Chapter 23
The upper mantle
Let's descend to that blind world mantle by combining known components of the below. I'll go first, and you can follow upper mantle- basalts, peridotites, recycled crust and so on. The MORB source is just part of the Dante upper mantle and it is not the only LIL-depleted part of the mantle. It is not necessarily convec Overview tively homogenized. The composition of the crust and the upper man Attempts to establish an average composi tle are the results of a series of melting and frac tion for 'the upper mantle' focus on the most tionation events, including the high-temperature depleted MORB lavas or abyssal peridotites and accretion of the planet. Early attempts to esti involve major assumptions about melt genera mate upper mantle (UM) chemistry started from tion, melt transport and differentiation processes the assumption that UM initially was the same that have affected these. The depleted upper as bulk silicate Earth (BSE) and differed mantle, that part of the mantle that is assumed from it only by the extraction of the continen to provide MORB by partial melting is variously tal crust, or that the most depleted - low in called DUM, OM, DMM and the convecting LIL, U, Th, K - midocean-ridge basalts plus their upper mantle. Simplified mass balance cal depleted refractory residues - unaltered abyssal culations suggested to early workers that this peridotites- constitute the entire upper mantle. depleted mantle constituted only ~ 30 % of the Traditionally, geochemists have assumed that the mantle; the 650-670 lan discontinuity was there lower mantle is undegassed BSE or primitive mantle fore adopted as the boundary between DUM and (PM). Geodynamicists have noted that there are 'the primitive undepleted undegassed lower man heat flow problems with this model, which they tle.' The starting condition for the upper mantle fixed up by putting a large amount of U, Th and (UM) was taken as identical to primitive man Kin a lower mantle stealth layer. tle (PM) - or bulk silicate Earth (BSE) - Large-scale melting and differentiation upon and the present lower mantle (LM). Estimates of accretion probably pre-enriched the upper man PM and BSE are based on cosmological and petro tle with incompatible elements, including the logical considerations. The hypothetical prim radioactive elements; the crust and the vari itive upper mantle -crust plus DUM- is ous enriched and depleted components sam labeled PUM. It is further assumed that the pled by current melting events were already in upper mantle - from the base of the plate to the upper mantle shortly after accretion and 650 Ian - after extraction of the crust, became solidification. well-stirred and chemically homogenous by vig Recycling of crust into the upper mantle is an orous convection. The non-MORB basalts that important current process. It is possible to esti occur at the initiation of spreading and at mate the composition of the fertile upper various locations along the global spreading 302 THE UPPER MANTLE
system are attributed to plumes from the chem Observed isotopic arrays and mixing curves of ically distinct lower mantle. basalts, including ocean-island basalts (OIB), can However, it can be shown that most or all of be generated by various stages of melting, mix the mantle needs to be depleted and degassed to ing, melt extraction, depletion and enrichment form the crust and upper mantle and to explain and do not require the involvement of unfrac the amount of 40Ar in the atmosphere; this tionated, primitive or lower-mantle reservoirs. was a major theme in the first edition of The However, the first stage of Earth formation - the ory of the Earth (Anderson, 1989). Deple accretional stage - does involve large degrees of tion and degassing of the upper mantle alone melting that essentially imparts an unfraction cannot explain the observations. In addition, ated - but enriched - chondritic REE pattern to the MORB reservoir and the CC are not exactly the upper mantle. Small-degree melts from this complementary; another enriched component is then serve to fractionate LIL needed. When this (Qcomponent) is added in, MORB + CC + Q require that most of the mantle must be processed and depleted. There must be What is the upper mantle? other components and processes beyond single stage small-degree melt removal from part of the On the basis of seismic data Bullen divided the primordial mantle to form CC. There are other mantle into regions labeled B. C and D. Region B enriched components in the mantle, probably in is the upper mantle and C is the Transition Zone the shallow mantle, and other depleted residues (410 to 1000 km). D. the lower mantle, starts at over and above the MORB-source. The upper man 1000 km depth. The upper mantle (Region B) was tle (UM) is still generally treated as if its composi subsequently found to contain a high-velocity tion can be uniquely determined from the prop lid and a low-velocity zone (LVZ), generally asso erties of depleted MORB - NMORB or DMORB - ciated with the asthenosphere. Region C, the and depleted peridotites, continental crust, and mantle transition region (TR), was subse an undifferentiated starting condition. quently found to contain two abrupt seismic The traditional hotspot and plume models of discontinuities near the depths of 410 and 650 OIB and enriched magma genesis and mantle km, and a region of high and variable gradi heterogeneity are unsatisfactory [mantle plu ent below 650 km depth. A depth of 670 km mes, plume paradoxes]. Traditional models was found for the deeper discontinuity in west for both OIB and MORB genesis involving ern North America and this was adopted for the only peridotitic protoliths are also being re PREM model. However, the average depth of the evaluated [olivine-free mafic sources]. discontinuity, globally, is 650 km, with a varia Although recycling has long been used as a mech tion of about 30 km. Some authors have referred anism for modifYing the isotopic character of to the 650 (or 670) km discontinuity as the base OIB, it is now becoming evident that it can also of the upper mantle and have suggested that create melting anomalies. The roles of eclog this represents a profound chemical and isotopic ite and garnet pyroxenite in petrogenesis and in boundary between depleted upper mantle and the formation of melting anomalies are becom primitive lower mantle, rather than primarily an ing evident (Escrig et al., 2004, 2005, Gao et a!., isochemical phase change, as originally inferred 2004, Sobolev eta!., 2005) [delamination man (Anderson, 1967). Others have suggested that the tle fertility]. Midocean-ridge basalts rep 1000 km level is a chemical boundary and should resent large degrees of melting of a large be retained as the definition of the top of the source volume, and involve blending of magmas lower mantle. Sometimes the TR is included as having different melting histories. The Central part of the upper mantle; sometimes it is defined Limit Theorem explains many of the differences as a separate region, Bullen's Region C. This con between MORB and other kinds of melts that fusion in terminology about what constitutes sample smaller volumes of the heterogenous the upper mantle and the lower mantle has mantle. led to the widespread view that there are WHAT IS THE UPPER MANTLE? 303 fundamental conflicts between isotope and a residual depleted peridotite or an unal geochemistry and geophysics, and confu tered abyssal peridotite. It has a simple one-stage sion about whether slabs penetrate into 'the history. lower mantle' or not. It appears that the upper In some models of upper-mantle chemistry 1000 km of the mantle -about 40% by mass of only the most depleted materials are used in its the mantle - differs from the rest of the mantle construction; hence Depleted Upper Mantle. and this also appears to be the active layer for plate tectonics. The majority of the incompat Heterogeno us upper mantl e ible trace elements that are not in the crust Convective stirring takes large blobs and shears may be confined to an even shallower depth and stretches them, folds them and stretches range. Bullen's nomenclature is precise and use them more, repeatedly, until the dimensions are ful and I will follow it. The terms upper mantle very small; for obvious reasons this is known and lower mantle are now fuzzy concepts because as the Baker's transformation, a funda of usage in the geochemical literature and the mental result of chaotic advection the decoupling of this usage from seismological ory. This theory may not be appropriate for the data. These terms will be used when precise mantle. It implies high Rayleigh number, stirring depths, volumes or masses are not needed. The or folding in one direction and low-viscosity pas term mesosphere has also been used for the mid sive particles. We must therefore pay attention mantle, Bullen's Region D'. The terms shallow to the materials that enter the mantle instead of mantle and deep mantle will be used to avoid relying on averages of the magmatic products. the conflict between the precise seismological We must keep an open mind about the possi definitions of mantle regions and geochemical bility of large fertile blobs in the man usage. tle, and extensive regions having high homologous temperature. Depleted upper mantle; the DUM idea From a petrological point of view, the man tle can be viewed as a multi-component sys Geochemists have ideas different from seismo tem. The known components of the upper man logical conventions about what constitutes the tle are recycled continental crust and other upper mantle. They are based on compositions mafic components, ultramafic rocks, MORB and of depleted midocean-ridge basalts, DMORB, and other basalts, depleted refractory residues and assumptions about how these form. The defini enriched components (Q, quintessence or fifth tion of the upper mantle adopted by isotope geo component) such as kimberlite; mixtures of chemists is the following. these satisfY most chemical constraints on the The upper mantle is that part of the mantle that composition of the mantle or BSE, including provides uniform and depleted midocean ridge major and trace elements. The compositions basalts and that formed by removal of the continental of basalts and the compositions of con crust; it extends from the Moho to the 670-lan mantle tinental and abyssal peridotites are discontinuity. It is also called 'the convecting mantle.' available in petrological databases. These can be used to reassemble the original petrology The upper mantle is usually viewed by and composition of the mantle, and with a few geochemists as homogenous because MORB other assumptions, the composition of the upper are relatively homogenous. The assumption is mantle. that homogenous products require homogenous Basalt and peridotite compositions represent sources. The composition of the inferred MORB the culmination of melt depletion and enrich reservoir has been attributed to the whole upper ment processes over the entire history of the mantle. Since MORB is depleted in LIL compared mantle, including the accretional process. There to other basalts, the above assumptions have are a variety of basalts and peridotites. The com led to the Depleted Upper Mantle, or DUM, con positions of many basalts and peridotites appear cept. DUM is a two-component system, DMORB to lie along mixing lines and the end-member 304 THE UPPER MANTLE
compositiOns have been interpreted as trapped Enrichment and depletion processes melts. depleted residues and recycled and dela Vigorous stirring can homogenize a fluid by a minated materials. On major element plots process !mown as chaotic advection. Dif (Chapter 15) the end-members are harzburgite fusive and thermodynamic processes, in the and MORE, or eclogite. Picrites. komatiites and absence of gravity, are homogenizers. Large primitive mantle have intermediate composi scale melting can be a homogenizer. Plate tec tions. On LIL and REE plots, the extreme com tonic and other petrological processes, however. positions are kimberlites and DMORE or abyssal create heterogeneities. Removal of small-degree peridotite. melts causes depletion of fertile regions (basalt From an isotopic point of view, oceanic sources)- or components- and refractory regions basalts are also treated as multi-component sys (depleted peridotites, cumulates). Sm.all-degree tems involving mixtures of DMM, EMl , EM2 melts are enriched in LIL and become the crust and HIMU and c or FOZO. These are short and the enriched components (EM) in the upper hand names for what are thought to be the mantle, such as ldmberlite and carbonatites, and various enriched (EM) and depleted (OM) iso in the sources of enriched magmas such as topic end-members of the mantle and there is EMORE and OIB. Large-degree melting occurs at a large literature on each. There is no agree spreading centers and at thin spots of the litho ment regarding the lithology or history that sphere from upwelling mantle that has been goes with each component. Trends of OIB and depleted (NMORE source) or enriched (EMORB MORE isotopic compositions approach - or con source) by the transfer of these small melt frac verge on- a hypothetical component of the man tions. Large-degree and large-volume melts blend tle referred to as FOZO (focal zone) or C together large- and small-degree melts from a (common) . It has been assumed that this reflects large volume of the mantle and give fairly the composition of the lower mantle. It is not uniform magmas with small variance in trace clear why the most common component in element and isotopic ratios, even if the shallow basalts should represent the deepest. rather than mantle is heterogenous. This is called melt aggre the shallowest, mantle. Melts pond beneath, and gation or blending. percolate through, the lithosphere, and inter Melt extraction from partially molten rocks act with it. A lithosphere or harzburgite com or crystallizing cumulates is not 100% effi ponent may therefore be involved in most mag cient and residual melts help explain some of mas. If so, ultramafic rocks (UMR) may anchor the trace element and isotopic paradoxes of the ends of both major element and isotope mantle magmatism, such as apparent contradic mixing arrays. UMRs are certainly the most tions between the elemental and isotopic com common or prevalent lithology of the shallow positions. Other sources of magmatic diversity mantle. include recycling, delamination and melting of The average or prevalent mantle com diverse lithologies such as eclogite and peri position has been referred to as PREMA in the dotite, which also have experienced various levels isotope literature. In contrast to the end-member of melt extraction and infusion. This heteroge components, PREMA, c and FOZO are interior nous upper mantle or statistical upper components - on isotope diagrams - and are mantle assemblage gives relatively homoge therefore either mixtures or sources. The one nous products when it experiences large degree extreme attribute of these average compositions melting. Magmas at ridges and thin spots is that basalts falling near these compositions represent blends of melts from vari tend to have higher variance in their 3 He/4 He ous depths, lithologies and extents of par ratios, and therefore contain some high 3 Hej4He tial melting. The compositions of ocean island samples. The most prevalent lithology of the basalts, ocean ridge basalts, and residual mantle mantle - peridotite - may be implicated in this reflect upper mantle processes of melt extraction, component while EM and HIMU may reside in migration and trapping - as well as recycling the fertile or mafic components. from the surface - and the sampling/melting WHAT IS THE UPPER MANTLE? 305
process that blends the various enriched and components are those that have relatively low depleted products. melting points and which provide the main com During accretion, melting is extensive and ponents for basalts. The incompatible LIL ele there should be good separation between the ments enter the melt. The refractory components elements - LIL - that enter the magmas and have higher melting temperatures and are left those that are retained by the residual solids. behind when partial melts are extracted. There The LIL include U, Th and K- the heat-producing is no requirement that the fertile and refractory elements - the concentrations of which were components are intimately mixed or remixed in much higher during the first Gyr after Earth for the mantle or that the fertile components are mation. The relative buoyancy of magma, and veins in a refractory matrix rather than km the self-heating tendency of potentially fertile sized blobs. When small amounts of melting are parts of the mantle combine to concentrate the involved the separation tendencies are quantified radioactive elements in the outer shells of the by partition coefficients, which give the parti Earth, including the crust and upper mantle. To tioning of a given element between the magma a good approximation the large-ion and large and the residual solid. When melting is exten charge elements - LIL and HFS elements - can sive, most of the incompatible and basalt form be considered to reside in the fertile parts of ing elements in the source end up in the magma the mantle and the crust. This is not necessar or the fertile components. ily true of non-LIL elements such as He and Os, DMORB and abyssal peridotites are, respectively, although these also are probably in the shallow the 'blank' slates - depleted basalt and refrac mantle. tory residue - upon which various enriched sig natures are written. These are among the most Composition of the upper mantle depleted of mantle materials and form the basis An estimate of the original composition of the for recent estimates of upper-mantle composition mantle (see Part IV) can be derived simply by (e.g. Donnelly et al., 2004, Salters and Stracke, mixing together all the products of mantle dif 2004, Workman and Hart, 2005). These are esti ferentiation. For example, the mantle produces mates of the most depleted parts of the upper MORB, EMORB, OIB, kimberlite (KlMB), island mantle, not the upper mantle as a whole. arc basalts and so on, and has also produced Kimberlites and continental crust are the ma the continental crust (CC). There is direct evi in enriched complements to the above. EMORB dence that the mantle contains a variety of and OIB are intermediate in trace element prop peridotites, pyroxenites and eclogites. One can erties - but extreme in some isotopes. Eclogite alternatively or in addition focus on the recy reservoirs/components are required to balance cled materials that are known to be entering the such trace elements as Re, Zr, Ti and Na but mantle - sediments, oceanic plates and delami are also implied by recycling models and by nated lower continental crust. All of these can mass balance using chondritic constraints. Mass be put into the mix. If one assumes that the mix balance calculations (e.g. Part IV and Chap ture must have chondritic ratios of the refrac ter 8 o f Theory of the Earth) imply that tory elements and that the present mass of the mantle contains less than about 10% of the the crust is a lower bound on the CC compo fertile - basalt, gabbro, eclogite, garnet pyroxen nent then one can estimate the mixing ratios ite - component; this can be regarded as poten of the components and the original composition tial basalt for ridges, islands, CFB and LIPs; some of BSE, or at least that part that has provided of this material may be recycled or delami material to the surface. This can be posed as nated crust/eclogite. This is about the amount a least-squares or geophysical inverse of oceanic crust recycled into the mantle over problem. billions of years at current subduction rates. The components in the mantle can be alter Delaminated lower continental crust also intro natively subdivided into fertile components and duces fertile material- eclogite cumulates- into refractory or residual-solid components. The fertile the upper mantle. The fertile components are 306 THE UPPER MANTLE
generally viewed as well-mixed with the refrac 0.001 - 0.002 kimberlite, or some combination. tory infertile components - depleted peridotites, All of this material fits easily into the upper lherzolites, ultramafic residues (UMR). They are mantle, and is potentially available for incor more likely to reside in large blobs. poration into ocean-ridge basalts, ocean-island Various combinations of materials give chon basalts, seamounts and continental basalts. Esti dritic or BSE ratios of the refractory LIL elements. mates of the composition of fertile mantle are The fertile components can account for all the LIL given in Table 23.1. and they occupy about 10% of the mantle; the Original unprocessed mantle may have average enrichment - of the fertile part of the included the equivalent of 7.5-9.5% NMORE. 1% m antle- is therefore a factor of 10 times the con OIB, 2% EMORE plus the present continental centration levels of primitive mantle. About crust (Chapter 13). This particular combination 0.5-2% melting is implied to obtain material gives chondritic ratios of the refractory LIL and as enriched as CC and KIMB from BSE, and to accounts for almost all the LIL and Na of BSE. deplete DM to the extent observed; up to approx The crust itself accounts for about 50-80% of the imately 25% melting of this still fertile residue, original LIL material in PM. Almost all the rest at a later time, is implied in order to generate the is contained in the various MORE components, more abundant depleted basalts. The time sepa in ldmberlites and in OIB. Other considerations ration of these events can be estimated from iso suggest that the mantle may contain from 6-15% topes to be of the order of 1.5 to 2.5 Gyr. This has eclogite. traditionally been taken as the convective over turn time of the mantle. From a plate-tectonic or top-down point of view it is the revisitation Melt generation time of a migrating spreading ridge. There is no contradiction between small-degree melting in Current upper-mantle conditions allow a wide the past, when the mantle was hotter, and large range of melt fractions, with the largest extents degree melting at the present, after the mantle of melting permitted at midocean ridges and has cooled down. Since the mantle is close to other thin-lithosphere spots where mantle near the melting point, the formation and removal the melting point can increase its melt content of very small-degree melts may have occurred in by adiabatic ascent to shallow levels, a mech the thermal boundary layer at the surface of the anism not available under thick plates. Large Earth, where the current temperature rises from degrees of melting can also occur in the more n ear 0 oc to about 1400 a c. Low-degree melting fertile - or eclogite-rich - regions of the man can occur in a surface or internal TBL or in recy tle. Small-degree melts occur on the wings of cled mafic material. High-degree melting requires midocean-ridge melting zones, and at greater special circumstances. depths, and under thick plates, and in the man Kimberlites, OIB and EMORE are probably all tle wedge above subducted plates. As slabs warm produced from the upper mantle; there is no rea up to ambient mantle temperature they can also son to suppose that they are not. Mass-balance experience small-degree melting. Thus, there are constraints on the composition of the mantle many opportunities for generating and removing can be achieved by adding EMORE, EM or KIMB enriched small-melt fractions, and for enriching to the very depleted components involved in and depleting various regions of the upper man DMORE genesis. For example, having 5% EMORE tle. On the present Earth, and probably through or 0.5% kimberlite plus MORE plus peridotite in out Earth history, small-degree and large-degree the upper mantle gives approximately chondri tic melting conditions can both operate. ratios of the refractory LIL. The simplest melt-generation scenerio is one Petrological and cosmochemical constraints in which the source rock is melted by a cer can be satisfied if the basaltic or fertile com tain amount and the melt is then completely ponents of the mantle are mainly DMORE but extracted. This single-stage process can be mod also include about 10% OIB, 10% EMORE and eled, and the melt and residue compositions Table 23.1 / Estimates of 'fertile mantle' compositions derived by mixing components (continental crust (CC), ultramafic rocks (UMR), MORB, om. kimberlite (Q)) in various proportions in order to match chondri tic ratios of refractory elements. BSE is bulk silicate Earth (CC, upper mantle, and lower mantle). Several estimates are given for some components % BSE BSE cc NMORB EM ORB OIB KIMB UMR UMR Fertile Mantle % Si02 48 48 59 50 50 49 35 44 45 44 45 45 Si02 MgO 34 34 4 8 8 8 28 41 39 39 37 37 MgO CaO 3.6 3.1 6.4 I 1.4 10.7 10 19 2.2 3.2 2.8 3.6 3.7 CaO AI203 4.5 3.8 15.8 15.7 16.4 14 1.3 2.3 4.0 3.2 4.6 4.7 AI203 K20 0.03 0.02 1.9 0.1 0.6 0.38 I 0.05 0.01 0.07 0.02 0.02 K20 Na20 0.4 0.3 3.2 3.1 3.3 2.1 0.8 0.2 0. 13 0.44 0.29 0.33 Na20 Ti02 0.2 0.2 0.2 1.5 1.7 2.5 0.5 0.1 0. 13 0.19 0.21 0.22 Ti02 ppm ppm Ba 6.99 5.22 390 6.8 100 350 2000 33.0 0.563 35.3 5.43 6.08 Ba Rb 0.64 0.39 58 0.7 12 31 200 1.9 0.05 2.3 0.61 0.71 Rb u 0.02 0.02 1.42 0.05 0.3 1.0 20 0.1 0.003 0.1 0.02 0.04 u Th 0.09 0.08 5.60 0.14 1.1 4.0 92 0.7 0.01 0.7 0.07 0.18 Th Ta 0.04 0.04 1.1 0.1 0.2 2.7 40 0.4 0.01 0.4 0.04 0.08 Ta Nb 0.71 0.97 12 2 16 48 400 4.8 0. 15 5.0 0.65 0.95 Nb Zr II 13 123 109 139 280 400 21.0 5.08 28.4 12.31 12.54 Zr La 0.7 0.6 18 3.5 10 37 200 2.6 0. 19 2.9 0.70 0.79 La Sm 0.4 0.3 3.9 3.6 4 10 30 0.5 0.24 0.7 0.48 0.51 Sm Yb 0.5 0.3 2 3.4 3 2 0.3 0.37 0.5 0.52 0.56 Yb 308 THE UPPER MANTLE
calculated, by use of partition coefficients for enrichment include fractionation during melt each element. This results in LIL-fractionated and ing or metasomatic events. Enriched MORE may enriched melts and strongly depleted residues. occur without any clear relationship The more likely situation is that melt is contin to plumes. uously extracted as melting proceeds, and some Many estimates of the composition of the melt is left behind. The extracted melt interacts upper mantle equate the NMORB source with with the matrix it is percolating through. Incom the whole upper mantle. The use of depleted plete melt extraction on the one hand, and trap MORB and ultra-depleted residues in the esti ping of small-degree melts on the other, result mates of DUM or DMM gives a lower bound on in source rocks that are enriched compared to the LIL content of the upper mantle. Use of BSE the theoretical residues of complete melt extrac as the starting composition for the upper man tion calculations. The small-degree melts act as tle and then removing CC from it leaves a very enriching or metasomatizing agents for both depleted composition for the subsequent gener magmas and mantle xenoliths and are essential ation of magmas. Calculations in Theory of components when attempting to do mass-balance the Earth, abbreviated in Chapter 13 of the calculations. Theoretical attempts to reconstruct present volume, suggest that accretional differen the composition of the primordial upper mantle tiation pre-enriched the upper mantle by a factor from the compositions of MORB, peridotite and of about 3 compared with primitive abundances, continental crust (CC) rely heavily on data selec and the CC, and the depleted and enriched com tion, melting models and partition coefficients. ponents evolved from that starting condition. Attempts are made to determine the average com The whole mantle was processed (mined) dur positions of the MORB and peridotite endmem ing accretion and the continental crust repre bers, and to make corrections for contamination. sents only part of the enriched material that was An alternative method (Chapter 13) just mixes in the starting UM. The NMORB 'reservoir' was together materials thought to represent a spec depleted by removal of small-degree melts, most trum of products of mantle melting and metaso of which remained in the shallow mantle and matism. some entered the crust. EMORB and OIB are prod ucts of those parts of the mantle that have been Enriched upper mantle? enriched by these small-degree melts, as well as The earliest studies of ocean-ridge basalts recog those parts of the mantle affected by recycling nized that some ridge segments display enrich and delamination of the lower continental crust. ments in highly incompatible elements. The ori Mantle peridotites, in general, have a wide gin of enriched midocean-ridge basalts (EMORB) range of compositions and can be viewed as is controversial. EMORB have been sampled both mixtures of the most depleted ones and an on- and off-axis at various ridges and fracture enriched fluid such as kimberlite. Continental zones, and at places not normally considered peridotites tend to have 1-2 orders of magnitude to be hotspots. EMORB have been attributed to higher LIL concentrations than theoretical interaction of plumes from the deep mantle abyssal peridotites from melt extraction with the depleted upper-mantle source of nor calculations. Infertile refractory peridotites can mal midocean-ridge basalts (NMORB). Enriched be enriched in trace elements by a low-degree basalts have also been attributed to preferen melt, or other metasomatic fluids, without nec tial melting of veins in the mantle, to small essarily being very fertile or a suitable source plumes dispersed as small-scale heterogeneities rock for basalts. Other enriched materials, such and to melting of enriched eclogitic veins from as recycled eclogite, may also reside at depth, e.g. subducted and stretched oceanic crust recycled in the transition region. into the upper mantle. EMORB along the EPR The outer 1000 km of the mantle - 40% of has been attributed to plume material that the mantle - appears not to be well stirred or was transported 5000 lan from Hawaii, through homogenous (Meibom and Anderson, 2003). Since the asthenosphere. Other proposed sources of the observed compositions of NMORB, EMORB, MASS BALANCE 309
peridotites, kimberlites and CC can be mixed depend on which ones are selected to be repre together to achieve chondri tic ratios of the refrac sentative. Even tholeiites have been subdivided tory trace elements (Chapter 13) or inferred BSE into DMORB, TMORB. NMORB, PMORB, EMORB, compositions of LIL, there is no compelling rea OIB and so on. Should one attempt to construct son to involve the lower mantle in mass bal averages, or pick endmembers. or pick represen ance calculations for the volatile and very incom tative values, or correct observed compositions patible elements, including U, Th and K. The for contamination? lower mantle, of course, was involved in the orig inal differentiation and is needed to balance the The MORB sources major elements and the chalcophiles and the Midocean ridge basalts range in composition more compatible elements such as Pb. In fact var from depleted (DMORB and normal. N-type ious geochemical paradoxes may be resolved with MORB) to transitional. enriched and plume-type a hidden isolated depleted (but not fertile) reser basalts (TMORB. EMORB, PMORB); NMORB are voir, such as parts of the mantle below 1000 km. tholeiites from normal ridge segments; DMORB are particularly depleted basalts that represent endmembers of this class of component. Even Mass balance MORB from normal ridge segments display a range of compositions. including EMORB; the In the standard model of mantle geochem definitions of N-type MORB and 'normal ridge istry. the continental crust, CC, is considered segments' are arbitrary. From a major element to be complementary to the MORB reservoir. and petrological point of view even OIB tholei MORB extraction leaves behind a complementary ites are similar to MORB and imply similar depleted residue, and the lower mantle remains amounts of melting. Estimates of the composi primordial (PM); tion of the upper mantle, however, focus on the BSE = CC +DUM+ PM most depleted products. CC+DUM =PM Salters and Stracke (2004) presented an esti DUM = MORB + UMR mate for the composition of depleted mantle (DM), tailored to be a suitable source for midocean In the RAdial ZOne Refining model, RAZOR, ridge basalts. The depleted mantle reservoir is for Earth accretion and differentiation, the man defined as mantle that can generate 'pure' MORB tle melts, fractionates and differentiates dur uncontaminated by enriched or 'plume components'; ing accretion. The proto-crust and proto-upper 'pure' MORB is then used to estimate upper mantle are the buoyant products of this mantle composition. Estimates for some ele accretional differentiation and a perovskite-rich ments are derived from elemental and isotopic residue (PV) makes up the deeper mantle. compositions of peridotites assumed to be com plementary to MORB. The concentrations of some BSE = CC + MORB + UMR + PV + Q elements are estimated by subtraction of a the There is a large literature on the composition oretical low-degree melt from an unfractionated of each of these components and attempts have bulk silicate Earth (BSE) composition. The meth been made to filter and correct values of natu ods used by Salters and Stracke to select and ral samples to be representative of uncontami filter the input data are typical of this forward nated material. There are several recent attempts approach to modeling compositions. to derive the composition of DUM from literature A variety of criteria are used to identify values of the composition of MORB, UMR and CC 'depleted' (i.e. lacking an enriched component) (Donnelly et al., 2004; Salters and Stracke, 2004; MORB and to eliminate enriched or potentially Workman and Hart, 2005). enriched samples: the choice of criteria influ Data selection is a problem in this approach. ences the calculated source region - DM - com Basalts and peridotites have a large range in com position. Only those MORB samples 'thought position. Estimates of upper mantle composition to be representative of DM' are considered. 310 THE UPPER MANTLE
Attempts are made to avoid accessory phases accounts for 4.5-6%. EMORB and OIB can be up and hydrothermal alteration. The net result is to 0.7% of the mix and KIMB can be up to 0.15%. that estimates of DMM, DM or DUM, and the CC is constrained to be 0.55%. Since the upper wh ole upper mantle in some models, are biased mantle is 30-40% of the mantle, there is no mass toward very depleted end members. Remaining balance reason to suppose that the enriched and questions are, what is the fraction of EM and recy fertile components - EMORB, OIB, KIMB - must cled components in the source regions of MORB come from the lower mantle. and OIB, what is the origin of EM, and what is the Primitive upper mantle (PUM) in the standard average composition of the upper mantle? The model of mantle geochemistry is standard model is that MORB derive from a vig orously convecting well-stirred reservoir and that PUM = CC + DUM enriched materials, and hidden heat-producing reservoirs are in the deep mantle. ln the current model, fertile mantle (FM), Although MORB and their presumed residues, prior to removal and stabilization of the conti abyssal peridotites, have some degree of heteroge nental crust, was neity in radiogenic isotope ratios (Sr-Nd-Pb-Hf), FM = CC + MORB + EM ORB + OIB + KIMB they have a small range of values relative to ocean island basalts and are, with some excep + residual refractory peridotites (UMR) tions, depleted, relative to bulk Earth values, thus The currently accessible outer reaches of the requiring a long-term history of low Rb/Sr, Hf/Lu mantle probably contain the MORE-source, the and Nd/Sm (i .e. incompatible element depletion). EMORB-source, the OlE-source and the KIMB The small range in composition is partly a result source. The non-fertile mantle (ESE minus FM), of the data selection and filtering operations just discussed and partly due to the averaging accom based on cosmic abundances of the major ele plished by natural processes at ridges (Meibom ments, has more silica and less magnesia than and Anderson, 2003). FM and is therefore less olivine-rich. Estimates of Donnelly et al. (2004), Salters and Stracke (2004) and Workman and Hart (2005) Enriched blobs for the NMORB source can be made consistent The NMORB-source region, DMM, is just part with chondritic and accretional differentiation of the mantle, probably just part of the upper constraints, and the compositions in Chapter 13 mantle. A representative mass-balance calcula simply by using different proportions of the com tion using published estimates of the composi ponents, and by adding enriched components tions of DMM (DMORB + abyssal peridotite), con representing small degrees of melt, or recycled tinental crust, recycled oceanic crust (now in the material. Mass-balance calculations provide no mantle as eclogite) and an OIB-source yields, in constraints on the sizes and locations of the var round numbers, 74% DMM, 5% recyling MORB ious components, reservoirs or blobs. Chondritic (eclogite), 21 % OlE-source and 0% primitive man constraints, however, can be satisfied even if all tle. The fraction of mafic material in this and sim the components are in a volume less than the ilar calculations is less than the mass of oceanic size of the upper mantle. crust generated throughout Earth history, assum Table 23.1 contains estimates of BSE, CC, ing current rates for 4.55 Ga (6-7% the mantle NMORB, EMORB, OIB, KIMB and various ultra mass). mafic rocks (UMR). The fertile-mantle values tab One point of this exercise is to show that ulated in Table 23 .1 are derived by mixing MORB. there is no mass-balance contradiction with the EMORB, OIB, KIMB, CC and various estimates hypothesis that most of the fertile and heat-producing of the compositions of spinel and abyssal peri elements are in the upper mantle. It is only by dotites in order to retain the UL and refrac assumption, in simple box-model calculations, tory element ratios in BSE (Chapter 13). Peri that the entire upper mantle is required to be dotites account for 92-94% of the mix and MORB depleted and low in radioactive elements. This MASS BALANCE 311 single assumption has created many geochemi Donnelly et al. (2004) estimate that about 2-10% cal paradoxes. of ridge samples are EMORB in which incom Mass-balance calculations make no statement patible elements such as Th and Ba are a fac about the origins, locations or shapes of the tor of 10 more abundant than in typical NMORB enriched domains. TI1ey are usually assumed to sources. The continental crust and kimberlites be large isolated regions of the mantle, such contain enrichment factors of up to 100 and as the lower mantle, or the sizes of grain 1000, respectively - relative to BSE - for the boundaries and veins. Fertile, or mafic, material more incompatible elements. End-member kim gets into the mantle via subduction of oceanic berlites can account for no more than 0.1% of crust and delamination of continental crust, the mass of BSE, by simple mass balance. This and fertile blobs can have dimensions typical of is about one-sixth of the present mass of the these tectonic elements. Delaminated lower crust continental crust and about one-tenth of the may have dimensions of tens of km. Subducted upper bound on the allowable amount of CC seamount chains may have dimensions of tens by in BSE. hundreds of km. If oceanic plateaus can subduct Estimates of the degree of melting required to they would generate fertile regions having lateral generate MORB and EMORB from a fertile peri dimensions of thousands of km. dotite - ultramafic source - range from 6-20%. The present crust is 0.566% of the mass of Estimates of the degree of melting obviously the mantle and contains more than 50% of the depend on the source composition. Magmas can Earth's inventory of some elements (Chapter 13). also be derived from mafic sources such as recy Therefore, there cannot be more than two crustal cled ocean crust, delminated lower continental masses in the BSE, or one in the mantle. TI1e max crust and eclogitic cumulates. Eclogite is a type imum thickness of the crust, except in actively of rock and also a metamorphic facies; it can converging areas, is about the depth to the have an original basaltic extrusive or gabbroic basalt-eclogite phase change, suggesting that cumulate protolith; or it can be a residue left the volume of the present crust may not be over from partial melting and melt extraction. controlled by the volume of potential crust but by TI1ese 'eclogites' are not expected to have the the depth of a phase change. TI1e potential crust same compositions as unprocessed ocean crust in the mantle may be about equal to the present unless the metamorphic transformation to eclog continental crust. Some of this may appear as ite is isochemical. Eclogites are cpx- and ga-rich enriched components - EM2, say - in midocean by definition but may also contain trace to abun ridge and ocean-island basalts dant oxide minerals (e.g. rutile) that replace orig EMORB is found far from any melting inal magmatic ilmenite and magnetite. Phases anomalies and bathymetric highs, showing that such as rutile, zircon, amphibole and micas (phlo enriched components are widely available in the gopite in particular) can dominate some trace shallow mantle; it is chemically similar to OIB. element patterns.