Concurrent magma mixing and crystallization processes revealed by clinopyroxene macrocrysts from Lamont guyot lavas in NW Pacific

• Published in: Lithos 2022-11, Vol.428-429, p.106833, Article 106833
• Main Authors: Wei, Xun, Zhang, Yan, Shi, Xue-Fa, Castillo, Paterno R., Xu, Yi-Gang
• Format: Article
• Language: English
• Subjects: Antecryst; Clinopyroxene macrocryst; Fractional crystallization; Magma mixing; Wake seamounts
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2022.106833

Minerals in volcanic rocks have been commonly used to reconstruct the physicochemical conditions of crystallization and magma chamber processes (magma mixing and recharge) prior to eruption which are fundamental to deduce the petrogenesis of volcanic rocks. Here we present mineral chemistry including in situ major-trace element and Sr isotopic data for clinopyroxene (Cpx) and plagioclase macrocrysts from lavas collected from Lamont guyot in the Wake seamount group, NW Pacific. The Cpx macrocrysts display normal and reverse zoning or complex zoning (e.g., combination of reverse, normal, sector and/or patchy zoning). The normally-zoned Cpx macrocrysts are euhedral and in FeMg equilibrium with the host rock compositions. In contrast, the cores of reversely- and complexly-zoned Cpx macrocrysts are in textural (e.g., resorption texture) and chemical (very low Mg#) disequilibrium with their host rock compositions. These reversed cores, however, form coherent compositional trends with and display parallel trace element patterns to the normally zoned Cpx crystals, suggesting that these resorbed cores are most likely antecrysts which crystallized from progenitor melts. The continuously changing major-trace element contents with Cpx Mg# values can be explained by crystallization of these Cpx crystals from melts that progressively fractionate Cpx, plagioclase and FeTi oxides. Together with those of the complexly-zoned plagioclase, the complex zoning patterns and large ranges of La/Yb and Sm/Yb ratios at a restricted Cpx Mg# range record magmatic processes of fractional crystallization and mixing in the magma plumbing system. The Cpx crystals from a single lava sample have various 87Sr/86Sr ratios (0.7025–0.7042) extending from HIMU (high μ = 238U/204Pb) to FOZO (focal zone) mantle components suggesting that the large 87Sr/86Sr variation in Cpx could be explained by mixing between HIMU and FOZO components. Our data suggest that frequent recharging of melts derived from different mantle sources and concurrent mixing and crystallization are important processes that shape the currently observed compositional variation of Lamont guyot lavas. •Textural and compositional zoning of Cpx from Lamont seamount lavas, NW Pacific•Cpx shows large trace element and 87Sr/86Sr variations at a restricted Mg# range.•Reverse and complex zoning textures of Cpx record magma recharge and mixing.•Mixing of multiple batches of HIMU- and FOZO-derived melts.•Concurrent mixing and crystallizatio