chapter 6. age and growth of continental crust from radiogenic isotopes pp. 321-348
TRANSCRIPT
Chapter 6.
Age and Growth of Continental Crust from Radiogenic Isot
opes
pp. 321-348
What is the crust? Continental crust?What is the crust? Continental crust?
1. Total mass = 0.6% silicate earth
2. Main reservoir for K-U-Th (heat) and mineral resources
3. Primary archive of the earth history
study of the continental crust is critical to our understanding of the origin & differentiation of the Earth.
•When did continental crust form? How to get the age?
•How did it form? What mechanism of its formation?
•What is its geodynamics?
1.Age of the continental crust
2. Mechanism of continental growth 2. Mechanism of continental growth and formation and formation
3. Geodynamics for continental 3. Geodynamics for continental growth: supercontinent and growth: supercontinent and continental growth continental growth
Three major topics
1. Age of the continental crust
The age of the continental crust is the amount of The age of the continental crust is the amount of time the crustal rocks has been isolated from the time the crustal rocks has been isolated from the mantle sources (DePaolo et al., 1991).mantle sources (DePaolo et al., 1991).
40Ar/39Ar
Rb–Sr Dating
Sm–Nd Dating
U–Pb Dating
TIMS(Thermal Ionization Mass Spectrometry)—The Isotope Dilution MethodSHRIMP (Sensitive High Mass Resolution Ion MicroProbe)
1.1 How to determine age of crust------ Isotopic dating
0.7
0.8
0.9
1.0
0 20 40 60 80 100 120
87Rb/86Sr
87S
r/8
6S
r
Age = 154.5 ± 7.6 Ma
Initial 87Sr/86Sr =0.7086 ±0.0064
MSWD = 10.0
水泉沟石榴石花岗岩全岩-黑云母 Bi
AGES OF METAMORPHISM
40Ar/39ArRb–Sr
U–PbTIMs
SHRIMP
How to determine initial age of crust?
How to constrain on age of lower crust? Rock probe
40Ar/39Ar Rb–Sr Dating Sm–Nd Dating
U–Pb Dating
Metamorphic events
Igneous eventsInherited or xenocrystzircon
?Age of crust
Lower crust
MC
Upper crust 200 Ma
1000 Ma 300 Ma
Neodymium Model AGE143Nd is produced by α-decay of 147Sm. Sm/Nd ratio of the mantle> the crust a
nd hence143Nd/144Nd ratio >crust.
Sm and Nd are not mobile, so ages and initial ratios are relatively insensitive to weathering and metamorphism.
εNd is simply the relative deviation of the 143Nd/144Nd ratio from the chondritic ratio:
where all 143Nd/144Nd values are specified at the age of interest (t). Preset-day (146Nd/144Nd)CHUR=0.512638.
εNd-- variations in143Nd/144Nd relative to the primitive mantle (CHUR).
εNd<0----crustεNd>0-----mantle
)(144
147
)(144
147
)(144
143
)(144
143
1ln1
ODMOSA
ODMOSADM
NdSm
NdSm
NdNd
NdNd
T
=0.00654/Ga ,( O )代表現在值。下標 SA 和 DM 分別表示樣品和虧損地幔(DePaolo, 1988), (143Nd/144Nd)DM(O) = 0.51325, (147Sm/144Nd)DM(O) = 0.2168 。
地殼樣品從虧損地幔中分離後,无 Sm/Nd變化, TDM值爲殼幔分異年齡或地殼形成年齡。若有 Sm/Nd值變化,它代表在不同的化學體系中演化時間的加權平均年齡。
Nd同位素模式年齡 TDM
TDM定義爲由樣品現在的 147Sm/144Nd和 143Nd/144Nd值反演到其 143Nd/144Nd值與虧損地幔源區( DM)的該比值一致的時間 :
-6
-4
-2
0
2
4
6
8
10
0 200 400 600 800 1000 1200
Age, Ma
E N
d
DM
CHUR
T C = TDM
Continental crust
TC
T CF
200 Ma1000 Ma
εNd = -40 Ma
εNd = -2200 Ma
εNd=DM1000 Ma
Model age can not be used if parental felsic melts Model age can not be used if parental felsic melts was generated by partial melting of mixed sources was generated by partial melting of mixed sources
of various ages and compositionsof various ages and compositions..
-6
-4
-2
0
2
4
6
8
10
0 200 400 600 800 1000 1200
Age, Ma
E N
d
DM
CHUR
Continental crust
T CF
Mixing ofvarious sources ?
V.I. Kovalenko et al. (2004)
Condie 1998
Tectonic agesTectonic ages
Patchett (2005)
Patchett (2005)
1.2 Growth events of continental crust
大陆的幕式生
长
时间
大陆生长
量
Condie 1998
K.C. CondieTectonophysics 322 (2000)
大陆生长模
式
大陆
体
积
时间
Models for continental growth 大陆模式生长
1.3 Phanerozoic continental growth1.3 Phanerozoic continental growth
It is widely agreed that the production of the contineIt is widely agreed that the production of the continental crust was essentially completed in the Precambntal crust was essentially completed in the Precambrian, and was minor in the Phanerozoic. rian, and was minor in the Phanerozoic.
However, recent isotope investigations in the westerHowever, recent isotope investigations in the western North America (Sierra Nevada, Peninsular Range, an North America (Sierra Nevada, Peninsular Range, and Canadian Cordillera) and eastern Australia (Lachlnd Canadian Cordillera) and eastern Australia (Lachlan and New England Foldbelts) have revealed that a an and New England Foldbelts) have revealed that a substantial proportion of the Phanerozoic crust is jusubstantial proportion of the Phanerozoic crust is juvenile.venile.
IGCP420 Phanerozoic continental growth: evidenIGCP420 Phanerozoic continental growth: evidence from central Asia (1998-2003, Bor-ming Jahn) ce from central Asia (1998-2003, Bor-ming Jahn) Two issues: Two issues:
Central Asian Orogenic Belt is the largest and typiCentral Asian Orogenic Belt is the largest and typical Phanerozoic juvenile crustcal Phanerozoic juvenile crust
Jahn (2004)
1.3 Phanerozoic continental growth1.3 Phanerozoic continental growth
NCCCOBC
Qinling belt
Dabei belt
CAOB
SCB(diamond)
Pacific
- 30
- 20
- 10
0
10
0. 7 0. 705 0. 71 0. 715 0. 72
EMI
DM CAOB
Pre-Mesozoic
εNd(t)
(87Sr / 86 Sr)i
NCB andDabei Blet
Dabei belt
Intermediate-acid rocks
Production of the continental cruProduction of the continental crust not only mainly occurred in the st not only mainly occurred in the Precambrian, but also in the PhanPrecambrian, but also in the Phanerozoic. erozoic.
Central Asian Orogenic Belt is the Central Asian Orogenic Belt is the largest Phanerozoic juvenile crustlargest Phanerozoic juvenile crust
2 How did continental crust form? 2 How did continental crust form?
Formation mechanism and Formation mechanism and processesprocesses
Juvenile continental crust is produced at two tectonic settings and ways:
Subduction upper crust
mantle plumes lower crust
Two major mechanism:Two major mechanism:
Horizontal growthHorizontal growth Vertical growth
Mantel oceanic crust continental crust
2.1 Horizontal growth2.1 Horizontal growth
Melting in subduction zoneMelting in subduction zone
Mantel oceanic crust continental crust Chemical process
Barr et al., 1999, Lithos
Physical process
Chemical process
Horizontal growthHorizontal growth
Arc magmatism and lateral accretion of arcs
2.2 Vertical growth from below2.2 Vertical growth from below
The plumes give rise to juvenile crust:by partial melting as they arrive at the base of the lithosphere,or by heating the upper mantle. Oceanic ridge subduction
Identify new juvenile compositions from beIdentify new juvenile compositions from belowlow
Probable sources for post-orogenic granitesProbable sources for post-orogenic granites
(a) Recycle from young crustal (including ocean cru(a) Recycle from young crustal (including ocean crust) by horizontal accretion----- horizontal growthst) by horizontal accretion----- horizontal growth
(b) New underplating mantle-derived magma---- po(b) New underplating mantle-derived magma---- post-orogenic vertical growthst-orogenic vertical growth
(c) pre-orogenic underplating mantle-derived magm(c) pre-orogenic underplating mantle-derived magm
a----- pre-orogenic vertical growtha----- pre-orogenic vertical growth
How to iHow to identify new juvenile compositions from belowdentify new juvenile compositions from below
εNd(t)>0
?
?
Jahn (2004)
Vertical evidence from Eastern Tianshan, NW ChinaVertical evidence from Eastern Tianshan, NW China
Vertical growthVertical growth
Vertical growthVertical growth
Vertical evidence from Eastern Tianshan, NW ChinaVertical evidence from Eastern Tianshan, NW China
What is the geodynamics for contiWhat is the geodynamics for continental growth? nental growth?
Why did continental grow?Why did continental grow?
Continental crustal growth Supercontinental cycle and are two of the most important subjects in the earth sciences. These two problems have long been studied separately.
Crustal growth: 3.6–3.5, 2.7–2.6, 2.0–1.8, 1.2–1.0, and 0.5–0.3 Ga.The supercontinental accumulation: 2.6, 1.8, 1.0, 0.65, and 0.25 Ga.
Continental crustal growth
Supercontinental cycle
Mantle convection
The Hercynian collision of Gondwanaland and Laurasia marked the initial coalescence of Pangea in the mid-Carboniferous (320 Ma), which finally became a uniformly emergent supercontinent in the Triassic. Starting in the mid-Jurassic (160 Ma), Pangea broke up intofragments (Veevers, 1994).
Pangea 450-250 Ma
Laurasia
Gondwanaland
CAOB
Condie 1998
Summary Summary 1. Age of the continental crust could be determined by N
d model age, zircon U-Pb dating. Formation of continental crust mainly took place before Precambrian (2.7, 1.9, 1,2Ga), but also during Phanerozoic (o.45-0.25Ga).
2. Mechanisms of continental growth are mainly horizont2. Mechanisms of continental growth are mainly horizontal (subduction) and vertical (underplating, plum) procal (subduction) and vertical (underplating, plum) processes. esses.
3. Continental growth may be related to supercontinental 3. Continental growth may be related to supercontinental
recycles (assemblage)recycles (assemblage)..
三、主要成果(在新疆)在新疆) 4 提出中亚造山带成矿物质来源与地幔源有关,与华南成矿背景不同,
显示了重要而特殊的成矿域,值得进一步研究,对找矿勘探有实际意义。