Workshop Program
Geochemical Earth Reference Model (GERM)

Daily Program -- La Jolla, 10-13 March 1998

Tuesday, March 10, 9:00 - 17:00
Geochemical Modeling Short course by Francis Albarede

Sun City/Apollo Room (Located on the lowest floor of IGPP: Take the elevator down and go straight ahead. The Apollo Room is the computer room at the end of this hallway.)

Morning Program

Introduction to an easy programming tool: MatLab.
One-box models.
Residence time.
Mean age.
Mixing time.
Heterogeneities and fluctuations.
Radioactive tracers.
Computer applications.

Lunch (Munk Conference Room)

Afternoon Program

Many-box models.
General dynamics.
Computer applications.
Heterogeneous reservoirs.
Advection and diffusion.
Mixing.
Computer applications.

17:00 - 19:00: Icebreaker and Registration
Spring Stirring/ IGPP Patio

Wednesday, March 11, 8:30 -10:00
Reservoir Definition

Global Geophysics & GERM (Hubbs Hall)
Keynote Speaker: J. Phipps-Morgan

This talk reviews geophysical evidence constraining the present-day structure and past thermal evolution of the mantle. Seismic images of subducting slabs imply present-day flow mixing between upper and lower mantle. Thermal arguments for the ratio of past to present heat loss imply that Earth is not a steady-state heat engine, but rather that heat loss and chemical differentiation rates have decreased with time. The talk reviews geochemical observations on the way the mantle melts to make oceanic basalts. It concludes that several independent lines of geochemical evidence (Os-isotopic disequilibria between MORB and abyssal peridotites; the tube like (1-D) isotopic structure of arrays if basalts from a common hotspot; the trace element and isotopic paradoxes in orogenic lherzolite massifs) favor the assertion that basalt melts are not in chemical/ diffusive equilibrium with a ~km-scale volume of the melting source, but rather that basalts form from pooled melts, each of which is only in local equilibrium with the phase assemblage from which it melts.

Discussion Topics
(D. L. Anderson -- Discussion Leader)

• Volatile and LIL elements in crust and atmosphere indicate 50-70 % extraction. Is this more likely to be 100 % efficient extraction from 50-70 % of mantle (primordial mantle scenario) or 50 - 70 % efficient extraction (plus recycling) from all the mantle?
• OIB show strong recycling or surface contamination signatures (oceanic sediments, continental crust or sediments, altered oceanic crust, seawater contamination (including noble gases), oxidation state, oxygen isotopes, iodine content). How deep into the mantle are these carried by the slab and what fraction get stripped off above 200 km?
• Do the LIL, volatiles and alteration products have to accompany the slab to depth (evident in tomography) or would linear blue things still be evident if the deep slab is mainly MgO, FeO, Al2O3, CaO, SiO2?
• Is it true that all the volatiles in the Earth can be accounted for by a 1 % late veneer and if so what does this imply about mantle reservoirs and differentiation ? Can most of the Earth be depleted and infertile (the MORB reservoir is depleted and fertile).
• Are komatiites due to damp or wet melting or can they form at 1400°C or less?
• Geophysics suggests that the upper mantle potential temperature is 1400 +/- 200°C. And is partially molten almost everywhere. Is this consistent with petrology; it is not consistent with McKenzie. Suppose that geophysics is right and the upper mantle (from shear velocities ) contains a melt phase. What then causes volcanoes (cracks or extensional stresses in the lithosphere?), continental flood basalts (transient drainage of ponded melts?) i.e. what is the role of the lithosphere -domineering or subservient?
• All high ³He/⁴He magmas are deficient in ³He compared to MORB (by 1 to 2 orders of magnitude), even the deepest Loihi samples cf. shallow MORB And there is no evidence from He/Ne isotopes that these low ³He (high ³He/⁴He) basalts have been degassed more than MORB. What is the evidence that high ³He/⁴He basalts are high ³He or is this just convention?
• The 3He flux from hotspots is 102-3times lower the predictions from the undegassed mantle scenarios and less than from ridges. Why (it is not in the basalts either)?
• Do peridotites suggest that MORB processes are cooler than OIB processes?
• Why are the hottest magmas (komatiites) isotopically depleted, similar to MORB?

Wednesday, March 11, 10:30 - 12:00
Reservoir Definition

Hydro-Lithosphere Systems (Hubbs Hall)
Keynote Speaker: D. J. De Paolo

Interactions between the lithosphere and the hydrosphere involve fluid-rock chemical interaction, typically in relatively complex fractured rock systems with variable temperature. This talk will review some recent advances and unresolved scientific issues that come into play in estimating chemical fluxes, particularly the fluid-rock reaction rates, and how the same issues restrict the ability to interpret the rock record in terms of paleoenvironments.

Discussion Topics
(F. Albarede -- Discussion Leader)

• Scales of chemical heterogeneities in the lithosphere.
• Length and time scales of runoff and groundwater chemical variability.
• The estuarine filter.

12:00 Lunch (T 29)

Wednesday, March 11, 01:30 - 03:00
Reservoir Definition

Core-Mantle Exchanges (Hubbs Hall)
Keynote Speaker: A. Brandon

Evidence from ¹⁸⁶Os/ variations for core-mantle interaction will be discussed. ¹⁹⁰Pt decays to ¹⁸⁶Os with a half life of 450 Ga. The ¹⁸⁶Os/¹⁸⁸Os ratios of Hawaiian picrites are enriched compared to those for representative upper mantle samples. The Pt concentrations of ancient recycled oceanic crust are too low to produce the ¹⁸⁶Os/¹⁸⁸Os enrichments in the picrites. The alternative is that these enrichments result from addition of small amounts (<1%) evolved outer-core metal with high Pt/Os and Os concentrations into plume source of the Hawaiian lavas.

Discussion Topics
(W. F. McDonough -- Discussion Leader)

• What is the composition of the Core and how is it established?
  Given a Silicate Earth composition and a Cosmo chemical based estimate for the composition of the Bulk Earth, one can calculate a core composition by the difference.
• When did Core formation begin and end?
  Core formation is likely to have begun early during accretion and finished at or near the final stages of planetary accretion.
• What is the nature of the light element component(s) in the Core? Considerable uncertainty continues to surround this question. Given the volatile element signature of the Earth, there is likely to be ~2wt% sulfur in the core. Likewise, other volatile elements (e.g., C, N, H, etc.) are also unlikely to be more than trace constituents. Silicon and oxygen are the likely candidates. Strong evidence is still lacking in support of Si and/or O as the component added to the core to explain its low density.
• Is the Inner Core compositionally similar to the Outer Core?
  The inner core is generally not assumed to be compositionally similar to the outer core, however, some authors have suggested otherwise. It is likely that the solid inner core is a complex alloy of Fe, Ni and other constituents. There is also some suggestions that there might be some significant chemical fractionation of siderophile elements between the inner and outer core.
• What is the nature and origin of the PGE (platinum group element) signature in the Earth's Mantle today?
  The PGE abundances in the mantle are often explained to be the result of a late stage, 1% addition to the mantle. The high concentrations and chondritic relative abundances of the PGE in mantle rocks is not explainable by any model of core-mantle equilibrium.
• Late Stage Veneer hypothesis versus Late Stage core contamination of the mantle?
  Subsequent to Core formation there is some suggestion of material exchange between the core and mantle: (1)Siderophile/Lithophile elemental ratios for peridotites, komatiites and basalt demonstrate that there has been little to no exchange of material between the core and mantle for the last 3.5 Ga. (2) Re-Os and Pt-Os isotopic signatures are suggestive of trace components of core material contaminating the sources of some basalts that are believed to originate from the core-mantle boundary.

Wednesday, March 11, 03:30 - 05:00
Reservoir Definition

Group Sessions

Mantle/Core (Hubbs)
Discussion Leaders: A. Zindler, W. McDonough

Crusts Continental and Oceanic (Munk)
Discussion Leaders: (C. Langmuir, R. Rudnik)

Hydrosphere/ Atmosphere (Revelle)
Discussion Leaders: J. Edmond, R. Keeling

Poster Sessions: 5:00 - 6:30
(with Wine and Cheese in the Munk Hall)

Anderson, Don L.
"The Helium Paradoxes", "The Philosophy of Reference Earth Modeling".

Booij, Ernst.
"Bulk Composition Estimates of Mature Oceanic Crust ..."

Calderwood, Arthur R.
"Sm-Nd Isotopic Modeling of the Evolution ..."

Janney, P.E.
"Using the Geochemistry of Old Oceanic ..."

Michael, Peter J.
"Constraints on Cl in Mantle Reservoirs ..."

Muehlenbachs, Karlis.
"The Oxygen Isotopic Composition of Oceanic ..."

Nielsen, R.L.
"The GERM Geochemical Model and Trace ..."

Vervoort, J.D.
"Behavior of Lu-Hf and Sm-Nd Isotopes in the ..."

Thursday, March 12, 08:30 -10:00
Flux Definition

Crust-Ocean-Atmospheric Geochemical Cycles (Hubbs Hall)
Keynote Speaker: Michael A. Arthur
Co-authors: Lee Kump, Mark Pagani, Kate Freeman

Two major topics will be discussed, a model of seawater oxygen isotope composition and the paleo-pCO₂ for the latest Oligocene through Late Miocene. A simple time-dependent numerical model of the seawater oxygen isotope system has been constructed and used to examine the possibility that dw varied significantly through the Phanerozoic. This model will be discussed and placed into the context of available data sets and previous modeling approaches. Paleo-pCO₂ has been determined for the latest Oligocene through Late Miocene (25 to 8 Ma) based on ep derived using the difference between the d¹³C of di-unsaturated alkenones and d13C of the calcite of surface-dwelling plank tonic foraminifers. A sharp decline in ep at about 25 to 23 Ma indicates a dramatic fall in [CO₂ (aq)] and calculated pCO₂ (ca. 270±15 ppmv). These data will be discussed in the context of major global tectonic events (i.e. the uplift of the Himalayan-Tibetan Plateau), climate and global weathering rates.

Discussion Topics
(Lou Derry -- Discussion Leader)

• How do the rates of exchange between the oceanic crust and the ocean vary with time? How well do we know it now? Typically, the crust-to-ocean flux is presumed to scale linearly with the rate of crustal production. Is this assumption correct? How can it be tested?
• What is the current magnitude of the ocean-to-crust exchange flux, and how does it vary in time? Are processes such as "reverse weathering" major sinks for alkalinity.
• Given significant complications in the interpretation of Sr isotopes as a record of continental crust-to-ocean fluxes (weathering), how can we constrain variations in the chemical weathering flux through time? Knowledge of both the total alkalinity flux and its chemical composition is necessary to estimate the magnitude of the weathering sink for CO₂
• How can we resolve the apparent dichotomies between theoretical considerations which appear to limit possible d¹⁸O variation in seawater and data that argue for significant, and in some cases rapid, changes in marine sediments?

Thursday, March 12, 10:30 - 12:00
Flux Definition

Core-Mantle-Crust differentiation (Hubbs Hall)
Keynote Speaker: H. St. C. O’Neill

The most important constraint on the differentiation of the Earth is its mean (i.e., whole Earth) composition, which may be deduced with a number of assumptions. These will be discussed. New evidence on core formation indicates that this major differentiation event occurred between 4.50 and ~4.0 Ga, by means which are presently under vigorous debate. Whether subsequent mass exchange between core and mantle has occurred is also contentious. The formation of oceanic crust is fairly well understood, that of continental crust remains entirely enigmatic.

Discussion Topics
(Alan Zindler -- Discussion Leader)

I. The starting point:

A. Chondrite- and Earth-Based Models: How good are they? Is this the strongest link in our understanding of Earth differentiation, or the weakest?

B. What should we infer from the fact that there is less contention over the state of the Earth 4.5 Ga than with regard to its present composition?

II. Post-core-formation mantle-core interactions:

A. Are these potentially significant to our understanding of the silicate portion of the Earth today? If so, what elements are involved?

B. Can we just forget about the core, and go on and worry about the rest of the Earth?

III. The importance of the continental crust:

A. Characterizing its bulk composition: The upper crust - a piece of cake? The middle crust - getting harder! The lower crust - do we know anything at all?

B. Without the lower crust, might we as well be back at ground zero? Just how much more must we learn about the lower crust before limitations in our understanding of the upper crust become important? Do we need to understand the processes that lead to cratonization too? And, how in the world does continental crust form anyway?

IV. The mantle:

A. Can we estimate the bulk composition of the mantle without introducing a strong dependence on the crust? Evidence from volcanic rocks and ultramafic rocks at the surface --do we need more data or are we just missing the forest for all the trees? How good is this? Are there parts of the mantle from which we see no melt products?

B. Evidence from process characterization: Fundamental processes - volcanism in all settings, subduction of oceanic crust. Really-hard-to-pin-down processes - crust formation (e.g., the high-silica kind), sub-continental lithosphere formation, and recycling old stuff back into the mantle (this, a very diverse family of processes...).

C. Is our understanding of these processes sufficient to support our present-day mantle composition based on initial estimates? Or, might we be able to estimate the present composition via process characterization from a good starting point & nothing more?

D. And, by the way (again), what should we learn from the fact that there is less disagreement concerning the composition of the mantle than the crust? (or is there?)

V. What are the most promising research directions for shedding new light on any of these interrelated issues?

Assuming that our primary goal is a quantifiable GERM, where should we focus our pitifully limited collective effort (considered in the grand scheme of things, of course): Mantle-derived volcanic products? Mantle-derived ultramafics? Quantifying our understanding of mantle mineralogy, phase transitions, physical structure, etc.? Better characterization of the crust? Trying to better quantify recycling fluxes (or is it more accurate to say quantify them for the first time, to first order?)

12:00 Lunch (T 29)

Thursday, March 12, 01:30 - 03:00
Flux Definition

From the Continents and Around the Oceans (Hubbs Hall)
Keynote Speaker: S. L. Goldstein

Erosion and weathering play important roles in the redistribution of elements on the earth. The detrital and authigenic fractions of marine sediments carry a barely tapped archive of information about climate conditions on the continents, wind and ocean circulation patterns and strength. This talk will review material and elemental fluxes from the continents to the oceans, how they are distributed by atmospheric and ocean circulation, and relationships to global climatic change.

Discussion Topics
(A. Spivack -- Discussion Leader)

• How accurately can global material fluxes (specifically of the various individual components of sediments) to the ocean be inferred from the analysis of specific sites?
• Can the sediment record be used to resolve questions concerning the physical processes that control weathering rates?
• How do weathering rates effect the ocean/atmosphere carbon system and how strong is the feedback between weathering and climate?
• On what time scales do fluxes vary? What is the variance in weathering rates (how important are catastrophic events?). What time-scales are of interest versus what can be resolved?
• Do evaporite mineral sequences contain a record of the relative magnitudes of continental weathering versus seafloor alteration?
• What is the role of submarine groundwater fluxes in the chemical budgets of the ocean? How can they be quantified and how might they respond to changes in sea level?
• What are the "key-measurements" that need to be made? Are bulk analyses and (or) logging measurements useful? Do isotopic proxies provide more useful data than bulk elemental analyses?

Thursday, March 12, 03:30 - 05:00
Flux Definition

Group Sessions (locations: Hubbs Hall, Munk Hall, Revelle)

Arcs (Hubbs)
Discussion Leaders: G. Bebout, T. Elliott

Hydrothermal Processes & Ocean Crust - Seawater Exchange (Revelle)
Discussion Leaders: D. Butterfield and M. Mottl

ODP and GERM (Munk)
Discussion Leaders: J. Pearce and M. Arthur

Poster Session 05:00 - 06:30
(with Wine and Cheese Munk Hall)

Barth, Matthias G.
"Tracking the Budget of Nb and Ta in the Continental ..."

Elliott, Tim.
"The Effect of Subduction Zone Processing ..."

France-Lanord, C.
"Riverine Alkalinity Flux and Atmospheric ..."

Galy, Albert.
"Processes of Weathering in the Himalaya ..."

Jochum, K.P.
"Constraints on the Abundances and the ..."

Kurtz, Andrew C.
"Reevaluating the Importance of Mineral ..."

Mottl, Michael J.
"Quantifying Hydrothermal Fluxes on Mid-Ocean ..."

Rose, E.F.
"Boron Transfer from the Continents to the ..."

Veizer, Jan.
"⁸⁷Sr/⁸⁶Sr, d¹³C and d¹⁸O Evolution of the ..."

Friday, March 13, 08:30 -10:00
Secular Evolution

Evolution of the Mantle (Hubbs Hall)
Keynote Speaker: Stan Hart

In oxygen isotope space, only enstatite chondrites are like the earth. Can the bulk earth equal to enstatite chondrites (Javoy, 1995)? Rare gas models argue for very little mass flux from the lower mantle. If true, how can we ever know what lower mantle composition to use in the modeling? Why does high ³He/⁴He mantle equate to more "primordial/ primitive/undegassed" mantle? If the mantle is really full of pyroxenite veins, how can we ever estimate a bulk earth composition?

Discussion Topics
(W. M. White -- Discussion Leader)

• Chemical structure of the mantle.
  • Is there major chemical layering in the mantle?
  • If so, what fraction does the depleted mantle represent?
  • If not, what is the reservoir of 3He?
  • Do plumes come from D"?
  • What fraction of the mantle does sub continental lithosphere compose?
• Chemical evolution of the mantle.
  • The "depleted mantle".
    • Is the apparent early depletion of the mantle real?
    • What fraction of the mantle was depleted by the early Archean?
    • Has the depleted mantle become more depleted with time?
    • Has the volume of depleted mantle grown with time?
    • Is the Depleted Mantle resupplied with incompatible elements & basaltic component?
    • Through recycling of oceanic crust?
    • Through input from below (e.g., mantle plumes)?
  • The plume reservoir.
    • Is D" the plume reservoir?
    • If so, what is its volume?
    • Is the entire lower mantle the plume reservoir?
    • What process or processes give rise to the compositions observed in mantle plumes?
    • Oceanic Crust/Sediment recycling?
    • Lower continental crust delamination?
    • Sub continental lithosphere delamination?
    • A primitive/undegassed component?
    • Are all the above involved?
    • If so, which process is associated with which compositional class?
  • Sub continental lithosphere.
    • How heterogeneous?
    • Mostly depleted or mostly enriched?
    • How is it enriched?
    • At subduction zones during crust generation.
    • By small degree melts from the asthenosphere.
    • By Plumes?
  • Lower mantle.
    • Is there a separate lower mantle reservoir?
    • Is the 660 km discontinuity a chemical boundary?
    • Is the lower mantle: primitive/more primitive? undegassed/less degassed? depleted.

Friday, March 13, 10:30 - 12:00
Secular Evolution

Evolution of the Atmosphere-Ocean-Biosphere Hydrosphere (Hubbs Hall)
Keynote Speaker: H. D. Holland

The pre-biotic atmosphere was probably mildly reducing. The d¹⁵N value of 3.4-3.5 Ga organic matter indicates that photosynthetic organisms used photosystem I and produced no free O₂. The d¹⁵N value of 2.7-2.5 Ga organic matter indicates that oxygenic photosynthesis was well established; however several sedimentologic criteria indicate that pO₂ was very low (ca. 10^-4 atm) and remained low until ca 2.3 Ga. pO₂ rose rapidly between ca. 2.25 and 2.05 Ga. These observations can be explained if pO2 was controlled by the marine nitrogen cycle until about 2.3 Ga and by the marine phosphate cycle since then.

Discussion Topics
(Lou Derry -- Discussion Leader)

• The organic carbon sub-cycle is a critical factor in determining long term atmospheric composition. What external factors (nutrient supply, sedimentation rate, ...) contribute to changes in the primary productivity and organic carbon preservation that lead to changes in the burial rate of organic carbon?
• What feedbacks exist in the system that control the crustal reservoirs of carbon and sulfur?
• What is the role of biotic evolution in driving changes in the fluxes and mass balances of the ocean-atmosphere-crust system?
• How does the long term thermal and tectonic evolution of the Earth act as a boundary condition for the evolution of biogeochemical systems? How do rates of Earth degassing and crustal recycling vary, and how do these variations affect the chemical stability of the ocean and atmosphere?

Friday, March 13, 01:30 - 03:00
Secular Evolution

The Continents through Time (Hubbs Hall)
Keynote Speaker: C. J. Hawkesworth
Co-author: Simon Turner

Bulk compositions of crust, estimated from surface sampling, and from heat flow and seismic refraction data suggest that the crust is broadly andesitic in composition, even though basaltic magmas dominate recent sites of crustal formation. Residual garnet was present during bulk crust formation. There is some evidence that the lower crust in Proterozoic and younger areas is significantly more mafic than that in the Archaean, and that post-Archaean crust has a greater average thickness. There is considerable uncertainty in estimates of how the volume of continental crust has changed through time. Sm-Nd model ages provide a minimum estimate, and suggest that 35-60% of the present crustal formed in the Archaean. This figure is 2-4 times the present area of Archaean crust. Estimates of maximum growth rates require information on the amounts of crustal recycling, and it has recently been argued that very little crustal recycling took place before ~2.0-1.6 Ga ago.

Discussion Topics
(R. L. Rudnick -- Discussion Leader)

• When did the continents (sialic crust) form?
  • Evidence: Initial epsilon Nd of early Archean rocks, Nb/U, Ce/Pb, Rb/Cs in the mantle through time.
• What is the composition of the current continental crust and how might this have changed through time?
• How did the continents form -- in what tectonic setting?

3:00 - 3:30 Coffee Break

Friday, March 12, 03:30 - 05:00
Plenary Session

GERM: Quo Vadis? (Hubbs Hall)

• Summary and Conclusions of Group Sessions.
• Publication Plans.
• Planning of future developments.

5:00: End of Workshop

Dinner/Farewell Party, 05:30 - 10:00
at Lisa Tauxe’s and Hubert Staudigel’s house, 4540 Chinook Ct.

Directions Take Ardath Road and Rte 52 East until Genesee Exit. Go North on Genesee (across Clairmont Mesa Bvd, major intersection after a mile and Bannok (100 yards further along). Turn right at the next street (Chickasaw Ave.) Which continues into Chinook Ct.

We will try to shuttle everybody there with the University van and private transportation. A Taxi ride should be on the order of about $10.00.

Saturday, March 14, 09:00 - 12:30
GERM Editors Meeting (Munk Hall)

We will initially meet in Munk Hall but we will probably split up into smaller groups that work on various computers in the Apollo room, HS lab or office in IGPP.

12:30 Catered Lunch (T 29)