Iran appointed a new head of its central bank on Thursday after the currency crashed to its lowest level ever against the dollar amid mass protests and ongoing Western sanctions.  Mohammad Reza Farzin, 57, a senior banker and former deputy finance minister, was tapped to replace Ali Salehabadi, who resigned after 15 months at the post, the official IRNA news agency reported.  The rial was trading at around 430,000 to the dollar on Thursday, down from 370,000 earlier this month. Already battered by years of Western sanctions over Iran's nuclear program, the rial was trading at 315,000 when anti-government protests erupted in mid-September.  The protests were ignited by the death of a woman who was detained by the country's morality police. The demonstrations rapidly escalated into calls for an end to more than four decades of clerical rule.  At least 508 protesters have been killed and more than 18,600 people have been arrested, according to Human Rights Activists in Iran, a group that has closely monitored the unrest. Iranian authorities have not provided an official death toll.  Iran’s currency was trading at 32,000 rials to the dollar at the time of the 2015 nuclear accord that lifted international sanctions in exchange for tight controls on Iran’s nuclear program. That deal unraveled after then-President Donald Trump unilaterally withdrew the United States from it in 2018.  The Biden administration had been trying to restore the agreement up until the protests broke out, but those talks hit a deadlock several months ago.