Geochemistry: Overview: the geochemist's Earth (reservoirs, budgets and processes) Mantle geochemistry: How geochemists see the deep Earth Don DePaolo/Stan Hart CIDER - KITP Summer School Lecture #1, July 2004 Geochemistry 50 years ago dealt with fewer questions and parameters, e.g. Birch (1952) How does meteorite chemistry compare with seismic properties of Earth s interior Is it Olivine+Pyroxene or other phases? How much Fe in the mantle? How much Al,Ca,Na,K ( sialic components ) is in the mantle? 11 elements of interest: O,Mg,Si,Fe,Ni,Al,Ca,S,Na,K,P Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 1
What can geochemistry do in 2004? The earth is made of 90 or so chemical elements, about 30 w/isotopic variations Chemical/isotopic characteristics can be tied to geological processes - mantle isotopic chemistry is a tracer We can tell where a particular piece of mantle has been in the past and/or what has happened to it Radiogenic isotopes provide clocks as well as tracers Questions for geochemistry How deeply does near surface material circulate into the mantle? On what time scale? Does the mantle have large scale chemical structure (layering?) Does the core exchange material with the mantle? (Do plumes come from the CMB?) What are the characteristics of mantle convection in terms of its ability to stir and homogenize heterogeneous materials? What features of mantle seismic heterogeneity are thermal and which are chemical? What aspects of mantle structure are congenital?, of recent origin?; steady-state features? Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 2
Components of geochemistry Petrology of the mantle (proportions of minerals or rock types - e.g. lherzolite, harzburgite, eclogite, pyroxenite) Melting of the mantle Trace element composition of the mantle (doesn t affect mineralogy, but can be indicative of history) Trace element composition II (water and CO 2 ) - affects melting behavior. Isotopic composition of the mantle (from radioactive decay, input from surface reservoirs, input from core?) Sampling of the mantle (scale of sampling by magmatism; sampling biases, invisible reservoirs) Material balance - the sum of the parts must equal the whole Earth for every element and isotope Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 3
(1) (2) Upper mantle (100) Lower mantle (300) (200) Mass in units of 10 25 g Oceanic lithosphere Continental lithosphere (1) (2) Upper mantle (100) Lower mantle (300) (200) Mass in units of 10 25 g Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 4
Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 5
Consider this: (1) Heterogeneities are introduced from the top and the bottom (2) Magmatism samples only the top and the bottom There are key elements of the system where chemistry is done. (Most of what we infer about the mantle depends on how well we understand the processes.) Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 6
Choose one: There are... (a) too few (b) too many (c) just the right number...of isotopic tracers Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 7
There are stable isotopes too! Why the crustal reservoirs matter... Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 8
Why the crustal reservoirs matter... Depleted mantle 10 3 10 2. ε = 10-14 sec -1 MORB Heterogeneity thickness (km) 10 1 10 0 10-1 10-2 10-3 Turbulent shear Laminar shear Other lava sequences UM rocks 10-4 Chemical Diffusion 0 500 1000 1500 2000 Age (Ma) Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 9
Thickness of geochemical anomalies Geochemically Anomalous Layer (Thickness = λh ) Isotopic contrast = ²R h Concentration = Ch Background mantle concentration = Cb Background mantle that must be averaged with anomalous material (effective anomaly thickness): Ch ²R h λb = λh Cb ²R a Making heterogeneity at a mid-ocean ridge... Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 10
Mid-ocean ridge factory Hydrothermally altered sediment basalt, gabbro harzburgite strongly depleted lherzolite Incipiently depleted lherzolite H 2 O-enhanced melting region unmodified lherzolite sediment basalt, gabbro harzburgite 0.1 b.y. later 1.0 b.y. later ε -20 Nd ε 0 +20 Nd -20 0 +20 strongly depleted lherzolite Incipiently depleted lherzolite unmodified lherzolite Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 11
Anything systematic about distribution of heterogeneities? Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 12
Anything systematic about distribution of heterogeneities? Bulk Earth Younger cont. crust Lower cont. crust Upper cont. crust Older cont. crust Anything systematic about distribution of heterogeneities? Bulk Earth Chondrites Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 13
Distribution of isotopic ratios among ocean islands is not entirely random Al Hofmann s analysis, 2003 Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 14
120 Material balance for Sm-Nd... 100 80 Mid-Atlantic Ridge (data from PETDB) Average Mantle Count 60 40 20 Bulk Earth 0-8 -4 0 4 8 12 16 ε Nd An example of heterogeneity on various scales - Nd isotopes in MAR basalts. 5 to 10 units of variation can be found over 10km or 10,000km along the ridge. The entire range of values observed worldwide (in all types of oceanic basalts) is found along the ridge Epsilon Nd 15 10 5 0-5 -10-60 -40-20 0 20 40 60 80 LATITUDE Mid-Atlantic Ridge (data from PETDB) DM AM PM Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 15
Recycled MORB Primitive The helium problem Sampling issues: Pt. 1 9.0 8.5 Melting region edge center Mahukona Hualalai Kohala Present 200 Ka 400 Ka 600 Ka 800 Ka Mauna Kea Kilauea Melt Supply max = 5 cm /yr 1.0 0.9 0.7 0.5 8.0 ε 7.5 Nd 7.0 6.5 HSDP Mauna Kea (Bryce & DePaolo, 2002) 6.0 100 200 300 400 500 600 25 20 HSDP Mauna Kea (Kurz, 2002) Model Age (ka) Mauna Loa Magma Capture Area Loihi N 0.3 0.1 0.05 0.001 3He/4He (R/Ra) 15 10 5 MORB Range 20 km 0 100 200 300 400 500 600 Model Age (ka) Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 16
Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 17
3 He/ 4 He Width of melting region 5 10 15 20 25 30 35 40 He-3 anomaly (Pb anomaly?) Sr anomaly Sr Core or core-mantling dense layer He 0.7025 0.7035 87 Sr/ 86 Sr Things may get even more interesting when we model the melting in the context of the flow - (M. Jull, unfinished, 2003) Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 18
Melting versus tracers... (Modeling from Jull & Ribe, 2002) Sampling issues, Pt. 2: Over what vertical distance are isotopic ratios averaged? DePaolo, JGR, 1996 Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 19
3 He/ 4 He (R/Ra ) Estimating dispersivity in the Hawaii melting region 14 12 10 8 6 4 200 400 600 800 1000 Depth (meters) 206 Pb/ 204 Pb 18.40 18.45 18.50 18.55 18.60 18.65 200 400 600 800 1000 Depth (meters) 10 4 Hydrodynamic Dispersivity (m) 10 3 10 2 He Pb Sr Nd Mauna Kea (380-1000m) 10 1 0 5 10 15 20 From DePaolo, JGR, 1996 w/w 1 km For MOR s the channeling instability may apply; makes for very large vertical dispersion - i.e. lots of averaging. May not be the case for plumes (?) Spiegelman et. al, 2001 (JGR, 106, 2061-2077) Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 20
OK, so what do we think we know...? Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 21
Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 22
Where we are going in the next 2 weeks... Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 23
Geochemistry Tutorials... Donald DePaolo, UC Berkeley (KITP CIDER 7/12/04) 24