Deep mantle matters Quentin Williams Science 344, 800 (2014); DOI: 10.1126/science.1254399
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 27, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6186/800.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6186/800.full.html#related This article cites 12 articles, 6 of which can be accessed free: http://www.sciencemag.org/content/344/6186/800.full.html#ref-list-1 This article appears in the following subject collections: Geochemistry, Geophysics http://www.sciencemag.org/cgi/collection/geochem_phys
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on May 27, 2014
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INSIGHTS | P E R S P E C T I V E S
GEOPHYSICS
Deep mantle matters Experiments reveal how some deep seismic anomalies near the core-mantle boundary might be generated By Quentin Williams
T
he lower mantle, lying between ~670and ~2890-km depth, comprises most of the rocky portion of Earth. Convection processes within this region transfer heat from the iron core upward, advecting the heat flow that drives the near-surface tectonic engine. As many of the largest volcanic events appear to be correlated with seismic features in the deep mantle (1), the deep lower mantle may also represent an occasional scourge of our sur-
decrease in shear velocity extend up to 1000 km above the CMB (large low-shear-velocity provinces, LLSVPs) (4). Regions with almost horizontal discontinuities in seismic velocity ~100 to 300 km are present above the CMB with velocity jumps of 2 to 3% (5) (these discontinuities define the anomalous D⬙ zone at the base of the mantle). There are also regions with large reductions in compressional and shear wave velocity in sporadic places in the lowermost ~5 to 40 km of the mantle, with shear velocity decreases as large as 45% (6) (ultralow-velocity zones, ULVZs). But all
Lower mantle
a possible increase in iron content (10). But discerning how these anomalies are generated is difficult because our direct information is confined to a seismic snapshot of the present-day structure of this region. The new studies provide process-oriented constraints on the genesis of ULVZs and chemical variations within the lowermost mantle (2) and shift the view on the perceived mineralogic simplicity of much of the lower mantle (3). Andrault et al. study the melting behavior of basalt (ocean crust) to pressures of the CMB. Basalt is the lowest– melting temperature rock likely to be present near the CMB, and is expected to melt when exposed to the temperatures of Earth’s outer core (11). The picture that emerges is that subducted plates may carry oceanic crust to depths near the base of the mantle, where it can partially melt, forming ULVZs. But Andrault et al. propose that the melt can only be retained within the unmelted residue of
Disproportionation reaction
1000 km LLSVP
PV D”
PPv
ULVZ CMB Outer core
face environment. In this issue, two sets of challenging experiments yield new pictures for how different deep seismic anomalies might be generated. On page 892, Andrault et al. (2) examine the melting temperature of oceanic crust (basalt) to core-mantle boundary (CMB) pressures and temperature, and use that to explain the genesis of areas with ultralow seismic velocities near the CMB. On page 877, Zhang et al. (3) report the startling discovery of a new, iron-rich silicate phase that may be a major component of the lowermost ~700 km of Earth’s mantle. Seismic probing has led to a richness of phenomena being recognized in the inaccessible lowermost ~1000 km of Earth’s mantle (see the figure). Large, blocky structures beneath Africa and the Pacific with a ~3% Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA. E-mail: [email protected]
800
of these features are volumetrically small relative to the bulk of the mantle (LLSVPs are at most a few percent of the lower mantle volume, while ULVZs represent a small fraction of a percent). These features reside within a lower mantle that has long been recognized to be dominated by a simple mix of (Mg,Fe)SiO3-perovskite and (Mg,Fe)O [and with possibly some exotic behavior associated with spin transitions in iron (7)]. Modeling and experiments at the extreme pressures and temperatures of the deep mantle (on the order of 100 GPa and 2000 K) reveal links between variations in phase, composition, and temperature and the observed seismic anomalies. One explanation for LLSVPs is that they are associated with slight iron and (Mg,Fe)SiO3 enrichment (8), the seismic discontinuities with a pressure-induced transition from (Mg,Fe)SiO3perovskite to a post-perovskite structure (9), and the ULVZ with partial melting and
the original basalt. Once it migrates into normal mantle, its chemistry is such that it will react and form solid magnesian perovskite, chemically remixing oceanic crust back into the mantle. As a result, ULVZs should be generated from, and associated with, subducted material at depth. Thus, the most abundant magma at Earth’s surface may also produce most of the melt near the bottom of Earth’s rocky mantle—and plate tectonics may play a fundamental role in the geochemistry of material juxtaposed with Earth’s metallic core. Zhang et al. observe the disproportionation of (Mgx,Fe1-x)SiO3-perovskite (where x within Earth’s mantle is ~0.9) to nearly pure MgSiO3 and a new, iron-enriched approximately (Mg0.6Fe0.4)SiO3 phase. This “Hphase” has a hexagonal structure and occurs at pressures corresponding to depths greater than ~2200 km. The synthesis of this phase occurred at high temperatures (2200 K and above), and this temperature and a lowsciencemag.org SCIENCE
23 MAY 2014 • VOL 344 ISSUE 6186
Published by AAAS
ILLUSTRATION: ADAPTED FROM E. GARNERO
The mantle region. Schematic of seismically characterized features occurring in the lowermost ~1000 km of Earth’s mantle, and experimentally proposed oceanic crustal-associated ULVZ (2) and perovskite disproportionation boundary (3). (Note that the ULVZ vertical scale is exaggerated for visibility.) PV, perovskite; PPv, post-perovskite.
stress environment appear to be critical for synthesizing this phase. So why has this new transition not been observed seismically? One possibility is that the velocity change across the transition may be too small and/or the boundary may undulate dramatically in its depth. Alternatively, the temperature of the deep mantle may lie below the temperature of the disproportionation reaction [which would require that the mantle be a few hundred kelvin cooler than currently inferred (12)—but this would also imply that disproportionation could have been important in the hotter past]. Another option is that the oxidation state of iron in the mantle may differ from those within the experiments. The provocative aspects of this discovery include not just changing the possible mineralogy of the deeper lower mantle, but also that two phases of markedly different densities are produced. Whether these phases could undergo partial segregation, thus enriching or depleting regions in the H-phase (particularly in an earlier, hotter, less viscous, and possibly partially molten mantle), is unknown. If such segregation did occur, a natural explanation for the genesis of LLSVPs might exist. Depending on its elasticity, an enrichment of H-phase within these regions might provide an avenue to explain their anomalous seismic signature. Each of these experiments is the direct result of developments in high-pressure, hightemperature techniques and the availability of high-intensity synchrotron sources. Probing the sensitivity of the pressure and temperature of melting and the phase transition to variable oxygen fugacities, shifts in major and minor elemental abundances, and volatile contents holds the prospect of mapping out the likely chemical behavior of the lower mantle. In doing so, the current void of information on the differentiation processes that govern the chemical variations, structural features, and evolution of Earth’s deepest rocky reaches will be filled. ■ REFERENCES
1. 2. 3. 4. 5.
ILLUSTRATION: V. ALTOUNIAN/SCIENCE
6. 7. 8. 9. 10. 11. 12.
J. Austermann et al., Geophys. J. Int. 197, 1 (2014). D. Andrault et al., Science 344, 892 (2014). L. Zhang et al., Science 344, 877 (2014). S. Ni, D. V. Helmberger, J. Tromp, Geophys. J. Int. 161, 283 (2005). M. Wysession et al., in The Core-Mantle Boundary Region, M. Gurnis, M. Wysession, E. Knittle, B. Buffett, Eds., Geodyn. Ser. (American Geophysical Union, Washington, DC, 1998), vol. 28, 319–334. M. S. Thorne, E. J. Garnero, G. Jahnke, H. Igel, A. K. McNamara, Earth Planet. Sci. Lett. 364, 59 (2013). J. F. Lin et al., Rev. Geophys. 51, 244 (2013). F. Deschamps, L. Cobden, P. J. Tackley, Earth Planet. Sci. Lett. 349-350, 198 (2012). M. Murakami et al., Science 304, 855 (2004). S. Rost et al., Nature 435, 666 (2005). S. Anzellini et al., Science 340, 464 (2013). T. Katsura, A. Yoneda, D. Yamazaki, T. Yoshino, E. Ito, Phys. Earth Planet. Inter. 183, 212 (2010). 10.1126/science.1254399
CANCER IMMUNOLOGY
Identifying the infiltrators Molecular characterization of macrophages reveals distinct types during tumorigenesis By Elisa Gomez Perdiguero and Frederic Geissmann
T
he mammalian immune system both suppresses and tolerates tumors, so understanding this complexity should benefit the development of cancer therapies. Macrophages are proposed to play an important role in suppressing the immune response to cancer cells, but it is not clear where these immune cells come from or whether there are distinct populations of macrophages with specific roles in this setting. On page 921 of this issue, Franklin et al. (1) forge a more coherent view of macrophages that are associated with tumor growth by assessing their origin, phenotype, and functions in an animal model of breast cancer. Tumor progression can be divided into three phases—initiation, growth, and metastasis (see the figure). The first phase is characterized by the cell-autonomous accumulation of genetic defects that leads to cell transformation. This is followed by clonal growth of transformed cells within the tissue—the primary tumor site (2). Metastasis results from the successful “engraftment” of circulating tumor cells into secondary locations where they proliferate after a dormancy phase in which metastatic cells remain quiescent (3). In both primary and secondary tumor sites, the stroma, which includes mesenchymal cells, macrophages, and extracellular matrix (3), is thought to play a role in the initial survival and proliferation of transformed cells. However, as a solid tumor grows and tumor cells acquire the potential to escape the primary site, the stroma becomes a more complex environment, with newly formed blood and lymphatic vessels and the recruitment and/or proliferation of lymphoid and myeloid immune cells (4). Immune cells are proposed to prevent tumor progression via the elimination of immunogenic tumor cells by T lymphocytes (CD8 subtype), a phenomenon known as immunosurveillance (also called immunoediting). During this process, tumors that display either reduced immunogenicity or enhanced immunosuppressive activity will escape elimination (5). Macrophages present in the tumor site can activate the immune response, but are mainly
SCIENCE sciencemag.org
thought to contribute to immunosuppression and tumor progression (6, 7), particularly in the mammary gland (8). A high density of macrophages in tumors is also associated with worse overall survival in patients with gastric, urogenital, and head and neck cancers, although it seems to be associated with better overall survival in patients with colorectal cancer (7). Franklin et al. carefully explore the contribution of macrophages to tumor growth in mice that develop a mammary cancer
that is genetically driven by the expression of an oncogene. In investigating the development and differentiation of macrophages in the normal mammary gland and during the progression of a mammary tumor, the authors identify a population of macrophages that accumulates during tumor growth called tumor-associated macrophages (TAMs). These cells develop from bone marrow–derived cells with the characteristics of inflammatory monocytes, which are recruited to the tumor where they differentiate into macrophages and subsequently proliferate. Franklin et al. observed that when signaling by the protein Notch is prevented in these TAMs, their differentiation is blocked. Interestingly, TAMs are distinct from macrophages present in normal mammary tissue, which develop independently of Notch signaling. Depletion of TAMs led to a reduced tumor burden in the animal and increased the cytotoxic potential of T lymphocytes present in the primary tumor site. Thus, monocyte-derived Notch-dependent TAMs are critical for tumor growth in this mammary gland tumor model, at least in 23 MAY 2014 • VOL 344 ISSUE 6186
Published by AAAS
801
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 27, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6186/800.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6186/800.full.html#related This article cites 12 articles, 6 of which can be accessed free: http://www.sciencemag.org/content/344/6186/800.full.html#ref-list-1 This article appears in the following subject collections: Geochemistry, Geophysics http://www.sciencemag.org/cgi/collection/geochem_phys
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on May 27, 2014
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
INSIGHTS | P E R S P E C T I V E S
GEOPHYSICS
Deep mantle matters Experiments reveal how some deep seismic anomalies near the core-mantle boundary might be generated By Quentin Williams
T
he lower mantle, lying between ~670and ~2890-km depth, comprises most of the rocky portion of Earth. Convection processes within this region transfer heat from the iron core upward, advecting the heat flow that drives the near-surface tectonic engine. As many of the largest volcanic events appear to be correlated with seismic features in the deep mantle (1), the deep lower mantle may also represent an occasional scourge of our sur-
decrease in shear velocity extend up to 1000 km above the CMB (large low-shear-velocity provinces, LLSVPs) (4). Regions with almost horizontal discontinuities in seismic velocity ~100 to 300 km are present above the CMB with velocity jumps of 2 to 3% (5) (these discontinuities define the anomalous D⬙ zone at the base of the mantle). There are also regions with large reductions in compressional and shear wave velocity in sporadic places in the lowermost ~5 to 40 km of the mantle, with shear velocity decreases as large as 45% (6) (ultralow-velocity zones, ULVZs). But all
Lower mantle
a possible increase in iron content (10). But discerning how these anomalies are generated is difficult because our direct information is confined to a seismic snapshot of the present-day structure of this region. The new studies provide process-oriented constraints on the genesis of ULVZs and chemical variations within the lowermost mantle (2) and shift the view on the perceived mineralogic simplicity of much of the lower mantle (3). Andrault et al. study the melting behavior of basalt (ocean crust) to pressures of the CMB. Basalt is the lowest– melting temperature rock likely to be present near the CMB, and is expected to melt when exposed to the temperatures of Earth’s outer core (11). The picture that emerges is that subducted plates may carry oceanic crust to depths near the base of the mantle, where it can partially melt, forming ULVZs. But Andrault et al. propose that the melt can only be retained within the unmelted residue of
Disproportionation reaction
1000 km LLSVP
PV D”
PPv
ULVZ CMB Outer core
face environment. In this issue, two sets of challenging experiments yield new pictures for how different deep seismic anomalies might be generated. On page 892, Andrault et al. (2) examine the melting temperature of oceanic crust (basalt) to core-mantle boundary (CMB) pressures and temperature, and use that to explain the genesis of areas with ultralow seismic velocities near the CMB. On page 877, Zhang et al. (3) report the startling discovery of a new, iron-rich silicate phase that may be a major component of the lowermost ~700 km of Earth’s mantle. Seismic probing has led to a richness of phenomena being recognized in the inaccessible lowermost ~1000 km of Earth’s mantle (see the figure). Large, blocky structures beneath Africa and the Pacific with a ~3% Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA. E-mail: [email protected]
800
of these features are volumetrically small relative to the bulk of the mantle (LLSVPs are at most a few percent of the lower mantle volume, while ULVZs represent a small fraction of a percent). These features reside within a lower mantle that has long been recognized to be dominated by a simple mix of (Mg,Fe)SiO3-perovskite and (Mg,Fe)O [and with possibly some exotic behavior associated with spin transitions in iron (7)]. Modeling and experiments at the extreme pressures and temperatures of the deep mantle (on the order of 100 GPa and 2000 K) reveal links between variations in phase, composition, and temperature and the observed seismic anomalies. One explanation for LLSVPs is that they are associated with slight iron and (Mg,Fe)SiO3 enrichment (8), the seismic discontinuities with a pressure-induced transition from (Mg,Fe)SiO3perovskite to a post-perovskite structure (9), and the ULVZ with partial melting and
the original basalt. Once it migrates into normal mantle, its chemistry is such that it will react and form solid magnesian perovskite, chemically remixing oceanic crust back into the mantle. As a result, ULVZs should be generated from, and associated with, subducted material at depth. Thus, the most abundant magma at Earth’s surface may also produce most of the melt near the bottom of Earth’s rocky mantle—and plate tectonics may play a fundamental role in the geochemistry of material juxtaposed with Earth’s metallic core. Zhang et al. observe the disproportionation of (Mgx,Fe1-x)SiO3-perovskite (where x within Earth’s mantle is ~0.9) to nearly pure MgSiO3 and a new, iron-enriched approximately (Mg0.6Fe0.4)SiO3 phase. This “Hphase” has a hexagonal structure and occurs at pressures corresponding to depths greater than ~2200 km. The synthesis of this phase occurred at high temperatures (2200 K and above), and this temperature and a lowsciencemag.org SCIENCE
23 MAY 2014 • VOL 344 ISSUE 6186
Published by AAAS
ILLUSTRATION: ADAPTED FROM E. GARNERO
The mantle region. Schematic of seismically characterized features occurring in the lowermost ~1000 km of Earth’s mantle, and experimentally proposed oceanic crustal-associated ULVZ (2) and perovskite disproportionation boundary (3). (Note that the ULVZ vertical scale is exaggerated for visibility.) PV, perovskite; PPv, post-perovskite.
stress environment appear to be critical for synthesizing this phase. So why has this new transition not been observed seismically? One possibility is that the velocity change across the transition may be too small and/or the boundary may undulate dramatically in its depth. Alternatively, the temperature of the deep mantle may lie below the temperature of the disproportionation reaction [which would require that the mantle be a few hundred kelvin cooler than currently inferred (12)—but this would also imply that disproportionation could have been important in the hotter past]. Another option is that the oxidation state of iron in the mantle may differ from those within the experiments. The provocative aspects of this discovery include not just changing the possible mineralogy of the deeper lower mantle, but also that two phases of markedly different densities are produced. Whether these phases could undergo partial segregation, thus enriching or depleting regions in the H-phase (particularly in an earlier, hotter, less viscous, and possibly partially molten mantle), is unknown. If such segregation did occur, a natural explanation for the genesis of LLSVPs might exist. Depending on its elasticity, an enrichment of H-phase within these regions might provide an avenue to explain their anomalous seismic signature. Each of these experiments is the direct result of developments in high-pressure, hightemperature techniques and the availability of high-intensity synchrotron sources. Probing the sensitivity of the pressure and temperature of melting and the phase transition to variable oxygen fugacities, shifts in major and minor elemental abundances, and volatile contents holds the prospect of mapping out the likely chemical behavior of the lower mantle. In doing so, the current void of information on the differentiation processes that govern the chemical variations, structural features, and evolution of Earth’s deepest rocky reaches will be filled. ■ REFERENCES
1. 2. 3. 4. 5.
ILLUSTRATION: V. ALTOUNIAN/SCIENCE
6. 7. 8. 9. 10. 11. 12.
J. Austermann et al., Geophys. J. Int. 197, 1 (2014). D. Andrault et al., Science 344, 892 (2014). L. Zhang et al., Science 344, 877 (2014). S. Ni, D. V. Helmberger, J. Tromp, Geophys. J. Int. 161, 283 (2005). M. Wysession et al., in The Core-Mantle Boundary Region, M. Gurnis, M. Wysession, E. Knittle, B. Buffett, Eds., Geodyn. Ser. (American Geophysical Union, Washington, DC, 1998), vol. 28, 319–334. M. S. Thorne, E. J. Garnero, G. Jahnke, H. Igel, A. K. McNamara, Earth Planet. Sci. Lett. 364, 59 (2013). J. F. Lin et al., Rev. Geophys. 51, 244 (2013). F. Deschamps, L. Cobden, P. J. Tackley, Earth Planet. Sci. Lett. 349-350, 198 (2012). M. Murakami et al., Science 304, 855 (2004). S. Rost et al., Nature 435, 666 (2005). S. Anzellini et al., Science 340, 464 (2013). T. Katsura, A. Yoneda, D. Yamazaki, T. Yoshino, E. Ito, Phys. Earth Planet. Inter. 183, 212 (2010). 10.1126/science.1254399
CANCER IMMUNOLOGY
Identifying the infiltrators Molecular characterization of macrophages reveals distinct types during tumorigenesis By Elisa Gomez Perdiguero and Frederic Geissmann
T
he mammalian immune system both suppresses and tolerates tumors, so understanding this complexity should benefit the development of cancer therapies. Macrophages are proposed to play an important role in suppressing the immune response to cancer cells, but it is not clear where these immune cells come from or whether there are distinct populations of macrophages with specific roles in this setting. On page 921 of this issue, Franklin et al. (1) forge a more coherent view of macrophages that are associated with tumor growth by assessing their origin, phenotype, and functions in an animal model of breast cancer. Tumor progression can be divided into three phases—initiation, growth, and metastasis (see the figure). The first phase is characterized by the cell-autonomous accumulation of genetic defects that leads to cell transformation. This is followed by clonal growth of transformed cells within the tissue—the primary tumor site (2). Metastasis results from the successful “engraftment” of circulating tumor cells into secondary locations where they proliferate after a dormancy phase in which metastatic cells remain quiescent (3). In both primary and secondary tumor sites, the stroma, which includes mesenchymal cells, macrophages, and extracellular matrix (3), is thought to play a role in the initial survival and proliferation of transformed cells. However, as a solid tumor grows and tumor cells acquire the potential to escape the primary site, the stroma becomes a more complex environment, with newly formed blood and lymphatic vessels and the recruitment and/or proliferation of lymphoid and myeloid immune cells (4). Immune cells are proposed to prevent tumor progression via the elimination of immunogenic tumor cells by T lymphocytes (CD8 subtype), a phenomenon known as immunosurveillance (also called immunoediting). During this process, tumors that display either reduced immunogenicity or enhanced immunosuppressive activity will escape elimination (5). Macrophages present in the tumor site can activate the immune response, but are mainly
SCIENCE sciencemag.org
thought to contribute to immunosuppression and tumor progression (6, 7), particularly in the mammary gland (8). A high density of macrophages in tumors is also associated with worse overall survival in patients with gastric, urogenital, and head and neck cancers, although it seems to be associated with better overall survival in patients with colorectal cancer (7). Franklin et al. carefully explore the contribution of macrophages to tumor growth in mice that develop a mammary cancer
that is genetically driven by the expression of an oncogene. In investigating the development and differentiation of macrophages in the normal mammary gland and during the progression of a mammary tumor, the authors identify a population of macrophages that accumulates during tumor growth called tumor-associated macrophages (TAMs). These cells develop from bone marrow–derived cells with the characteristics of inflammatory monocytes, which are recruited to the tumor where they differentiate into macrophages and subsequently proliferate. Franklin et al. observed that when signaling by the protein Notch is prevented in these TAMs, their differentiation is blocked. Interestingly, TAMs are distinct from macrophages present in normal mammary tissue, which develop independently of Notch signaling. Depletion of TAMs led to a reduced tumor burden in the animal and increased the cytotoxic potential of T lymphocytes present in the primary tumor site. Thus, monocyte-derived Notch-dependent TAMs are critical for tumor growth in this mammary gland tumor model, at least in 23 MAY 2014 • VOL 344 ISSUE 6186
Published by AAAS
801
