7/31/2019 Geochem Exercise Set 1
2/6
2. A second worksheet in the Excel workbook contains major
element, trace element, and Sr, Nd,
and Pb isotope analyses from 15 samples of lava flows erupted
from three different volcanoes
during the Quaternary (last 1.8 Ma).
A. The geochemical behavior of K and U differ markedly when
temperatures are hot enough to
volatilize K (leaving U as a refractory solid). In contrast, K
and U behave similarly at lowertemperatures (
7/31/2019 Geochem Exercise Set 1
4/6
3. U-Pb geochronology. Using the equations:
D = N(e8t -1)206Pb = 238U(e
8t -1); 8 = 1.5512x1010 yr-1
207Pb = 235U(e8t -1); 8 = 9.8485x1010 yr-1
Construct a U-Pb concordia diagram for 4560 Ma of Earth History.
Begin at an intial time of
0 years and increment time in 200 million year steps.
Add the following data points obtained by ion microprobe from
zircon crystals in the Jack Hills
conglomerate, Western Australia:
206Pb/238U 207Pb/235U.928 69.5
.919 68.2
.797 58.8
.304 21.4
.496 36.2
.200 14.9
.965 71.9
.929 68.6
.768 54.5
.897 67.2
.498 35.4
.356 25.2
.968 74.6
.683 49.6
Draw a best fit line through the array of data. What can you
conclude about the age and
history of these zircons?
7/31/2019 Geochem Exercise Set 1
5/6
4. Partial melting of the mantle
A third spreadsheet in the workbook available on the course
website is set up to calculate the
chondrite-normalized REE patterns in liquids generated from the
batch modal melting of
peridotite (equation 1 from lecture 5). It does this by
tabulating the bulk distribution coefficient,
D, for each rare earth relement based on the modal percentage of
the minerals comprising thesource rock. The modal percentages can
be adjusted in the spreadsheet (they should total 100%)
to model the melting of different mineral assemblages. Once you
have chosen the modal
percentages, you can vary F, the fraction of melt produced, to
generate an REE pattern (black
values in diagram). The spreadsheet uses a batch melting
equation (Wilson, p. 63, Eq. 1)
Using the spreadsheet, answer the following questions:
A. MORB that forms the Pacific Ocean crust typically has a light
rare earth depleted pattern
(NMORB, see Wilson, 1989, p. 142, Fig. 5.42). Given the REE
concentrations of N-MORB
in the spreadsheet (values and REE pattern in red), is it
possible to melt peridotite with
chondritic abundances of REE (values in green) to produce
Pacific MORB? Use thefollowing modal % of minerals in the
peridotite: 50% Olivine, 10% Opx, 40% Cpx, and try
varying the percent melt between 10 and 30% (F = 0.1 to
0.3).
B. Try adding 10% modal garnet to the source and reducing the
olivine to 40%. What is the
effect of garnet on the REE patterns of the liquid produced?
What controls the observed
effect? Are these patterns more, or less, similar to N-MORB than
the liquids you were able to
produce in in part A?
C. The column in blue color allows you to adust the
concentration of the different REE in thesource rock by factors
either less than or greater than the condritic values. At a fixed
value of
F = 0.25 (as we will see later, most petrologists think about
20-30% melting of the mantle is
necessary to generate MORB), determine the factors that you need
to adjust the source
concentrations by, in order to generate the MORB.
D. Explain why the chondritic mantle fails to produce MORB
(hint: in lecture 2 and lecture 4
we reviewed the connection between chondritic meteorites and the
earth and radiogenic
isotopic and trace element evididence concerning the long-term
evolution of the earths
mantle.
