Global variation in Cr-spinel trace elements

The major element composition of Cr-spinel [(Mg,Fe 2+ )(Cr,Al,Fe 3+ ) 2 O 4 ] has long been used to infer the petrogenesis and tectonic setting of its host rocks. However, overlaps in major element compositions and susceptibility of Mg and Fe 2+ to secondary alteration limit its diagnostic utility. In contrast, some trace elements may be particularly diagnostic of certain petrogenic processes or retain primary signatures even when major elements are modified, offering a complementary tool for petrogenetic interpretation. However, a comprehensive evaluation of the systematics of trace elements in Cr-spinel has not yet been undertaken. This study compiles major and trace element data from Cr-spinel-bearing samples worldwide to assess global variability and to identify robust geochemical indicators. Based on a larger, more representative set of Mid-Ocean Ridge Basalt (MORB) spinel data we present new MORB spinel normalization values and a recommended element order for spinel multi-element plots. We document systematic variations in trace elements (Sc, Ga, V, Ni, Mn) with Cr# (Cr# = Cr/(Cr + Al) mol.), particularly in mantle-derived spinels, and show that these trends reflect mantle depletion. Melting models and regression of literature partition coefficients demonstrate that partitioning of Sc, Ga, Ni, Mn, and Ti is likely compositionally controlled. We also show that Cr-spinels from lavas and intrusions can be distinguished based on late-stage re-equilibration with interstitial melt, evident from the low Ru and Cu contents and Sc/Ga ratios in Cr-spinels from intrusions. Our findings highlight the potential of trace element systematics in Cr-spinel as a powerful tool for reconstructing the petrogenesis of mafic-ultramafic rocks. • New MORB normalization and element order for Cr-spinel multi-element plots. • Modeling Cr-spinel trace elements requires varying partition coefficients with Cr#. • Sc, Ga, V, Ni, and Mn in disseminated mantle Cr-spinels reflect mantle depletion. • Intrusion Cr-spinel is characterized by late equilibration with interstitial melt.