1. Chapter 16. Island Arc Magmatism
Arcuate volcanic island chains along subduction zones
Distinctly different from mainly basaltic provinces thus far
Composition more diverse and silicic
Basalt generally subordinate
More explosive
Strato-volcanoes most common volcanic landform

2. The initial petrologic model:
Igneous activity related to convergent plate situations- subduction of one plate beneath another
The initial petrologic model:
Subducted oceanic crust is partially melted
Partial melts- more silicic than source
Melts rise through the overriding plate ® volcanoes just behind leading plate edge
Unlimited supply of oceanic crust to melt
This simple elegant model fails to explain many aspects of subduction magmatism
Partial melts should be less mafic than their parent
Ultramafic mantle  mafic basalt
Basaltic oceanic crust  intermediate andesites

3. The Subduction Factory
From Tatsumi, Y. (2005) The subduction factory: How it operates in the evolving Earth. GSA Today, 15, 4-10.

4. Active Continental Margin (ACM)
Ocean-ocean  Island Arc (IA)
Ocean-continent  Continental Arc or
Active Continental Margin (ACM)
Figure Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding plate. PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.

5. Subduction Products Characteristic igneous associations
Distinctive patterns of metamorphism
Orogeny and mountain belts
Complexly
Interrelated
Orogenic and Subduction-related synonyms when referring to the common association of basalts, basaltic andesites, andesites, dacites, and rhyolites produced at subduction zones
= “orogenic suite”

6. Structure of an Island Arc
Note mantle flow directions (induced drag), isolated wedge, and upwelling to  back-arc basin spreading system
Benioff-Wadati seismic zone (x x x x)
Volcanic Front
h is relatively constant  depth is important
Figure Schematic cross section through a typical island arc after Gill (1981), Orogenic Andesites and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6 joules/cm2/sec)

7. Volcanic Rocks of Island Arcs
Complex tectonic situation and broad spectrum of volcanic products
High proportion of basaltic andesite and andesite
Most andesites occur in subduction zone settings
Basalts are still very common and important!

8. Major Elements and Magma Series
Tholeiitic (MORB, OIT)
Alkaline (OIA)
Calc-Alkaline (~ restricted to SZ)
All three series occur in SZ setting, yet something about SZ is different that  CALC-ALKALINE
Calc-alkaline magma series is used as yet another synonym to orogenic suite by some workers
Since other magma series can occur at subduction zones, I recommend that we use the term calc-alkaline strictly to denote a chemical magma series, not a tectonic association

9. Major Elements and Magma Series
a. Alkali vs. silica
b. AFM
c. FeO*/MgO vs. silica
diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics
Figure Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90,

10. Figure 16. 4 The three andesite series of Gill (1981)
Figure 16.4  The three andesite series of Gill (1981). A fourth very high K shoshonite series is rare. Contours represent the concentration of 2500 analyses of andesites stored in the large data file RKOC76 (Carnegie Institute of Washington).

11. Figure a. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K, diamonds = low-K series from Table Smaller symbols are identified in the caption. Differentiation within a series (presumably dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.

12. Figure b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series.

13. Figure c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series. The gray arrow near the bottom is the progressive fractional melting trend under hydrous conditions of Grove et al. (2003).

14. 6 sub-series if combine tholeiite and C-A (some are rare)
May choose 3 most common:
Low-K tholeiitic
Med-K C-A
Hi-K mixed
Figure Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calc-alkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The points represent the analyses in the appendix of Gill (1981).

15. Tholeiitic vs. Calc-alkaline differentiation
These Harkers represent > 1 volcano from each arc, but give the general idea nonetheless
C-A shows continually increasing SiO2 and lacks dramatic Fe enrichment (note Guatemala and PNG in FeO*/MgO)
TiO2 decr due to Fe-Ti oxide
CaO/Al2O3 should increase with Plag FX, but decreases in all types -> Cpx responsible for CaO
SiO2 doesn’t show up in Harker, but it increases progressively in CALC-ALKALINE (although ~ constant in Thol)
Figure From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

16. Calc-alkaline differentiation
Early crystallization of Fe-Ti oxide
Probably related to the high water content of calc- alkaline magmas in arcs, dissolves  high fO2
High PH2O also depresses plagioclase liquidus  more An-rich
As hydrous magma rises, DP  plagioclase liquidus moves to higher T  crystallization of considerable An- rich-SiO2-poor plagioclase
The crystallization of anorthitic plagioclase and low- silica, high-Fe hornblende may be an alternative mechanism for the observed calc-alkaline differentiation trend

17. Other Trends Spatial Temporal
“K-h”: low-K tholeiite near trench  C-A  alkaline as depth to seismic zone increases
Some along-arc as well
Antilles  more alkaline N  S
Aleutians is segmented with C-A prevalent in segments and tholeiite prevalent at ends
Temporal
Early tholeiitic  later C-A and often latest alkaline is common
Many exceptions to any trend!

18. Trace Elements
REEs
Slope within series is similar, but height varies with FX due to removal of Ol, Plag, and Pyx
(+) slope of low-K  DM
Some even more depleted than MORB!
Others have more normal slopes
® heterogeneous mantle sources
HREE flat, so no deep garnet
Figure REE diagrams for some representative Low-K (tholeiitic), Medium-K (calc-alkaline), and High-K basaltic andesites and andesites. An N-MORB is included for reference (from Sun and McDonough, 1989). After Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag.

19. MORB-normalized Spider diagrams Intraplate OIB has typical hump
Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp

20. MORB-normalized Spider diagrams
IA: decoupled HFS - LIL (LIL are hydrophilic)
What is it about subduction zone setting that causes fluid-assisted enrichment?
Figure Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp
Figure 16-11a. MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Pearce (1983) with LIL on the left and HFS on the right and compatibility increasing outward from Ba-Th. Data from BVTP. Composite OIB from Fig 14-3 in yellow.

21. Isotopes
New Britain, Marianas, Aleutians, and South Sandwich volcanics plot within a surprisingly limited range of DM
Figure Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures and After Wilson (1989), Arculus and Powell (1986), Gill (1981), and McCulloch et al. (1994). Atlantic sediment data from White et al. (1985).
The principal source of island arc magmas is very similar to MORB source (DM)
Trace element data still require enriched components
The data for the other arcs extend along 2 enrichment trends, one for the Banda arc and the other for the Lesser Antilles extend beyond the OIB field
Antilles (Atlantic) and Banda and New Zealand (Pacific) can be explained by partial melting of a MORB-type source + the addition of the type of sediment that exist on the subducting plate (Pacific sediment has 87Sr/86Sr around and 143Nd/144Nd around 0.715)
The increasing N-S Antilles enrichment probably related to the increasing proximity of the southern end to the South American sediment source of the Amazon

22. Pb is quite scarce in the mantle
Low-Pb mantle-derived melts susceptible to Pb contamination
U, Pb, and Th are concentrated in continental crust (high radiogenic daughter Pb isotopes)
204Pb non-radiogenic: 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb increase as U and Th decay
Oceanic crust also has elevated U and Th content (compared to the mantle)
Sediments derived from oceanic and continental crust
Pb is a sensitive measure of crustal (including sediment) components in mantle isotopic systems
93.7% of natural U is 238U, so 206Pb/204Pb will be most sensitive to a crustal-enriched component
Mantle-derived melts are susceptible to contamination from U-Th-Pb-rich reservoirs which can add a significant proportion to the total Pb
U, Pb, and Th are concentrated in sialic reservoirs, such as the continental crust, which develop high concentrations of the radiogenic daughter Pb isotopes
204Pb is non-radiogenic, so 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb increase as U and Th decay
Oceanic crust has elevated U and Th content (compared to the mantle) as will sediments derived from oceanic and continental crust
Pb is perhaps the most sensitive measure of crustal (including sediment) components in mantle isotopic systems
Since 93.7% of natural U is 238U, equation 9-20 will dominate over equation 9-21, and thus 206Pb/204Pb will be most sensitive to a crustal-enriched component
U  234U  206Pb
U  207Pb
Th  208Pb

23. H I M U
Geochron = line on which all undisturbed/unreset Pb isotopic systems (such as BSE) plot.
~ NONE of the arc volcanics fall on the geochron.
Pb in some arcs overlap with the MORB (DM component = major subduction reservoir)
Majority of data enriched in radiogenic lead (207Pb and 206Pb), trending toward the appropriate oceanic marine sedimentary reservoir
Several arcs could = mixing of DM, PREMA, and sedimentary sources
Sunda extends to EMII
Antilles follow mixing line between DM or PREMA and Atlantic sediments, perhaps extending beyond toward HIMU (Fig. 14-6)
Pb data clearly indicate a sedimentary component in arc magmas
But WHEN?? Ancient EM component or recent sedimentary additions to the mantle?
Nd-Sr-Pb isotopes cannot distinguish
Figure Variation in 207Pb/204Pb vs. 206Pb/204Pb for oceanic island arc volcanics. Included are the isotopic reservoirs and the Northern Hemisphere Reference Line (NHRL) proposed in Chapter 14. The geochron represents the mutual evolution of 207Pb/204Pb and 206Pb/204Pb in a single-stage homogeneous reservoir. Data sources listed in Wilson (1989).

24. 10Be created by cosmic rays + oxygen and nitrogen in upper atmos.
 Earth by precipitation
Readily  clay-rich oceanic sediments
Half-life of only 1.5 Ma
Long enough to be subducted
After about 10 Ma 10Be is no longer detectable
Not a part of main mantle systems
10Be/9Be averages about 5000 x  in the uppermost oceanic sediments
In mantle-derived MORB and OIB magmas, & continental crust, 10Be is below detection limits (<1 x 106 atom/g) and 10Be/9Be is <5 x 10-14
9Be is a stable natural isotope & used as a normalization factor

25. Boron is a stable element
Very brief residence time deep in subduction zones
B in recent sediments is high ( ppm), but has a greater affinity for altered oceanic crust ( ppm)
In MORB and OIB it rarely exceeds 2-3 ppm
Very brief residence time deep in subduction zone source areas & cycles quickly through to shallow crustal and hydrospheric systems
B concentrations in recent sediments is high ( ppm), but has a greater affinity for altered oceanic crust ( ppm)
In MORB and OIB it rarely exceeds 2-3 ppm

26. 10Be/Betotal vs. B/Betotal diagram (Betotal  9Be because 10Be so rare)
Figure Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88,
Each arc formed unique linear array
Other known reservoirs: typical mantle (virtually no 10Be or B), hydrated and altered oceanic crust (high B, low 10Be), and young pelagic sediments (low B and 10Be/Be extending off the diagram up to 2000)
Simplest explanation: each arc represents a mixing line between a mantle reservoir (near the origin) and a fluid (or melt) reservoir, that is specific for each arc and itself a mixture of slab crust and sediment
Hypothetical fields for each arc are illustrated, but the exact location along the extrapolated line is unknown

27. Petrogenesis of Island Arc Magmas
Why is subduction zone magmatism a paradox?
Paradox: Great quantities of magma are generated in regions where cool lithosphere is being subducted into the mantle and isotherms are depressed, not elevated
No adequate petrogenetic model can be derived without considering the thermal regime in subduction zones

28. Main variables that can affect the isotherms in subduction zone systems:
1) Rate of subduction
2) Age of subduction zone
3) Age of subducting slab
4) Extent to which subducting slab induces flow in the mantle wedge
5) Effects of frictional shear heating along W-B zone
Other factors, such as:
Dip of slab
Endothermic metamorphic reactions
Metamorphic fluid flow
are now thought to play only a minor role

29. Typical thermal model for a subduction zone
Isotherms will be higher (i.e. the system will be hotter) if
a) Convergence rate is slower
b) Subducted slab is young and near the ridge (warmer)
c) Arc is young (< Ma according to Peacock, 1991)
yellow curves = mantle flow
This is sufficiently representative
Figure Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, ) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

30. The principal source components  IA magmas
1. Crustal portion of the subducted slab
1a Altered oceanic crust (hydrated by circulating seawater, and metamorphosed in large part to greenschist facies)
1b Subducted oceanic and forearc sediments
1c Seawater trapped in pore spaces
Figure Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, ) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

31. The principal source components  IA magmas
2. Mantle wedge between slab and arc crust
3. Arc crust
4. Lithospheric mantle of subducting plate
5. Asthenosphere beneath slab
The last three sources on the list are unlikely to play much of a role
Lithospheric mantle of subducting plate (4) is already refractory (extraction of MORB at the ridge) and it heats very little in the upper 200 km of the subduction zone
Asthenosphere beneath the slab (5) flows with it, but does not heat much at all, since the isotherms are essentially parallel to the flow lines at these depths
Arc crust: isotherms at the base of the (predominantly andesitic) crust  the temperature is too low for melting, even under hydrous conditions
(much more important in active continental margins)
Figure Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, ) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

32. Left with the subducted crust and mantle wedge
Trace element and isotopic data ® both contribute to arc magmatism.
How, and to what extent?
Dry peridotite solidus too high
LIL/HFS ratios of arc magmas  water plays a significant role in arc magmatism
Since we know what the general composition of the constituents in Fig are, it is a matter of combining this information with the other information in the figure showing us the pressure-temperature conditions to which the constituents will be subjected as they move through the subduction zone, and considering the consequences

33. Sequence of pressures and temperatures a rock subjected to during burial, subduction, metamorphism, uplift, etc. is called a pressure-temperature-time (P-T-t) path
Oceanic crust, as it subducts, will begin to heat at about km depth, and will continue to heat with rising pressure, although slowly due to the depressed isotherms
Mantle wedge material will follow the path of drag-induced flow, also illustrated in Fig This is less well known, but should follow a path (like the arrows beginning in the center of the right edge of the figure) of initial cooling from about 1100oC to about 800oC at nearly constant pressure, and then heat up toward 1000oC as pressure increases

34. P-T-t paths for subducted crust Based on subduction rate of 3 cm/yr
(length of each curve = ~15 Ma)
Subducted Crust
Yellow paths = various arc ages
Red paths = different ages of subducted slab
The yellow P-T-t paths represent various arc ages
A newly formed arc will not have subducted sufficient cool slab material to fully depress the initial ocean geotherm that existed prior to the initiation of subduction
It will thus follow a path toward higher temperatures at low pressure
Only after Ma will a steady state equilibrium be attained between subduction and heating of the slab
Figure Subducted crust pressure-temperature-time (P-T-t) paths for various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc) assuming a subduction rate of 3 cm/yr (Peacock, 1991, Phil. Trans. Roy. Soc. London, 335, ).

35. Add solidi for dry and water-saturated melting of basalt
and dehydration curves of likely hydrous phases
Subducted Crust
Figure Subducted crust pressure-temperature-time (P-T-t) paths for various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc) assuming a subduction rate of 3 cm/yr (Peacock, 1991). Included are some pertinent reaction curves, including the wet and dry basalt solidi (Figure 7-20), the dehydration of hornblende (Lambert and Wyllie, 1968, 1970, 1972), chlorite + quartz (Delaney and Helgeson, 1978). Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
None of the P-T-t paths intersect the dry solidus -> NO partial melting of dry basaltic crust
Melting of wet basalt will depend upon the availability of “free” (non-bonded) water
Numerous metamorphic reactions involve dehydration of the hydrated oceanic crust and/or subducted sediments
Because chlorite and hornblende are the two dominant hydrous minerals in metamorphosed hydrated basalt, the breakdown curves for chlorite + quartz and hornblende are included
Biotite and other hydrous minerals will mostly dehydrate between these curves

36. Mature arcs (lithosphere > 25 Ma): Dehydration D releases water in
No slab melting!
Slab melting M in arcs subducting young lithosphere.
Dehydration of chlorite or amphibole releases water above the wet solidus ® (Mg-rich) andesites directly.
Subducted Crust
What happens to intermediate cases, such as the curve for the 5 Ma old arc?
If dehydration occurs before the subducted material reaches the wet melting point, will it melt?
Depends upon the fate of the water released by dehydration
If it remains in place, melting will occur when the wet solidus is reached
Many workers believe that the water rises and leaves the system, and the remaining crust will no longer have free water, and no melting will occur at the wet solidus
An anhydrous eclogite soon forms
Will it melt, or will the water escape first? This is the subject of great controversy, and models diverge at this point into those that perceive the melting of the oceanic crust (as eclogite plus water) as the dominant source of arc magma, and those that favor the mantle wedge.

37. Newer models allow for temperature and stress dependence of mantle wedge viscosity. Indicates much higher temperatures in the shallowest part of the subducted slab.
Figure P-T-t paths at a depth of 7 km into the slab (subscript = 1) and at the slab/mantle-wedge interface (subscript = 2) predicted by several published dynamic models of fairly rapid subduction (9-10 cm/yr). ME= Molnar and England’s (1992) analytical solution with no wedge convection. PW = Peacock and Wang (1999) isoviscous numeric model. vK = van Keken et al. (2002a) isoviscous remodel of PW with improved resolution. vKT = van Keken et al. (2002a) model with non-Newtonian temperature- and stress-dependent wedge viscosity. After van Keken et al. (2002a) © AGU with permission.
What happens to intermediate cases, such as the curve for the 5 Ma old arc?
If dehydration occurs before the subducted material reaches the wet melting point, will it melt?
Depends upon the fate of the water released by dehydration
If it remains in place, melting will occur when the wet solidus is reached
Many workers believe that the water rises and leaves the system, and the remaining crust will no longer have free water, and no melting will occur at the wet solidus
An anhydrous eclogite soon forms
Will it melt, or will the water escape first? This is the subject of great controversy, and models diverge at this point into those that perceive the melting of the oceanic crust (as eclogite plus water) as the dominant source of arc magma, and those that favor the mantle wedge.

38. Slab melting in mature arcs no longer precluded by models.
Subducted Crust
Slab melting in mature arcs no longer precluded by models.
Debate renewed.
What happens to intermediate cases, such as the curve for the 5 Ma old arc?
If dehydration occurs before the subducted material reaches the wet melting point, will it melt?
Depends upon the fate of the water released by dehydration
If it remains in place, melting will occur when the wet solidus is reached
Many workers believe that the water rises and leaves the system, and the remaining crust will no longer have free water, and no melting will occur at the wet solidus
An anhydrous eclogite soon forms
Will it melt, or will the water escape first? This is the subject of great controversy, and models diverge at this point into those that perceive the melting of the oceanic crust (as eclogite plus water) as the dominant source of arc magma, and those that favor the mantle wedge.

39. Flat HREE pattern argues against a garnet- bearing (eclogite) source
LIL/HFS trace element data underscore the importance of slab-derived water and a MORB-like mantle wedge source
Flat HREE pattern argues against a garnet- bearing (eclogite) source
Modern opinion has swung toward the non- melted slab for most cases
McCulloch (1993) suggests that young, warm subducted crust is more likely to melt before it dehydrates. Thus slab melting may be more common where the a subduction zone is close to a ridge, or during the Archean, when heat production was greater

40. Mantle Wedge P-T-t Paths
Start at X
Temperature decreases followed by pressure increase with only slight temperature increase (clockwise P-T-t)

41. Amphibole-bearing hydrated peridotite should melt at ~ 120 km
Phlogopite-bearing hydrated peridotite should melt at ~ 200 km
 second arc behind first?
Figure Calculated P-T-t paths for peridotite in the mantle wedge as it follows paths similar to the flow lines in Fig Included are dehydration curves for serpentine, talc, pargasite, and phlogopite + diopside + orthopyroxene. Also the P-T-t path range for the subducted crust in a mature arc, and the wet and dry solidi for peridotite. Subducted crust dehydrates, and water is transferred to the wedge (labeled arrows). Areas in which the dehydration curves are crossed by the P-T-t paths below the wet solidus for peridotite are stippled and labeled D for dehydration. Areas in which the dehydration curves are crossed above the wet solidus are hatched and labeled M for melting. Note that although the slab crust usually dehydrates, the wedge peridotite melts as pargasite dehydrates (Millhollen et al., 1974) above the wet solidus. An alternative model involves dehydration of serpentine  chlorite nearer the wedge tip (lower-case d) with H2O rising into hotter portions of the wedge (gray arrow) until H2O-exess solidus is crossed (lower-case m). A second melting may also occur as phlogopite dehydrates in the presence of two pyroxenes (Sudo, 1988). After Peacock (1991), Tatsumi and Eggins (1995). Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Crust and Mantle Wedge
Areas in which the dehydration curves are crossed by the P-T-t paths below the wet solidus for peridotite are blue and labeled D for dehydration
Areas in which the dehydration curves are crossed above the wet solidus are purple and labeled M for melting.
Note that although the slab crust usually dehydrates, the wedge peridotite melts as amphibole dehydrates above the wet solidus
A second melting may also occur as phlogopite dehydrates in the presence of two pyroxenes.

42. Island Arc Petrogenesis
Figure A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.
Altered oceanic crust begins to dehydrate at depths ~ 50 km or less, as chlorite, phengite, and other hydrous phyllosilicates decompose
Further dehydration takes place at greater depths as other hydrous phases become unstable, including amphibole at about 3 GPa.
The slab crust is successively converted to blueschist, amphibolite, and finally anhydrous eclogite as it reaches about km depth
In most (mature) arcs, the temperature in the subducted crust is below the wet solidus for basalt, so the released water cannot cause melting, and most of the water is believed to rise into the overlying mantle wedge, where it reacts with the lherzolite to form a pargasitic amphibole and probably phlogopite (yellowish area)
Slightly hydrous mantle immediately above the slab is carried downward by induced convective flow where it heats up, dehydrates, and melts at A (120 km)

43. A multi-stage, multi-source process
Mantle wedge ® HFS and other depleted and compatible element characteristics
Slab dehydration (and perhaps melting) ® LIL, 10Be, B, etc. enrichments + enriched Nd, Sr, and Pb isotopic signatures
These components, plus other dissolved silicate materials, are transferred to the wedge in a fluid phase (or melt in some cases?)
Mass-balance calculations suggest that the contribution of the subducted sediments is only a few percent in most arc systems, but the result is the high LIL/HFS patterns
The nearly closed-cell induced flow in the wedge may result in progressive depletion of the wedge as arc magmas are extracted. This provides an explanation of some HREE and more compatible trace element data, which in many cases is more depleted than MORB

44. Phlogopite is stable beyond amphibole breakdown
Wedge P-T-t paths reach phlogopite dehydration at ~ 200 km depth
If this occurs above the wet peridotite solidus, a second phase of melting will occur at B a position appropriate for the secondary volcanic chain that exists behind the primary chain in several island arcs
The P-T-t paths are nearly parallel to the solidus, and may be above it or below it. Thus dehydration may or may not be accompanied by melting, so that the development of a second arc will depend critically upon the thermal and flow regime of a particular arc
Melting initiated by the breakdown of the potassium-rich mica will probably be more potassic, as is true in most secondary arc occurrences.
The K-h relationship probably more complex, reflecting the decreasing quantity of H2O with depth and thus the degree of partial melting, as well as the depth of melting (which becomes more alkaline with depth), and perhaps the vertical length of the rising melt diapir column within the mantle wedge

45. Fractional crystallization takes place at a number of levels
In the shallower chambers the calc-alkaline fractionation trend takes place in a hydrous magma with the fractionation of magnetite, hornblende, and/or a highly anorthitic plagioclase, as discussed previously
The restriction of calc-alkaline magmas to subduction zones may thus result from the uniquely high water content or the thickened arc crust that causes the primary tholeiites to pond and fractionate
The SZ environment is a complex one, and the generation of arc magmas reflects a number of sources and stages
The wedge may be heterogeneous and variably depleted/enriched, the subducted material has a variety of crustal and sedimentary constituents
The thermal and flow patterns are variable, and the nature of the fluid is poorly constrained and probably variable as well

46. From Peacock (2003) Geophysical Monograph 138 Am. Geophys. Union