GEOS 310 Geos communications
Scientific papers
1. abstract
2. introduction
3. (optional) background for research – and in the case of geology papers, geologic
background
4. techniques, methodology
5. results
6. interpretations and/or discussion
7. (optional) conclusions
Figures, tables, supplementary data.
IMPORTANT RULES:
Give all the major results and upshot of interpretations in abstract
Set the problem up in introduction – why should the reader care?
Separate the results from interpretations – what you find from what you think that
means.
Clearly present what the paper has resolved – hypotheses that can be ruled out.
Separate that from “maybe’s”, things that could be interpreted from your results.
If you have room for conclusions, tell the reader again what he/she should have
gotten out of the abstract.
Figures – don’t make them too complicated – try to keep them readeable
EXAMINE the paper handed out last week.
The paper we all read- questions:
Is it understandable, does it make a clear point?
Is it explaining : why is the problem important?
How is this paper new and important?
What was the hypothesis to test?
How was it tested (what innovative ideas did the authors bring into the
research)
Why was the project conducted there as opposed to somewhere else?
What have we learned from this research?
An example of an introduction. Give it a five minute read and tell me if it’s:
a. awesome, b. very good, c- average, d- hard to understand, e- it stinks.
Introduction
Cordillera-type batholiths not only represent tremendous tracts of petrogenetically young sialic basement
extending along the entire western margin of the Americas, but also the best Phanerzoic analogues to the average
upper-crustal composition in Archean cratons. Observations of continental arc magmatism and petrogenetic models are
made principally from upper- to mid-crustal level exposures at the Earth’s surface, and complementary experimental
studies of plausible source materials and the liquid line of decent of arc magmas (e.g., Wyllie, 1984). However, these
observations can not uniquely answer a number of first order questions with regard to the origin of large granitic
batholiths such as the Sierra Nevada batholith. The relative importance of mantle wedge, pre-existing continental crust,
or subducting oceanic slab components in the formation of granitic batholiths still remains an outstanding question. The
Sierra Nevada and other Cordilleran batholiths have characteristic and systematic geographic variations of isotopic
ratios (Sr, Nd, Pb and O), trace and some major elements which have been attributed to variation in such source
components (Kistler and Paterman, 1973, 1978; Silver and Taylor, 1978; DePaolo, 1981; Farmer and DePaolo, 1983;
Solomon and Taylor, 1986; Ague and Brimhall, 1988a; Silver and Chappel, 1988; Kistler, 1990; Chen and Tilton,
1991; Pichett and Saleeby, 1994). The interpretations of such isotopic patterns fall into two fundamentally different
schools: (1) the isotopic heterogeneities reflect mainly the deep-crustal and/or upper-mantle source regions from which
the batholith were generated (Kistler and Paterman, 1973, 1978; Silver and Chappel, 1988; Hildreth and Moorbath,
1988; Ducea and Saleeby, 1998c), and (2) the isotopic heterogeneities are derived primarily from fractionation,
crystallization and assimilation of heterogeneous mid- to upper-crustal rocks in shallow- to mid-crustal
magma(DePaolo, 1981; Saleeby, 1990; Clement-Knott, 1992). In support of this second school of thought, for example,
Halliday (1983) demonstrated the contamination of mid- to high-crustal level granitic plutons of Scotland by
incorporating partial melt of country rock metasediments. By studying migmatitic, high-grade metamorphic pendant
rocks that are in direct contact with the invading batholithic rocks which promoted the partial melting, we can evaluate
the importance of shallow- to mid-crustal level assimilation of metasedimentary rocks, one of classic geologic
problems in batholithic granite-bearing and high-grade metamorphic terranes (Sederholm, 1967).
In the Lake Isabella area of the southern Sierra Nevada, a well-preserved roof pendant composed of
continental margin metasedimentary rocks is engulfed in granodiorite plutons emplaced over a relatively short time
span (~2Ma). Excellent exposures, a well-defined migmatite complex, and a coherent protolith stratigraphic sequence
for the pendant provide an unparalleled opportunity to characterize the geochemical and isotopic compositions of the
invading plutons and pre-existing continental crustal rocks, and to further constrain the relative importance of each
component in the formation of the batholith. In this paper, We present new detailed Sr and Nd isotopic data from this
igneous and high-grade metamorphic complex. These data are used in conjunction with regional published isotopic
data of the batholith to evaluate the extent of the anatectic contributions from metasedimentary roof pendant rocks in
the formation of the greater Sierra Nevada batholith. Specifically, we have examined the geochemical relationships
between the partial-melting products of the roof pendant represented by leucosomes in an extensive migmatitic
metapelite unit, and the adjacent Goat Ranch granodiorite. We will address: (1) what is the extent of mixing between
metasedimentary-derived melts and batholithic magmas; and (2) Does this process have a significant effect in
generating within-pluton isotopic heterogeneities at the observed crustal level?
Another Introduction:
Significant areas of the continents expose metamorphic rocks that at some stage were buried to depths equal
or greater than the normal thickness of the continental crust (30-40 km). The processes by which high-pressure rocks
reach great depths are generally accepted to be distributed shortening and thickening of the crust, and lithospheric
subduction [e.g., 4]. How rocks subsequently return to the surface is an intriguing question that has generated
considerable interest among geologists [5]. Understanding the exhumation mechanisms of orogenic roots requires
quantification of the pressure-temperature-time (P-T-t) paths of these rocks. Commonly the P-T paths are deciphered
without age constraints. The cooling and the related exhumation rates are commonly derived from plots of selected
mineral ages versus assumed or inferred closure temperatures of the specific geochronologic systems. Exhumation rates
determined this way vary by approximately two orders of magnitude [e.g. 6, for a review]. Some of this variability is
related to uncertainties in closure temperatures. Other additional problems are whether a mineral age dated by a
particular system corresponds to peak metamorphic age, cooling age, or in some cases, is an average of various ages in
rocks with complex, polyphase histories [7,8].
Closed form analytical expressions of closure temperature (Tc) and Tc-profile in a mineral, which is
surrounded by a homogeneous infinite matrix during cooling, were derived by Dodson [9,10]. However, although it is
commonly overlooked, Dodson’s Tc formulations are applicable only to cases where the composition of the mineral,
even at the core, is significantly removed from what it was at the onset of cooling. Consequently this formulation is not
applicable to the cases of slowly diffusing species or when the diffusion distance is small compared to the grain size.
Ganguly and Tirone [11] extended Dodson’s formulation to include the cases of the slowly diffusing species, such as
Sm and Nd in garnet, and developed [1,11] mathematical formulations and algorithms that permit retrieval of cooling
rate from the cooling age profile, or from the difference between the core and bulk ages of a single crystal, and its
initial temperature (To), provided that the diffusion kinetic properties of the decay system are known. The diffusion
kinetics of Sm and Nd in almandine garnet have recently been determined by Ganguly et al. [3]. These data, along with
the formulation in [1], permit quantitative use of the spatial variation of Sm-Nd age within a garnet crystal as a high
temperature thermo-chronometer.
A significant practical problem in the application of the Ganguly-Tirone formulation [1] is the difficulty in
obtaining single garnet age profiles, or the average (Sm-Nd) age of the garnet core on a sufficiently small spatial scale
(50% of the grain size) such that it is distinctly different from the bulk or rim (Sm-Nd) age. In this study we overcome
this difficulty by (a) using a milling device that can sample to micron-scale resolution, and (b) analyzing small Nd
quantities as NdO+ via thermal ionization mass spectrometry.
Spatially resolved Sm-Nd dating of garnet single crystals has been conducted previously [e.g. 12-15].
However, these studies utilized large crystals that preserved prograde growth zoning with the objective of determining
the rates of crystal growth or unraveling the timing of poly-metamorphic events. This is the first study aimed at
recovering high temperature cooling rate from dating core and rim of single garnet crystals.
Better?
Assignment for next time:
1. Read a Geology paper of your choice and give it a one paragraph review on
presentation
2. Write a one paragraph “abstract” of the definition and significance of basalt on
our planet, as if you had just discovered this rock and are writing a paper trying to
convince the reader that it is a relevant rock.