Abstract The processes and time scales of magma mixing in shallow silicic magma chambers prior to eruptions in the Okinawa Trough (OT) are still poorly understood. Here, we employed in situ major-trace elements and Sr isotopic analyses, coupled with textural investigations, on plagioclase phenocrysts in the mafic magmatic enclaves (MMEs) and their host dacites from the southwestern OT to address this issue. The plagioclase phenocrysts in the MMEs have high An% contents (82–88) and Sr/Ba ratios (7.2–17.2) and relatively unradiogenic 87Sr/86Sr ratios (0.70536–0.70595), suggesting that they crystallized in the chilled basaltic andesitic magma. In contrast, plagioclase phenocrysts in the host dacites show a wide composition range, with An% contents of An47 to An93, Sr/Ba ratios of 3.4–18.0, and 87Sr/86Sr ratios of 0.70535–0.70791. The plagioclase phenocrysts are zoned, with high-An% cores characterized by resorption textures and mantled by euhedral low-An% rims. The transition between cores and rims is very abrupt, with a decrease of 30 to 40 mol% An. This shift coincides with large and abrupt changes in Sr/Ba ratios. The plagioclase phenocryst rims have low Sr/Ba values (3.4–5.1) and 87Sr/86Sr ratios (0.70632–0.70675) that are similar to those of the whole-rock dacites, suggesting that the plagioclase rims grew from the host dacitic magma. In contrast, Sr isotopes in the plagioclase cores are either radiogenic (0.70724–0.70791) or unradiogenic (0.70535–0.70595) compared with rims, combined with glomerocryst textures, consistent with their derivation from distinct deep mafic crystal mush zones, and were then entrained by ascending melts and transported rapidly to the shallow silicic magma chamber and underwent decompression during ascent, leading to plagioclase core resorption. Mg diffusion calculations indicate that incorporation of the calcic cores into the shallow silicic magma chamber experienced a long pre-eruptive storage (~600 years). The distinct Sr isotopes of the plagioclase, particularly the different compositions of titanomagnetite in the MMEs and host dacites, reflect the mafic magma injection into the silicic magma chamber and formation of the MMEs must have occurred over very short time scales before eruption to inhibit complete re-equilibration, which likely triggered the dacitic magma to finally erupt. Hence, the silicic magma in the southwestern OT contains a crystal cargo with a complex, open-system crystallization history and that magma mixing/mingling is an important process for controlling the chemical and textural diversity of these silicic magmas as well as a likely eruption trigger.

中文翻译：

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冲绳海槽西南部英安岩斜长石中的元素和 Sr 同位素分带：对岩浆混合过程和时间尺度的洞察

摘要 冲绳海槽（OT）喷发前浅层硅质岩浆房中岩浆混合的过程和时间尺度仍知之甚少。在这里，我们采用原位主要痕量元素和 Sr 同位素分析，再加上质地调查，对基性岩浆飞地 (MME) 中的斜长石斑晶及其来自西南部 OT 的寄主英安岩进行了研究，以解决这个问题。MME 中的斜长石斑晶具有高 An% 含量 (82-88) 和 Sr/Ba 比值 (7.2-17.2) 和相对无辐射的 87Sr/86Sr 比值 (0.70536-0.70595)，表明它们在冷的玄武质岩浆岩中结晶。相比之下，寄主英安岩中的斜长石斑晶显示出较宽的组成范围，An% 含量为 An47 至 An93，Sr/Ba 比率为 3.4-18.0，87Sr/86Sr 比率为 0.70535-0.70791。斜长石斑晶被分区，具有以吸收纹理为特征的高 An% 核，并被自形低 An% 边缘覆盖。核心和轮辋之间的过渡非常突然，An 减少了 30 至 40 mol%。这种转变与 Sr/Ba 比率的大而突然的变化相吻合。斜长石斑晶边缘具有较低的 Sr/Ba 值 (3.4-5.1) 和 87Sr/86Sr 比值 (0.70632-0.70675)，与全岩英安岩相似，表明斜长石边缘是从宿主英安岩浆中生长出来的。相比之下，与边缘相比，斜长石核心中的 Sr 同位素要么是放射成因 (0.70724-0.70791) 要么是非放射成因 (0.70535-0.70595)，结合肾小球晶体纹理，与它们源自不同的深基性晶体糊状区一致，然后被上升的熔体夹带并迅速输送到浅层硅质岩浆房，并在上升过程中减压，导致斜长石岩心吸收。镁扩散计算表明，将钙质核并入浅层硅质岩浆房经历了长时间的喷发前储存（约 600 年）。斜长石的独特 Sr 同位素，特别是 MME 和宿主英安岩中钛磁铁矿的不同成分，反映了镁铁质岩浆注入硅质岩浆房，MME 的形成必须在喷发前的很短时间尺度内发生，以抑制完全重新-平衡，这很可能触发英安质岩浆最终喷发。因此，OT 西南部的硅质岩浆含有一种复杂的晶体物质，