperatures look low, you are possibly not looking at magmatic crystallization temperatures.
Dejar Srinjoy: I think you understood wrongly my answer. I just you could use the Al in olivine geothermometer, but you hace to make sure that the olivine is cumulative, and Aldo take care of the pressure effect, since dolerites are shallow intrusions, and mantle peridotites are rocks formed at deeper levels.
Regaras, Sebastián
As I I see @S.Datta want to analyse phenocysts of olivine found in dolerite sill/dike of late stage emplaced in country rock gabbro.
Regards
Ijaz
Dear Srinjoy: excuse me for the orthographic mistakes, but it is to blame the "corrector" in my phone. Here is the corrected message:
I think you understood wrongly my answer. I just said you could use the Al in olivine geothermometer, but you have to make sure that the olivine is cumulative, and also take care of the pressure effect, since dolerites are shallow intrusions, and mantle peridotites are rocks formed at deeper levels.
Regards, Sebastián
@ Sebastian Grande any suggestions on what might be the balancing factors if I consider a lower pressure for De Hoog's formula?
Ok, let’s take this from another angle shall we? What are you try to prove by understanding the geothermometry of the olivine “phenocrysts” in this dolerite? How do you know they are phenocrysts and not xenocrysts? Are they cumulative? Or do you think that understanding the temperature of formation will tell you about their origin? I’m sorry, playing devil’s advocate here!!
From petrography, and inter grain textural analysis be believe the olivine are pgenocrysts, the Olivine Trace element and major oxide analysis has been conducted, and we can use the data to understand for sure weather the Olivines r mantle xenoliths or magmatic Olivines refering to Sobolev, Foley and other workers. I want to conduct thermometry using Olivine, CPX and Opaques( Cr- Spinel) and compare the Temperatures to find the probable range of crystallisation temperature of the primary grains. I also want to compare the crystallisation temperatures with mantle potential temperatures. Lastly I'd want understand the mangma source characteristics using mineral chemistry of the above mentioned minerals.
PS. The Olivines in my samples do not seem to be cumulates.
Dear Srinjoy,
You can follow the article and references therein: https://www.sciencedirect.com/science/article/abs/pii/S0024493720305028
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