lherzolite - This is a type of peridotite that consists chiefly of olivine but which also contains pyroxene and Al-bearing phases such as plagioclase, Al-spinel, or Al-garnet. Lherzolites are considered to be "fertile" mantle rocks because they can produce basaltic magmas during partial melting. Wehrlites are less common but potentially also fertile peridotites that are dominated by olivine and clinopyroxene. In the paper, nakhlites are described as wehrlites, but technically wehrlites contain more olivine than clinopyroxene, and the opposite is true for nakhlites. The latter are more accurately described as olivine clinopyroxenites.
harzburgite - This is a type of peridotite that consists chiefly of olivine and low-Ca pyroxene. Harzburgites are considered to be "infertile" mantle rocks that formed as residues of previous melt extraction. Dunites are also infertile mantle rocks that consist mainly of olivine.
REE, LREE, HREE - These acronyms stand for "rare earth elements", "light- rare-earth elements" (La to Eu, atomic numbers 57 to 63), and "heavy-rare earth elements" (Eu to Lu, atmic numbers 63 to 71). These elements are commonly used to study melting processes because of the regular variation of mineral-melt partition coefficients (D) from light to heavy REE. Commonly, their abundances are shown in REE diagrams which plot CI-chondrite-normalized concentrations against atomic number. During partial melting of an ultramafic mantle rock, all REE (except for divalent Eu in plagioclase) are incompatible (D < 1), and thus all become enriched in the melt. This is especially true for the LREE, which have lower D-values than for the HREE in such phases as olivine, orthopyroxene, clinopyroxene, and garnet. (Only plagioclase is different in that it prefers LREE compared to HREE). The residues of partial melting, such as olivine and orthopyroxene, will have HREE-enriched patterns, complementary to the the HREE-depleted patterns of the partial melts.
time-integrated LREE enrichments or depletions - This concept is discussed on p. 767 in connection to epsilon Nd-143 values and gives us information about how the source region (e.g., mantle) composition of an igneous rock evolved with time. The epsilon Nd-143 value refers to the 143Nd/144Nd ratio in a sample relative to what a rock with chondritic Sm/Nd would have at the same time:
epsilon Nd-143 = [(143Nd/144Nd)sam/(143Nd/144Nd)ch - 1] * 10,000
where sam = sample with a given age, and ch = chondrite value at the same age.
143Nd/144Nd ratios change with time because 147Sm decays to 143Nd with a half-life of ~ 700 Ga. The rate of change will depend on the Sm/Nd ratio in the source region of the rock.
An epsilon Nd value of ~0 represents a chondritic
143Nd/144Nd ratio; an epsilon Nd value < 0 represents a lower-than-chondritic
143Nd/144Nd ratio; and an epsilon Nd value > 0 represents a greater-than
chondritic 143Nd/144Nd ratio. So how is this related to Sm/Nd ratios?
An igneous rock with epsilonNd > 0 must have been derived from a source
region that a high Sm/Nd over time, to account for the excess 143Nd.
This would imply a source that was LREE-depleted over a large part of its
history, because Nd has a lower atomic number than Sm. Conversely,
low values of epsilon Nd imply derivation of an igneous rock from a source
that was LREE-enriched over a large part of its history. We can summarize
these relationships as follows:
| high time-integrated Sm/Nd in source... | ... is recorded by large positive values of epsilon Nd | ... and signifies LREE depletion in source relative to chondrites |
| chondritic time-integrated Sm/Nd in source... | ... is recorded by chondritic values of epsilon Nd | ... and signifies chondritic source composition |
| low time-integrated Sm/Nd in source... | ... is recorded by large negative values of epsilon Nd | ... and signifies LREE enrichment in source relative to chondrites |
Note that epsilon values do not necessarily have any relationship
to the current Sm/Nd ratio of a rock. Rather, they refer to the time-integrated
Sm/Nd values of the source region that produced the rock. So we can
utilize them to see how the composition of the source changed over time.
The same kind of analysis can be applied to other radiometric systems such
as Rb-Sr. This is a powerful technique to understand how the compositions
of mantles on Earth, Mars, and other differentiated bodies evolved.