Need help with some Geology (petrology) homework

[Yuriman]

Got a set of questions I need to work through for a class, and thought I'd post them here for help. I should be able to do them all myself but a second opinion would be useful, and might save me some time.

 

[deadlyapp]

AT is not your tutor (or mom).

 

[FleshLight]

Change your major

 

[Ruptga]

AT is not your tutor (or mom).

Yep. There's "Hey guys the problem is XYZ and I think it works this way because of abc, what do you think?" And then there's posting your entire assignment with no comments at all.

 

[Yuriman]

Yep. There's "Hey guys the problem is XYZ and I think it works this way because of abc, what do you think?" And then there's posting your entire assignment with no comments at all.

Perhaps I'll come back later tonight with some specific questions, then.

It's not uncommon to see people asking for explanations of how to do specific math problems, I didn't consider that it would be an issue posting this assignment. It's not as though I'd use an answer from a Random Internet Stranger without looking it up myself first.

EDIT: Maybe I need mothering, then? I've been moved out for close to 8 years now.

 

[norseamd]

how about you start by asking a question and then we will say something and then ask you what you think

 

[Yuriman]

Alright, here's one:

As I understand it, it's difficult to represent continuous reactions on ACF diagrams, because you need a series of diagrams to show snapshots of what's going on at different pressures and temperatures. Discontinuous reactions usually plot well on a single diagram.

Am I correct in this understanding?

 

[BladeVenom]

42

 

[JManInPhoenix]

42

Win - was going to say that but you beat me to it.

 

[Yuriman]

Thanks!

I'm reading some conflicting things about the differences between primary and primitive magmas. My text says that primitive magmas are those which have moved to the surface unmodified and undifferentiated from their source, while primary magmas are those which are unmodified at their source of generation. My interpretation of this is that primary magmas are local, while primitive are from mantle sources and have moved, unmodified, to where we can study them.

Any comments?

 

[norseamd]

Thanks!

I'm reading some conflicting things about the differences between primary and primitive magmas. My text says that primitive magmas are those which have moved to the surface unmodified and undifferentiated from their source, while primary magmas are those which are unmodified at their source of generation. My interpretation of this is that primary magmas are local, while primitive are from mantle sources and have moved, unmodified, to where we can study them.

Any comments?

seems like it

 

[norseamd]

Primary melts

When a rock melts, the liquid is a primary melt. Primary melts have not undergone any differentiation and represent the starting composition of a magma. In nature it is rare to find primary melts. The leucosomes of migmatites are examples of primary melts. Primary melts derived from the mantle are especially important, and are known as primitive melts or primitive magmas. By finding the primitive magma composition of a magma series it is possible to model the composition of the mantle from which a melt was formed, which is important in understanding evolution of the mantle.
Parental melts

Where it is impossible to find the primitive or primary magma composition, it is often useful to attempt to identify a parental melt. A parental melt is a magma composition from which the observed range of magma chemistries has been derived by the processes of igneous differentiation. It need not be a primitive melt.
For instance, a series of basalt flows are assumed to be related to one another. A composition from which they could reasonably be produced by fractional crystallization is termed a parental melt. Fractional crystallization models would be produced to test the hypothesis that they share a common parental melt.
At high degrees of partial melting of the mantle, komatiite and picrite are produced.

 

[norseamd]

so primitive melts are a form of magma from the mantle

 

[dennilfloss]

Damn that exam brings back memories. Igneous & metamorphic petrology was not one of my favourite courses.

 

[Yuriman]

For #13, "Justify the characterization of the Al2SiO5 and CaCO3 systems as consisting of only one component, even though there are three chemical elements in each."

Frankly, I don't know and I'm not finding anything in my text. My best guess is that it has something to do with them effectively being solid solutions.

 

[Yuriman]

Also, for #19, "How does Earth bring granulite facies metamorphic rocks back to the Earth’s surface? Explain the processes."

I can imagine granulite clasts being carried up by volcanism, since they have very high melting points. I understand them to be formed deep within continental crust. I don't know what processes might move large bodies of it upward without breaking them apart.

Some useful info on granulites: http://www.britannica.com/EBchecked/topic/377777/metamorphic-rock/80340/Granulite-facies

EDIT: My best guess here would be subduction zone mountain building followed by tons of erosion.

 

[norseamd]

Pressures calculated from fragments of granulite-facies metamorphic rocks carried to the surface in young volcanic eruptions suggest that the fragments were derived from the lower crust. It is likely that the lower crust is currently composed largely of granulite-facies rocks that may be exposed by future episodes of mountain building, but it is also possible that these granulites will prove to be different from their Precambrian counterparts. In order to resolve some of the controversies surrounding the origin and composition of granulites, it is necessary that considerable studies of these rocks be conducted in the future.

from brittanica

 

[Yuriman]

from brittanica

I might be overthinking this, but I'm imagining the question implying that large bodies of this rock can be found at the surface in some areas.

 

[norseamd]

Large areas of granulite facies rocks are confined almost entirely to Precambrian areas of the continents (those areas that were formed more than 542 million years ago), with well-developed areas exposed in Canada, India, Africa, Antarctica, Greenland, and the Adirondack Mountains of New York in the northeastern United States. Smaller areas of granulite facies rocks occur in younger mountain belts, with Paleozoic examples in New England (U.S.) and Brittany and Paleogene and Neogene examples (those formed between about 65.5 million and 2.6 million years ago) in British Columbia (Canada) and Timor. The apparent decrease in the volume of granulite facies rocks with decreasing age of metamorphism has led some geologists to postulate, as mentioned above, that plate-tectonic processes might have changed significantly with time—specifically that steady-state continental geotherms were hotter in the Precambrian than at the present time. Some work on pressure-temperature-time paths in granulites also suggests that Precambrian granulites were metamorphosed along distinctly different paths than younger granulites, lending credence to models invoking changes in tectonic processes. An alternative hypothesis is that large volumes of granulites have been formed throughout Earth history but that they have not yet been exposed by erosion. Pressures calculated from fragments of granulite-facies metamorphic rocks carried to the surface in young volcanic eruptions suggest that the fragments were derived from the lower crust. It is likely that the lower crust is currently composed largely of granulite-facies rocks that may be exposed by future episodes of mountain building, but it is also possible that these granulites will prove to be different from their Precambrian counterparts. In order to resolve some of the controversies surrounding the origin and composition of granulites, it is necessary that considerable studies of these rocks be conducted in the future.

read through the paragraph

sourced from brittanica

you might have to look for more explanation for precambrian granulites

 

[Yuriman]

For #18, "Tremolite+ 3 Calcite + 2 Qtz = 5Diopside + 3CO2+H2O. At what fluid composition is the equilibrium temperature for this reaction the highest? Explain why."

I don't know if he wants an exact composition, or something like "deficient in volatiles". I'm thinking it's going to be in the range of 0.1-0.3H2O but I'm not sure where to track this information down.

EDIT: I appreciate your help.

 

[norseamd]

For #13, "Justify the characterization of the Al2SiO5 and CaCO3 systems as consisting of only one component, even though there are three chemical elements in each." Frankly, I don't know and I'm not finding anything in my text. My best guess is that it has something to do with them effectively being solid solutions.

molecules?

 

[Yuriman]

I'm a bit stumped on 9, 10, 12 (the first 12, he numbered it wrong), and 21. I'm pretty satisfied with my answers to the rest.