Earth's Mantle and Continental Crust

Question 1

Following the formation of the core, what was the next/second major differentiation event within Earth?

Answer 1

the extraction of the continental crust (CC) from the primitive mantle (BSE).

Question 2

Why do we have very detailed understanding of the continental crust?

Answer 2

1. we can sample it directly as it is the uppermost layer
2. exhumed terranes and crustal xenoliths expose the lower continental crust which we can directly study

Question 3

What do the BSE and primitive mantle denote?

Answer 3

the silicate portion of the Earth following volatile depletion and core formation

Question 4

What term describes the source of mid ocean ridge basalts (MORB)?

Answer 4

the depleted mantle. hence why it is called the depleted MORB mantle (DMM)

Question 5

why is the continental crust and island arc andesites similar in composition?

Answer 5

island arc andesites form by small partial melting of the mantle during subduction. the continental crust is also thought to be formed from small degrees of partial melting of the mantle producing a melt rich in incompatibles.

Question 6

which element is relatively enriched in the continental crust than expected from its partition coefficient?

Answer 6

Lead (Pb)

Question 7

CC + DMM = ???

Answer 7

BSE

Question 8

the major element composition of the primitive and depleted mantle are very similar. why?

Answer 8

the continental crust is such a small geochemical reservoir (0.4%), whilst the mantle is the largest 65% therefore any extraction of major elements will barely make a difference

Question 9

compared to the BSE, the DMM is depleted in what kind of elements?

Answer 9

incompatible trace elements, with increasing depletion in the DMM with increasing incompatibility

Question 10

is the continental crust compositionally similar to the mantle?

Answer 10

no

Question 11

does the continental crust have a high or low Si/Mg ratio?

Answer 11

high

Question 12

which elements is the CC enriched in compared to the mantle? these elements are also expected to be enriched in mantle partial melts too.

Answer 12

CC rich in:
Si, Ca, Al, Ti, Na, K

Question 13

how can the size of the earths depleted mantle reservoir be estimated?

Answer 13

by exploiting the complementary relationship between the CC and the depleted mantle

Question 14

what are the 3 reservoirs used when estimating the size of earths depleted mantle?

Answer 14

the continental crust, the depleted mantle, and the primitive mantle (aka the BSE). the isotope ratio of Nd (which has a known partition coefficient) is calculated for each reservoir and used to estimate a size of DM

Question 15

the conclusion that 50% of the mantle was depleted by the extraction of CC led to the view that the mantle was…???

Answer 15

stratified, with an upper depleted mantle and lower primitive mantle with limited chemical communication due to a phase boundary (~670km depth)

Question 16

why is the idea of a stratified upper and lower mantle not feasible anymore?

Answer 16

seismic tomography models indicate that slabs subduct past the 670km phase transition, indicating that whole mantle convection enables at least some chemical communication between the upper and lower mantle

Question 17

what region of the mantle do MOR sample?

Answer 17

the upper depleted mantle

Question 18

the deeper mantle may have two sources primitive mantle reservoirs bc of two reasons (think of subduction and semi-stratification)

Answer 18

1. subducted slabs reach the CMB but do not fully mix, they slowly heat up and become buoyant. they mix with lower mantle primitive magma on their ascent and form a hot spot which forms enriched OIB
2. whole mantle convection dominates but there is still likely some regions of the lower mantle which have not mixed with the upper mantle and so remain primitive in patches

Question 19

the differentiation process of Earths mantle produced two complementary reservoirs. what are they, and what elements are they enriched/depleted in?

Answer 19

1. a continental crust formed by low-degree partial melting of the mantle. this has high Si/Mg, enriched in Ca, Al, Na, K, Ti and in incompatible trace elements (makes sense because it is formed from low partial melt - most incompatible)
2. a residual mantle with low Si/Mg depleted in incompatible trace elements. This depleted upper mantle is now the source of MORB (the DMM)