Insights into the Evolution of the Solar System from Isotopic Investigations of Samples. Lars Borg

Transcription

1 Insights into the Evolution of the Solar System from Isotopic Investigations of Samples Lars Borg

2 Harold Masursky Harold Masursky was a stalwart of the U.S. planetary exploration program for nearly three decades, a key participant in nearly every mission from the pre-apollo era of lunar exploration through Voyager, Pioneer Venus, and Magellan. He is remembered as much for his passion for explaining planetary science to the public as for the innumerable working groups and planning committees on which he strived tirelessly to ensure that the scientific return from every mission on which he worked was maximized. Sean Solomon (2001)

3 Pillars of Scientific Exploration of the Solar System Remote Observations What Sample Analysis When & What Theoretical Investigations & Analog Studies How

4 Changing role of sample analysis Scientific questions being asked are becoming progressively more sophisticated Some can only be addressed with samples Technology development Remote measurements often interpreted in the context of sample data Missions to return samples to Earth more feasible However, samples are variably suited to address specific questions Goal: Illustrate the strengths and weaknesses of sample analysis to constrain Solar System evolution

5 Martian Meteorites Figures from Shih et al. (1982) First isochrons yielded ~180 Ma Rb-Sr ages and disturbed Sm-Nd isotopic systematics Suggested samples derived from large body and ages recorded either: ~180 Ma metamorphic event (true age was either 1300 Ma or 4500 Ma) or ~180 Ma age was crystallization event

6 Technique Development Leads to Clearer Insights Development of better analytical techniques led to concordant ages from multiple isotopic systems Required ~15 years of analytical development Strongly suggests ages recorded a crystallization event Indicates Mars likely to be volcanically active today Borg et al (1997) Borg et al (1997) QUE94201

7 More Insights from Sample Suites 146 Sm- 142 Nd isotopic measurements suggest shergottites formed at same time Planetary differentiation in a magma ocean? The Nd isotopic composition of Mars appears to be closer to Earth than to primitive meteorites ε 142 Nd Source (present-day) Age = /-5.0 Ma 142 Nd\ 144 Nd= ± 2 Slope = ± 52 MSWD = Earth Chondrites Borg et al (in review) ε 143 Nd Source (present-day) 0.32

8 Petrogenetic Relationships Can Be Developed for a Sample Suite Old Shergottites (~340 Ma) Old Shergottites (>474 Ma) Young Shergottites (~175 Ma) Low Mg# Basalts Ol-Px Cumulates High Mg# Basalts Ol-Px Cumulates Ol Cumulates Ol-Px Cumulates Ol Cumulates Low Mg# Basalts Ol Cumulates Enriched Mantle (contains trapped liquid) Enriched Source Intermediate Source Depleted Mantle Source Depleted Suite Intermediate Source Enriched Source Compositions Depleted Basaltic Shergottites Olivine-Opx Shergottites Olivine Shergottites Symes et al (2008)

9 Summary Significant insights into planetary scale process from a single sample Present-day martian volcanism likely Mantle source region differentiation occurred early Suites of related samples dramatically increases the constraints on planetary evolution processes Provide petrogenetic framework for the interpretation of planetary geology More precise, less model dependent ages of differentiation Difficult to determine physical size and location of planetary reservoirs from samples

10 Background for the Study of Lunar Samples Petrogenetic framework for lunar differentiation is the magma ocean model Deduced from: Widespread anorthosites on surface Composition of anorthosites and KREEP-rich lunar samples Experimental investigations of basalt source regions Predicts: Anorthosites, KREEP, & mafic cumulates should be same age Anorthosites older than Mg-suite rocks

11 Rb-Sr, Sm-Nd, & U-Pb Highland Rock Ages All ages from 1975 to present No temporal distinction between anorthosite and Mgsuite samples Implies extended period of lunar crust formation involving contemporaneous production of multiple crustal lithologies Single samples yield multiple, discordant ages , / , , Y Mg-Suite FANs Borg et al. (2014) Age (Ma)

12 New Techniques Permit Concordant Ages to be Determined Using Multiple Isotopic Systems Orange circles are average of ages determined in single samples using multiple isotopic systems 146 Sm 142 Nd ages and 147 Sm 143 Nd ages determined on same samples 146 Sm 142 Nd t 1/2 = 103 Ma is ideal to identify samples older than 4.45 Ga , / , , Mg-Suite Only recently applied to lunar chronology due to technical challenges Ages imply anorthosite and Mg-suite magmatism was contemporaneous around 4.35 Ga Y FANs Age (Ma) Borg et al. (2011, 2014), Absts. Gaffney et al. (this meeting), Marks et al. (2013)

13 Ramifications for Lunar Evolution Best rock ages are contemporaneous with ages defined for several other lunar lithologies and reservoirs Some of these reservoirs are thought to reside in the lunar mantle Implies there was widespread igneous activity in mantle and crust around 4.30 to 4.37 Ga Age of LMO solidification or other widespread igneous process? Mg-suite 78236, 76535, 77215, Crystallization Ages Peak Zircon Age Mare Basalt Source Model Age Average ur- KREEP Model Age Age (Ma) FAN Crystallization Ages Borg et al. (2014), Gaffney & Borg (2014)

14 Summary New techniques can be applied to old samples Lunar crustal rock ages do not support classic Lunar Magma Ocean Model Ages provide basis for development of more sophisticated petrogenetic models Non-LMO anorthosites Late LMO crystallization Caveat Only a few samples are amenable to chronology so results are constrained by sampling bias

15 Initial Solar System Isotopic Composition Chondritic Meteorites LL L H EH EL C Boyet & Carlson, 2005; Carlson et al., 2007; Andreasen & Sharma, 2006; Gannoun et al., 2011; LLNL unpublished data ε 142 Nd T e r r e s t r i a l V a l u e Problem: Why do Earth and chondritic meteorites have different heavy element isotopic compositions? Significance: Heavy elemental composition of chondrites thought to be representative of planets Key assumption for theoretical studies of planet formation and chronological investigations of planetary differentiation

16 New Capability Applied to Old Samples (1977) Marks et al, (2014)

17 142 Nd Evolution Plot planetary bodies 1. T-I diagram all 146 Sm- 142 Nd isochron data for planetary reservoirs 2. Evolution curves calculated assuming 147 Sm/ 144 Nd = Red = calculated from terrestrial 142 Nd/ 144 Nd Yellow = calculated from chondritic 142 Nd/ 144 Nd 3. Data for CAI, Moon, and Mars fall closer to the terrestrial evolution curve 4. Indicates initial 142 Nd/ 144 Nd of these bodies is more similar to Earth than to chondritic meteorites

18 Conclusions Three facets of Solar System science: (1) Remote Observations (what) (2) Theoretical and analog investigations (how) (3) Sample analyses (when and what) Strengths of samples analysis: Single samples can provide fundament information on planetary evolution Sample suites provide basis of development of broad scale petrogenetic models Only mechanism to address temporal relationships Allows new measurements to be completed as scientific theories mature and as analytical capabilities increase Weakness of sample analysis: Difficult to place physical constraints on planetary reservoirs Rosetta stone effect often can t predict which samples will be the most useful to address specific questions Adopting an exploration strategy that employs all facets of exploration science has been the basis for maximizing scientific yield

19 Co-authors Relevant to Discussion Ian Hutcheon Amy Gaffney Naomi Marks Larry Nyquist Chip Shearer James Connelly Ben Jacobsen Rick Carlson T. Kruijer Steven Symes Maud Boyet G. Brennecka Thorsten Kleine Carl Agee C. Burkhardt T. Kayzar Chi-Yu Shih

20 LLNL Auspices Statement This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA LLNL release number: LLNL-PRES