Magmas and Igneous Rocks 4S
.....-1—
Spo     nwo     t-oo hoo
(a)
Figure 2-11 fa| Densities of some common igneous compositions in the liquid state as functions of temperature. |b] Variations of density (above) and viscosity (below) of the three main types of magmas as they evolve toward more feisic compositions The differences result mainly from their differing fates
AAafi£ _Inter n?fJ täte__FeIjit
Otffaftntttttitnt
(b)
of enrichment of silica, rroa and alkalies. Tne curves for thotentes are based on Javas of Hawaii and the Galapagos islands. The calc-aikalrne curves are for rocks of the Cascade Range, and tne alkaline ones are for the phonoJftic series Of Tahrri.
36   Igneous Petrology
1000°
^5   5    eto
(a)
6      8 10
Weight % H-,0
m
Figure 2-S The solubility of volatiles in magmas is a function of pressure, tempera-lure, and the compositions of the liquids and gases, (a] shows the solubility of H?0 in basalt (B), granite |G], and andesire (A), all at 1, IOO°C, as a function of pressure of H20. Note that although the three compositions have about the same solubilities at the same temperature, the three types of magmas are liquid at very different temperatures, and their solubilities under natural conditions will differ accordingly. As shown in (b), the solubility in granite increases with falling temperature; basalt and andesite show similar effects. Hence natural basalts have the lowest solubility as natural liquids and granites the highest. The pressure of another gas, such as CO,,, decreases the solubility of H20. |c) shows solubilities in a basaltic melt at 1.2Q0°C and two different total pressures, one and three kilo-bars. (Data from Hamilton and Anderson. 1967, Basalts 1:445-482; Goranson. 1931, Amer. J. Sci.. 22:481-502: and Kadik et al.. 1972, Geoch. Internal-. 9:1041-1050,1
42   Igneous Petrology
Figure 2-10 Viscosities of some common igneous melts |a| as func-rions of temperature in and above the melting range |bj shows the increase in viscosity of a basalt after cooling for different periods of time. The Infection point is dose to the temperature at which the basalt begins to crystallize Tne effect of water on viscosity is illustrated by the example of a granitic Jiquid shown in |c). The effect Of pressure is to reduce me vis-cosily of most silicate melts [d|. ja. b. and c are from Muraseand McBirney. 1973. Geot. Joe Amer Butt.. 84-3563-3592. d is from Scarfe. My-sen, and Virgo, 1987. Geocn Soc. Spec Pubt No !. 59-67,)
Two ininiiíiiiliiť
metis oľdiľfenertl composition
fD..;
o-
■■m
Four phase system Meli pi lis huhhles o\ volatile fluid iinü crysliils lit'olivine ItlHl plilgilU'liiJik'
4.1    Sthemutie possible imi^iniis.
Igneous Petrology
LIQUID SILICA
O Bringing oxygen    O N,mbri Jging oxygen    • Network tonnenSij    • Network modifier (Caj    • NťiiMitk modifier < Mel
Í .oiiccjUiiiiE models q| at<wnic sTrucrures ol plicate melts annpared with rlie syiTiraeiric laitice of a crysiallme solid l■&} CrysialLfle aBa 'high íridyrttiteí. Layersof hexagonal hup* ol Si-O tefrahedra with alternating apices pofourtg up and down are slacked on topotoai another, creating a rh ree-dimensiuraal structure in which each oxygen is shared Iw Euro silicons. Tctrcahedra with apices, poantin^ injkw the upjKrr jpicd oxygen tell out of the drawing so as to reveal underlying silicon. Dashed I me indjcaics outline cA <>nc unii cull in the Es- J rice, tbi MixJelot liquid silica. Si-O rc-irahedra aec sliphdy distorted relative to die crystalline lattice. Long-range order is abscni. Sircc-lure b highly poLyntemcd because all icrrahedra arc interconnected bj bridging oxygen anions. <cl Model of liquid CjM|gSJj04 shimir$ less rH>lymerizaiion dian that or liquid silica. Note presence of network-modifying cations iCa and Mgi and nonbridging oxygen, nriiaf I or which occurs in the silica melt. U<cdrawn from Carmiehael et aL. 1974. tt. l}_3.)
Si-O polymer in anhydrous melt
Broken Si-O polymer in hvrtrous meh
Water molecule
4- o
4.S
Some dissolved water in siHeare meJts forms: hydroxy! inns, (QH) . which break O-Si-0 poismers. nnLuana :hc decree of polymerization.
T CO
3 Specific volume (cubic centimeters per gram) of pure water as ■a Function of P and 1 Fo: reference, goorhcrmaJ gradients oE 10X, 3C)°C. and 50°Gykm are shown as dashed lines. Pilled circle is die critical point of pure water. {Data from Burnham et al.. 1969.)
4A    Specific volume of pure water as a function of P (and depth) for two temperatures.
		
2.5 r	-1-1 1	
		s 8
2,0 1.5 i L0	# / J / Water*uiutersaturated mell / 7 /	-6 II 1 ■a * Q
0.5	/   /    Water-satmated / y/         melt + water jf(water oversaturated s/^s                 magma system)	
I	L            1               1               1--1-	
	a--2 Concentration Of dissolved water (wt.	
4.7	Splubiliry of water in silicate melts. (Mm from Moore «	
	ill., 1998.)	
Total concentration of dissolved C02 (wt. %)
Solubility of carbon dioxide in some silicate melts. Note that more CO? can dissolve in less polymerized mafic and especially silica-Lindersamrated melts. (From Holloway and Blank,
1994. >
8,-r
s	-1-1-1       1 "	—1-F      i i
!l		/
4		o/ ^ / V /          (OHr 1G00°C
I		___— — — _
		
		
n		■    i    i    i —i-
4 6 H 10
Tnt.il concentration of dissolved water (wL %)
Dissolved skater in rhyolitic melts exists in ionic, lOH) , and molecular, H20 form. The concentration of (OH)~ fs dependent upon X but molecular H20 concentration is independent of 7 For example, in a melt at 1000'C that contains A wt.% total water, about half is dissolved as (OH) ions and half as molecular H20. For a 1000aC melt containing 2 wt.% total water, about 1.5 wt.% is dissolved as (OH)" and 0.5 wt.% as. molecular H20. (From Silver et al., 1990.)
Water imdersaiuralcil melt
-X
c 4 Q
(Water oversaturated magma system)
- i
4 (l Concentration of witter (wt. %>
4.11 Evolution of a hypothetical dosed magma system during decompression from mi initially water-undersaturated state. The Initial magma is a crystal- and bubbk-free melt at 1.6 kbar, corresponding to a deprh of 6 km, and contains 4 wt.% d]S-solvcd water and no other volatile.
Unstable loam
with inctpmBteod
\ t /
[>ISPKKSE[> PYROCLASTS IN A GREATLY EXPANDED CONTINUOUS GAS PHASE
Larper, more abundant bubbles {+ crystals) in meh
DISPERSED 1*1 .'BULKS IN A CONTINUOUS M [-:[.T
Spsirsc, small bubbles (+ cryslals) in melt
Schematic crow section through die volcanic conduit of an explnciinp. inagmi* system Circular ^wgraros are "snapshots" of [he stale of :hi-.. -.:i:tn*linp ma^mit ji* a (unciion of depth and P in the conduit, Because nj limitations of the diagram area, (he Jlimdreds-fokl expansion ot inc vnlaiUe Huid phase cannot be jitciir.itclv represented.
Table 2-2  Average Chemical and Mineral Compositions of Selected Plutonic Rocks										
					Chemical composition					
	Granite	Syenite	Granodiorite	Quartz Die rite	Diorite	Gabbro	Olivine Diabase	Diabase	Dunite	Lherzolite (Peridotite)
Si02	70.18	60.19	65.01	61.59	56.77	48.24	48.54	50.48	40.49	43.95
Ti02	0.39	0.67	0.57	0.66	0.84	0.97	1.31	1.45	rj.02	0.10
	14.47	16.28	15.94	16.21	16.67	17.88	15.24	15,34	0.86	4.82
Fe203	1.57	2.74	1.74	2.54	3.2Ó	3.1ft	3.06	3.84	2.84	2.20
FeO	1.78	3.28	2.65	3.77	4.40	5.95	8.88	7.78	5,54	6.34
MnO	0.12	0.14	0.07	0.10	0.13	0.13	0.21	0.20	0.16	0.19
MgO	0.88	2.49	1.91	2.80	4.17	7.51	8.08	5-79	46.32	36.81
CaO	1.99	4.30	4.42	5.38	6.74	10,99	9.38	8.94	0.70	3.57
Na,0	3.48	3.98	3.70	3.37	3.39	2.55	2.69	3.07	Ü.10	6.63
K,0	4.11	4.49	2.75	2.10	2.12	0.89	0.98	0.97	0.04	0.21
H20	0.84	1.16	1.04	1.22	1.36	1.45	1.35	1.89	2.88	1.08
PA	0.19	0.28	0.2Ü	0.26	0.25	0.23	0.28	0.25	0.05	0.10
Density	2.667	2.757	2.716	2.806	2.839	2.976	2.970	2.965	3.289	3.330
Mineral composition										
Quartz	25	—	21	20	2	_	_	J_,		
K Feldspar	40	72	15	é	?■	—	—	_	_.	__
Oligoclase	26	12	..—	—	-	—	_	_	_	_
Andesine	--	—	46	56	64	._	_.	_		_
Labradorite	—	í—	—	—	—	65	63	62	_-	
Biotite	5	2	3	4	5	i		1		
Amphibole	1	7	13	8	12	3	—	1	_	;__
Orrhopyroxene		—	—	1	3	6	-r-	—	2	15
Clinopyroxene	—	4	—	3	8	14	21	29	,_.	10
Olivine		—	—	—	—	7	12	3	95	71
Magnetite	2	2	1	2	2	2	2	2	2	1
Itmenite	1	1	—	—	i—;	2	2	2		
Apatite	t]	tr	tr	tr	tr	—	—		_	_
Sphene	tr	tr	1	tr	tr	—	_	_	._	_
Spinel		—	—	—	—		—	—	1	3
Source: After Daly and Larsen, Gcol. Sac. Am.			Special Pafier 36, 1942 with modifications and reduced to 100%.							
Magmas and Igneous Rocks 57
52   igneous Petrology
(e)
Figure 2-14 Magma generated in trie mantle fa) is thought to rise when the buoyant mass of crystal-liquid mush becomes gravita-tionally unstable. It moves as plume-shaped diapirs through the plastic mantle (bj but oil reaching more brittle rocks of the lithosphere. it may stope its way upward, either by dislodging blocks in a piecemeal fashion [c| or' wedging its way upward in "bell-jar" shapes intrusions fdj. At some stage in this sequence, the fluid part of the magma may separate from.| its crystalline residue. In the uppermost levels of the crust, magmas rise through dilational fractures that tend to become cylindrical condjits as they become established vents for surface' eruptions |e|,