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Extracting Bulk Rock Properties from Microscale
Measurements: Subsampling and Analytical Guidelines

M.C. McCanta, Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA; M.D. Dyar, Dept.
of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, USA; and P.A. Dobosh, Dept. of Computer Science,
Mount Holyoke College, South Hadley, Massachusetts 01075, USA

ABSTRACT terminology based on bulk rock character- formation conditions, as well as pseudo-
istics persists even in the twenty-first section analysis (e.g., Nutman et al., 1997;
Geologists are commonly faced with century. Thus an ironic modern conundrum Powell et al., 1998; Bucher and Frey, 2002).
questions relating to representative sam- is this: how many microanalyses of a
pling at all scales: outcrop to formation, rock are needed to accurately represent Despite the importance of bulk rock
hand sample to bulk rock, microanalysis its bulk composition? data, they are surprisingly complicated to
to overall chemistry. A new computer measure. For glassy or fine-grained rocks
model allows quantitative answers to the The problematic issue is that of scale, (e.g., pumice or shale), direct microanaly-
question of how many different micro- i.e., the ratio of sampling size to that of the ses and bulk techniques easily yield com-
analysis spots are needed to determine feature being measured. Field geologists parable results. Complications arise when
different bulk properties of a rock for any encounter this problem when they set out a rock contains xenocrysts or rock frag-
type and scale of measurement, including to sample an outcrop: how many hand ments that are not in equilibrium, or when
whole rock composition and oxidation samples will represent the bulk character- mineral chemical zonation is present. It
state. The relationships among grain size, istics of the outcrop, or even the entire should be obvious why bulk composition
glass ordering, and microbeam size, the formation? For geochemists, the scale of calculations are rarely attempted on
composition and heterogeneity of the rocks interest is that of mineral grain size rela- coarse-grained samples. For porphyritic or
studied, and the location of the analyses tive to analytical beam size. As microbeam most metamorphosed rocks, determining a
relative to textural features are all impor- techniques continue to sample smaller vol- bulk composition is possible but tedious.
tant. These variables can be grouped into umes, the scale may be that of individual Igneous rocks can be crushed and hand-
those that affect the heterogeneity (H) of atoms. Increasing resolution only exacer- picked to separate the glass for melt com-
the material versus the scale of measure- bates the understanding of bulk geological position analysis, or mass balance calcula-
ments (M) being used. For rocks where H properties. tions can be run using glass and crystalline
(grain size, glass long- or short-range compositions from electron probe micro-
ordering, or composition) <<M (beam Why are bulk rock analyses important? analysis (EPMA). Alternatively, material
size), an average of fewer than ten analy- Because magma composition is rarely, if can be ground and fused experimentally
ses will yield a representative bulk rock ever, measured in its liquid state, data from prior to bulk or microanalysis. These are
composition no matter how heterogeneous the resulting solidified materials must be time-consuming tasks, and the accuracy of
the phase assemblage. For rocks where used to back-calculate original compositions these estimation methods is difficult to
H  M, hundreds of analyses may be and conditions. In an era when microanal- quantify. In addition, the total sample vol-
needed to result in acceptable analytical ysis is routine, bulk rock composition is ume may be prohibitively small to apply
precision. Guidelines for how many sam- still an important parameter because it these methods to, as is often the case for
ples/analyses are needed to represent geo- permits correlations with other rocks and extraterrestrial materials, thereby requir-
logic materials at any scale are presented. geologically related regions (e.g., Philpotts ing a microanalytical technique.
and Ague, 2009). On an even broader
INTRODUCTION scale, knowledge of magma source region Moreover, “bulk analysis” means differ-
conditions and compositions helps define ent things for varying scales of geologic
For more than a century, geologists have the state of the mantle, provides insight processes and analytical instruments; a
used bulk analyses (e.g., Bowen, 1928; into the geochemistry of crystallization “bulk” analysis for one application may
Daly, 1933; Yoder and Tilley, 1962; BVSP, and ascent, and characterizes processes not be useful for another (e.g., Potts et al.,
1981) to develop frameworks and classifi- affecting composition and redox, such as 1995; Martin, 2003). EPMA routinely mea-
cations for understanding rock paragenesis assimilation or injection of a new melt sures sample sizes of 1 × 1 m; handheld
and properties. This practice has its origins (e.g., Cox et al., 1979; BVSP, 1981; Raman or laser-induced breakdown spec-
in the tradition of wet chemistry, which Asimow, 2000). Bulk rock compositions troscopy (LIBS) beam sizes can be nano-
required grams of material for analyses. and properties may also be important in meters up to centimeters; an atom probe
Despite the now-widespread availability of sedimentary and metamorphic rock studies may have sub-nanometer spatial resolution
modern microanalytical techniques, use of to provide information on protoliths and (Fig. 1). When beam size shrinks to the

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