GEOL 567
Global Geochemical Systems
Spring 2003
Name ___________________________
Problem Set Three
Isotope and Trace Element Models: Bulk Assimilation and AFC
50 points
The data for a hypothetical suite of comagmatic lava flows are given on Data Page 1.
These samples are all less than 1 Ma old. (This means that the measured isotopic ratios can be
considered to be initial values and no age correction must be applied before modeling. But see
Part Three before starting to rejoice!) Reasons to suspect a comagmatic relationship could be
that these hypothetical lavas all erupted into the same hypothetical basin and show similar major
element, trace element, and isotopic trends.
Your assignment is to decide what magmatic process was important in the evolution of
this suite. Closed-system fractional crystallization can be ruled out because of the isotopic
variation. And for simplicity, we will rule out magma mixing, so that you can concentrate on
two processes: bulk assimilation (mixing of parental basalt and crust) and assimilation-fractional
crystallization (AFC).
You are also expected to decide which of three hypothetical crustal lithologies is the most
likely assimilant. The assimilants are young (100 Ma) upper crust, old (1700 Ma) upper crust,
and old (1700 Ma) lower crust.
Your assignment has three sections. First, calculate six isotopic-trace element models for
this suite of lavas, as described below. Second, write a paper that discusses the results. The
paper must contain the following sections:
Abstract
Introduction (be creative! You may make up a name and geologic setting for these rocks
if you want!)
Data presentation
Results: presentation and comparison of the models, selection of the best model
Discussion: Implications of your models and a discussion of the stratigraphic-chemical
relationships described below.
Conclusion: List your major conclusions
A reference list is not necessary (unless you want to include a hypothetical one, in the
spirit of the problem set....)
The third part of the set is to calculate the initial values for a set of isotopic data. Turn these in
with your paper, but you don’t need to include it as a topic in your paper.
Assimilation vs. AFC Models
Calculate assimilation and AFC curves using the basalt and each of the three crustal
lithologies as end-members. This can be done with a calculator or speadsheet. For AFC
calculations, use the following parameters.
DSr = 1.5 (plag-dominated)
DNd = 0.1 (no LREE-bearing minerals)
DRb = 0.1 (if you really get into this)
DSm = 0.11 (if you really get into this)
rupper crust = 0.2
rlower crust = 0.5
Stratigraphic-Chemical Relationships
As presented, each lava type (basalt, andesite, and dacite) erupted during much of the life
of the system. How would the physical interpretation of your model be different if the basalts
were erupted only early (1 - 0.5 Ma), the andesites only intermediate in time (0.5 - 0.3 Ma), and
the most evolved lavas late in the life of the system (0.3 - 0.1 Ma)?
Initial Isotopic Ratios and εNd
The isotopic data is presented on Data Page 2 as measured ratios for samples between 70
and 72 Ma old. Calculate initial 87Sr/86Sr, 143Nd/144Nd, and εNd values for the ages given (crustal
lithologies at 71 Ma). Use the trace element concentrations on Data Page 2 and the following
information.
Rb = 1.42 X 10-11 /yr
87
Rb/86Sr = 2.9 (Rb/Sr)
Sm = 0.654 X 10-11 /yr
147
Sm/144Nd = 0.604 (Sm/Nd)
(143Nd/144Nd)CHUR,t = 0 = 0.512638
(147Sm/144Nd)CHUR = 0.1967
εNd = 104*((143Nd/144Ndi/143Nd/144NdCHUR,t)- 1)
PROBLEM SET THREE EVALUATION CRITERIA
Technical aspects
____ Three assimilation models are correctly calculated. (6)
____ Three AFC models are correctly calculated. (6)
____ All models are plotted on at least the following diagrams: 87Sr/86Sr - Nd (or 143Nd/144Nd),
Sr - 87Sr/86Sr, Nd - Nd (or 143Nd/144Nd). (6)
____ Initial values of the isotopic data in part three are correctly calculated. (5)
Content
____ The abstract introduces the problem, presents data, presents the models, selects a model,
discusses the stratigraphic-chemical relationships, and draws a conclusion. There are lots
of numbers in the abstract! (4)
____ The paper presents the 6 models. (3)
____ The paper supports the writer’s choice of models with calculations, figures, and text. (4)
____ The differences between AFC and assimilation models are explained. (4)
____ The difference in interpretation of the two possible stratigraphic-chemical relationships is
discussed. (4)
____ The data, models, and interpretations are all presented separately. (4)
____ Standard edited English is used. (2)
____ The paper has been proofread and edited. (2)