Subduction Zones
(continued) &
Organic
Geochemistry I
Lecture 49
Sedimentary Component
•
•
•
We saw in the last lecture that trace
elements, Ce anomalies in particular,
and isotope ratios provide hints of a
sedimentary component in IAV
magma sources. 10Be (Chapter 8)
provides unequivocal evidence of
this.
Plank and Langmuir found they
could relate the degree of
enrichment of most incompatible
elements to the sediment flux of that
element. For example, the Ba/Na
and Th/Na ratios (after correction for
fractional crystallization) correlate
strongly with the Ba and Th sediment
fluxes.
Different arcs are enriched to
different degrees in these elements:
for example, the Lesser Antilles arc
has moderate Th/Na ratios but low
Ba/Na ratios. The difference appears
to be due to the difference in the
sediment flux.
Genesis of Subduction Zone Magmas
• Arc magmas are produced primarily within the ‘mantle
wedge’ overlying the subducting slab. The evidence for
this is as follows:
o Primary arc magmas differ only slightly in major element chemistry from
oceanic basalts (the typical andesitic composition results from fractional
crystallization). Thus IAV are partial melts of peridotite rather than subducted
basalt or sediment.
o Radiogenic isotopic and trace element systematics generally allow only a
small fraction of sediment (generally a few percent or less) to be present in arc
magma sources. Relatively high 3He/4He ratios in arc lavas confirm a mantle
source.
o REE patterns of IAV are consistent with partial melting of peridotite, not of
eclogite (high pressure basalt). Because the heavy rare earths partition strongly
into garnet, melts of eclogite should show steep rare earth patterns, with low
concentrations of the heavy rare earths. (Rare high-magnesium andesites, or
“adakites” with steep rare earth patterns may represent exceptions to this
rule.) It is possible that such “slab melts” were more common several billion
years ago.
Role of Dehydration
•
•
•
•
If IAV magmas are not melts of the slab, how do they
acquire the geochemical signature or “flavor” of
subducting oceanic crust and sediment?
Dehydration and migration of the evolved hydrous fluid
has long been suspected as the primary means by which
the subducting slab influences the composition of IAV
magmas.
Ba/La ratios in Marianas arc lavas plot systematically
above a mixing line between MORB and sediment
subducting beneath the arc (ODP Hole 801) on a plot of
Ba/La vs. La/Sm. The same is true of Pb/Ce vs. Th/Nb
ratios.
Elliott et al. (1997) concluded that both a hydrous fluid
and a silicate melt were involved in transport of the
sediment component. They proposed that the melt was
a hydrous partial melt of the subducted sediments.
Melting is necessary to account for the fractionation
between Th and Nb, neither of which is particularly
soluble in aqueous fluids. Furthermore, lavas with the
highest Th/Nb also show the greatest light rare earth
enrichment. Subsequent dehydration and melting
experiments confirmed the need for melting to transport
elements such as Th into the magma genesis zone of the
mantle wedge.
Melting and the Role of Water
•
•
•
•
This brings us to what is perhaps the most
fundamental question: why does melting
occur at all in an area where cold lithosphere
is descending?
The answer is water. Water lowers the solidus
of rock and leads to enhanced melting at
any given temperature compared with dry
conditions; water released by the subducting
slab migrates into the overlying hotter mantle
wedge where it induces melting.
Under water-saturated conditions, the
peridotite solidus is depressed by hundreds of
degrees compared with the “dry solidus”. At
1.5 GPa (50 km depth), peridotite begins to
melt at over 400˚C cooler temperatures than
under “dry” conditions. The effect is even
larger at higher pressure.
At pressures above ~2 GPa, ilmenite &
chlorite are stable at and above the solidus.
Nb and Ta strongly partition into ilmenite. Thus
the characteristic Nb-Ta depletion of island
arc lavas and the continental crust may be
due to residual ilmenite present during the
initial states of melting deep within the arc.
Curved dashed lines are the chlorite +
ilmenite- and amphibole-out curves.
Straight dashed lines illustrate the
progressive replacement of spinel with
garnet. The broad stippled arrow shows
the path the melts take in T-P space as
they rise through the mantle wedge.
Magma Genesis in Subduction Zones
•
•
•
•
•
Subducting sediment and hydrothermally
altered oceanic crust carry water and
incompatible elements into the mantle.
Compression during the early phases of
subduction drives off much of the unbound
water occupying pores and veins in the
subduction lithosphere. This water sometimes
emerges as “seeps” in accretionary prisms.
The subducting lithosphere is metamorphosed
as it encounters higher T and P, with water-rich
minerals progressively replaced by water-poor
ones and anhydrous ones.
The water released in these reactions rises into
the overlying mantle wedge. The wedge
immediately above the subducting slab has an
inverted thermal gradient. 10 km above the
slab, temperatures approach 1000˚C, well
above the wet solidus and melting begins.
These initial melts may contain as much as 28%
water, but as they rise, continued melting
progressively dilutes the water content.
Work over the past couple of decades has
produced evidence directly relating water
content to melting in subduction zones: the
smallest extents of melting (about 5%) occur in
H2O-poor sources and give rise to incompatible
element-rich basalts, while the highest extents
(over 20%) give rise to H2O-rich and
incompatible element-poor basalts.
Refining the Continental Crust
• Conundrum: nearly all mantle-derived magmas are mafic
(basaltic) and are poorer in SiO2 and generally richer in MgO
and FeO than the continental crust.
• If the continental crust has been produced by partial melting
of the mantle, why then is it not basaltic in composition, as is
the oceanic crust? Four possibilities:
o
✔
o
o
✓
o
Magmas have already evolved, by fractional crystallization to andesitic composition
by the time they cross the crust–mantle boundary (the Moho). The complementary
mafic cumulates are left behind in the upper mantle. This idea is not supported by
observation.
Lower crustal floundering, or delamination, may occur when continental crust is
thickened in compressional environments, such as convergent plate boundaries.
when the lower crust is transformed into eclogite. This process would preferentially
remove the mafic part of the crust, leaving a residual crust that consequently
becomes more silicic. A related process is subduction erosion. Lower crust is more
likely to be removed by subduction erosion than upper crust.
Preferential loss of Mg and Ca from continents by weathering and erosion. Mg is
then taken up by the oceanic crust during hydrothermal alteration; Ca is
precipitated as carbonate sediment. Both are returned to the mantle by subduction.
Under hotter conditions of the Archean, melting of subducting oceanic crust may
have been much more common, giving rise to silicic melting, particularly the
trondhjemite, tonalite, and granodiorite (TTG) suites common to the Archean.
However, Taylor and McLennan’s estimate of Archean crustal composition is slightly
more mafic that their estimate of present composition, which is inconsistent with this
idea.
Some Topics in Organic
Geochemistry & the Carbon
Cycle
Organic Chemistry
Organic compounds can be
thought of as a basic hydrocarbons
(C-H compounds) - which can be a
branched or unbranched chains or
rings with one or more functional
groups attached.
Simple hydrocarbons are called
alkanes had have names ending in
‘ane’.
Unsaturated Compounds
•
•
•
•
•
Compounds where all carbon atoms
have single bonds to 4 other atoms
are said to be saturated
hydrocarbons (i.e., the carbon is
hydrogen-saturated).
Carbon atoms that are double
bonded are termed olefinic units.
Compounds containing one or more
pairs of doubly bonded carbons are
said to be unsaturated hydrocarbons.
Simple unsaturated, hydrocarbons
having one double bond are named
by replacing the suffix “-ane” by “ene”.
If there are more than two double
bonds the ending becomes “adiene”, “-atriene”, and so on.
Generic names are alkene,
alkadiene, for example.
Triple carbon bonds are also possible,
in which case the suffix becomes
“yne”.
Organic Compounds in the
Environment
•
•
•
•
Almost all organic compounds
on this planet are of biological
origin.
Living matter continually sheds
these into the environment in a
variety of ways, including death
and metabolic biproducts.
Primary biological compounds,
lipids, amino acids, nucleic acids,
carbohydrates, etc. are
generally metabolized or
converted to other forms fairly
rapidly.
Those compounds that do
survive are quickly reconfigured
into humic substances.
Humic Substances
•
•
•
•
Humic substances are high molecular weight (>500 u)
compounds that are produced by partial
degradation of complex biomolecules and
recombination of these with simple biomolecules and
their breakdown products. Their exact structures are
not known, and in any case are variable.
Soluble humic substances in waters are divided into
fulvic acid and humic acid. The definition of these
two is again analytical. Humic acids are defined as
those humic substances that precipitate when the
solution is acidified with HCl to a pH of 1. Fulvic acids
are those substances remaining in solution at this pH.
Hydrophilic acids are closely related to humic
substance, but simpler with a greater number of acid
functional groups than humic substances. They are
slightly colored, highly branched, and highly
substituted organic acids.
Soil organic matter, collectively called humus,
includes biomolecules as well as humic substances.
The relatively high proportion of aromatic units
(carbon rings with alternative double C-bonds)
suggests the most important contributors to humic
substances are lignins and tannins. These are
polyaromatic substances that are quite refractory in
a biological sense. These are partially degraded by
soil microbes. Monomer or smaller polymer units may
then condense, perhaps catalyzed by clays, metal
ions, or bacteria.
Hypothetical structure of
aquatic fulvic acid.
Marine vs. Terrestrial
•
•
•
There are differences between marine
and aquatic humic substances.
Derivatives of lignin (high molecular
weight polyphenols) appear to be
important in the backbone of aquatic
humic substances, but not marine.
Marine humic acids appear to have
an even smaller proportion of
aromatic carbon than aquatic ones,
and marine fulvic acids have
essentially none (results from a
difference in marine and terrestrial
autotrophs).
Marine fulvic acids may arise by
autoxidative cross-linking of
polyunsaturated lipids, perhaps
catalyzed by light and transition
metals. Doubly bonded carbons
(olefinic groups) may be particularly
susceptible to autoxidation.
Organo-Metallic Complexes
•
•
•
•
Organic molecules readily form
complexes with metals, especially
transition metals and aluminum.
Complexation between metal ions and
organic anions is similar, for the most part,
to complexation between metals and
inorganic anions. One important
difference is that many organic
compounds have more than one site
that can bind to the metal, these are
called chelators.
An example of a natural chelator specific
to iron is enterobactin. Such Fe-specific
chelators may have stability constants in
excess of 1030.
A large fraction of at least some trace
metals (particularly Fe, Cu, and Zn) is
complexed by organic ligands in streams,
lakes, and ocean surface water. In some
cases, more than 99% of the metal in
solution is present as organic complexes.
Hydrophobic Absorption
•
•
Since water molecules normally orient
themselves in a manner that reduces
electrostatic repulsions and minimizes
interaction energy, the presence of a
large nonpolar molecule is energetically
unfavorable. As a result, solution of such
substances, called hydrophobic
substances, in water is associated with a
large ∆Hsol and large ∆Gsol. Thus
hydrophobic substances have low
solubility in water. A second characteristic
is they are readily absorbed on to
nonpolar surfaces, such as those of
organic solids.
Hydrophobic adsorption occurs because
of incompatibility of the hydrophobic
compound with water. When a
hydrophobic molecule is located on a
surface, water molecules are present on
one side only, and there is less disruption of
water structure than when water
molecules are located on both sides. Thus
the interaction energy is lower when the
substance is located on a surface rather
than in solution.
Origin of Petroleum &
Natural Gas
Sedimentary Organic Matter
• Despite the abundance of life in water, most sedimentary
rocks contain rather little organic matter (<1%) because
virtually all organic is subsequently to CO2 by respiration, a
process called remineralization. Indeed, most of the organic
carbon synthesized in the oceans and deep lakes never
reaches the sediment; it is consumed within the water column.
• Organic carbon that does to reach bottom is consumed by
organisms living on and within the sediment. Concentrations
of bacteria in the surface layers of marine sediments are
typically in the range of 108 to 1010 cells per gram dry weight.
• These observations raise the question of why any organic
matter survives. Why do most sediments contain some organic
matter? How does it escape bacterial consumption? And why
do some sediments, particularly those that give rise to
exploitable petroleum and coal, contain much more organic
matter? What special conditions are necessary for this to
occur?
Preservation of Sedimentary Organic Matter
• particulate remains of phytoplankton are the main source
organic matter in most marine and many aquatic sediments.
• Factors that affect preservation of these remains include the flux
of organic matter to the sediment, bulk sediment accumulation
rate, grain size, and availability of oxygen.
o
o
o
o
Flux of organic matter depends on productivity, which depends on nutrient abundance
(N, P, Si, Fe, etc). plus the flux of ‘imported’ or ‘allochthonous’ organic matter (e.g.,
supplied by a river to a coastal sea).
Organic matter falling though the water column is rapidly remineralized in the water
column. Hence the greater the water depth, the less organic matter reaches the
sediment.
Organic carbon concentrations are inversely correlated with grain-size for several
reasons. First, low-density organic particles can only accumulate where water velocities
are low enough to them to settle out. Second, a significant fraction of the organic
matter in sediments may be present as coatings on mineral grains. Small grains have
higher relative surface areas and therefore higher organic content. Third, the low
permeability of fine-grained sediments limits the flux of oxygen into the sediments into
the sediment.
The availability of oxygen is among the most important factors. Simply put, the
preservation of significant amounts of organic matter in sediment requires that the burial
flux of organic matter exceeds the flux of oxidants. Where the burial flux of organic
carbon exceeds the downward flux of oxygen aerobic respiration must cease. Such
conditions are unlikely in the open ocean.
Diagenesis of Marine Sediment
• Marine sedimentary organic matter is the source of
most (not all) petroleum.
• Diagenesis in the context of organic matter refers to
biologically induced changes in organic matter
composition that occur in recently deposited
sediment.
o These changes begin before organic matter reaches the sediment, as
organic matter sinking through the water column is fed upon by both the
macrofauna and bacteria. Roughly 98% of the organic matter reaching
the sediment is already degraded.
o Burial by subsequently accumulating sediment eventually isolates it from
the water. Where the burial flux of organic matter is high enough, oxygen
is eventually consumed, but respiration continues through fermentation, in
which reactions use an internal source of oxidants. An example familiar to
brewers and vintners is the fermentation of glucose to alcohol:
C6H12O6 → 2 C2H5OH + 2 CO2
Methane Clathrates
• In anoxic environments,
methanogenic bacteria
produce methane, e.g.:
• 2CH2O → CH4 + CO2
• Methane can react with
water to form solid icelike methane clathrate,
which consists of a
methane molecule
locked in a cage of
hydrogen-bonded water
molecules with the overall
composition of
approximately
CH4(H2O)5.75.
Clathrate Stability
•
•
•
•
Depending upon water temperature,
methane clathrate becomes stable at water
depths > 300–400 m, and is favored by
increasing pressure and decreasing
temperature.
An enormous mass of methane clathrate, in
the range of 500–2500 carbon gigatons
(700–3000 Gt methane), appears to exist in
continental margin sediments). A smaller
mass is also present in the deep permafrost
of the Arctic tundra.
On the positive side, the amount of methane
clathrate likely exceeds the total
recoverable natural gas in other deposits by
a factor of 2 to 10 and compares with an
overall global inventory of fossil fuels of 5000–
15,000 Gt carbon.
On the negative side, methane clathrate is
only marginally stable and destabilization of
methane clathrate, through ocean or
atmospheric warming or geologic events
such as submarine landslides, could release
massive quantities of methane, a powerful
greenhouse gas, and this could have
profound climatic consequences.