Chapter 14
Magmas: windows into the mantle
If our eye could penetrate the Earth if one mixes together all the materials known to and see its interior from pole to enter or leave the mantle, one can obtain cosmic ratios of the refractory elements except for Mg/Si pole, from where we stand to the and other elements likely to enter dense refrac antipodes, we would glimpse with tory residues of mantle differentiation. Likewise. horror a mass terrifying riddled with this mix is deficient in the siderophile elements, fissures and caverns which are plausibly in the core, and the volatile elements, which were probably excluded from Tel/us Theoria Sacra ( 1694), Thomas Burnet the beginning. Magmas are an important source of infor Various ldnds of magmas, ranging from mido mation about conditions and composition of cean-ridge basalts (MORE) to ldmberlite (KIMB) the Earth's interior. The bulk composition. trace emerge from the mantle. Material is also recycled element chemistry, isotope geochemistry and into the mantle; sediments, oceanic crust, delam volatile content of magmas all contain infor inated continental crust, water and peridotite. mation about the source region and the pro Parts of the mantle are inaccessible to direct cesses that have affected the magmas before sampling and we can only infer their composi their eruption. Mantle fragments, or xenoliths, tions by subtracting off the sampled components found in these magmas tell us about the mate from what is thought to be the original composi rial through which the magmas have passed on tion. There is no assurance that we are currently the way to the surface. Representative composi receiving samples from all parts of the Earth, tions of various magmas are given in Table 14.1. although this is the hope of many geochemists. Three principal magma series are recognized: There is reason to believe that the chem.ical strat tholeiite, calc-alkaline and alkali. TI1e various ification of the mantle is irreversible, and that rock types in each series may be related by vary there exist hidden 'reservoirs' (a bad term for a ing degrees of partial melting or crystal sepa permanent repository). Nevertheless, the accre ration. The dominant rock type is tholeiite, a tional stratification of the Earth may have placed fine-grained dark basalt containing little or no most of the low-melting point lithophile ele olivine. Tholeiites are found in both oceanic ments within reach. In fact, one can construct a and intraplate settings. Those formed at mido plausible compositional model for the mantle by cean ridges are low-potassium and LIL-depleted isolating the dense refractory depleted products and relatively high in AI, while those found on of differentiation in the deep mantle. This would oceanic islands and continents are generally LIL make the Mg. Si and Fe contents of the mantle enriched. Ridge tholeiites differ from continental uncertain but these can be constrained by geo and island tholeiites by their higher contents of physics, mineral physics and cosmology. In fact, AI and Cr. low contents of large-ion lithophile MAGMAS: WINDOWS INTO THE MANTLE 169
Table 14.1 I Representative compositions of basalts and andesites. Tholeiite Oceanic Cont. Rift Continental Alkali Alkali Andesite Ridge Arc Rift Island High-AI Basalt Basalt Arc Low-K High-K
Si02 49.8 51.1 50.3 49.4 51.7 47.4 47.8 57.3 59.5 60.8 Ti02 1.5 0.83 2.2 2.5 1.0 2.9 2.2 0.58 0.70 0.77 AI 20 3 16.0 16.1 14.3 13.9 16.9 18.0 153 17.4 17.2 16.8 Fe20 3* 10.0 I 1.8 13.5 12.4 11.6 10.6 12.4 8.1 6.8 5.7 MgO 7.5 5.1 5.9 8.4 6.5 4.8 7.0 3.5 3.4 2.2 CaO 11.2 10.8 9.7 10.3 11.0 8.7 9.0 8.7 7.0 5.6 Na20 2.75 1.96 2.50 2.13 3.10 3.99 2.85 2.63 3.68 4.10 K20 0.14 0.40 0.66 0.38 0.40 1. 66 1. 31 0.70 1.6 3.25 Cr 300 50 160 250 40 67 400 44 56 3 Ni 100 25 85 ISO 25 50 100 15 18 3 Co 32 20 38 30 50 25 60 20 24 13 Rb I 5 31 5 10 33 200 10 30 90 Cs O.Q2 0.05 0.2 0.1 0.3 2 >3 ~0.1 0.7 1.5 Sr 135 225 350 350 330 800 1500 2 15 385 620 Ba II 50 170 100 115 500 700 100 270 400 Zr 85 60 200 125 100 330 800 90 110 170 La 3.9 3.3 33 7.2 10 17 54 3.0 12 13 Ce 12 6.7 98 26 19 50 95 7.0 24 23 Sm 3.9 2.2 8.2 4.6 4.0 5.5 9.7 2.6 2.9 4.5 Eu 1.4 0.76 23 1.6 1.3 1.9 3.0 1.0 1.0 1.4 Gd 5.8 4.0 8.1 5.0 4.0 6.0 8.2 4.0 3.3 4.9 Tb 1.2 0.40 1.1 0.82 0.80 0.8 1 2.3 1.0 0.68 1.1 Yb 4.0 1.9 4.4 1.7 2.7 1.5 1. 7 2.7 1.9 3.2 u 0.10 0.1 5 0.4 0.18 0.2 0.75 0.5 0.4 0.7 2.2 Th 0.1 8 0.5 1.5 0.67 1.1 4.5 4.0 1.3 2.2 5.5 Th/U 1.8 3.3 3.8 3.7 5.9 6.0 8.0 3.2 3.1 2.5 K/Ba 105 66 32 32 12 28 16 58 49 68 K/Rb 1160 660 176 630 344 420 55 580 440 300 Rb/Sr 0.007 0.022 0.089 0.014 0.029 O.Q45 0. 13 0.046 0.078 0.145 La/Yb 1.0 1.7 10 4.2 3.7 II 32 1.1 6.3 4.0
*Total Fe as Fe 2 0 3 . (After Candie, 1982.)
(LIL) elements (such as K, Rb, Cs, Sr, Ba, Zr, U, The world-encircling midocean-ridge system Th and REE) and depleted isotopic ratios (i.e. accounts for more than 90% of the material flow- low integrated Rb/Sr, U/Pb etc. ratios). Tholeiitic ing out of the Earth's interior. The whole ocean basalts have low viscosity and flow for long dis- floor has been renewed by this activity in less tances, constructing volcanic forms of large area than 200 Ma. Hotspots represent less than 10% of and small slope. The major- and trace-element dif- the material and the heat flow. The presence of a ferences between the tholeiitic and calc-alkaline crust and core indicates that the Earth is a differ- suites can be explained by varying proportions entiated body. Major differentiation was contem- of olivine, plagioclase and pyroxene crystallizing poraneous with the accretion of the Earth. The from a basaltic parent melt. high concentrations of incompatible elements in 170 MAGMAS: WINDOWS INTO THE MANTLE
the crust and the high argon-40 content of the reservoirs and the geophysical evidence for gross atmosphere indicate that the differentiation and layering suggest that differentiation and chemi degassing has been relatively efficient. The pres cal stratification may be more important in the ence of helium-3 in mantle magmas shows, how long run than mixing and homogenization. ever, that outgassing has not been 100% effi Fluids and small-degree melts are LIL cient. The evidence fi·om helium and argon is enriched, and they tend to migrate upward. Sedi not contradictory. Helium dissolves more read ments and altered ocean crust, also LIL-enriched, ily in magma than argon and the heavy noble re-enter the upper mantle at subduction zones. gases so we expect helium degassing to be less TI1us there are several reasons to believe that the effective than argon degassing. We also have no shallow mantle serves as a scavenger of incompat constraint on the initial helium content of the ible elements, including the radioactive elements mantle or the total amount that has degassed. (U, Th and K) and the key tracers (Rb, Sr, Nd, Sm
So helium may be mostly outgassed and what we and, possibly, Pb and C02 ). The continental crust see is just a small fraction of what there was. TI1e and lithosphere are commonly assumed to be the evidence for differentiation and a hot early Earth main repositories of the incompatible elements, suggest that much of the current magmatism is but oceanic islands, island arcs and deep-seated a result of recycling or the processing of already kimberlites also bring LIL-enriched material to processed material. TI1e presence of helium-3 in the surface. Even a moderate amount of LIL in the mantle suggests that a fr-action of the magma the upper mantle destroys the arguments for a generated remains in the mantle; that is, magma primitive lower mantle or the need for a deep removal is inefficient. radioactive-rich layer. It is becoming increasingly clear that all Generalities magma are blends of melts from an inhomoge In most models of basalt genesis, it is assumed nous mantle. In fact, the source of magma has not only that olivine and orthopyroxene are con been described as a statistical upper man tained in the source rock but that these are tle assemblage, or SUMA. Various apparent the dominant phases. Petrologic and isotope data inconsistencies between the trace element ratios alone, however, cannot rule out a source that and isotopic ratios in basalt can be understood is mainly garnet and clinopyroxene. The eclogite if (1) partial melting processes are not 100% (garnet plus clinopyroxene) source hypothesis dif efficient in removing volatiles and incompati fers only in scale and melt extraction mechanism ble elements fi·om the mantle; (2) basalts are from the fertile peridotite hypothesis. In the fer hybrids or blends of magmas from depleted and tile peridotite, or pyrolite, model the early melt enriched components; and (3) different basalts ing components, garnet and clinopyroxene, are represent different averages, or different volume distributed as grains in a predominantly olivine sampling. In a chemically heterogenous man orthopyroxene rock. On a larger scale, eclogite tle mixing or contamination is inevitable. Even might exist as pods or blobs in a peridotite rnan with this mixing it is clear fi·om the isotopes tle. Since eclogite is denser than peridotite, at that there are four or five ancient components least in the shallowest mantle, these blobs would in the mantle, components that have had an tend to sink and coalesce. In the extrerne case an independent history for much of the age of the isolated eclogite-rich layer might form below the Earth. lighter peridotite layer. Such a layer could form Some components in a heterogenous man by crustal delamination, subduction or by crystal tle melt before others. Material rising from one settling in an ancient magma ocean. Melts from depth level advects high temperatures to shallow such blobs or layers still interact with olivine and levels, and can cause melting from the material orthopyroxene. If eclogite blobs are surrounded in the shallow mantle. It is not necessary that by refractory peridotite they can extensively melt material melt itself in situ. Cold but fertile sub upon adiabatic decompression, without the melt ducted or delaminated material will melt as it draining out. The isotopic evidence for isolated warms up to ambient mantle temperature; excess MAGMAS: WINDOWS INTO THE MANTLE 171 temperatures are not required. Thus, there are a the only complement - to the depleted MORB variety of ways of generating melting anomalies. source. Early mass-balance calculations based on In a stratified mantle, without transfer of this premise, and on just a few elements and material between layers, convection in a lower isotopes, suggested that most of the mantle layer can control the location of melting in the remains undepleted or primitive. The depleted overlying layer. In a homogenous mantle, the reservoir was assumed to occupy most or all of high temperature gradient in the thermal bound the upper mantle. Since the continental crust is ary layer between layers is the preferred source the only enriched reservoir in this three-reservoir of magma genesis since the melting gradient is model, magmas that have high LIL-contents and larger than the adiabatic gradient in a homoge 87Srf86 Sr ratios are assumed to be contaminated nous convecting fluid (Figure 14.5). In a homoge by the continental crust, or to contain a recycled nous mantle, if the melting point is not exceeded ancient crustal component. in the shallow mantle, then it is unlikely to A depleted reservoir (low in LIL, low in Rb/Sr, be exceeded at greater depth since the melt 87Srf86 Sr and so on) can still be fertile, i.e. it can ing point and the geotherm diverge with depth. provide basalts by partial melting. A clinopyrox Material can leave a deep source region by several enite or gabbro cumulate, for example, can be mechanisms. depleted but fertile. Similarly, an enriched reser voir can be infertile, being low in Ca, Al, Na and (1) Melting in the thermal boundary at the bot so on. The fact that most of the mantle must tom or the top of a region because of the high be depleted as implied by mass-balance calcula thermal gradient. tions, does not mean that it is fertile or similar (2) Melting, or phase changes, due to adiabatic to the MORB reservoir. Most of the volume of the ascent of hotter or lower melting point mantle is depleted infertile- or barren-refractory regions. residue of terrestrial accretion, and most of it is (3) Entrainment of material by adjacent convect magnesium perovskite in the lower mantle. ing layers. (4) Heating of fertile blobs by internal radioac Ocean-island basalts tivity or by conduction of heat from the sur The trace element and isotope ratios of basalts rounding mantle. from ocean islands (OIB) differ from otherwise Some of these mechanisms are illustrated in Fig similar MORB. There is still no general agreement ure 7.1. on how such variations are produced; chemical and isotope variability of the mantle is likely to Midocean ridge basalts be present everywhere, not just in the source TI1e most voluminous magma type, MORB, has regions of OIB. Both geochemical and geophys low LIL-content and depleted isotopic ratios, with ical observations, and plate-tectonic processes, very low variance, that is, they are homogeneous. require the upper mantle to be inhomogenous, The Rb/Sr, Nd/Sm and U/Pb ratios have been low and a variety of mechanisms have been suggested since > 1 Ga. Since these ratios are high in melts to produce such inhomogeneities. Perhaps the and low in residual crystals, the implication is most obvious model for generating a variably fer that the MORB source is a cumulate or a crys tile and inhomogeneous mantle is subduction, talline residue remaining after the removal of delamination and incomplete melt extraction. a melt fraction or residual fluid. Extraction of Variability is generated by recycling of oceanic very enriched, very small-degree melts ( < 1 %) is and continental crust, and of seamounts, aseis implied. In some geochemical models the LIL mic ridges, oceanic islands and sediments, all enriched melt fraction is assumed to efficiently of which are being transported into subduction leave the upper mantle (the depleted MORE zones. Recycling of the oceanic crust involves source in these models) and enter, and become, the greatest mass flux of these various compo the continental crust. TI1e continental crust is nents but this does not require that such mate therefore regarded as the complement - often rial is the dominant source of OIB. A popular 172 MAG MAS : WINDOWS INTO THE M A NTLE
model involves transport of cold slabs, includ tle is polluted by delaminated lower ing the crust, to the core-mantle boundary (CMB) continental crust . Enriched MORB (EMORB) where they heat up and generate ~ 3000 km is also common along the global spreading ridge deep narrow plumes that come up under oceanic system and on near-ridge seamounts. There is islands. Enrichment of the lower part of the litho a continuum between depleted and enriched sphere by upward migrating metasomatic melts tholeiites (DMORB, NMORB, TMORB, EMORB, and fluids, and inefficient extraction of resid PMORB, OIB). Veins in mantle peridotites and ual melts could produce extensive fractionation, xenoliths contained in alkali basalts and kim and also allow the isotopic anomalies to form berlites are also commonly enriched and, again, within a region that was not being stirred by crustal contamination is unlikely unless conti mantle convection. Many oceanic islands and vol nental crust somehow gets back into the man canic chains - often called hotspots and hotspot tle. In many respects enriched magmas and xeno tracks - are on pre-existing lithospheric features liths are also complementary to MORB (in LIL such as fracture zones, transform faults, conti contents and isotopic ratios), suggesting that nental sutures, ridges and former plate bound there is ancient enriched material in the mantle. aries. Volcanism is also associated with regions of Island-arc basalts are also high in LIL, 87Srf86Sr, lithospheric extension and thinning, and swells. 143 Ndf144 Nd and 206 Pbf204 Pb, suggesting that The cause and effect relations are often not obvi there are shallow - and global - enriched com ous. Oceanic islands are composed of both alkali ponents. Back-arc-basin basalts (BABE) are similar and tholeiitic basalts. Alkali basalts are subor to MORB in composition and, if the depth of the dinate, but they appear to dominate the early low-velocity zone and the depths of earthquakes and waning stages of volcanism. The newest sub can be used as a guide, tap a source deeper than marine mountain in the Hawaii chain, Loihi, 150 km. Many BABBs are intermediate in chem is alkalic. Intermediate-age islands are tholeiitic, istry to MORB and OIB . This and other evidence and the latest stage of volcanism is again alka indicates that enriched components- or enriched lic. The volcanism in the Canary Islands in the reservoirs - may be as shallow or shallower than Atlantic changes from alkalic to tholeiitic with the depleted MORB reservoir. On average, because time. of recycling and crustal delamination, enriched Ocean-island basalts, or OIB, are UL-rich and and fertile components may be preferentially col have enriched isotopic ratios relative to MORB. lected in the shallow mantle. However, they may Their source region is therefore enriched, or be dispersed throughout much of the upper man depleted parent magmas have suffered contami tle. It is quite probable that the upper man nation en route to the surface. The larger oceanic tle is also lithologically stratified. This would islands such as Iceland and Hawaii generally show up as scattered energy in short-period have less enriched magmas than smaller islands seismograms. and seamounts. A notable exception is Kergue The trace-element signatures of some ocean len, which may be built on a micro-continent. island basalts and some ridge basalts are con Other volcanic islands may involve continental sistent with derivation from recycled lower crust in a less obvious way. The islands in the continental crust. Others are apparently derived Indian ocean and the south Atlantic are from from a reservoir that has experienced eclogite mantle that was recently covered by Gondwana fractionation or metasomatism by melts or other and may contain fragments of delaminated con fluids from an eclogite-rich source. Kimberlites tinental crust. are among the most enriched magmas. Although Enriched magmas such as alkali-olivine they are rare, the identification of a kimberlite basalts, OIB and nephelinites occur on oceanic like component in enriched magmas means that islands and have similar LIL and isotopic ratios they may be volumetrically more important, or to continental flood basalts (CFB) and continen more wide-spread, than generally appreciated. tal alkali basalts. Continental contamination is Simple mass balance calculations suggest that unlikely in these cases unless the upper man- kimberlite-like components may account for up APPARENTLY HOT MAGMAS 173 to 0.05% of the mantle, or about 10% of the plume head. Suitable stress conditions of the crustal volume. A small amount of such enriched plate and fertility of the mantle may be more magma can turn a depleted MORB into an important than the absolute temperature of the enriched melt. mantle. The final stages of ocean closure at a Trace-element and isotopic patterns of OIB convergent margin or suture and the presence overlap continental flood basalts (CFB), and com of batholiths with cumulate roots serve to fertil mon sources and fractionation patterns can be ize the mantle and lower the melting point of inferred. Island-arc basalts (lAB) also share many the mantle and the hydrated crust. of the same geochemical characteristics, suggest ing that the enriched character of these basalts may be derived from the shallow mantle. This is Apparently hot magmas not the usual interpretation. The geochemistry of CFB is usually attributed to continental contami Magmas that are associated with so-called nation or to a plume head fi:om the lower mantle, hotspots differ in various ways from what are that of lAB to sediment and hydrous melting of thought to be normal mid-ocean ridge basalts (i.e. the mantle wedge, and that of OIB to 'primitive' along normal portions of the ridge system). There lower mantle. In all three cases the basalts have is little evidence, however, that they are hotter evolved beneath thick crust or lithosphere, giving than normal basalts. High MgO contents are usu more opportunity for crystal fractionation and ally considered to be a diagnostic of high man contamination by crust, lithosphere and shallow tle temperatures but such magmas are rare at mantle prior to eruption. CFB and continental hotspots. These magmas include the following. island arcs are on continental or craton bound Boninite: A high-MgO, low-alkali, andesitic aries, or on old sutures, and may therefore tap rock with textures characteristic of rapid over-thickened continental or arc crust. crystal growth. Boninites are largely restric ted to Pacific island arcs, and are more Rhyolites and silicic LIPs abundant in the early stages of magmatism. Silicic volcanism is a component of large igneous Picrite: A high-MgO basalt extremely rich in provinces (LIPs) such as continental flood basalt olivine and pyroxene. In some petrological provinces related to continental breakup and schemes, picrite is the parent magma of assembly. A crustal source is the main differ other basalts, prior to olivine separation. ence in silicic and mafic LIP formation. Large Picrites are common at the base of conti degrees of crustal partial melting, implied by nental flood basalt sequences. They occur in the large volumes of rhyolitic magma, are con Hawaii, and, possibly, in Iceland. Sometimes trolled by the water content and composition of they have had olivine added by crystal set the crust, and the thermal input from the mantle tling and are not true high-MgO melts. via basaltic magmas. Rhyolites are more common Komatiite: An ultramafic lava with 18-32% and voluminous on the continents and continen MgO. The more highly magnesian varieties tal margins than in the ocean basins. Sili are termed peridotitic komatiite. c i c vol c anis m i s associate d with most Meimechite: A high-MgO, TiOz-rich ultramafic CFB p rovinc e s , and some oceanic plateaus (e.g. volcanic rock composed of olivine, clinopy Kerguelen, Wallaby and Exmouth). roxene, magnetite and glass occurs in Silicic LIPs - SLIPs - have erupted volumes association with flood basalts, Ni-Cu-PGE similar to CFB, but are produced over time peri bearing plutons, and diamond-bearing kim ods extending up to 40 Myr. The Kerguelen berlite and carbonatite. and Ontong-Java oceanic plateaus were erupted, episodically, over similar time-spans (30--40 Myr). All of these, plus kimberlites and carbon The generation of SLIPs and oceanic plateaus atites, have been taken by many authors as requires the sustained input of basaltic mantle diagnostics of deep mantle plumes. Ocean-island melts rather than the transient impact of a hot basalts, almost by definition, and certainly by 174 MAGMAS: WINDOWS INTO THE MANTLE
convention, are considered to be hotspot mag most gases and other volatiles are probably con mas. Although island-arc magmas and oceanic centrated into the upper mantle by magmatic island magmas differ in their petrography and processes, only a fraction manages to get close inferred pre-eruption crystal fractionation, they enough to the surface to outgas and enter the overlap almost completely in the trace elements atmosphere. High helium-3 contents relative to and isotopic signatures that are characteristic of helium-4, however, require that the gases evolved the source region. It is almost certain that island in a relatively low uranium-thorium reservoir for arc magmas originate above the descending slab, most of the age of the Earth. Shallow depleted although it is uncertain as to the relative con reservoirs such as olivine cumulates may be the tribution of the slab and the intervening man traps for helium. . tle wedge. Water from the descending slab may be important, but most of the material is pre Komatiites sumed to be from the overlying 'normal' mantle. Komatiites are ultrabasic melts that occur mainly Whether the subducted oceanic crust also melts in Archean rocks. The peridotitic variety appar below island arcs is controversial. It is curious ently require temperatures of the order of that the essentially identical range of geochemi 1450-1500°C and degrees of partial melting cal signatures found in oceanic-island and conti greater than 60-70% in order to form by melting nental flood basalts have generated a completely of dry peridotite. At one time such high temper different set of hypotheses, generally involv atures and degrees of melting were thought to ing a 'primitive' lower-mantle plume source. If be impossible. Even today, similar degrees of par island-arc magmas and boninites originate in the tial melting of eclogite, at much lower tempera shallowest mantle, it is likely that island and tures, are considered unlikely. At high pressure it continental basalts do as well. Subduction, and is possible to generate MgO-rich melts from peri bottom side erosion and delamination, rather dotite with smaller degrees of partial melting. It than plumes, are probably the primary causes of is even possible that olivine is replaced as the liq shallow mantle enrichment. uidus phase by the high-pressure majorite phase The main diagnostic difference between some of orthopyroxene, again giving high-MgO melts. islands and most other magmas is the higher Komatiites may represent large degrees of melt maximum values and the greater spread of ing of a shallow olivine-rich parent, small degrees the 3 Hej4He ratios of the former. High ratios of melting of a deep peridotite source, melting are referred to as 'primitive,' meaning that the of a rock under conditions such that olivine is helium-3 is assumed to be left over from the not the major residual phase, or melting of wet accretion of the Earth and that the mantle is not peridotite at much lower temperatures. At depths fully outgassed. The word 'primitive' has intro greater than about 200 km, the initial melts of a duced semantic problems since high 3 Hef4 He peridotite may be denser than the residual crys ratios are assumed to mean that the part of tals. This may imply that large degrees of partial the mantle sampled has not experienced partial melting are possible and, in fact, are required melting. This assumes that (1} helium is strongly before the melts, or the source region, become concentrated into a melt (relative to U and Th), (2} buoyant enough to rise. The ratios of some trace the melt is able to efficiently outgas, (3} mantle elements in some komatiites suggest that garnet that has experienced partial melting contains no has been left behind in the source region or that helium and (4} helium is not retained in or recy high-pressure garnet fractionation occurred prior cled back into the mantle once it has been in a to eruption. The existence of komatiites appears melt. As a matter of fact it is difficult to outgas to refute the common assumption that melt a melt. crystal separation must occur at relatively small Magmas must rise to relatively shallow depths degrees of partial melting. The rarity of komati before they vesiculate, and even under these ites since Precambrian times could mean that the circumstances gases are trapped in rapidly mantle has cooled, but it could also mean that quenched glass or olivine cmnulates. Although a suitable peridotite parent no longer exists at APPARENTLY HOT MAGMAS 175 about 300 km depth, or that the currently rela relatively high Ti02 even in the most LREE tively thick lithosphere prevents their ascent to depleted varieties. Uncontaminated komatiites the surface. Diapirs ascending rapidly from about and picrites have similar isotopic ratios overlap 300 km from an eclogite-rich source region would ping MORB, indicating generation from a man be almost totally molten by the time they reach tle source with a long-term depletion in LREE. shallow depths, and basaltic magmas would pre Geochemical characteristics of the komatiite dominate over komatiitic magma. picrite association, including REE and NbfY-ZrfY Komatiites may be the result of particularly systematics, indicate chemical heterogeneities in deep melting. By cooling and crystal fractiona the source region. TI1e high MgO contents of the tion komatiitic melts can evolve to less dense rocks has been used to support a mantle plume picritic and tholeiitic melts. Low-density melts, model for their genesis but they do not have of course, are more eruptable. It may be that the enriched or primitive isotopic signatures komatiitic melts exist at present, just as they did expected for plumes and significant regional in the Precambrian, but, because of the colder uplift before volcanism is lacking. shallow mantle they can and must cool and fractionate more, prior to eruption. In any case komatiites provide important information about Hot mantle the physics and chemistry of the upper mantle. Hot mantle intersects the solidus at different The major-element chemistry of komatiites depths and it melts over a larger pressure interval and related magmas (komatiitic basalts) are more and to greater extents than cooler mantle. This is similar to modern mafic subduction magmas why melting anomalies have been called hotspots, than to magmas thought to be produced by high and why they are attributed to hot upwelling temperatures. Komatiites with high Si02-contents plumes. The melts produced at high-temperature show similarities to modern subduction magmas have higher MgO and FeO and lower Na 20, Al 2 0 3
(boninites). Si02 contents of magmas generally and Si02 and lower LIL-contents than melts of decrease as the pressure of melting increases cooler mantle. therefore high Si02 is difficult to explain in a Archean volcanism produced high-MgO mag plume scenario. Boninites have similarly high mas called komatiites that seemed to fit the crite
Si02 at high MgO contents because they are ria for being the products of super-hot plumes. high-degree melts produced at shallow depths. In These magmas, however, may be produced by an anhydrous, high-temperature decompression hydrous melting processes in the upper mantle, melting regime, high degrees of melting begin possibly associated with subduction (Parman at great depth; in the case of boninites, large and Grove, 2001). [mantleplurnes) extents of melting can occur at shallow depth Some modern subduction related magmas, because of high H2 0 contents (Crawford et al., boninites, show a large compositional overlap 1989; Parman et al., 2001). with Archean basaltic komatiites. Komatiites The chemical similarities between have a wide range of major and minor element boninites, basaltic komatiites and composition. High-Si02 komatiites resemble mod komatii tes suggests that komatiites were ern boninite magmas that are produced by produced by similar melting processes that hydrous melting, while low-Si02 komatiites produce modern boninites, with the primary resemble more closely modern basalts produced difference being that the mantle was about by anhydrous decompression melting. If high 100 °C hotter in t he Archean. Rather than MgO magmas represent deep plume sources being products of deep mantle thermal plumes, then the isotopic and trace element patterns komatiites may be products of normal plate tec should resemble ocean island basalts, but they do tonic processes. not. The low-Ti02 contents of komatiite magmas Komatiitic rocks are depleted in both light requires their source to have been depleted (i.e. and heavy rare earth elements (LREE and HREE) to have had a melt extracted). The isotopic com relative to middle REE (MREE) and possess positions of komatiites overlap those of depleted 176 MAGMAS: WINDOWS INTO THE MANTLE
MORB rather than being similar to those exhib Table 14.2 J Kimberlite composition com- ited by OIB or hotspots. pared with ultrabasic and ultramafic rocks Melting temperatures can be lowered by the presence of water, carbon dioxide, and eclog Average Average Average ite. Boninites are high-degree melts produced Oxide Kimbe rlite Ultrabasic Ultramafic at shallow depths. High degrees of melting in hot upwellings necessarily must begin at great Si02 35.2 40.6 43.4 depth; in the case of boninites, large extents Ti02 2.32 0.05 0.1 3 of melting occur at shallow depth because the AI20 3 4.4 0.85 2.70 melting is facilitated by high water contents. 6.8 Fertile mantle, such as eclogite, also melts at FeO 2.7 12.6 8.34 higher pressure and at lower temperature than MnO 0. 11 0. 19 0.1 3 peridotite, and can therefore have a long melt MgO 27.9 42.9 41.1 ing column, and produce large amounts of CaO 7.6 1.0 3.8 Na20 0.32 0.77 0.3 basalt. K20 0.98 O.Q4 0.06 7.4 Kimberlite H20 C02 3.3 0.04 Some rocks are rare but are so enriched in certain P20 s 0.7 O.Q4 0.05 key elements that they cannot be ignored in any attempt to reconstruct the composition of the Wederpohl and Muramatsu (1979). mantle. Kimberlite is a rare igneous rock that is Dawson (1980). volumetrically insignificant compared with other igneous rocks. Kimberlite provinces themselves, however, cover very broad areas and occur in been subjected to kimberlite intrusion over long most of the world's stable craton, or shield, areas. periods of geological time, suggesting litho Kimberlites are best known as the source rock for spheric control. diamonds, which crystallized at pressures greater Compared to other ultrabasic rocks than about 50 kilobars. They carry other samples lherzolites, dunites, harzburgites - kimberlites from the upper mantle that are the only direct contain high amounts of K, AI, Ti, Fe, Ca, C, P samples of mantle material below about 100 km. and water. For most incompatible trace elements Some kimberlites appear to have exploded from kimberlites are the most enriched rock type; depths as great as 200 km or more, ripping off important exceptions are elements that are samples of the upper mantle and lower crust retained by garnet and clinopyroxene. Carbon in transit. The fragments, or xenoliths, provide atites may be enriched in U, Sr, P, Zr, Hf and the samples unavailable in any other way. Kimber heavy REEs relative to kimberlites. Since kimber lite itself is an important rock type that provides lites are extremely enriched in the incompatible important clues as to the evolution of the mantle. elements, they are important in discussions of It contains high concentrations of lithophile ele the trace-element inventory of the mantle. Such ments (Table 14.2) and higher concentrations of extreme enrichment implies that kimberlites the most incompatible trace elements (Pb, Rb , Ba, represent a small degree of partial melting of a Th, Ce, La) than any other ultrabasic rock. Kim mantle silicate or a late-stage residual fluid of a berlite is also enriched in elements of ultramafic crystallizing cumulate layer. The LIL elements in affinity (Cr and Ni). kimberlite show that it has been in equilibrium Diamond-bearing kimberlites are usually with a garnet-clinopyroxene-rich source region, close to the craton's core, where the lithosphere possibly an eclogite cumulate. The LIL contents may be thickest. Barren kimberlites (no dia of kimberlite and MORB are complementary. monds) are usually on the edges of the tecton Removal of a kimberlite-like fluid from an ically stable areas. Kimberlites range in age from eclogite cumulate gives a crystalline residue that Precambrian to Cretaceous. Some areas have has the required geochemical characteristics APPARENTLY HOT MAGMAS 177 of the depleted source region that provides peridotite). The bulk modulus in the transition MORE. Adding a kimberlitic fluid to a depleted zone for olivine in its spinel forms is higher than basalt can make it similar to enriched magmas. observed. Carbonatites and other ultramafic alka Alkali basalts have LIL concentrations that are line rocks, closely related to kimberlites, are intermediate to MORE and ldmberlite. Although widespread. They are probably common in the kimberlite pipes are rare, there may be a upper mantle, although they have been argued ldmberlite-like component (Q) dispersed through to come from the lower mantle because they are out the shallow mantle. Indeed, alkali basalts can not MORE. Kimberlites provide us with a sam be modeled as mixtures of a depleted magma ple of magma that probably originated below (MORE) and an enriched magma (kimberlite), as about 200 km and, as such, contain informa shown in Table 14.3. Peridotites with evidence tion about the chemistry and mineralogy of of secondary enrichment may also contain a the mantle in and below the continental litho kimberlite-like component. sphere. Kimberlites are anomalous with respect to other trace-element enriched magmas, such Carbonatite as nephelinites and alkali basalts, in their trace Carbonatites are igneous rocks that typically element chemistry. They are enriched in the very occur in regions of lithospheric extension of incompatible elements such as rubidium, tho thick continental plates, i.e. continental rift rium and LREE, consistent with their represent zones (e .g. East Africa) and on the margins of ing a small degree of partial melting or the continental flood-basalt provinces (e.g. Parana final concentrate of a crystallizing liquid. The Etendeka, Deccan). They have also been found extreme enrichments of ldmberlitic magmas in on ocean-islands on continental margins (e.g. incompatible elements (e.g. compared with a typ Cape Verde a n d Can ary I s l a n d c a r bon ical undepleted mantle) are usually attributed a t ites). The close spatial and temporal asso to low degrees of melting andjor metasomatized ciation of kimberlites and carbonatites suggests source compositions. The obser ved enri c h that these rocks are genetically related, although me nt of k i mberlitic magmas wi t h rare diamond-bearing ldmberlites may come from ear th elements (REE )can b e explained in deeper in the upper mantle. Kimberlites are terms of melt mi grati o n throu g h source rare on oceanic islands. This may be because rock s h aving t h e composition of norma l the adiabatic ascent of the parent rock through mantle. the asthenosphere generates large-degree melt Kimberlites are relatively depleted in the ele ing, while small-degree melts can be better pre ments (Sc, Ti, V, Mn, Zn, Y, Sr and the HREE) that served if the eruption is through cold continental are retained by garnet and clinopyroxene. They mantle. Both carbonatites and kimberlites have are also low in silicon and aluminum, as well as enriched trace-element patterns, consistent with other elements (Na, Ga. Ge) that are geochemi small-degree melts. Carbonatites have depleted cally coherent with silicon and aluminum. This isotopic signatures while some kimberlites are suggests that kimberlite fluid has been in equi enriched. Kimberlites and carbonatites provide librium with an eclogite residue. Kimberlites are an opportunity to study low-degree partial melt also rich in cobalt and nickel. ing in the mantle. This is important since the Despite their comparative rarity, dispropor removal of low-degree partial melts seems to be tionately high numbers of eclogite xenoliths have the mechanism for depleting the mantle. Since been found to contain diamonds. Diamond is much of this depletion apparently occurred in extremely rare in peridotitic xenoliths. Eclogitic early Earth history, when the mantle was much garnets inside diamonds imply a depth of ori hotter, the implication is that this melting and gin of about 200-300 km if mantle tempera melt removal occurred at shallow depths, prob tures in this depth range are of the order of ably in the thermal boundary layer and the
1400- 1600 oc. Seismic velocities in the transition lithosphere. The solidi of C02-bearing rocks is region are consistent with piclogite (eclogite plus much lower than volatile-poor rocks; this allows 178 MAGMAS: WINDOWS INTO THE MANTLE
Table 14.3 I Trace-element chemistry of MORB, kimberlite and intermediate (alkali) basalts (ppm)
Alkali Basalts Kimberlite
Element MORB EPR* Australia Hawaii BCRt Theoretical** Average Max
Sc 37 27 25 33 34-37 35 30 v 210 297 260 170 399 197-214 120 250 Co 53 41 60 56 38 57-61 77 130 Ni 152 113 220 364 IS 224-297 1050 1600 Rb 0.36 13 7 24 47 10-45 65 444 Sr 110 354 590 543 330 200-289 707 1900 y 23 37 20 27 37 23-28 22 75 Zr 70 316 Ill 152 90 97-133 250 700 Nb 3.3 24 24 14 19-48 110 450 Ba 5 303 390 350 700 150-580 1000 5740 La I .38 21 24 23 26 16-31 ISO 302 Ce 5.2 46 47 49 54 31-57 200 522 Nd 5.6 28 24 23 29 16-26 85 208 Sm 2.1 6.7 6.8 5.5 6.7 Jl-4.8 13 29 Eu 0.8 2.1 2.1 2.0 2.0 1.1-1.4 3 6.5 Tb 0.5 1.2 0.92 0.87 1.0 0.6-0.7 2.1 Yb 2. I 3.4 I .6 I .8 3.4 2.0-2.1 1.2 2.0 Lu 0.34 0.49 0.28 0.23 0.55 032-0.34 0.16 0.26 Hf 1.4 5.3 2.8 3.9 4.7 2.2-4.3 7 30 Ta 0.1 9 I .8 2.1 0.9 IJ-2.5 9 24 Pb 0.08 4.0 5 18 I .6-5.1 10 50 Th 0.35 2.7 2.0 2.9 6.8 2.4-5.4 16 54 K 660 8900 4700 8800 1200 3200-4770 17600 41 800 u 0.014 0.51 0.4 0.6 I .8 0.48-1.84 3. I 18.3 *EPR = East Pacific Rise. tBCR = Basalt, Columbia River (USGS Standard Rock). ** Mixture ranging from 85 percent MORB plus 15 percent Average Kimberlite to 90 percent MORB plus 20 percent Max Kimberlite.
relatively cold rocks to have very low seismic identified in carbonatites. HIMU may represent velocities. The low-velocity zone in the upper recycled crust, or lower continental crust, dehy- mantle and the low-velocity regions under some drated and converted to eclogite. HIMU sources so-called hotspots may be due to this rather than are enriched in U, Nb and Ta relative to MORB. to high absolute temperatures. HIMU islands include Mangaia, and St. Helena. There is no particular reason for supposing EMl has extremely low 143Ndj144Nd and very low that carbonatites come from deep in the mantle, lead isotopic ratios compared to other OIB compo- although their isotopic similarity with some OIB nents. Pitcairn and the Walvis Ridge are the type has led to suggestions that they come from the locations for EMl basalts. EMl sources may be lower mantle. This is circular reasoning and is recycled crust plus pelagic sediment or metasom- based on the assumption that only MORB origi- atized continental lithosphere. The EMl compo- nates in the upper mantle. nent has 818 0 similar to MORB sources and peri- The isotopic components HIMU, EMl, FOZO dotite xenoliths, ruling out a large contribution and DMM (depleted MORB mantle) have been of subducted sediment. APPARENTLY HOT MAGMAS 179
The most likely place to generate low-degree melting in a low-melting point mantle with
C02 is in the outer reaches, probably in the conduction layer or above 200 km. The EMl, HIMU and FOZO components may all originate in 100 • KIMBERLITE the upper and lower crust, the sub-continental c 0 lithosphere and the shallow mantle. They can CFB ~c enter the source region of OIB and carbonatites Q) (.) by erosion, subduction or delamination. In the 5 10 (.) "0 absence of subduction, the lower crustal delam Q) ination explanation is preferred. A wide-spread cr;- ~ E but sporadic, and shallow depleted FOZO source 0 provides an explanation for carbonatites. The z 'depleted mantle' signatures may be attributed to differentiation events in the mantle that pro duced depleted sub-continental lithosphere and continental crust. Carbonatite isotope data sug gest that a shallow source in the lithosphere or perisphere could be FOZO mantle. It could have existed for billions of years as an approxi Trace-element concentrations in MORB, mately closed system if it is buoyant and of high continental flood basalts (CFB) and kimberlites. The elements viscosity. to the right are incompatible in all major mantle phases Carbonatites may represent the initial melt (olivine, pyroxene and garnet) while those to the left are ing of carbonated mantle peridotite and lumber retained in the eclogite minerals (clinopyroxene and garnet). lites may represent larger degrees of partial melt Note the complementary pattern between MORB and kimberlite and the intermediate position of CFB. ing after the C02 is exhausted. There is also the possibility that carbonated eclogite produces car Concentrations are normalized to estimates of mantle composition. bonatitic melt under various depth conditions in the upper mantle and that this melt metasoma tizes mantle peridotite. The source of the eclogite as the solid/liquid partltlon coefficient for that could be delaminated lower continental crust, or element. This is illustrated in Figure 14.2. The subducted oceanic crust. solid line is a profile of the MORE/kimberlite ratio for a series of incompatible elements. The verti The Kimberlite-MORE connection cal lines bracket the solid/liquid partition coef Kimberlites are enriched in the LIL elem.ents, ficients for garnet and clinopyroxene. Although especially those that are most depleted in MORE. MORE is generally regarded as an LIL-depleted Figure 14.1 gives the composition of kimberlites, magma and kimberlite is ultra-enriched in most MORE and continental tholeiites. The comple of the incompatible elements, MORE is enriched mentary pattern of l relatively depleted in strontium and europium, I D . _ eclogite residue I elements that have been removed from the eclog1te- me:ORB ft~~ KREEP source region by plagioclase fractiona tion. Kimberlite is depleted in sodium, HREE, KIMB // hafnium, zirconium and yttrium, elements that I are removed by garnet-plus-clinopyroxene frac 0.1 I tionation. It appears that the differences between / / / KIMB and KREEP are due to difference in pressure / / between the Earth and the Moon; garnet is not /~ stable at pressures occurring in the lunar man --' tle. The plagioclase and garnet signatures show MORB +50% 01 KIMB through such effects as the different volatile refractory ratios of the two bodies and expected differences in degrees of partial melting, extent Solid line is ratio of concentrations in MORB and of fractional crystallization and other features. kimberlite (KIMB). Vertical bars are solid/liquid partition The similarity in composition extends to metals coefficients for garnet plus clinopyroxene. The dashed line of varying geochemical properties and volatilities gives the ratio for MORB plus 50 percent olivine, a possible such as iron, chromium, manganese as well as picrite parent magma for MORB. If the MORB or pic rite phosphorus. KREEP is depleted in other metals (V. source region is the crystalline residue remaining after Co, Ni, Cu and Zn), the greater depletions occur removal of a kimberlitic fluid, the ratio of concentrations, MORB/KIMB or picrite/KIMB, should equal the solid-liquid ring in the more volatile metals and the met partition coefficient, which depends on the crystalline (xi) als that are partitioned strongly into olivine. This phases in the residue. suggests that olivine has been more important in the evolution of KREEP than in the evolution of kimberlite, or that cobalt, copper and nickel are and low-degree melts. If diapirs initiate in a deep more effectively partitioned into MgO-rich fluid depleted layer, they must traverse the shallow such as kimberlite. mantle during ascent, and cross-contamination It appears from Figure 14.3 that KREEP and seems unavoidable. The more usual model is that kimberlite differ from chondritic abundances in the whole upper is depleted and enriched mag similar ways. Both are depleted in volatiles rel mas originate as plumes in the deep mantle. ative to refractories, presumably due to preac cretional processes. Strontium is less enriched The Kimberlite-KR.EEP Relation than the other refractories. although this is KREEP is a lunar material having very high con much more pronounced for KREEP. The pro centrations of incompatible elements (K. REE, P, nounced europium and strontium anomalies U, Th). It is thought to represent the r e sidual for KREEP are consistent with extensive plagio l i qui d o f a crys t a lliz ing ma gma ocean. clase removal. The HREE, yttrium, zirconium and Given proposals of a similar origin for kim hafnium are relatively depleted in kimberlite, berlite, it is of interest to compare the com suggesting eclogite fractionation or a garnet-rich position of these two materials. An element source region for kimberlite. The depletion of by element comparison gives the remarkable scandium simply indicates that olivine, pyroxene result that for many elements (K, Cs, P. S, Fe, or garnet have been in contact with both KREEP Ca. Ti, Nb, Ta, Th, U, Ba and the LREE) kim and KIMB. The depletion of sodium in KIMB is berlite is almost identical (within 40%} to the also consistent with the involvement of eclogite composition of KREEP (Figures 14.3 and 14.4}. in its history. The depletion of sodium in KREEP, This list includes compatible and incompati relative to the other alkalis. is presumably due ble elements, major, minor and trace elements. to the removal of feldspar, and the greater rela and volatiles as well as refractories. KREEP is tive depletion of sodium in kimberlite therefore APPAR EN T LY HOT MAGMAS 181 Volatiles Chondrite-normalized Refractories trace-element compositions for to3 - kimberlite and lunar KREEP. 2 .~ 10 - -o<:: 0 .<:: u Q; Ci. E (f)"' 10-1- Eclogite elements 10 2 - Li Na K Rb Cs P Nb Ta Th U Sr Ba La Ce Nd Sm Eu Yb Y Zr Hf Sc requires another process, such as removal of a including HREE and sodium. Kimberlite may rep jadeite component by eclogite fractionation. resen t the late-stage residual fluid of a crystalliz Chromium, manganese and iron are approx ing terrestrial magma ocean. A buried eclogite imately one-third the chondritic level in both rich cumulate layer is the terrestrial equivalent KREEP and kimberlite. This is about the level in of th e lunar anorthositic crust. the mantles of these bodies. Nickel and cobalt in Removal of a kimberlite-like fluid from a kimberlite are about the same as estimated for garnet-clinopyroxene-rich source region gives a the Earth's mantle. These elements are extremely crystalline residue that has the appropriate trace depleted in KREEP, indicating that they have element chemistry to be the reservoir for Lit been removed by olivine or iron extraction from depleted magmas such as MORE. Enriched fluids the source region. permeate the shallow mantle and are responsible The similarity of kimberlite and KREEP is for the LIL-enrichment of island-arc and oceanic shown in Figure 14.4. For many elements (such island basalts. as Ca, Ba, Nd, Eu, Nb, Th, U, Ti, Li and P) the con centrations are identical within 50%. Kimberlite Alkali basalt is enriched in the volatiles rubidium, potassium Continental alkaline magmatism may persist and sulfur, reflecting the higher volatile content over very long periods of time in the same region of the Earth. Kimberlite is also relatively enriched and may recur along lines of structural weakness in strontium and europium, consistent with a after a long hiatus. The age and thickness of the prior extraction of plagioclase from the KREEP lithosphere play an important role, presumably source region. by controlling the depth and extent of crystal Kimberlite and lunar KREEP are remarkably fractionation and the ease by which the magma similar in their minor- and trace-element chem can rise to the surface. istry. The main differences can be attributed to The non-random distribution of alkaline plagioclase fractionation in the case of KREEP provinces has been interpreted alternatively as and eclogite fractionation in the case of kimber hotspot tracks and structural weaknesses in the lite. KREEP has been interpreted as the residual lithosphere. What have been called hotspots may fluid of a crystallizing magma. In a small body be passive centers of volcanism whose location the Al-content of a crystallizing melt is reduced is determined by pre-existing zones of weak by plagioclase crystallization and flotation. In a ness, rather than manifestations of deep man magma ocean on a large body, such as the Earth, tle plumes. Alkaline basalts both precede and the Al 20 3 is removed by sinking or residual gar follow abundant tholeiitic volcanism. In general net. Kimberlite is depleted in eclogite elements alkaline basalts erupt through thick lithosphere 182 I MAGMAS: WINDOWS INTO THE MANTLE Trace-element concentrations in kimberlite relative Zn Refractories to KREEP. Elements that are 10 Volatiles .. Garnet : ; }Olivine fractionated into olivine, plagioclase, garnet and jadeite are noted. The C..u solid/liquid partition coefficient for Co n_ garnet is shown. If kimberlite has w w been in equilibrium with garnet, or a: :.:: an eclogite cumulate, the concentrations in kimberlite will be proportional to the reciprocal of the h. Metals 1 Garnet / garnet partition coefficient. partition ~ 1 0.1 coefficient 'v/ I I I I I I I 0 0 · \i Na K Rb Cs P S Ca Ti Nb Ta Th U Sr Ba La Ce Nd Sm Eu Tb Dy Ho Er Yb Lu and are early when the lithosphere is thinning midocean ridges, may be tapping the flanks of (rifting) and late when the lithosphere is thicken a mantle heterogeneity, where crystal fractiona ing (flanks of ridges, downstream from midplate tion and contamination by shallow mantle are volcanic centers). most prevalent. In all cases lithospheric thick On Kerguelen, the third largest oceanic ness appears to play a major role. This plus the island, and on Ascension Island, the youngest long duration of alkalic volcanism, its simultane basalts are alkalic and overlie a tholeiitic base. ity over large areas and its recurrence in the same In Iceland, tholeiites dominate in the rift zone parts of the crust argue against the simple plume and grade to alkali basalts along the western concept. and southern shores. Thus, there appears to be Alkali-rich basalts generally have trace a relation between the nature of the magma element concentrations intermediate between tism and the stage of evolution. The early basalts MORE and highly alkalic basalts such as nepheli are tholeiitic, change to transitional and mixed nite. In fact, they can be treated as mixtures of basalts and terminate with alkaline composi MORE , or a picritic MORE parent, and nephelin tions. It was once thought that Hawaii also fol ite. Alkaline rocks are the main transporters of lowed this sequence, but the youngest volcano, mantle inclusions. Loihi, is alkalic and probably represents the earli est stage of Hawaiian volcanism. It could easily be covered up and lost from view by a major tholei Continental flood basalts itic shield-building stage. Thus, oceanic islands Continental flood basalts (CFB), the most copi may start and end with an alkalic stage. ous effusives on land, are mainly tholeiitic In continental rifts, such as the Afar trough, flows that can cover very large areas and are the Red Sea, the Baikal rift and the Oslo graben, typically 3-9 km in thickness. These are also the magmas are alkalic until the rift is mature called traps, plateau basalts and fissure basalts, and they are then replaced by tholeiities, as in a and large igneous provinces (LIPs). Examples are widening oceanic rift. A similar sequence occurs the Deccan traps in India, the Columbia River in the Canary Islands and the early stages of province in the USA, the Parana basin in Brazil. Hawaii. The return of alkalies in the Hawaiian the Karoo province in South Africa, the Siberian chain may occur when the islands drift away traps, and extensive flows in western Australia, from a fertile blob. Seamounts on the flanks of Tasmania, Greenland and Antarctica. They have APPARENTLY HOT MAGMAS 183 great similarity around the world. Related basalts plate beneath the North American plate and may are found in recent rifts such as East Africa and have involved interaction with old enriched sub the Basin and Range province. Rift and flood continental mantle. The old idea that continen basalts may both form during an early stage of tal flood basalts are derived from a primordial continental rifting. 'chondritic' source does not hold up when tested In some cases alkali-rich basalts occurring on against all available trace-element and isotopic the edge of rifts were erupted first, while tholei data, even though some of these basalts have ites occur on the floor of the rift. Continental neodymium isotopic ratios expected from a prim alkali basalts and tholeiites are highly enriched itive mantle. in LIL elements and have high 87Srf86Sr ratios. The relationships between some continen There are many similarities between continen tal basalts and oceanic volcanism are shown in tal and ocean-island basalts. Fractional crystal Figure 14.5. The basalts in the hatched areas lization or varying degrees of partial melting can are generally attributed to hotspot volcanism. produce various members of the alkali series. The The dashed lines show boundaries that appar large range of isotopic ratios in such basalts indi ently represent sutures between fragments of cates that their source region is inhomogeneous continents. or that mixing between magma types is involved. The similarity between many continental basalts Andesite and ocean-island basalts indicates that involve Andesites are associated primarily with island ment of the upper continental crust is not neces arcs and other convergent plate boundaries and sary to explain all continental flood basalts. have bulk compositions similar to the continen Continental flood basalts have been the sub tal crust. Tholeiites and high-MgO magmas are ject of much debate. Their trace-element and iso also found in island arcs. The accretion of island topic differences from depleted midocean-ridge arcs onto the edges of continents is a primary basalts have been attributed to their derivation mechanism by which continents grow or, at least, from enriched mantle, primitive mantle, magma maintain their area. Delamination of the lower mixtures or from a parent basalt that experi mafic arc crust may be the main mechanism of enced contamination by the continental crust. crustal recycling and an important mechanism In some areas these flood basalts appear to be for creating fertile heterogeneities, or melting related to the early stages of continental rifting anomalies, in the mantle. or to the proximity to an oceanic spreading cen Andesites are lavas with > 54 wt.% Si02 . ter. In other regions there appears to be a con Andesites can, in principle, originate as primary nection with subduction, and in these cases they melts from the mantle, as melts of continental may be analogous to back-arc basins. In most crust, by contam.ination of melts by continen isotopic and trace-element characteristics they tal crust, by partial fusion of hydrous peridotite, overlap basalts from oceanic islands and island by partial fusion of subducted oceanic crust, arcs. by differentiation of basalt by crystal fraction Chemical characteristics considered typi ation, or by magma mixing of melts from a cal of continental tholeiites are high Si02 variety of sources. Andesites may involve inter and incompatible-element contents, and low actions between mantle peridotite and partial MgOfFeO and compatible-element concentra melts of subducted basalt, eclogite and sedi tions. The very incompatible elements such as ments. Arc magmas are enriched in thorium and cesium and barium can be much higher than lanthanum, and depleted in niobium and tan in typical ocean island basalts. Many CFB, gener talum - relative to thorium and lanthanum - ally spealdng, are in back-arc environments. The this may involve dehydration of subducted Columbia River flood basalts are not necessar crust. ily typical but they appear to have been influ Melting of hydrous peridotite and eclog enced by the subduction of the Juan de Fuca ite and low-pressure differentiation of basalt 184 MAGMAS: WINDOWS INTO THE MANTLE shallow mantle and may be responsible for the islands, and oceanic plateaus, with ages in millions of years. magmatism associated with continental breakup The period between 130 and 65 Ma was one of extensive and the formation of oceanic plateaus some volcanism and most of this occurred inside geoid highs 40-80 Myr after breakup. (contours) and areas of high elevation. Some of the basalt Volcanic or island arcs typically occur 100-250 provinces have drifted away from their point of origin km from convergent plate boundaries and are (arrows). (See also Fig. 5.5.) of the order of 50-200 km wide. The volca noes are generally about 100-200 km above the by crystal fractionation may all be involved. earthquake foci of the dipping seismic zone. Andesites probably originate in or above the sub Crustal thicknesses at island arcs are generally ducted slab and, therefore, at relatively shallow 20-50 km thick. The upper bound may be the depths in the mantle. The thick crust in most basalt-eclogite phase change boundary and the andesitic environments means that shallow-level level of crustal delamination. Andesitic volcan crystal fractionation and crustal contamination ism appears to initiate when the slab has reached are likely. In their incompatible-element and iso a depth of 70 km and changes to basaltic vol topic chemistry, andesites are similar to ocean canism when the slab is no longer present or island basalts and continental flood basalts. They when compressional tectonics changes to exten differ in their bulk chemistry. sional tectonics. Figure 14.6 shows that island-arc Calc-alkalic andesite production involves mix basalts (hatched) overlap ocean-island and conti ing of two end-member magmas, a mantle nental basalts in isotopic characteristics. A simi derived basaltic magma and a crust-derived fel lar mantle source is indicated, and it is probably sic magma. The bulk continental crust possesses shallow. compositions similar to calc-alkalic andesites. If Back-arc-basin basalts (BABE) are similar to continental crust is made at arcs, the mafic midocean-ridge basalts but in many respects they residue after extraction of felsic melts must are transitional toward island-arc tholeiites and be removed - delaminated from the initial ocean-island basalts (Tables 14.4, 14.5 and 14.6). basaltic arc crust - in order to form andesitic They are LIL-enriched compared with normal crust. Chemically modified oceanic materials and MORE and generally have LREE enrichment. High delaminated mafic lower crust materials, cre 3 Hef4 He ratios have been found in some BABE, ate chemical and melting anomalies in the in spite of the fact that they are cut off from APPARENTLY HOT MAGMAS 185 and the density of the mantle under the spread ing centers is the same. The marginal basins of Southeast Asia, however, tend to be deeper 0.5132 than similar age oceanic crust elsewhere, by one half to one kilometer, and to deepen faster with 0.5130 age. TI1e presence of cold subducting material underneath the basins may explain why they are "z 0.5128 deeper than average and why they sink faster. TI1e ;!" =o low-density material under the ridge axis may be ~ more confined under the major midocean ridges. 0.5126 On average the upper 200-300 km of the mantle in the vicinity of island arcs and marginal basins 0.5124 has slower than average seismic velocities and the deeper mantle is faster than average, probably reflecting the presence of cold subducted mate rial. The depth of back-arc basins is an integrated effect of the thickness of the crust and litho Neodymium and strontium isotopic ratios for sphere, the low-density shallow mantle and the oceanic, ocean island, continental and island-arc (hatched) presumed denser underlying subducted material. basalts and diopside inclusions from kimberlites. The numbers It is perhaps surprising then that, on average, are the percentage of the depleted end-member if this array the depths of marginal basins are so similar to is taken as a mixing line between depleted and enriched equivalent age oceans elsewhere. The main dif end members. Mixing lines are flat hyperbolas that are ference is the presence of the underlying deep approximately straight lines for reasonable choices of subducting material, which would be expected parameters. The fields of MORB and CFB correspond to > 97% and 70-95%, respectively, of the depleted to depress the seafloor in back-arc basins. Basalts end-member. The enriched end-member has been arbitrarily in marginal basins with a long history of spread taken as near the enriched end of Kerguelen (K) basalts. The ing are essentially similar to MORB, while basalts most enriched magmas are from Kerguelen, Tristan da Cunha generated in the early stages of back-arc spread (T) and Brazil (Br). Other abbreviations are Sc (Scotia Sea), A ing have more LIL-enriched characteristics. A sim (Ascension), lc (Iceland), G (Gouch), Or (Oregon), Sb ilar sequence is found in continental rifts (Red (Siberia), P (Patagonia) and E (Eifel). Along top are strontium Sea, Afar) and some oceanic islands, which sug isotopic data for xenoliths and kimberlites. gests a vertical zonation in the mantle, with the UL-rich zone being shallower than the depleted the lower mantle by a slab. Yellowstone is not in zone. TI1is is relevant to the plume hypothesis, a classic back-arc basin but it has high 3 He/4 He which assumes that enriched magmas rise from in spite of the underlying thick U-rich crust and deep in the mantle. The early stages of back no deep low-velocity zone. BABE are similar to, arc magmatism are the most LIL-enriched, and or gradational toward, ocean-island basalts and this is the stage at which the effect of hypo other hotspot magmas. thetical deep mantle plumes would be cut off In most respects active back-arc basins appear by the presence of the subducting slab. Conti to be miniature versions of the major oceans, nental basalts, such as the Columbia River basalt and the spreading process and crustal structure province, are also most enriched when the pres and composition are similar to those occurring ence of a slab in the shallow mantle under west at midocean ridges. The age-depth relationships ern North America is indicated. The similarity of of marginal seas, taken as a group, are indistin the isotopic and trace-element geochemistry of guishable from those of the major oceans. To island-arc basalts, continental flood basalts and first order, then, the water depth in marginal ocean-island basalts and the slightly enriched sea basins is controlled by the cooling, contrac nature of the back-arc-basin basalts all suggest tion and subsidence of the oceanic lithosphere, that the enrichment occurs at shallow depths, 186 MAGMAS: WINDOWS INTO THE MANTLE Table 14.4 1 Composition of marginal basin basalts compared with other basalts North Mariana Scotia Lau Bonin Fiji Arc Island Oxide MORB MORB Trough Sea Basin Rift Plateau Tholeiites Tholeiite Si02 49.2 1 S0.47 S0 .56 S l.69 Sl.33 Sl. ll 49.5 48.7 Sl.l 49.4 Ti02 1.39 1.58 1.21 1.41 1. 67 1.10 1.2 0.63 0.83 2.5 AI20 3 IS.81 IS.31 16.53 16.23 17.22 18.20 IS.S 16.S 16.1 13.9 FeO 7.19 10.42 8.26 8.28 S.22 8. 18 6.2 8.4 Fe20 3 2.2 1 270 3.9 3.4 11.8 12.4 MnO 0.1 6 0.10 0.17 O.IS 0.1 7 0.1 0.29 0.2 MgO 8.53 7.46 7.2S 6.98 S.23 6.2S 6.7 8.2 S.l 8.4 CaO 11.14 I 1.48 I 1.59 11.23 9.9S I 1.1 8 11.3 12.2 10.8 I 0.3 Na20 2.71 2.64 2.86 3.09 3. 16 3.12 2.7 1.2 1.96 2.1 3 K20 0.26 0.1 6 0.23 0.27 1.07 0.38 0.3 0.23 0.40 0.38 P20 s O.IS 0.1 3 O.IS 0.18 0.39 0. 18 0.1 0.10 0.14 H20 1.6 1.18 1.4 87 Sr/86Sr 0.7023 0.7028 0.7028 0.7030 0.7036 Gill (1976), Hawkins (1977). Table 14.5 I Representative properties of basalts and kimberlites Midocean Alkali- Ridge Back-Arc Basin Island-Arc Ocean-Island Olivine Ratio Basalt Basalt Basalt Basalt Basalt Kimberlite Rb/Sr 0.007 0.0 I 9-0.04 3 0.0 1-0.04 0.04 O.OS 0.08 K!Rb 800-2000 SS0-860 300-690 2S0-7SO 320-430 130 Sm/Nd 0.34-0.70 0.31 0.28-0.34 O.IS-0.3S 0.37 0.18 U/Th 0.11-0.7 1 0.21 0.1 9-0.77 O. IS-0.3S 0.27 0.24 K!Ba 110-120 33-97 7.4-36 28-30 13.4 7.5 Ba/Rb 12-24 9-2S 10-20 14-20 IS IS La/Yb 0.6 2.5 2-9 7 14 12S Zr/Hf 30-44 43 38-S6 44 41 36 Zr/Nb 3S-40 4.8 20-30 14.7 2.7 2.3 87 Sr/86Sr 0.7023 0. 702 7-0. 70S 0.703-0.706 0.7032-0.706 0.71 206 Pb/204pb 17.5 -1 8.5 18.2- 19.1 18.3-18.8 18.0-20.0 17.6-20.0 perhaps by contamination of MORB rising from attenuation. The crusts are typically oceanic and a deeper layer. The high 3Hej4He of Lau Basin basement rocks are tholeiitic. The spreading cen basalts also suggests a shallow origin for 'primi ter in the back-arc basins, however, is much tive' gases. less well defined. Magnetic anomalies are less Both back-arc basins and midocean ridges coherent, seismicity is diffuse and the ridge have shallow seafloor, high heat flow and thin crests appear to jump around. Although exten sedimentary cover. The upper mantle in both sion is undoubtedly occurring, it may be oblique environments has low seismic velocity and high to the arc and diffuse and have a large shear TRA C E- ELE M ENT MO DELING 187 Table 14.6 1 Represen tative values of large-ion lithophile and high-field-strength elements (HFSE) in basalts (ppm) Midocean- Back-Arc-Basin Island-Arc Ocean-Island Element Ridge Basalt Basalt Basalt Basalt Rb 0.2 45-7.1 5-32 5 Sr 50 146- 195 200 350 Ba 4 40- 174 75 100 Pb 0.08 1.6-3 9.3 4 Zr 35 121 74 125 Hf 1.2 2.8 1.7 3 Nb I 25 2.7 8 Ce 3 32 6.7-32. 1 35 Th om 1- 1.9 0.79 0.67 u O.G2 0.4 0.19 1.18 component. The existence of similar basalts at the residual solid, respectively; D is the bulk dis midocean ridges and back-arc basins and the tribution coefficient for the minerals left in the subtle differences in trace-element and isotopic residue, and F is the fraction of m elting. Each ratios provide clues as to the composition and element has its own D that depends not on the depth of the MORE reservoir. initial mineralogy but on the residual minerals, and, in some cases, the bulk composition of the melt. For the very incompatible elements (D « I); Trace-element modeling essentially all of the element goes into the first melt that forms. The so-called compatible ele An alternative to mixing known components to ments (D » 1) stay in the crystalline residue, and in~r t h e composition of the depleted the solid residual is similar in composition to MORB mantle is to calculate the partitioning of the original tmmelted material. The above equa elements with a particular model in mind. For tion is for equilibrium partial melting, also called example, instead of using MORE and Q, one can batch melting. The reverse is equilibrium crystal suppose that the MORE source was the result of a fractionation, in which a melt crystallizes and certain kind of melting process, and calculate the the crystals remain in equilibrium with the melt. properties of the melt and the residue. This usu The same equations apply to both these situati ally gives a more depleted result than the mixing ons. The effective partition coefficient is a weigh method. ted average of the mineral partition coefficients. The trace-element contents of basalts contain The Rayleigh fractionation law information about the composition, mineralogy and depth of their source regions. When a solid partially melts, the various elements composing applies to the case of instantaneous equilibrium the solid are distributed between the melt and precipitation of an infinitesimally small amount the remaining crystalline phases. The equation of crystal, which immediately settles out of the for equilibrium partial melting is simply a state melt and is removed from further equilibration ment of mass balance: with the evolving melt. The reverse situation is C _ Co Cr called fractional fusion. m- D(1 -F ) +F D Nickel and cobalt are affected by olivine and where C111 , C0 and Cr are the concentrations of orthopyroxene fractionation since D is much the element in the melt, the original solid and greater than 1 for these minerals. These are 188 MAGMAS: WINDOWS INTO THE MANTLE called compatible elements. Vanadium and scan imply. As semi-molten diapirs or melt pack dium are moderately compatible. Rare-earth pat ets rise in the mantle, the equilibrium phase terns are particularly diagnostic of the extent of assemblages change. The composition of a melt garnet involvement in the residual solid or in the depends on its entire previous history, including fractionating crystals. Melts in equilibrium with shallow crystal fractionation. Melts on the wings garnet would be expected to be low in HREE, Y of a magma chamber may represent smaller and Sc. MORE is high in these elements; kimber degrees of partial melting. Erupted melts are lite is relatively low. hybrids of low- and high-degree melts, shallow Melt-crystal equilibrium is likely to be much and deep melting, and melts from peridotites more complicated than the above equations and eclogites.