A heterogenous mantle and crustal structure formed during the early differentiation of Mars

Highly siderophile element abundances and Os isotopes of nakhlite and chassignite meteorites demonstrate that they represent a comagmatic suite from Mars. Nakhlites experienced variable assimilation of >2 Ga altered high Re/Os basaltic crust. This basaltic crust is distinct from the ancient crust represented by meteorites Allan Hills 84001, or impact-contaminated Northwest Africa 7034/7533. Nakhlites and chassignites that did not experience crustal assimilation were extracted from a depleted lithospheric mantle distinct from the deep plume source of depleted shergottites. The comagmatic origin for nakhlites and chassignites demonstrates a layered martian interior comprising ancient enriched basaltic crust derived from trace element-rich shallow magma ocean cumulates, a variably metasomatized mantle lithosphere, and a trace element-depleted deep mantle sampled by plume magmatism.

Methods
Major and trace element abundance determinations and sources

New major and trace element abundance data are reported for nakhlite Y-000802 and ALH 84001 (Table S2). The data were obtained in an identical way to previously reported data by our group on nakhlites and chassignites (e.g., [14,16,25,32]) at the Scripps Isotope Geochemistry Laboratory (SIGL). This included the digestion of a 50 mg aliquot from a larger mass of sample powder (1 to 2g) also used for Re-Os isotope and HSE abundance analysis. Complete digestion was accomplished by sealing the sample with Teflon-distilled 27.5M HF (4 mL) and 15.7M HNO3 (1 mL) in closed-capped Teflon beakers for >72 hrs on a hotplate at 150°C, along with total procedural blanks and terrestrial basalt and andesite reference materials (BHVO-2, BCR-2, BIR-1a, AGV-2). Samples were sequentially dried and taken up in concentrated HNO3 to destroy fluorides, followed by doping with indium to monitor instrumental drift during analysis and then diluted to a factor of 5,000. Trace-element abundances were determined using a ThermoScientific iCAP Qc quadrupole inductively coupled plasma mass spectrometer (ICP-MS) and all data are blank-corrected. Long-term reproducibility of abundance data is better than 6% for most elements, except for Mo, Te, and Se (>10%).

Osmium isotope and highly siderophile element (Re, Pd, Pt, Ru, Ir, Os) abundances

Osmium isotope and HSE abundance analyses were performed at the SIGL on precisely weighed aliquots of homogenized powder that were then digested in 10 cm sealed borosilicate Carius tubes with isotopically enriched multi-element spikes (99Ru, 106Pd, 185Re, 190Os, 191Ir, 194Pt), and 7 mL of a 1:2 mixture of multiply Teflon distilled HCl and HNO3 purged of excess Os by repeated treatment and reaction with H2O2. Samples were digested to a maximum temperature of 270˚C in an oven for 72 hours. Osmium was triply extracted from the acid using CCl4 and then back extracted into HBr, prior to purification by micro-distillation. Rhenium and the other HSE were recovered and purified from the residual solutions using standard anion exchange separation techniques (47). Isotopic compositions of Os were measured in negative-ion mode using a ThermoScientific Triton thermal ionization mass spectrometer in peak-jumping mode on the secondary electron multiplier. Rhenium, Pd, Pt, Ru, and Ir were measured using a Cetac Aridus II desolvating nebuliser coupled to a ThermoScientific iCAPQc ICP-MS. Offline corrections for Os involved an oxide correction, an iterative fractionation correction using 192Os/188Os = 3.08271 and assuming the exponential law, a 190Os spike subtraction, and an Os blank subtraction. Precision for 187Os/188Os, determined by repeated measurement of a 35 pg UMCP Johnson-Matthey standard solution was better than ±0.2% (2 SD; 0.11381 ±12; n = 12). Rhenium, Ir, Pt, Pd, and Ru isotopic ratios for sample solutions were corrected for mass fractionation using the deviation of the standard average run on the day over the natural ratio for the element. External reproducibility for HSE analyses was better than 0.5% for 0.5 ppb solutions and all reported values are blank corrected. The total procedural blanks (n = 3) run with the samples gave 187Os/188Os = 0.150 ± 0.012, with quantities (in picograms) of 2.5 [Re], 27 [Pd], 3.1 [Pt], 16 [Ru], 0.3 [Ir] and 0.3 [Os]. Blanks contributions are listed in Table S3 and resulted in negligible corrections for most elements within samples (<5%), unless noted in the table.