Gyarub Zangbo        33.126°N, 87.079°E

Xizang, China   Find: 2020 Oct

Classification: Pallasite

History: In October 2020 exploration by Mr. G. Tulga on the Qinghai-Tibet Plateau in the Qiangtang uninhabited area NE of Gyarub Zangbo resulted in the discovery of many disaggregated fragments composed of olivine+metal at 33.126°N, 87.079°E, as well as a much larger single metal-rich specimen nearby at 33.127°N, 87.080°E. All of this material was purchased by Ziyao Wang in October and November, 2020.

Physical characteristics: The material is composed of dark brown olivine and metal in variable proportions from specimen to specimen.

Petrography: (A. Irving, UWS and P. Carpenter, WUSL) Coarse grained aggregate (mean grainsize ~5 mm) with simple mineralogy of olivine and metal (both kamacite and taenite).

Geochemistry: Olivine (Fa21.6-22.8, FeO/MnO = 61-72, N = 4).

Classification: Pallasite.

Gyarub Zangbo, reclassification

Gyarub Zangbo was announced in Met. Bull. 110 simply as a pallasite, without assignment to a pallasite group. In addition to the kamacite, taenite and olivine reported in the original description, J. Boesenberg (BrownU) and T. Gao (PMO) subsequently identified other minor phases including troilite, schreibersite, chromite, phosphates (stanfieldite and farringtonite), as well as pyroxenes. Additional mineral analyses (T. Gao and Y. Jiang, PMO; J. Boesenberg, BrownU) include olivine (Fa21.6±0.4, Fe/Mn = 66.9±3.9, n=41), orthopyroxene (Fs19.1±2.3Wo2.0±0.8, N=21), augite (Fs8.2±0.4Wo43.0±1.6, N=6) and two kinds of chromite (Mg# = 21.5±0.6, 8.0±3.2; Cr# = 87.8±0.6). Trace element compositions of the metal were published by Jiang et al. (2023) and Bosenberg et al. (2023). Oxygen isotopes measured by H. Bao (NJU) are δ17O = -1.294±0.006, -1.3210±0.0036; δ18O = 1.824±0.005, 1.7293±0.0046; Δ17O = -2.262±0.004, -2.2385±0.0060 (all in ‰). Oxygen isotopes measured by D. Ibarra [Ed: and R. Havel] (BrownU) are δ17O =-1.408; δ18O = 1.563; Δ17O = -2.230 (all in ‰). The updated total mass is now >200 kg.

These data taken together indicate that Gyarub Zangbo (GZ) is an ungrouped pallasite. Explanation (J. Boesenberg, BrownU, W. Hsu, PMO): Olivine (with Mg# of 21-22) in GZ is similar in composition to that in the Eagle Station (ES) pallasites, but with lower Fe/Mn ratios (55-65 in GZ vs 95-100 in ES), and is significantly distinct from PMG olivine compositions (Fa12). The presence of pyroxene in GZ contrasts with its complete absence in either ES or Main Group (PMG) pallasites. Oxygen isotopes for GZ plot along an array between values for the PMG and ES pallasites, but do not appear to overlap with values for any other ungrouped pallasites. Metal in GZ differs from that in IIIAB irons and instead shows affinity with IIF irons. Finally, the Ni content in GZ metal is higher than in metal in the IVB irons, ES, and PMG pallasites, and the Ga-Ge contents separate GZ from the PMG or ES groups.