Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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fractionation can be approximated by consider-

ing just two trace elements—Rb and Sc. (a) Are

these elements compatible? Incompatible?

(b) Using the data in Table 12.7, make a plot

of Rb versus Sc for the sequence of rocks.

magma for the sequence? (d) Determine bulk

distribution coefﬁcients for Rb and Sc that,

when used in equation 12.1, yield a model

Rb-Sc curve ﬁtting the data points as closely as

possible. Can the entire sequence be modeled

by one set of coefﬁcients? Discuss. (e) What

weight proportions of clinopyroxene (Cpx),

titaniferous magnetite (Ti-Mag), and plagioclase

(Pl) account for differentiation of the basanite-

phonotephrite part of the sequence? Discuss

your results. (f) Can the tephriphonolite-

phonolite suite be produced by fractionating

the same minerals? Others? Discuss.

12.7 If the immiscible Fe-rich glass in column 1,

Table 12.3, were to crystallize, of what minerals

would it be composed and what would be their

modal proportions? Assume an oxygen fugacity

buffered by QFM (Figure 3.14).

12.8 For the shonkinite-syenite association in Table

12.3 plot the ﬁrst 13 trace elements listed on a

primitive-mantle normalized diagram (Table

2.7). How do the two patterns compare with

those for other rocks in this textbook? Is it pos-

sible that these two rocks could be related by

fractional cystallization, rather than by separa-

tion of immiscible melts? Critically evaluate

whether fractionation of common rock-forming

minerals might account for the trace element

contents of the two rocks. For example, are the

Co and Ni contents consistent with olivine frac-

tionation? Ti and V with Fe-Ti oxide fractiona-

tion? And so on.

Differentiation of Magmas

347

UNDAMENTAL

UESTIONS

ONSIDERED IN

HIS

HAPTER

1. What rock types and rock suites form petrotec-

tonic associations with particular tectonic settings?

2. How do trace element and isotopic compositions

of petrotectonic associations, combined with other

compositional, fabric, and ﬁeld data, provide

insights into magma evolution?

3. What role does global tectonic setting play in the

way magmas are generated and differentiated?

INTRODUCTION

Global magma production is inextricably linked with

movement of lithospheric plates and mantle plumes—

the two manifestations of convection in the cooling

mantle. Despite the stirring action of these two modes

of convection, the mantle has a long-standing chemi-

cal and isotopic heterogeneity. Hence, mantle-derived

magmas have varying elemental and isotopic signatures

that depend on the part of the mantle where plate- and

plume-related conditions allow partial melting. Con-

trasting mantle as well as continental sources, together

with variable processes of magma generation and dif-

ferentiation, depend upon the global tectonic setting in

which they function. Consequently, different global

settings yield suites of comagmatic rocks having dis-

tinctive compositional attributes, called petrotectonic

associations. A striking contrast between two such as-

sociations is the copious production of tholeiitic basalt

magma along oceanic rifts versus the creation of calc-

alkaline granitoid batholiths along continental margin

subduction zones.

As in any categorization of geologic systems, some

transitional associations are to be expected, such as the

back-arc setting where rocks have attributes of both is-

land arcs and oceanic spreading ridges. Additionally,

some tectonic settings can evolve over geologic time

from one to another. For example, a continental in-

traplate regime overlying a rising mantle plume might

evolve into a continental rift and then into an oceanic

rift; such is the well-documented continental breakup

experienced by landmasses surrounding the Atlantic

Ocean.

This chapter begins with associations of chieﬂy

basaltic magmas generated by decompressing mantle

where it wells up beneath oceanic spreading ridges and

ascends in deep mantle plumes. Following sections

deal with more varied arc associations where plates

converge and ﬁnally with associations in continental

rifts and stable cratons.

The intent of this chapter is to summarize the rock

types and rock suites that occur in major global tec-

tonic settings and the creation of the magmas in these

petrotectonic associations. Trace element and isotopic

data furnish insights into the origin of the magmas—

their sources, partial melting conditions, and subse-

quent differentiation.

Plate tectonic settings where petrotectonic associa-

tions develop are displayed schematically in Figures 1.5

and 11.2. Additional details can be found in many

books dedicated to speciﬁc associations, as referenced

herein, and in overall summaries by Hess (1989),

Wilson (1989), and McBirney (1993).

CHAPTER

Magmatic

Petrotectonic

Associations

Magmatic Petrotectonic Associations

349

Sediment

Seawater

CrustMantle

LITHOSPHERE

0.3−5.0 km

0.1−4.0 km

0.5−2.3 km

0.5−2.0 km

Magma

ASTHENOSPHERE

Metamorphic

ultramafic rocks

Ultramafic cumulates

Isotropic (minor cumulus)

gabbroic rocks

Sheeted diabase-

basalt dikes

Pillow lavas

(a)

13.1 Oceanic lithosphere consisting of crust and underlying mantle. (a) Highly idealized cross section at a spreading ridge. Note that the seis-

mic M-discontinuity between crust and higher-velocity ultramaﬁc mantle is here drawn at the top of the ultramaﬁc cumulates whereas

the petrologic discontinuity between magmatic crust and metamorphic mantle lies 0.5–2.0 km deeper. (b) Heat ﬂow from the seaﬂoor as

a function of its age. One heat ﬂow unit (HFU)  0.0418 W/m

. Shaded area approximates distribution of observed values of heat ﬂow

perturbed by advective circulation of seawater through the crust. Solid line is the theoretical heat ﬂow according to a model in which the

lithosphere is cooling by conduction only. (c) Schematic geologic cross section through the upper lithosphere (ignoring surface topo-

graphic characteristics and structural complications) showing increasing thickness of sediment with age away from crest of ridge; deep-

ening of seaﬂoor away from ridge crest due to cooling and thermal contraction of lithosphere; approximately uniform thickness of lay-

ers 2 and 3 of basalt and gabbro, which together with sediment constitute the oceanic crust; and the underlying ultramaﬁc rocks of the

uppermost mantle. Advective circulation of seawater through the upper part of hot crust diminishes away from ridge crest. (d) The litho-

sphere beneath the basalt-gabbro crust is a thickening lid of cooling peridotitic mantle covering the hotter asthenosphere. Light lines are

highly schematic isotherms suggesting that the base of the lithosphere, deepening with respect to age, is essentially an isothermal surface.

Dashed heavier lines indicate possible directions of ﬂow of the asthenosphere away from axis of the rise: Stippled area under the crest is

the approximate location of a zone of partial melting (compare Figure 13.4). Note difference in depth scales in (c) and (d). (b–d Redrawn

from Anderson et al., 1977.)

13.1 OCEANIC SPREADING RIDGES

AND RELATED BASALTIC ROCKS

The 65,000-km-long system of oceanic spreading

ridges encircling the Earth is by far the most signiﬁ-

cant expression of its magmatism (Figure 1.1). Act-

ing in concert with seaﬂoor spreading, magmatism at

oceanic rifts has produced in 200 My, since the early

Jurassic, the entire present-day oceanic crust, covering

nearly 70% of the Earth. The continental rock record

indicates that similar ridge magmatism and seaﬂoor

spreading occurred in pre-Mesozoic time, probably

well into the Precambrian.

Spreading junctures in the oceanic lithosphere

(Plate I) are marked by broad rises that are thousands

of km wide and stand about 2.5 km above the average

level of adjacent abyssal plains. Beneath these oceanic

rifts, upwelling, decompressing, and partially melting

mantle generates basaltic magma, which rises and

solidiﬁes as oceanic crust. Seismic data, augmented

by ﬁeld study of subaerially exposed segments of the

oceanic lithosphere (ophiolite; Section 13.6), indicate

the typically 6–7 km thick crust is layered (Figure

13.1a). Beneath a veneer of sediment, which thickens

away from the oceanic rift, are the following layers,

from the top down:

1. Basaltic lava ﬂows, commonly pillowed (Section

10.2.1).

2. Sheeted mass of their feeder dikes.

3. Massive gabbro that becomes more layered down-

ward and merges into layered ultramaﬁc cumulates;

these form in crustal magma storage chambers.

4. Locally in sharp contact with the cumulates is man-

tle peridotite that has a high-T, solid-state strain

fabric.

Advective percolation of seawater into the cooling,

pervasively fractured basaltic rocks at oceanic rifts pro-

duces widespread and locally intense alteration, or

ocean-ridge metamorphism (Special Interest Box 11.3).

Outcroppings of mantle peridotite on the seaﬂoor,

called abyssal peridotite, are also variably hydrated. In

addition to advective cooling of the oceanic crust by

circulating seawater, the lithosphere as a whole cools

conductively so that farther from the rift it progres-

sively cools and thickens (Figure 13.1d).

13.1.1 Mid-Ocean Ridge Basalt (MORB)

Basaltic rocks constituting the upper part of the

oceanic crust have been called many names, including

abyssal tholeiite, seaﬂoor and ocean-ﬂoor basalt, sub-

marine basalt, and mid-oceanic ridge basalt, or MORB.

Although not all ridges are centrally positioned in

oceans, the MORB label has become well entrenched.

Although most of the oceanic crust is subalkaline

tholeiite basalt, there are local, evolved rock types and

rare alkaline basalts.

The fabric of MORB reﬂects rapid cooling of gener-

ally near-liquidus magma extruded in cool seawater as

ponded sheet ﬂows, pillow lavas, and associated hydro-

clastic breccias. Quenched glassy rinds on pillows (Fig-

ure 7.3) and vitroclasts have very sparse phenocrysts

of bytownitic plagioclase with or without magnesian

olivine, Fo

80–90

, that contains minute Cr-Mg spinel

inclusions. Glass is commonly altered to palagonite.

Phenocrysts of augite are rare and usually conﬁned to

more extensively crystallized lavas that contain abun-

dant olivine and plagioclase. Glomeroporphyritic clots

represent accumulations of fractionated crystals. Local

disequilibrium attributes include embayed phenocrysts

in chemically more evolved groundmasses, phenocrysts

having corroded cores of higher-T solid-solution com-

position, and phenocrysts containing melt inclusions

different from groundmass glass. These attributes

likely reﬂect mixing of fractionated and more primitive

magmas (Section 12.4.3).

The elemental composition of normal MORB is rel-

atively uniform compared to that of other basaltic asso-

ciations. Typical is subalkaline olivine or quartz tholei-

ite, which contains normative olivine and hypersthene

or quartz and hypersthene, respectively (Table 13.1).

The range of SiO

is only 47–51 wt.%. The most dis-

tinctive attribute is the low concentration of incompat-

ible elements, including Ti and P, and large-ion-

lithophile elements, such as K, Rb, Ba, Th, and U,

compared to that of tholeiites in oceanic islands and

continental plateaus (see Tables 13.2 and 13.5). On a

Theoretical conductive model

Convective cooling

by sea water

Conduction

Observed values

Age (My)

80 120

160

Heat flow (HFU)

Sediment

Basalt, gabbro

Ultramafic rocks

Convecting

seawater

Age (My)

Water

120

160

S.L.

Depth (km)

Age (My)

040

120 160

Depth (km)

Lithosphere

Asthenosphere

S.L.

160

(b)

(c)

(d)

13.1 (Continued).

350 Igneous and Metamorphic Petrology

Magmatic Petrotectonic Associations

351

primitive-mantle-normalized diagram, the trace ele-

ment pattern is a relatively smooth, positively sloping

line (Figure 13.2) that shows lesser concentrations of

incompatible elements than, for example, oceanic is-

land tholeiitic basalt.

Origin of MORB Magmas and the Mantle Source. Be-

cause ocean-ridge magmas ascend through the thinnest

and compositionally least contrasting crust of any

petrotectonic association, they potentially provide the

clearest “window” into the mantle source. Normal

MORB is commonly used as a reference composition

for comparison with other maﬁc magmas. Nonetheless,

most MORB cannot represent primary magma from a

peridotite source (Section 11.3.5) for at least two rea-

sons: First, MORB has only about 5–10 wt.% MgO

and 300 ppm Ni and most have Mg/(Mg  Fe)

0.7. Second, most MORB magmas are not multiply

saturated near their liquidus with olivine  orthopy-

roxene  clinopyroxene at mantle pressures, as they

would be if they were in equilibrium with a near-

solidus peridotite source. Instead, compositions are

displaced from 10- to 20-kbar beginning-of-melting

invariant points in Figure 12.24 by apparent olivine

fractionation. Compositional variations in fresh glasses

corroborate olivine control (Figure 12.23). Primary

magmas are believed to be olivine-rich picrite that on

ascent and decompression lose olivine, reducing their

MgO, Ni, and Mg/(Mg  Fe) to values observed in

most MORB. Partial melting appears to occur in the

stability ﬁeld of spinel peridotite at depths of about

30–75 km (P  10–25 kbar) because the trace element

pattern of MORB (Figure 13.2) indicates that neither

plagioclase nor garnet was a residual phase in the

source; their absence is suggested by the lack of a neg-

ative Eu anomaly and low Sm/Yb ratio, respectively.

However, if large melt fractions, 20%, were gener-

ated, these aluminous phases would be consumed and

their associated geochemical indicators eliminated.

Large degrees of partial melting yield trace element

signatures for the melts like that of the source (Figure

11.15). Indeed, spinel-bearing Cr-diopside peridotite

xenoliths (Figure 11.6b) and normal MORB (N-

MORB in Figure 13.2) have similar light rare earth el-

ement and incompatible element depleted patterns.

This depletion in incompatible elements in the mantle

source before the generation of MORB partial melts

is conventionally interpreted to be a consequence of

long-term (billions of years) extraction of enriched

melts that formed the continents (Hofmann, 1988).

One facet of this complementary relation between

MORB and continental crust is their Sr and Nd isotope

compositions (Figure 2.27).

The compositions of MORB and of mantle (abyssal)

peridotite sampled at ocean ridges reﬂect varying de-

grees of partial melting and melt extraction, respec-

tively. Global variations in MORB shown in Figure

13.3a reveal an inverse correlation between CaO/Al

ratio and Na normalized at MgO  8 wt.%. The ratio

is essentially unaffected by fractionation of olivine, py-

roxene, and plagioclase but increases with increasing

degrees of partial melting as Ca-rich clinopyroxene is

dissolved out of the peridotite source. Normalized Na

decreases as the degree of partial melting increases:

That is, Na behaves as an incompatible element in

source peridotite. Modal variations in abyssal peri-

dotites collected on the ocean ﬂoor are consistent with

varying amounts of melted and extracted pyroxene

components from this mantle source (Figure 13.3b).

After polybaric fractionation of olivine during their

ascent from the mantle source, MORB magmas are

Table 13.1. Average Chemical and Normative

Composition of N-MORB and Trace Elements (ppm)

for N-MORB and E-MORB

N-MORB N-MORB

SiO

49.93 Or 1.00

TiO

1.51 Ab 22.06

15.90 An 31.14

FeO 10.43 Di 21.04

MnO 0.17 Hy 15.55

MgO 7.56 Ol 2.47

CaO 11.62 Mt 3.89

O 2.61 Il 2.87

O 0.17 Ap 0.19

0.08

N-MORB E-MORB

Cs 0.007 0.063

Rb 0.56 5.04

Ba 6.3 57

Th 0.12 0.6

U 0.47 0.18

Nb 2.33 8.3

Ta 0.132 0.47

La 2.5 6.3

Ce 7.5 15

Sr 90 155

Nd 7.3 9

Sm 2.63 2.6

Zr 74 73

Eu 1.02 0.91

Gd 3.68 2.97

Dy 4.55 3.55

Y28 22

Yb 3.05 2.37

Lu 0.455 0.354

Top, major oxides from McKenzie and O’Nions (1991); bottom, data

from Sun and McDonough (1989).

352 Igneous and Metamorphic Petrology

0.7

Basalt/primitive mantle

E-MORB

N-MORB

Hawaiian late-shield tholeiitic basalt

13.2 Primitive-mantle-normalized trace element patterns of oceanic tholeiitic basalts. Decreasing element incompatibility in maﬁc magmas

from left to right. MORB data and normalizing values from Sun and McDonough (1989). Hawaiian tholeiite from Table 13.3.

DUNITE

01 90 80 70

HARZBURGITE

Modal proportions

in olivine

Increasing Fo

Increasing amount of

extracted melt

LHERZOLITE

Cpx

1.5 2.0 2.5 3.0 3.5 4.0

8.0

0.6

0.7

0.8

0.9

CaO

(a)

Increasing degree of partial

melting in

MORB source

Opx

(b)

13.3 Compositions of MORB and abyssal peridotite reﬂect varying degrees of partial melting of the mantle source. (a) Global MORB com-

position. Variable on the x axis is the concentration of Na at 8 wt.% MgO on a Na-MgO variation diagram drawn for 84 sites at spread-

ing ridges around the world (775 samples in all). The CaO/Al

ratio is taken from a CaO/Al

-MgO variation diagram for samples

at each site; intrasite variation in the ratio is negligible. (Redrawn from Klein and Langmuir, 1987.) (b) Mineral proportions in abyssal

peridotite show that varying amounts of basaltic partial melt have been extracted. Data from Dick et al. (1984). Each point is the aver-

age modal proportion of Ol, Cpx, and Opx in 266 samples from 36 dredge-haul sites along rifts in the Atlantic Ocean, Indian Ocean,

and Caribbean Sea. The classiﬁcation diagram is for ultramaﬁc rocks (Figure 2.10b). The most extensively melted peridotite, that is, the

most sterile, has least Cpx and Opx and greatest Fo content of olivine (compare Figure 11.9).

differentiated at low P in crustal storage chambers be-

neath the rift. As discussed in Section 12.4.3, the lim-

ited compositional variation in most MORB reﬂects

recurring replenishment and mixing of relatively prim-

itive mantle-derived magmas with fractionating magma

in rift chambers, moderating the effects of crystal-melt

fractionation. However, uncommon seaﬂoor basalts

follow a fractionation trend of extreme Fe enrichment

and limited enrichment in silica and alkalies.

Conﬁguration of Magma Bodies and Partial Melt Zones.

There has been considerable speculation, but few hard

data, regarding the size, shape, longevity, and evolution

of crustal magma chambers in the oceanic crust. Addi-

tional uncertainties relate to how chamber character

and the shape of partial melt zones in the underlying

mantle might explain compositional characteristics of

MORB and subtle contrasts between MORB at slow

and fast spreading rifts (Wilson, 1989). Slow spreading

systems, such as the Mid-Atlantic Ridge, have rates less

than about 5 cm/y, whereas the fast spreading East

Paciﬁc Rise has a spreading rate of more than about

8 cm/y. The zone of partial melting in the mantle be-

neath slow spreading rifts is presumed to be narrowly

focused, perhaps only a few kilometers wide. This

shape is believed to be governed by buoyancy-driven

ﬂow in the slowly diverging asthenospheric mantle be-

low the rift axis. On the other hand, below the faster

spreading East Paciﬁc Rise, a detailed geophysical in-

vestigation of a segment near 17S latitude revealed a

broad, partially melted zone in the upper mantle that is

hundreds of kilometers wide and up to 150 km deep

(Figure 13.4). This wide zone reﬂects passive upwelling

driven by viscous drag of the separating lithospheric

plates. In other words, convection in the asthenosphere

is forced by plate movement, not the other way round.

Signiﬁcantly, there is no indication of asthenospheric

upwelling ﬂow from the deeper mantle, below the 410-

km seismic discontinuity.

13.1.2 Iceland

Iceland is a large volcanic island positioned astride the

actively spreading Mid-Atlantic Ridge and built of

older, Tertiary olivine tholeiite lava ﬂows ﬂanking a

central rift zone where more recent volcanism is fo-

cused (Figure 13.5). This relationship mimics that of

the submarine oceanic ridge to the north and south-

west of Iceland. Despite its topographic elevation (as

much as 2 km above sea level) and unusual thickness

(20–35 km), the Icelandic crust is not sialic as large

continents are. The central rift zone has numerous

northeast-trending, elongate extensional fault blocks

Magmatic Petrotectonic Associations

353

410 km discontinuity

500 km

300

200

100

200

300

West

Pacific plate

10 cm/y

Rise

crest

Nazca plate

4.5 cm/y

East

500

400

300

150

100

Depth (km)

Pressure (kbar)

13.4 Geophysical cross section through the East Paciﬁc Rise near 17°S. Electrical, magnetic, and seismic velocity data indicate a broad vol-

ume of slightly melted mantle peridotite (about 1–2% melt fraction) in the heavily stippled region and less in the lightly stippled region.

Because there appears to be no deﬂection of the 410-km seismic discontinuity beneath the rise, it is believed that asthenospheric mantle

ﬂow (arrows) is conﬁned to the shallow mantle above 410 km without ascending ﬂow from the deeper mantle. Note the asymmetry of

the partially melted zone, skewed to the west under the faster-moving (10 cm/y) Paciﬁc plate relative to the slower (4.5 cm/y) Nazca plate.

Lithosphere is shaded dark. P, depth to which plagioclase peridotite is stable; S, spinel peridotite; G, garnet peridotite; shaded bands in-

dicate transition depths (Figure 11.5). (Redrawn from Forsyth et al., 1998.)

354 Igneous and Metamorphic Petrology

13.5 Generalized geological features of Iceland. Older (>3 Ma) vol-

canic rocks light shaded; younger (3 Ma) rocks not shaded;

alkaline basalts dark shaded. Eruptive ﬁssures, including Laki,

shown by heavy lines; major central volcanic complexes

(Kraﬂa, Askja, Hekla, Torfajökull) by large ﬁlled circles; glaci-

ers enclosed by dotted lines. (Redrawn from McBirney, 1993.)

Torfajokull

Laki

Ice field

0 km

Askja

15°W

Krafla

66°

65°N

Oceanic

ridge

Surtsey

19°

Reykjavik

Hekla

and ﬁssures. The 12 km

of basaltic lava from the

25 km-long Laki ﬁssure in 1783 was the largest

recorded historic extrusion of lava, covering 565 km

in 8 months. Innumerable feeder dikes can be seen in

the topographically lowest exposures, but their num-

bers diminish upward in subhorizontal lava ﬂows.

Picrites were erupted locally, and increasingly more

alkaline basalts occur farther from the rift zone. More

evolved tholeiitic-suite magmas—including quartz

tholeiite, ferrobasalt, icelandite (low-Al, high-Fe an-

desite)—have erupted chieﬂy from numerous central

volcano complexes. In these central volcanoes, as many

as half of the rocks are silicic, commonly mildly peral-

kaline. The origin of the exceptionally abundant (for an

oceanic island) silicic rocks is controversial. The sig-

niﬁcantly lower 

O values of the silicic rocks than of

coeval basaltic rocks suggest partial melting of the hy-

drothermally altered, isostatically subsiding maﬁc Ice-

landic crust with heat supplied by intrusions of basalt

magma along extensive rift-ﬁssure systems (Gunnars-

son et al., 1998).

An order-of-magnitude-higher rate of magma pro-

duction in Iceland than in submarine oceanic rifts im-

plies an unusually large volume of underlying hot de-

compressing mantle—a plume (Figure 1.5). Seismic

imaging has indeed revealed a column of hotter, lower-

velocity mantle rock beneath Iceland in which partial

melting occurs to at least 410 km and probably deeper

(Figure 13.6). Melting in an ascending plume of hotter

mantle begins at greater depth than in upwelling man-

tle beneath an oceanic ridge and yields a greater melt

volume to build thicker crust (Figure 13.7).

Many studies have found that MORB northward

along the Mid-Atlantic Ridge, particularly primitive

tholeiitic basalts in Iceland, is increasingly enriched in

light REE and large-ion lithophile (LIL) elements. The

implication of this trend is examined next.

13.1.3 Mantle Reservoirs

If the seismic images of the shallow convecting man-

tle beneath the East Paciﬁc Rise (Figure 13.4) and the

ascending plume from the deep mantle beneath Ice-

land (Figure 13.6) are credible, they support the exis-

tence of two fundamentally distinct mantle reservoirs.

Widely espoused by geochemists, these mantle regions

are the source of basaltic partial melts that have distinct

trace element and isotopic signatures. One reservoir is

relatively depleted in incompatible elements, has rela-

tively nonradiogenic Sr and radiogenic Nd isotope ra-

tios, and is the source of normal MORB, or N-MORB;

this source corresponds to the upper mantle underly-

ing the global oceanic spreading ridge system. The

other reservoir is the deeper, relatively enriched mantle

near bulk silicate Earth in its isotopic composition

(Figure 2.27) that constitutes plumes. Many basalts

from the Mid-Atlantic Ridge north of 30°N appear to

be derived by mixing of the two mantle reservoirs or of

partial melts from them. These mixed-source basalts

are enriched in incompatible trace elements relative

to N-MORB and are sometimes called E-MORB

(Figure 13.2 and Table 13.1). LIL elements are en-

riched by about an order of magnitude, and the light-

REE-to-heavy-REE ratio (e.g., La/Yb) is greater in

E-MORB than in N-MORB.

A wide range of

Sr/

Sr and

143

Nd/

144

Nd ratios in

oceanic basalts (Figure 13.8) can be produced by mix-

ing of components from the two mantle reservoirs de-

scribed. However, some ocean island rocks, such as

those of Kerguelen in the Indian Ocean, have isotopic

ratios extending to more enriched levels, even beyond

bulk silicate Earth, implying derivation from sources

with higher Rb/Sr and lower Sm/Nd ratios. Small de-

grees of partial melting of the mantle generate melts

with higher Rb/Sr and lower Sm/Nd, because of the

differing compatibilities of the element pairs (Section

2.6.2). Where the partial melts migrate and metasoma-

tize the mantle, especially if the ﬁxation is ancient (say,

1 Ga),

Sr/

Sr is higher than in bulk Earth and

143

Nd/

144

Nd lower.

Additional end-member sources are necessary to ac-

count for the entire range of isotopic ratios (including

Pb and He) found in mantle-derived oceanic basalts

(e.g., Hart et al., 1992).

13.2 MANTLE PLUMES AND OCEANIC

ISLAND VOLCANIC ROCKS

Upwards of one million intraplate volcanoes are esti-

mated to dot the ocean. Many are shields. Some

emerge above sea level as islands, but the majority is

Magmatic Petrotectonic Associations

355

13.6 Seismic image of the Iceland mantle plume. (Courtesy of Cecily Wolfe.) The approximately 300-km-diameter plume to a depth of 410

km beneath Iceland. Other seismic data suggest a perturbed 660-km discontinuity that distinguishes the upper from the lower mantle

(Figure 1.3), indicating that the plume ascends from the lower mantle, in contrast to the convective pattern beneath normal oceanic ridges

(Figure 13.4).

0 10 20

1100

1300

1500

1700

1900

Liquidus

T (°C)

P (kbar)

Thickness of

melt (km)

120

160

200

240

Depth (km)

Upwelling mantle beneath

oceanic ridge

Solidus

Ascending mantle plume

25%

44% melt fraction

13.7 Partial melting of a model dry mantle peridotite. Compare Figure 11.5. Two hypothetical adiabatic P (depth)-T paths, A and B, of as-

cending, decompressing mantle are shown by heavy solid lines. The low-T path A, appropriate to upwelling mantle beneath oceanic

ridges, reaches the solidus at a little over 50-km depth and about 1330°C and during continued ascent partially melts out the aluminous

phase and clinopyroxene in the peridotite, yielding ultimately near the base of the crust about 20% melt whose T is about 1180°C. The

subsidiary diagram on the left indicates that the generated melt forms a layer of melt about 7 km thick, matching the thickness of the

oceanic crust. An ascending hotter mantle plume (path B) generates as much as 30% melt in the uppermost mantle at T 1300°C, form-

ing a layer about 25 km thick, matching that in the Hawaiian Islands. (Redrawn from McKenzie and O’Nions, 1991.)

356 Igneous and Metamorphic Petrology

0.702 0.703 0.704 0.705 0.706 0.707

Sr/

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

0.5134

143

Nd/

144

−8

−4

Ker-

guelen

Oceanic

sediment

Hawaiian

Islands

East

Pacific

Rise

Mid-Atlantic Ridge

Iceland

13.8 Isotope ratio diagram for oceanic volcanic rocks showing ﬁelds of oceanic island groups, East Paciﬁc Rise MORB, Mid-Atlantic Ridge

MORB, and oceanic sediment (off diagram). BE, isotopic ratios in bulk silicate earth. Compare Figure 2.27. (Redrawn from Wilson,

1989.)

submerged below sea level as seamounts, either be-

cause magma supply was limited and growth subdued

or because, once emergent as islands, they were eroded

off ﬂat to sea level and then submerged as the underly-

ing lithosphere cooled, contracted, and subsided (Fig-

ure 13.1c). Most volcanoes appear to form at or near

spreading ridges but others, especially in the Paciﬁc, lie

in linear volcanic chains that have monotonically

changing age along the chain (Plate I). The best known

is the northwesterly trending, elbow-shaped Hawaiian

Island–Emperor Seamount chain in the north Paciﬁc.

From its disappearance in the Kurile-Aleutian trench

in the north, the chain includes 100 ediﬁces culmi-

nating 6000 km to the southeast in the active volcanoes

on the island of Hawaii (Figure 13.9). The rate of mi-

gration of volcanic activity along this chain has been

about 8–9 cm/y. The proposal of Wilson (1963) that

this chain originated by movement (at that rate) of the

oceanic lithosphere over an essentially stationary man-

tle plume is now accepted by most geologists.

In contrast to the Hawaiian plume positioned be-

neath the interior of the Paciﬁc plate, plumes under the

Mid-Atlantic Ridge have produced paired chains of ex-

tinct volcanoes or aseismic ridges on each side of the

divergent juncture (see Section 13.3.3). The V shape of

the paired chains reﬂects a northerly component of

plate motion during ocean opening. The displaced po-

sition of some supposed plume-related islands off the

ridge axis, such as Tristan da Cunha, has been caused

by westward drift of the ridge over the plume in the last

few tens of millions of years.

13.2.1 Character of Volcanic Rocks

Oceanic island volcanoes are more diverse in composi-

tion than the predominantly tholeiitic basalts (MORB)

of submarine ridges. Rocks range from tholeiitic to

alkaline and from ultramaﬁc to felsic. Whereas the

Hawaiian Islands appear to have mostly tholeiitic and

lesser alkaline underpinnings and small late alkaline

cappings, subaerial exposures in other islands are

wholly alkaline (e.g., Tahiti) or wholly tholeiitic (e.g.,

Galapagos).

Subalkaline tholeiites and alkaline basaltic rocks

grade one into the other without any well-deﬁned

break. Mineralogical and modal contrasts (Wilson,

1989) are as follows: Olivine is exclusively a pheno-

crystic phase in tholeiites and is quite Mg-rich, Fo

90-70

whereas in alkaline basalts it is both a phenocrystic and

a groundmass phase and is more variable in composi-

tion, Fo

90-35

, even within a single hand sample. (Rims

of olivines are commonly replaced by orange-brown

“iddingsite”—a complex mixture of clay minerals and

hydrous ferric oxides—in subaerially oxidized lavas,

regardless of compositional afﬁnity.) Plagioclase is

more commonly phenocrystic in tholeiites than in

alkaline basalts, in which it has higher K

O content.

The alkali concentration may be sufﬁcient in alkaline

magmas to stabilize alkali feldspar; this phase appears

in the groundmass of alkaline basalts, together with

feldspathoids or analcite in more silica-undersaturated

rocks; vesicle linings may be of these same minerals.

Tholeiitic lavas, on the other hand, may contain one or

more silica polymorphs (quartz, cristobalite, and/or