Harry Becker's Web Page
 
 

Assistant Research Scientist Geo- and Cosmochemistry

Department of Geology

University of Maryland

College Park, MD 20742, USA

E-mail: hbecker@geol.umd.edu

Fax:       (301) 314 9661

Phone:   (301) 405 4088

Office:  0214A (ChemistryBuilding)

Research
 

The application of isotopes to problems in geo- and cosmochemistry is a center piece of my research. Radioactive decay systems are particularly useful, because they can date processes that affected rocks such as weathering, melting and metamorphism. Radiogenic isotopes can be used as tracers of element fractionation that occurred in the past and constrain mixing processes between different reservoirs. Our main research tools are the thermal ionization and ICP mass spectrometers, and the cleanlab facility in the Department of Geology. The scope of our research often requires the development of new analytical protocols for chemical and isotopic analyses.

My research has focused on the following themes:

(1) Element fractionation and chronology of events in the early solar system. Formation of refractory inclusions, chondrules, and chondritic metal. Scale and causes of isotopic heterogeneity in the solar system.

(2) Differentiation processes in the deep Earth such as mantle melting, deep recycling of subducted crust, and core-mantle interaction.

(3) Global mass balances and element transfer at plate margins.

(4)  Hydrothermal and melting processes in the continental crust and in subduction and collision zones.

(5) Geochronology, metamorphism, cooling histories and exhumation of high-grade and high-pressure metamorphic rocks.
 
 
 
 

COSMOCHEMISTRY 
 

Siderophile element fractionation in the early solar system

Re-Os systematics of Ca-Al-rich inclusions and other components of carbonaceous  chondrites (w. R. J. Walker et al.).
    As a part of our studies of the ¹⁸⁷Re-¹⁸⁷Os systematics of chondrites (see also Rich Walker's web site and Walker et al., LPSC XXX), we have investigated the Re-Os systematics of Ca-Al-rich inclusions (CAIs) and other components of carbonaceous chondrites. The goal of this research was to understand chronology and processes that were affecting refractory siderophile elements in the early solar system. More information: Becker et al., 2001 Geochimica et Cosmochimica Acta

    Significant variability in Re/Os and Os isotopic composition were also found for other components of the carbonaceous chondrite Allende. Fine-grained matrix shows substantially lower Re/Os than bulk rock analyses and is balanced by the relatively high Re/Os of coarse-grained troilite and chondrules. These differences most likely result from the presence of several types of troilite and metal grains in chondrites that originally condensed in different locations (and at different times?) of the early solar nebula. Alternative, but less likely explanations include variable redox conditions during condensation of matrix grains, coarse troilite, and chondrule precursors (Re-oxide in the gas phase requires much more oxidizing conditions than assumed for the nebula), or partial evaporation and loss of Re from fine-grained matrix during parent-body metamorphism (K isotope data by Humayun and Clayton (1995) does not indicate significant K loss by evaporation).

The short-lived technetium-ruthenium-molybdenum chronometers and early solar system processes (w. R. J. Walker).
The short-lived Tc-Ru-Mo chronometers (⁹⁷Tc-⁹⁷Mo t_1/2= ca. 4 M. y.; ⁹⁸Tc-⁹⁸Ru, t_1/2= ca. 10 M. y.; ⁹⁹Tc-⁹⁹Ru, t_1/2= 0.21 M. y.) may provide new insight about time scales of fractionation of refractory siderophile elements (Re, Os, Ir, Tc, Ru, W, Mo) in the early solar system, and may provide clues on the initial abundance of Tc isotopes when the solar system formed. Because of the short half life of ⁹⁹Tc, anomalies in ⁹⁹Ru, if confirmed, would provide stringent constraints on the time interval between the formation of s-process ⁹⁹Tc, possibly in nearby red giant stars (Wasserburg et al., 1994), and Tc-Ru fractionation in the early solar system.

    Requirements for the use of these decay systems as relative chronometers are (1) that the parent-daughter ratios of two or more reservoirs were significantly fractionated by chemical and physical processes that occurred while sufficient amounts of the parent nuclide were still extant, and (2) that  such reservoirs were not remixed and equilibrated at a later time. Refractory siderophile elements have high half-mass condensation temperatures (high boiling points), and under reducing conditions they preferentially partition into a metal phase. Processes that may have fractionated the Tc/Ru and Tc/Mo ratios in the early solar system include condensation and evaporation processes in the nebula or on planetesimals, silicate melt-liquid FeNi metal partitioning (e. g., core formation in asteroids and planets), solid metal-liquid metal partitioning (e.g., formation of a solid core), low-temperature aqueous alteration, and, on a small scale, solid metal-solid metal (or sulfide) partitioning.

    Recent isotopic studies of iron meteorites and chondrites reported positive deviations for ⁹⁷Mo, and ⁹⁸Ru and ⁹⁹Ru in some of these meteorites at the 0.7-40 e (=0.007-0.4 %) level compared to terrestrial standards (Yin, 1995; Yin and Jacobsen, 1998; Smoliar, 1998; Yin et al., 1999). We have developed chemical separation methods and analytical protocols for precise measurement of Ru and Mo isotopic compositions using negative thermal ionization mass spectrometry. Multiple analysis of terrestrial Ru solutions from three different sources and a natural terrestrial OsIr alloy show within analytical precision similar Ru isotopic compositions. Six iron meteorites (IIA, IIB and IIIA groups) and two chondrites (Allende CV3, Allegan, H5) show Ru isotopic compositions indistinguishable from average terrestrial Ru at the 2s level (±0.3 e for ⁹⁹Ru, ±0.8 e for ⁹⁸Ru).  More information: Gold 2001

Multiple analyses (n = 4) of the IIA iron Negrillos yielded e⁹⁷Mo = 0.54±0.47, indistinguishable from terrestrial Mo (see also Yin et al., 2002). The significance of the small positive anomaly of Toluca (IA iron) needs to be investigated further. For the p-process chronometers (⁹⁷Tc, ⁹⁸Tc), low abundances in the seed material that formed our solar system and limited Tc-Ru fractionation may be the reason for the absence/scarcity of deviations in meteorites. For s-process ⁹⁹Tc, limited Tc-Ru fractionation and the short half life may be responsible for the homogeneous distribution of ⁹⁹Ru (More information: Manuscript submitted to Chemical Geology)
 

Scale of non-mass dependent isotopic fractionation in solar system materials (w. R. J. Walker).

Non-mass dependent isotopic heterogeneities for various elements in trace components of primitive chondrites (presolar grains) and in some refractory inclusions from carbonaceous chondrites are now well documented (Anders and Zinner, 1993). These isotopic anomalies are usually restricted to the sub-cm scale and mostly reflect lack of equilibration of their primitive components at this scale. Large-scale isotopic heterogeneities in solar system materials that do not reflect radiogenic ingrowth exist for O (R. N. Clayton et al.), S (Farquhar et al.) and possibly Cr (e. g., Podosek et al., Lugmair et al.). Some of these signatures reflect photochemical processes in planetary atmospheres (see web page of James Farquhar). The idea that intense irradiation processes may have melted and evaporated dust swept close to the young sun and in the course of these processes produced ¹⁶O and possibly some other nuclides finds increasing acceptance (Lee et al., 1999 Astrophysical Journal; Clayton, 2002, Nature).

Until recently, little evidence for large-scale non-mass dependent isotopic heterogeneities has been found for heavier elements. Recent Zr and Mo isotopic studies reported evidence for differences in isotopic compositions for whole rocks of some primitive (Sanloup et al., 2000; Yin et al. 2002) and differentiated meteorites (including IIAB and IIIA irons, mesosiderites and pallasites, Dauphas et al., 2002), relative to terrestrial materials. Enrichments of r- and p-process isotopes over s-process dominated isotopes are the most prominent features of these isotopic abundance patterns. Qualitatively, these patterns look complementary to patterns of Mo isotopes in presolar SiC grains isolated from carbonaceous chondrites (Nicolussi et al., 1998) and it has been argued that bulk meteorites that show such complementary patterns have a variable deficit in s-process nucleosynthetic components and their carriers (Dauphas et al. 2002, Yin et al. 2002).

These results are surprising in so far as high-precision data for the stable Ru isotopes in IIAB and IIIA iron meteorites obtained by N-TIMS overlap with terrestrial Ru at the 0.3 to 1.0 e level (Becker and Walker, 2002, LPSC 33). The Ru data show no resolvable and systematic enhancement in the p- (⁹⁶Ru, ⁹⁸Ru), mixed s-,r- (⁹⁹Ru, ¹⁰¹Ru, ¹⁰²Ru) and pure r-process ¹⁰⁴Ru, relative to pure s-process ¹⁰⁰Ru. The same is true for the ordinary chondrite Allegan (H5). The data for the carbonaceous chondrite Allende (CV3) is somewhat less precise and is consistent with a normal isotopic composition or a <1-2 e deficit in s-process isotopes.
More information: LPSC33 and Goldschmidt Conference 2002
 
 

GEOCHEMISTRY
 

Re-Os systematics of the Earth's upper mantle

    Because of improvements in analytical techniques the 187Re-187Os decay system has become an extremely versatile tool to constrain mass exchange and transfer processes in and between deep Earth and near-surface reservoirs. Yet, in spite of the increasing amount of Os isotopic data on mantle-derived rocks that accumulated over the years, the average composition of the modern convecting upper mantle remains difficult to constrain. Mass balance constraints indicate that the amount of Re in the continents is balanced by the average Re depletion in the lithospheric mantle. If no significant redistribution of Re occurred within the mantle over geologic time, the large reservoir of the convecting upper mantle should show no or only minor Re depletion. Os isotopic compositions should reflect this behavior, and should be in the range of chondrites (¹⁸⁷Os/¹⁸⁸Os = 0.126-0.130). Significant redistribution of Re may have occurred, if large quantities of Re-rich recycled oceanic crust were isolated in the lower mantle. In such a scenario, the ¹⁸⁷Os/¹⁸⁸Os may be lower than the chondritic range. Thus, constraining the ¹⁸⁷Os/¹⁸⁸Os of mantle reservoirs will provide us with information on efficiencies of convective mixing and their time scales in the Earth's mantle.

    Various materials that are believed to be derived from convecting upper mantle yielded conflicting results. [1] Some believe that the average ¹⁸⁷Os/¹⁸⁸Os of abyssal peridotites (0.125, Snow and Reisberg, 1995) reflects the average ¹⁸⁷Os/¹⁸⁸Os of the convecting upper mantle. Although these peridotites have high Os abundances (typically 3 ppb), they are often strongly  altered, and some believe that the more radiogenic values of some abyssal peridotites were caused by these alteration processes and interaction with seawater (Standish et al., 2002).  [2] Mid-ocean-ridge-basalts (MORB) have chondritic to suprachondritic ¹⁸⁷Os/¹⁸⁸Os (Schiano et al., 1997). The problem of the MORB data, however, are the low Os abundances in MORB (a few ppt or less) that make these materials very susceptible to late-stage alteration and modification by radiogenic Os from seawater (¹⁸⁷Os/¹⁸⁸Os = 1.05). [3] Chromites from podiform chromitites in ophiolites are crystal fractionation products of Mg-rich melts that, like MORB, provide an average of relatively large domains of the upper mantle. This, and their high Os abundances (reducing the effects of alteration) make chromites from podiform chromitites perhaps the best materials to constrain the composition of the convecting mantle. In a study of a large number of chromites from many ophiolites, Walker et al. (2002) determined a mean ¹⁸⁷Os/¹⁸⁸Os of 0.1281±0.0009 for the modern convecting upper mantle. If this estimate is roughly correct, the difference to the mean value for abyssal peridotites (0.125) and occurrence of some very uradiogenic peridotites in the ocean basins (e. g., Parkinson et al., 1998) require the presence of a component in the upper mantle that is characterized by suprachondritic ¹⁸⁷Os/¹⁸⁸Os and high Re.

Re-Os systematics of alkaline basalts from seamounts in the North Atlantic (w. K. Haase, R. J. Walker)
    In this study we evaluate whether the upper mantle contains a minor component with high Re and radiogenic Os isotopic composition that might balance the Re depletion in abyssal peridotites. Results indeed show suprachondritic ¹⁸⁷Os/¹⁸⁸Os and also higher Re abundances and higher Re/Y compared to values typical for alkaline ocean island basalts. To see our AGU Spring Meeting 2000 abstract, click here. A more up to date summary: here

Radiogenic Os in pyroxenites and peridotites from Jurassic seafloor (Totalp ultramafic massif, Swiss Alps).
    The Totalp serpentinite is a classic "alpinotype" ultramafic massif that was emplaced ca. 160 Ma ago on the seafloor of the Tethys ocean. The rocks are serpentinized fertile lherzolites topped by ophicalcitic breccias and pelagic sediments in primary contact. Re-Os data on whole rocks of serpentinized lherzolites show subchondritic to slightly suprachondritic initial ¹⁸⁷Os/¹⁸⁸Os (gOs at 160 Ma of -1 to +11). Geochronological data and Alpine kinematics and geodynamics suggest a short residence time for this mantle material in the oceanic lithosphere, followed by emplacement on the seafloor in a transform fault setting (Weissert and Bernoulli, 1985, Peters and Stettler, 1987). The massif also contains foliation-concordant pyroxenites, some of which show highly radiogenic initial ¹⁸⁷Os/¹⁸⁸Os (up to 2). Hence, it could provide compelling evidence regarding the presence of a high-¹⁸⁷Os/¹⁸⁸Os component in the convecting upper mantle. We currently study the significance of the radiogenic ¹⁸⁷Os/¹⁸⁸Os in terms of alteration vs. primary magmatic origin.
 

Melt percolation in the upper mantle and the effects on the Re-Os system

Compared to the Earth's mantle, the continents contain insignificant amounts of Re and Os. Hence, the most significant redistribution of these elements after accretion and core formation occurred by partial melting of the mantle and recycling of oceanic crust back into the mantle. In the past, the behavior of Re, Os and other platinum group elements during the interaction of ascending mantle melts and mantle wallrock has received little attention.

Re-Os systematics of pyroxenites and peridotites from a Paleozoic convergent plate margin in lower Austria. Transport of slab-derived Os and melt-peridotite interaction in sub-arc mantle (w. S. B. Shirey, R. W. Carlson).
Re-Os data on peridotites and layered dunite-pyroxenite rocks, in conjunction with major and trace element and Sr-Nd isotopic data  shows the effects of melt percolation on Re and Os abundances and 187Os/188Os in the peridotites at low and moderately high melt/rock ratios. More information Becker et al. EPSL 2001 and a figure from this paper
 

Element fluxes in subduction zones

The quantification of the fluxes of incompatible elements (K, U, Th, Pb, Sr, Nd etc) in subduction zones is crucial to understand the mass balance of these elements in the silicate Earth and the long-term isotopic evolution of silicate Earth reservoirs. Of particular importance are fluxes from basaltic oceanic crust via low- and high-temperature alteration near the ocean ridges, dehydration or melting in subduction zones, and recycling back into the mantle. While the alteration fluxes near ocean ridges have been studied by Staudigel et al. (1995, 1996) and Hart et al. (1999), we have focused on trace element fractionation and fluxes during dehydration of metabasalts and metagabbros in paleo-subduction zones.

Fractionation of incompatible elements of eclogites and  blueschists. Relevance for element fluxes into arcs, the continental crust and into the mantle (w. K.-P. Jochum, R. W. Carlson). Click here and here for details.

Re-Os systematics of eclogites and blueschists and the implications for recycling  of MORB and plume sources. Click here for details.

U-Th-Pb systematics of garnet pyroxenites from lower Austria. Evidence for slab- derived components in the pyroxenites and element fractionation in the slab (w. R. W. Carlson).
 

Magma chamber processes

Sr-Nd isotopic constraints on interaction of basaltic and granitic melt in stratified mafic- silicic magma chambers in the coastal Maine igneous province (w. R. Wiebe et al.).
Details here.
 

Laboratory Work: past and present 

Positive and negative thermal ionization mass spectrometry (SECTOR 54, NBS-designed 6", 9", 12", 15" instruments, VG 354, Finnigan MAT 261): Rb-Sr, Sm-Nd, U-Th-Pb, Re-Os, Pt-Os, Ru and Mo isotopes, isotope dilution techniques for Ba, K, Cs
Multi collector inductively coupled plasma mass spectrometry (Nu Plasma): Click here for paper on Ru isotopes
Element separation techniques such as ion exchange and solvent extraction.
High-performance-liquid-chromatography (REE separation, MPI Mainz 1992-1995).
Electron microprobe analysis CAMECA Microscan and SX50 (1990-1992 Universities of Karlsruhe and Mainz, Germany)
H₂O determination by Karl Fischer titration, C and S determination by CSA, Ferrous iron determination by titration, XRF pellet preparation (1987-1992, University of Karlsruhe)