Dear Mr. Baghban:

Bivalent Sr can substitute for bivalent Ca. It is not an abnormally high amount of Sr in your andradite-dominated garnet s.s.s. and I advise to be very cautious and do not over-interpret one trace element alone. You have to look at these values in context with other elements chemically affiliated to them and the overall composition of the garnet. Extremely high values may be expected, as alkaline magmatic activity is involved in the skarn formation. Are there LREE-bearing minerals such as monazite or bastnaesite among the minerals ? On the other hand, take also a closer look at the limestone that provides the parent material for the exoskarn. What is the background.

Sr/Sr isotopes have often been used for genetic interpretation of garnet emplacement. See one paper below.

Sousa et al. (2013) Strontium isotope zoning in garnet: implications for metamorphic matrix equilibration, geochronology and phase equilibrium modeling.

Best regards

Harald G. Dill

Hi Saeid:

It is likely that Sr in garnet - a mineral in skarn - is derived mainly from magmatic fluids related to igneous intrusions. This is because: 1). Sr has relatively high partition coefficient between magmatic fluid and melt; 2) early (dry) skarnization is dominated by magmatic fluids; 3) sedimentary carbonate rocks contain very low Sr.

Regards,

Xue-Ming Yang

Hi Saeid,

As Dr. Dill states, Sr2+ substitutes for Ca2+, and this is true also in carbonate rocks.  I have seen background Sr values in limestone in the low 100's of ppm in the Terrazas skarn deposit in Chihuahua, Mexico and also in regional prospecting farther south near Cuernavaca, Mexico (in unmineralized and unaltered limestones, where [Sr] is commonly in the 50-100 ppm range).  Most likely Sr2+ came out of the limestone during skarn formation and was preferentially incorporated into andradite.  Acquire some whole rock (XRF) data for your wall rocks and see where the high Sr background values are.  You could also compare the [Sr] in endoskarn garnet with exoskarn garnet and see if there are clear differences.

cheers,

Jason Price

Sorry, Saeid, I meant ICP-MS or ICP-OES data for [Sr] in your wall rocks.  We used to get 35 element analyses from an assay lab as part of our regional exploration, and these reports showed [Sr].  You should do the same thing for your samples from the skarn deposit.  Analyze a few of the wall rocks far from the deposit to get an idea of what background values are.

best of luck,

Jason

It is quite reasonable that the Sr is derived from the magmatic fluids but limestone can also have considerably higher Sr values than those quoted here. I have worked  on Neoproterozoic limestone with 1200ppm. Do you have any isotopic information on the Sr?

Hope this helps

Jana

That is a pity as isotopes are a useful tool Yes this was Cryogenian limestone that we were using the Sr initial ratio as a proxy for age dating. You might wish to look at this reference for more detail on another high Sr limestone .

https://www.researchgate.net/publication/249546274_Constraints_on_87Sr86Sr_of_Late_Ediacaran_seawater_insight_from_Siberian_high-Sr_limestones

Jana

Article Constraints on 87Sr/86Sr of Late Ediacaran seawater: insight...

Not too much to suggest here after lot of useful ideas have already been shared and discussed. It is for sure that Sr is replacing Ca in the garnet ( assuming you did hit garnet during EPMA point analysis and digested garnet for preparing solution for ICP measurements), so I wonder if you check the chemical zonation of Sr and Ca  in garnet using non destructive techniques such as SEM-EDX or EPMA etc. if you don't have isotope work done which is expensive and time consuming compared to Elemental profiles. It might shed some light on the Sr activity in your garnet during the course of its geologic history.

Sincerely, Arshad

What is the highest Sr concentation that has been observed in andradite?

Is it geochemically plausible for Sr and REE substitution to be detectable as an increase in RI?