Secular Evolution of Highly Siderophile Elements in the Oceanic Mantle Mantle section of the 497 Ma Lekaophiolite, Norway (left) and sheeted dike complex of the 1.95 Ga Jormuaophiolite, Finland (right) Richard J. Walker University of Maryland
Outline of Talk: • Overview of the oceanic mantle (“convecting” or “DMM” mantle). • Discuss why highly siderophile element abundances (HSE) and Os isotopes may play an important role in further characterizing the oceanic mantle. • Review current attempts to study the chemical evolution of the oceanic mantle using HSE as accessed via mantle peridotites.
Oceanic Mantle The structural, chemical and isotopic properties of the oceanic mantle are important parameters for understanding the long-term evolution of the mantle, as well as the current workings of Earth. Old view Less old view
Oceanic Mantle Heterogeneity Analysis of trace elements in MORB has revealed that that the oceanic mantle is chemically heterogeneous on meso and global scales. From: Shirey et al. (1987)
Oceanic Mantle Heterogeneity The level of long-term chemical heterogeneity in the oceanic mantle (as observed via basalt compositions) extends to isotopic distinctions among ocean basins. From Hofmann (2003 TOG)
1st Order Cause of Global Depletions Present in Oceanic Mantle Global “depleted” nature of oceanic mantle has been convincingly attributed to extraction of Continental Crust. From Hofmann (2003 TOG)
Causes of 2nd Order Oceanic Mantle Heterogeneities (enrichments) • Plume interactions. • Metasomatic processes. From: Hofmann (2003 TOG) Dunite channel in Lekaophiolite
Causes of 2nd Order Oceanic Mantle Heterogeneities (depletions) Effects of additional melt depletion events (reprocessing) at spreading centers. From: Liu et al. (2008)
Secular Evolution of Oceanic Mantle? Remains poorly defined. Are inflections in Nd isotope evolution real? Do ancient samples even derive from oceanic mantle? From Bennett (2003 TOG)
Most information about the oceanic mantle has come from basalts, but the basalt record is both incomplete for the present, and sparse for the ancient mantle. We must also directly interrogate mantle materials to answer some important questions about the mantle. Basalt Peridotite vs.
Key Questions • How are oceanic mantle heterogeneities physically manifested? • When did they originate? • How rapidly are they mixed away? From Helffrich & Wood (2001)
Palette of Mantle Materials to Work With • Abyssal peridotites • Mantle sections of ophiolites • (including some peridotite massifs)
Penalties • Abyssal peridotites are usually highly altered & do not permit the study of field relations. • Ophiolite peridotites are also usually highly altered, and can be contaminated with subduction zone materials.
Studies of Abyssal Peridotites & Ophiolites Reveal…. Key Observation: Isotopic compositions of mantle peridotites range to well outside those of MORB.
Is This Important? As much as 10% of the oceanic mantle may be relatively unexplored from a chemical and isotopic standpoint!
Highly Siderophile Elements (HSE) Include: Re, Os, Ir, Ru, Pt, Rh, Pd, Au Typical 1 bar metal-silicate D values of >10,000. Relative abundances of HSE in primitive mantle are in roughly chondriticrelative abundances. Figure from Cottrell
Highly Siderophile Element Isotopic Systems The HSE includes the Re-Os radiogenic isotope system. Crustal evolution • 187Re 187Os + b- • t½ = 41.6 x 109yr Chondritic Evolution Evolution of residual mantle
Why study HSE in mantle rocks? • High abundance in mantle relative to crust (especially for Os, Ir & Ru). Typically little affected by fluid transport and serpentinization. • Geochemical behavior of HSE resulting from mantle processes (melting, metasomatism) are generally well understood. • Os isotopes may allow new constraints on melt depletion history of portions of the oceanic mantle.
Some facts of life about the Geochemistry of HSE in the Mantle • Os, Ir, Ru tend to reside in sulfides (e.g., Mss) and alloys that remain stable during low extents of partial mantle melting. • Pt, Pd and Re tend to reside in base metal sulfides that are removed via minor extents of partial melting.
Os, Ir and Ru typically compatible. • Pt, Pd and Re typically incompatible. • Os/Ru highly affected by metasomatism Melt depletion Primitive mantle normalized patterns for Chinese xenoliths from Liu et al., 2011
HSE reside in isolated “islands” within the mantle. • They are not constantly homogenized via diffusion, despite high temperatures (as appears true for Nd, Hf, Sr). Retain melt history. Laurite
Osmium Isotopic Model Ages Osmium TRD model ages can be used to constrain the age of a substantial melting event. Chondritic Evolution TRD Model Age
Important Questions HSE May Help to Answer • Are events that are not manifested in lithophile elements recorded by HSE? • How and when were the enriched and depleted domains present in the oceanic mantle created, and what is their physical form? • Has the intensity of such processes as melting, metasomatism and convective mixing in the mantle changed through time?
Average Os isotopic composition of abyssal peridotites is lower than chondritic average, and lower than Primitive Mantle estimates.
AP offset from chondritic average likely reflects a long-term melt (Re) depletion history for the oceanic mantle.
Os isotopic compositions of abyssal peridotites from the Gakkel Ridge are highly variable (Liu et al., 2008) and show weak trend between Os IC and Al2O3, suggesting a correlation between 187Os/188Os and melt depletion history. Samples with lowest 187Os/188Os likely reflect a Proterozoic melt depletion event and subsequent isolation.
Other HSE? Liu et al. (2008) reported that HSE ratios for Gakkel Ridge peridotites that show characteristics of variable melt depletion, as well as characteristics that likely reflect bulk silicate Earth.
HSE Studies of Abyssal Peridotites Reveal… • Average 187Os/188Os is at the low end of the chondritic range, consistent with long-term melt (Re) depletion. • Evidence for ancient (mid-Proterozoic) melt depletion events is common in every suite of AP examined. • HSE ratio variations present in abyssal peridotites reflect variable melt depletion and metasomatic effects. Some ratios are suggestive of global characteristics of mantle HSE.
OphiolitePeridotites Tectonically obducted portions of oceanic lithosphere (at least in some instances). Most samples are captured by field work. Spatial relations of samples can be well constrained. Ophiolites span ages ranging from present to Archean.
Josephine Ophiolite, Oregon, USA (162 Ma) Large numbers of data for Os-Ir-Ru alloy grains show nearly Gaussian distribution. Chondritic Avg. @ 162 Ma From: Meibom et al. (2002)
TroodosOphiolite, Cyprus (90 Ma) Melt Depletion Metasomatism From: Büchl et al. (2002, 2004)
HSE Studies of Ophiolite Peridotites Reveal… • Ancient (mid-Proterozoic) melt depletion events common in every suite of ophiolite peridotites examined. • HSE ratio variations present in ophiolite peridotites reflect variable melt depletion and metasomatic effects.
Is Any of This Important? • Long term survival of ancient melt residues suggests convective mixing on the outcrop scale is not very effective in the oceanic mantle. • Evolution of Os isotopic composition of the oceanic mantle away from that of Primitive Mantle provides key insights into the proportion of recycled, unmixed oceanic crust present in the mantle today. • HSE document complex melting and refertilization processes occurring throughout the oceanic mantle.
TaitaoOphiolite, Chile (6 Ma) Osmium isotopic and HSE abundance results anchored to field observations. 187Os/188Os = 0.117 from Schulte et al. (2009)
Chondritic-like Os/Ir & Ru/Ir; but more variable Pt/Ir, Pd/Ir, and Re/Ir. Variations for Pd/Ir and Re/Ir broadly consistent with partial melting + re-enrichment via melt-rock reaction. Pt/Ir requires melt-rock reaction.
Correlation between Mg# of relict olivines in peridotites and bulk 187Os/188Os consistent with an ancient melt depletion event (similar to SCLM).
Taitaoperidotites define a crude trend consistent with a melt depletion age of ~ 1.6 Ga. A relict mantle isochron?
Multi-stage model required for TaitaoPeridotites Stage 1: Early melting event at ~1.6 Ga established long-term, variable Al2O3 (and Re/Os).
Multi-stage model required for TaitaoPeridotites Stage 1: Early melting (~1.6 Ga) established long-term, variable Re/Os.
Multi-stage model required for TaitaoPeridotites Stage 2: 1.6 billion years of variable growth in 187Os/188Os. Pseudo isochron (alumochron) present before melting at ridge ~6 million years ago.
Multi-stage model required for TaitaoPeridotites Stage 3: Remelting at ridge at ~ 6Ma, further lowers Al2O3. metasomatism Metasomatism may lead to increase in Al2O3, and change in 187Os/188Os.
TaitaoPeridotites Show Evidence for: • Variable extents of partial melting, at a minimum of two different times. • Recent melt/rock reactions apparent in HSE. • Similar HSE and Os isotopic systematics to abyssal peridotites.
Additional Questions • What is the scale and shape of early depleted zones? (resampling of Taitao w/ GPS needed !) • Did melt/rock reactions occur as a result of common processes at MOR, or elsewhere? • How did an ancient Re-Os pseudo isochron survive a second melting event?
JormuaOphiolite, Finland (1.95 Ga) Well preserved Proterozoic ophiolite. How similar/different is it to Taitao?
HSE Patterns for Jormua Peridotites HSE patterns for Jormuaperidotitesare “normal”. Depletions in Pd and Re are consistent with prior melt depletion history.
Secular Evolution of HSE in Oceanic Mantle? In comparison to Taitao, HSE patterns for Jormuaperidotites appear somewhat more depleted in incompatible HSE. No major change in melting or metasomatic histories. Jormua (1.95 Ma)