Imagine you have some volcanic rock samples from a given area and about 30 km southwest there is an acidic pluton which is the same age as your rocks. Let's say that both your rocks and samples from the intrusion show perfect fractional crystallization trend on the La vs. La/Sm diagram with only several samples deviating from the trend line. Their common La/Sm ratio is constant, in this case, and let's say it is around 7, while La contents vary from 20 to over 60 ppm with one sample reaching up to 90 ppm. In this case, it seems reasonable to argue that they evolved together from the same source, I guess.
My question is, if we assume a hypotethical situation where the La/Sm ratio of the volcanics is, say, 25, whereas that of the samples from the acidic pluton is 7, would that imply that they evolved from different source regions?
This may imply the acid pluton is derived from a different primitive magma with lower La/Sm. Differences can depend on the La/Sm ratio of the source composition in the melting region and differences in the extents of melting.
Unfortunately, elemental ratios do not provide robust constraints on protolith composition. Especially, if we deal with silicic magmas where various accessory minerals may strongly affect the bulk partition coefficient. To solve questions of magma sources, isotopic data are preferable, in the general case.
Thanks for the replies.
Petr Tikhomirov That’s right. Even though calculated La/Sm ratios and observed horizontal trends of these two groups coincide with one another, it’s still conceivable that one may have originated from a source area with high La/Sm or started as a primitive magma with high La/Sm, but later allanite may have fractionated from the melt to such extent that it may have brought down that initially high La/Sm ratio to the same value as the group that evolved from a primitive magma with a low La/Sm ratio, thereby causing confusion.
I am old-fashioned (geology-mineralogy-chemistry-advanced-level geosciences, .e.g., isotopes).. Therefore, in this case where only a chemical ratio forms centerpiece of a discussion of such a far-reaching issue, I can only respond in a way like that:
"One swallow does not make summer"
HGD
Hi dear.
Thank you for sharing your results. I must tell please send your diagram for me.when I see your diagrams i can able to put better comment for you.
Also, when you want to conclude that the samples are from the same source you need more data, isotopic and etc.
About different trace element you must attention to fluid role in subduction region. Mobility and imobile element as you know!
Sometimes you have sample from same source but several trace element is different. You may Refer to different reason such as degree of partial melting, fluid role or etc. So for reach to best conclude you need more data.
Sincerely yours.
Habib
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