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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. 10 Be (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 3 He/ 4 He 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 H 2 O-poor sources and give rise to incompatible element-rich basalts, while the highest extents (over 20%) give rise to H 2 O-rich and incompatible element-poor basalts.
Refining the Continental Crust Conundrum: nearly all mantle-derived magmas are mafic (basaltic) and are poorer in SiO 2 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 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. o 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. o 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. o 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 10 30. 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 ∆H sol and large ∆G sol. 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 CO 2 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 10 8 to 10 10 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 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). o 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. o 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. o 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: C 6 H 12 O 6 → 2 C 2 H 5 OH + 2 CO 2
Methane Clathrates In anoxic environments, methanogenic bacteria produce methane, e.g.: 2CH 2 O → CH 4 + CO 2 Methane can react with water to form solid ice- like methane clathrate, which consists of a methane molecule locked in a cage of hydrogen-bonded water molecules with the overall composition of approximately CH 4 (H 2 O) 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.