Geoscience Reference
In-Depth Information
several lithologies. Mixing arrays are therefore
likely to be very complex.
sections, and areas on the wings, having much
smaller degrees of melting. MORB are blends
of these various magmas. The blending process
erases the diversity of the components that go
into MORB, but nevertheless different species
of MORB have been identified (DMORB, NMORB,
EMORB, TMORB and PMORB). MORBs typically
contain orders of magnitude more
3
He than
other basalts. Lead isotopes suggest that MORB
has experienced contamination prior to erup-
tion. MORB itself is a hybrid magma.
Basalts with high Rb/Sr, La/Yb and Nd/Sm gen-
erally have high
87
Sr/
86
Sr and low
143
Nd/
144
Nd.
These can be explained by binary mixing of
depleted and enriched magmas or by mixtures of
a depleted magma and a component represent-
ing varying degrees of melting of an enriched
reservoir ore blob. The variable LIL ratios of such
an enriched component generates a range of
mixing hyperbolas or 'scatter' about a binary
mixing curve, even if there are only two iso-
topically distinct end-members. Thus a model
with only two isotopically distinct reservoirs
can generate an infinite variety of mixing lines.
In some regions, however, the inverse relation-
ship between LIL and isotopic ratios cannot be
explained by binary mixing. These regions are all
in midplate or thick lithosphere environments,
and sublithospheric crystal fractionation involv-
ing garnet and clinopyroxene might be expected
prior to eruption.
Magma mixing can happen in many different
ways. A heterogenous mantle can partially melt.
A depleted diapir can cause variable degrees of
melting of enriched components. An undepleted
or enriched magma can ascend and partially
melt a depleted layer. A fractionating (cooling
and crystallizing) depleted magma can interact
with the shallow mantle. To fix ideas, consider
a depleted magma from a depleted-mantle reser-
voir to be the parent of MORB. If this magma
is brought to a near-surface environment, it
may crystallize olivine, plagioclase and orthopy-
roxene.Ifarrestedbythicklithosphereitmay
precipitate garnet and clinopyroxene. Melts can
represent varying degrees of crystal fractiona-
tion, or varying degrees of partial melting. These
are called
evolved magmas
. A fertile blob may be a
garnet- and clinopyroxene-rich region, such as a
Mixing arrays involving ratios
Data arrays in isotope space reflect mixing
between distinct mantle or melt components,
with different isotopic and elemental concentra-
tions. The curvature of two-component mixing
curves is related to differences in the elemen-
tal abundance ratios of the components and as
such,mixingcurvescan,inprinciple,beused
to estimate the relative abundances of Sr, Nd,
Pb, He, and Os in various magmas. Linear mix-
ing arrays with little scatter can indicate sim-
ilar relative abundances of these elements in
the mantle components. For isotope arrays of
Sr, Nd, Pb and Hf, the assumption is usually
made that all components have similar elemental
concentrations.
In order to estimate trace-element concen-
trations from isotopic mixing arrays, simple
assumptions have to be made about the rela-
tive abundances of Sr, Nd, Pb and so on in the
mixing end-members. The same is true for mix-
ing arrays involving trace-element ratios. Often
it is assumed that the elemental abundances are
the same in all end-members. Such assumptions
are not valid if the mixing arrays reflect mix-
ing of melts from sources with various propor-
tions of these components. They are not valid if
the mixing components are fractionating melts,
blends of variable partial melts, or are lithologi-
cally very different (e.g. peridotite, recycled mafic
crust, sediment).
Oxygen isotopes play a key role in identifying
components because oxygen is not a minor or
trace element; the abundance of oxygen varies
little among various lithologies. Mixing rela-
tionships with oxygen isotopes therefore can
constrain the absolute abundances of various
elements in the end-members of mixing arrays.
Midocean-ridge basalts (MORB) are among the
most depleted (low concentrations of LILs, low
values of Rb/Sr, Nd/Sm,
87
Sr/
86
Sr,
144
Nd/
143
Nd,
206
Pb/
204
Pb) and the most voluminous magma
type. They erupt through thin lithosphere and
have experienced some crystal fractionation prior
to eruption. The melting region under ridges
involves sections of extensive melting and deeper
